energies

Article Modelling a Hypersonic Single Expansion Ramp Nozzle of a Hypersonic Aircraft through Parametric Studies

Andrew Ridgway, Ashish Alex Sam * and Apostolos Pesyridis

College of Engineering, Design and Physical Sciences, Brunel University London, London UB8 3PH, UK; [email protected] (A.R.); [email protected] (A.P.) * Correspondence: [email protected]; Tel.: +44-1895-267-901



Received: 26 September 2018; Accepted: 7 December 2018; Published: 10 December 2018


Abstract: This paper aims to contribute to developing a potential combined cycle air-breathing engine integrated into an aircraft design, capable of performing flight profiles on a commercial scale. This study specifically focuses on the single expansion ramp nozzle (SERN) and aircraft-engine integration with an emphasis on the combined cycle engine integration into the conceptual aircraft design. A parametric study using computational fluid dynamics (CFD) have been employed to analyze the sensitivity of the SERN’s performance parameters with changing geometry and operating conditions. The SERN adapted to the different operating conditions and was able to retain its performance throughout the altitude simulated. The expansion ramp shape, angle, exit area, and cowl shape influenced the thrust substantially. The internal nozzle expansion and expansion ramp had a significant effect on the lift and moment performance. An optimized SERN was assembled into a scramjet and was subject to various nozzle inflow conditions, to which combustion flow from twin strut injectors produced the best thrust performance. Side fence studies observed longer and diverging side fences to produce extra thrust compared to small and straight fences.

Keywords: scramjet; single expansion ramp nozzle; hypersonic aircraft; combined cycle engines

1. Introduction Hypersonic flight is considered the next great advancement within the aviation industry. In recent years there has been an increase in demand for commercial supersonic and hypersonic travel as aerospace companies have been looking to replace long haul subsonic flights with hypersonic aircraft designs that could be economically and environmentally viable. More aerospace companies are investing in new engine technologies and developing concept aircraft designs capable of supersonic and hypersonic travel. Even though this has been investigated for over half a century, there have been only few breakthroughs which have made hypersonic flight available on a commercial level. This is mainly due to the challenges ranging from structural demands, fuel economy, maintenance issues, direct operating costs, and supersonic combustion. The most successful supersonic commercial airliner was the Concorde which cruised at Mach 2. Concorde’s retirement in 2003 was a combination of inflated fuel prices and the infamous accident in which 113 people lost their lives. The loss of confidence in the safety of the aircraft was the same reason for the Tupolev Tu-144’s early retirement, the only other commercial supersonic aircraft equivalent to the Concorde. Hypersonic vehicles have been around for several decades but have never been utilized for commercial aviation. Rocket-powered vehicles were the main bearers of hypersonic flight as, they would propel vehicles beyond the atmosphere and exploit gravity to re-enter the atmosphere, achieving

Energies 2018, 11, 3449; doi:10.3390/en11123449 www.mdpi.com/journal/energies Energies 2018, 11, x FOR PEER REVIEW 2 of 29 achieving phenomenal speeds of up to Mach 36. The V-2 rocket, designed by Germany, flew at five timesEnergies the2018 speed, 11, 3449of sound, the first vehicle to be considered hypersonic. Other vehicles to have2 offlown 29 at hypersonic speeds are intercontinental ballistic missiles, Apollo space capsules, the space shuttle, and the experimental X-planes [1]. There has never been a hypersonic commercial aircraft phenomenal speeds of up to Mach 36. The V-2 rocket, designed by Germany, flew at five times manufactured despite these vehicles exceeding Mach 5. the speed of sound, the first vehicle to be considered hypersonic. Other vehicles to have flown at hypersonicThough speedsthis being are intercontinentalthe case, numerous ballistic experimental missiles, Apollo unmanned space capsules, aircraft the have space attempted shuttle, and and achievedthe experimental hypersonic X-planes speed by [1]. various There has means never of beenpropulsion, a hypersonic makin commercialg hypersonic aircraft flight manufactured of Mach 5 and abovedespite a reality. these vehicles exceeding Mach 5. AirThough-breathing this hypersonic being the case, propulsion numerous can experimentalhelp put aircraft unmanneds into orbit aircraft or sustain have attemptedflight within and the atmosphereachieved hypersonic with lower speed weight by and various propulsion means of required propulsion, compared making to hypersonic rockets, as flight the ofoxidizer Mach 5 (air) and is takenabove from a reality. the environment as opposed to being carried onboard. Hypersonic air-breathing commercialAir-breathing flight will hypersonic potentially propulsion offer great can advancements help put aircrafts in reliability, into orbit reusability or sustain, and flight economies within of thescale atmosphere for high-speed with loweratmospheric weight cruis and propulsioning. These proposed required compared vehicles a tond rockets, methods as theof propulsion oxidizer remove(air) is the taken need from to carry the environment oxidizers and as other opposed limitations to being which carried rocket onboard. engines Hypersonic incur and air-breathing increase the possibilitycommercial of achieving flight will the potentially desired offerrange, great payload advancements, and efficiency in reliability, [2]. reusability, and economies ofSeveral scale for air high-speed-breathing atmospheric propulsion cruising.methods These like turbojet, proposed turbofan, vehicles andramjets methods, and ofscramjets propulsion have beenremove adopted the needin the to past carry to oxidizers utilize high and- otherspeed limitations air-breathing which flight. rocket The engines limitations incur and observed increase within the thepossibility turbojet are of achievingthat the maximum the desired rotational range, payload, speed of and the efficiency rotor and [2 ].the maximum temperature of the gases inSeveral front of air-breathing the turbine a propulsionffect the durability methods of like theturbojet, turbine blades. turbofan, Consequently, ramjets, and engine scramjets reliability have is riskedbeen adopted during inoperation the past and to utilize the flight high-speed speed is air-breathing limited [3]. flight. The limitations observed within theRamjet turbojet and are s thatramjet the engines maximum utilize rotational the “ram speed” effect, of the by rotor exploiting and the maximumthe high speeds temperature and dynamic of the gases in front of the turbine affect the durability of the turbine blades. Consequently, engine reliability air pressure to compress the intake air, and thus, avoid the need for rotational components [4]. In is risked during operation and the flight speed is limited [3]. addition, ramjets can only produce thrust when the vehicle is moving, meaning other propulsion Ramjet and sramjet engines utilize the “ram” effect, by exploiting the high speeds and dynamic air systems are necessary to accelerate the vehicle to a speed where the ramjet becomes operable (Mach pressure to compress the intake air, and thus, avoid the need for rotational components [4]. In addition, 2.5–3) [5–7]. ramjets can only produce thrust when the vehicle is moving, meaning other propulsion systems are The scramjet (supersonic combustion ramjet) is a variant of the ramjet engine and similarly relies necessary to accelerate the vehicle to a speed where the ramjet becomes operable (Mach 2.5–3) [5–7]. on the vehicleThe scramjet’s speed (supersonic to compress combustion the air prior ramjet) to combustion. is a variant of Scramjets the ramjet are engine usually and utilized similarly at relies Mach 5 andon theabove vehicle’s because speed of tothe compress ramjet’sthe inability air prior to toovercome combustion. the Scramjetsdrag at Mach are usually 5 (Figure utilized 1). In at scramjets, Mach 5 theand airfl aboveow remains because supersonic of the ramjet’s as it inabilitypasses through to overcome the engine the drag including at Mach the 5 (Figure combustor1). In, scramjets, whereas in thethe ramjet airflow, flow remains is decelerated supersonic to assub itsonic passes velocity through for the combustion. engine including A scramjet the combustor, operates whereasefficiently in at hypersonicthe ramjet, speeds flow is and decelerated allows tofor subsonic supersonic velocity combustion for combustion. by compressing A scramjet operatesthe incoming efficiently airflow at withouthypersonic decelerating speeds and it bel allowsow Mach for supersonic 1. Although, combustion a number by compressingof research tests the incoming have been airflow performed without on scramjets,decelerating there it belowis still Macha long 1. Although,way to go a for number this oftechnology research tests to be have commercialized been performed [7,8 on]. scramjets, A typical scramjetthere is consists still a long of waya converging to go for this inlet, technology isolator, to combustion be commercialized chamber [7,,8 ].and A typicala divergi scramjetng nozzle. consists The basicof aschematic converging and inlet, the isolator, combustion combustion process chamber, in a scramjet and a engine diverging are nozzle. described The in basic deta schematicil by Neill and and Pesyridisthe combustion [4]. process in a scramjet engine are described in detail by Neill and Pesyridis [4].

Figure 1. Operation envelopes of air breathing engines. Figure 1. Operation envelopes of air breathing engines.

Energies 2018, 11, 3449 3 of 29

Air-breathing hypersonic vehicles require a scramjet system in a combined-cycle propulsion architecture that also involves conventional gas turbines and ramjets. The reason for this is that each engine can operate within a different flight speed regime and all three are required for successful operation at hypersonic speeds [2].

1.1. SERN The single expansion ramp nozzle (SERN) is an essential component for air-breathing hypersonic vehicles to achieve maximum thrust. The SERN is an essential component of the scramjet engine due to its qualities of minimizing weight and frictional drag while producing substantial thrust from high-pressure flow, reflected in its roles in the X-43 vehicle. The SERN operates by only having the gas pressure working on one side, unlike the traditional, axial symmetric nozzles [9]. Single expansion ramp nozzles can be comparable to a single-sided aerospike engine due to its single expansion ramp. Successful hypersonic flights have incorporated the SERN into the scramjet engine such as the X-43. More advanced nozzles are utilized at high speeds and are blended into the airframe which can reduce weight at large expansion ratios, reduce drag, controllability with thrust vectoring, and improve reliability [10]. The SERN shows self-adaptability at off-design conditions by managing the changing static pressure over a range of Mach numbers. This adaptability reduces the need to vary the exit area over different operating conditions. The SERN internal thrust is virtually unaffected by external flow if the pressure ratio remains high but at low-pressure ratios, the nozzle flow field is strongly influenced by external flow [11]. The expansion ramp is one with the aircraft fuselage, producing complex problems such as limited thrust vectoring controlled by throttling. This requires more control of elevators and complex control systems. In addition, flow leakage and cooling are issues. However, integrating the SERN into the airframe reduces the drag of the propulsion system [12]. Single expansion ramp nozzles tend to be more efficient than conical nozzles and produce less drag as the bottom surface is non-existent. Divergence loss needs to be considered within the SERN design. Larger angles of divergence will increase the flow speed due to the production of expansion fans but the streamlines at the nozzle exit will be at different angles relative to the flight path [8]. This paper aims to analyze the performance of a SERN for a scramjet engine and to optimize its performance and integrate the refined design into a scramjet engine. This scramjet engine will be constructed into a combined cycle engine design to be integrated with an aircraft. The paper will provide insight into performance characteristics of SERN, scramjets, and integration techniques of an aircraft and engine through 2D and 3D computational fluid dynamics (CFD) analysis.

1.2. SERN Analysis Wind tunnel experiments have investigated SERN’s performance at different altitudes and speeds capturing data through pressure measurements and Schlieren photographs [9,13,14]. Replicating the experimental data and hypersonic phenomena in CFD is complex and challenging as it has to combine all the variables of external and nozzle flow. Numerical modelling studies of SERN’s have been conducted in Reynolds-Averaged Navier-Stokes (RANS) for various geometry configurations and operating conditions using a variety of models. Reduced order CFD is accurate in predicting plume and flow features such as expansion fans but lacks accuracy in modelling boundary layers [15,16]. Single expansion ramp nozzles have been researched thoroughly including interaction between the nozzle exhaust and external flow [17]. Data mining studies have highlighted the rarity of optimizing the SERN geometry based on sensitivity analysis to highlight the design variables which have minimal impact on improving optimization efficiency. The length of the inner nozzle, cowl expansion, and expansion ramp have a significant influence on thrust and lift [18]. FLUENT was used to investigate SERN divergent angles, length, area ratios, cowl angles, and scramjet nozzle performance [19], and another study investigated ramp angles and side fence influence [20]. The nozzle Energies 2018, 11, 3449 4 of 29 thrust augmentation [21] and the aforementioned CFD investigations utilized the two equation turbulenceEnergies 2018 models, 11, x FOR of PEER either REVIEW K- or shear stress transport (SST) K-ω due to the robustness4 of 29 and accuracy in modelling high-speed exhaust flows. A few complicating factors in analyzing hypersonic air-breathingand accuracy nozzles in modelling are continuing high-speed chemical exhaust reactions, flows. A flowfew separation,complicating flow factors unsteadiness, in analyzing and cooling,hypersonic which air are-breathing similar parametersnozzles are analyzed continuing in rocket chemical nozzles. reactions, Computational flow separation, fluid dynamics flow RANSunsteadiness codes have, and developedcooling, which into are robust, similar high-fidelity parameters analyzed models whichin rocket are nozzles. capable Computational of numerically simulatingfluid dynamic the complexs RANS flowcodes characteristics have developed of into hypersonic robust, high flows-fidelity [22–24 models]. which are capable of numerically simulating the complex flow characteristics of hypersonic flows [22–24]. 2. Methodology 2. Methodology 2.1. Geometry 2.1. Geometry The 2D geometry for the scramjet nozzle, scramjet engine, and the integrated aircraft and combinedThe cycle2D geometry engine was for created the scramjet using ANSYSnozzle, designscramjet modeler engine, (Figureand the2). integrated aircraft and combinedA parametric cycle engine analysis was of created the single usin expansiong ANSYS design ramp nozzlemodeler was (Figure performed 2). at various operating conditions.A parametric Table1 analysis shows of the the different single expansion geometric ramp parameters nozzle was andperformed the corresponding at various operating values conditions. Table 1 shows the different geometric parameters and the corresponding values analyzed. analyzed. The final design will be selected based on the thrust capabilities of the nozzle. All the The final design will be selected based on the thrust capabilities of the nozzle. All the geometry geometry components parameterized were changed simultaneously rather than one component components parameterized were changed simultaneously rather than one component being being investigated individually, as this will help to understand its effect on the SERN performance. investigated individually, as this will help to understand its effect on the SERN performance. Cowl Cowl length was not varied as the design objective was to use the minimum amount of material as length was not varied as the design objective was to use the minimum amount of material as possible possiblefor lower for drag lower and drag hence and the hence shortest the shortestlength was length opte wasd. opted.

FigureFigure 2. 2. 2D2D geometries geometries of of(a) (anozzle,) nozzle, (b) scramjet (b) scramjet engine engine,, and (c) and aircraft (c) and aircraft scramjet and engine scramjet engineintegration integration..

TheThe 3D 3D geometry geometry of of the the nozzlenozzle waswas alsoalso developeddeveloped using using ANSYS ANSYS Design Design Modeler Modeler and and the the geometriesgeometries for for the the scramjet scramjet engine engine and and thethe integratedintegrated air aircraftcraft and and combined combined cycle cycle engine engine was was created created usingusing Solidworks Solidworks (Figure (Figure3 ).3). Periodic Periodic boundary boundary conditionsconditions were employed employed to to all all the the geometries, geometries, to to improveimprove the the computational computational efficiency. efficiency.

TableTable 1. 1.2D 2D nozzle nozzle geometric geometric parametersparameters with corresponding values values investigated investigated.. Geometrical Parameters Parameter ID Range Geometrical Parameters Parameter ID Range Nozzle inlet height (m) P1 1, 1.5, 2 NozzleNozzle inlet outlet height height (m) (m) P1P2 5.3, 7.3, 1, 1.5,9.3 2 ExpansionNozzle outlet ramp height contour (m) radius (m) P2P3 50–5000 5.3, (conical) 7.3, 9.3 Expansion ramp contour radius (m) P3 50–5000 (conical) Initial expansion ramp length (m) P4 1.6, 3.2 Initial expansion ramp length (m) P4 1.6, 3.2 InitialInitial expansion expansion ramp ramp angle angle (degree) (degree) P5P5 10, 10,30 30 NozzleNozzle and afterand after body body length length (m) (m) P6P6 30, 30,40 40 CowlCowl length length (m) (m) P7P7 3 3 CowlCowl thickness thickness (m) (m) P8P8 0.3 0.3 CowlCowl ramp ramp angle angle (degree) (degree) P9P9 5.72, 5.72, 33 33

Energies 2018, 11, 3449 5 of 29

EnergiesEnergies 20182018,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 2929

FigureFigure 3.3. 3D3D3D geometriesgeometries geometries ofof ( of(aa)) nozzle, (nozzle,a) nozzle, ((bb)) scramjetscramjet (b) scramjet engine,engine, engine, andand ((cc and)) aircraftaircraft (c) aircraft andand scramjetscramjet and scramjet engineengine integration.engineintegration. integration.

2.22.2.2.2.. Mesh Mesh FigureFigure 4 44 shows showsshows the thethe 2D 2D2D mesh meshmesh for forfor the thethe nozzle, nozzle,nozzle, scramjet scramjetscramjet engine, engineengine,, and andand integratedintegratedintegrated aircraftaircraftaircraft and andand scramjet scramjetscramjet engine.engine. The The specifications specificationsspecifications of of the the 2D 2D mesh mesh employed employed is is described described in in Table Table 2 2.2.. Each EachEach mesh meshmesh underwent underwentunderwent meshmesh refine refinementrefinementment and and convergence convergence studies. studies. The The mesh mesh for for the the nozzle nozzle parametric parametric studies studies was was setup setup to to ensureensure thatthat thethe mesh mesh adapted adapted toto each each change change in in geometry. geometry. TheThe 2D2D mesh mesh did did not not reduce reduce the the orthogonal orthogonal qualityquality belowbelow 0.1 0.1 and and the the skewnessskewness remainedremained minimal,minimal, facilitatingfacilitating accuraccur accurateateate resultsresults results forfor for eacheach each geometry.geometry. geometry. EdgesEdges were were blendedblended toto ensureensure thethe meshmesh qualityquality waswas notnot degradeddegraded throughthrough sharpsharpsharp edges.edges.edges. ProximityProximity andand curvaturecurvature meshingmeshing techniquetechnique waswas appliedapplied toto ensureensure thatthat thethe 3D3D meshmesh waswas ofof high high qualityquality betweenbetween thethe wallswalls andand thethe curved curved surface surface ofof the the expansion expansion ramp.ramp. Inflation InflationInflation layers layers were were added added alongalong thethe wallswalls ofofof the thethe nozzle nozzlenozzle to toto capture capturecapture the thethe flow flowflow at atat the thethe boundary boundaryboundary layer layerlayer accurately. accurately.accurately. The TheThe 3D 3D mesh3D meshmesh for theforfor thedifferentthe differentdifferent components componentscomponents is shown isis shownshown in Figure inin FigureFigure5 and 55 theandand mesh thethe meshmesh statistics statisticsstatistics are described areare describeddescribed in Table inin TableTable3. 3.3.

Figure 4. Sections of 2D mesh for the nozzle (top left), scramjet engine (top right), and integrated Figure 4. SectionsSections of of 2D 2D mesh mesh for for the the nozzle ( toptop left left), scramjet engine engine ( (toptop right )),, and integrated aircraft and scramjet engine ( ). aircraft and scramjet engine ( bottombottom))..

Energies 2018, 11, 3449 6 of 29

Energies 2018, 11, x FOR PEER REVIEW 6 of 29 Table 2. 2D mesh statistics. Table 2. 2D mesh statistics. Parameters Nozzle Only Scramjet Integrated Scramjet Parameters Nozzle Only Scramjet Integrated Scramjet ElementsElements 93,919–151,95693,919–151,956 360,702 360,702 453,185 453,185 Average skewness 0.432 0.617 0.138 Average skewness 0.432 0.617 0.138 Average orthogonal quality 0.895 0.830 0.973 Average orthogonal quality 0.895 0.830 0.973

Figure 5. 3D mesh for the nozzle ( toptop),), scramjet scramjet engine engine ( (middle)),, and integrated aircraft and scramjet engine (bottom).).

Table 3. 3D mesh statistics.statistics.

ParametersParameters NozzleNozzle Only Only Scramjet Scramjet Integrated Integrated Scramjet Scramjet Engine Engine ElementsElements 3,325,4053,325,405 4,524,968 4,524,968 5,591,991 5,591,991 AverageAverage skewness skewness 0.2590.259 0.234 0.234 0.215 0.215 AverageAverage orthogonal orthogonal quality quality 0.8580.858 0.821 0.821 0.866 0.866

2.3.2.3. Numerical Methodology The 2D and 3D CFD analysis of the compressible flowflow was performed using the Reynolds Averaged Navier–StokesNavier–Stokes equation. The use of the RANS equations was found to produce accurate results for the performance analysis of hypersonic propulsion components components and and s systemsystems [9,25 [9,25––2828].]. Density-basedDensity-based coupled solver was was utilized utilized as as this this will will provide provide strong strong couplings couplings between between density, density, energy, momentum,momentum, andand speciesspecies presentpresent inin hypersonichypersonic flow.flow. The shear-stressshear-stress transporttransport (SST) k k--ω model was chosen over the standard model as it uses a blended function to activate the k-k-ω model for the near wall region and the k-k- model for flow flow away from surfaces. The energy equation was applied to ensure ensure that that the the aerodynamic aerodynamic heating at hypersonic hypersonic ϵ speeds is accounted for and the heat produced from combustion is accurately modelled. An implicit approach waswas usedused forfor the the solution solution of of the the governing governing equations equations with with the the Roe-flux Roe-flux difference difference splitting splitting for forcalculating calculating the the convective convective flux. flux. The The flow, flow, turbulent turbulent kinetic kinetic energy, energy and, and turbulent turbulent dissipation dissipation were were set setas secondas second order order upwind upwind to obtain to obtain more more accurate accurate solutions. solutions. Single Single step, step, volumetric volumetric reactions reactions and finite and finiterate reaction rate reaction for the for turbulence-chemistry the turbulence-chemistry interaction interaction were enabled were enabled with the with hydrogen-air the hydrogen mixture-air (Hmixture2 + O2 (H), to2 + model O2), to the model scramjet the scramjet combustion. combustion. Table 4 describes the boundary conditions for computational analysis and Table 5 describes the input parameters for the nozzle inlet condition. Two-dimensional parametric studies of the scramjet nozzle geometry with different freestream (FS) conditions representing different stages of the flight profile and the expected nozzle inlet (NI) conditions at these stages were investigated. The input

Energies 2018, 11, 3449 7 of 29

Table4 describes the boundary conditions for computational analysis and Table5 describes the input parameters for the nozzle inlet condition. Two-dimensional parametric studies of the scramjet nozzle geometry with different freestream (FS) conditions representing different stages of the flight profile and the expected nozzle inlet (NI) conditions at these stages were investigated. The input parameters were obtained through analyzing different literature (discussed in the Introduction) which conducted experimental and computational studies, where suitable values were established for the input parameters at certain operating conditions. A parametric analysis of the scramjet nozzle geometries was performed to arrive at a parametrically optimum design.

Table 4. Boundary conditions for computational analysis.

Boundary Conditions Altitude, 80,000 ft Altitude, 90,000 ft Altitude, 100,000 ft Freestream pressure 2761 1730 1216 Freestream temperature 221 224 224 Freestream Mach number 5 6.5 8

Table 5. Input parameters for the nozzle inlet conditions.

Boundary Conditions BCC 1.1 BCC 1.2 BCC 2.1 BCC 2.2 BCC 3.1 BCC 3.2 (BCC) 80,000 ft 80,000 ft 90,000 ft 90,000 ft 100,000 ft 100,000 ft NI pressure 471,112 425,000 425,000 365,000 365,000 335,000 NI temperature 2972 3030 3030 3033 3033 3048 NI Mach number 2 2.4 2.4 2.7 2.7 3 FS (freestream) pressure 2761 2761 1730 1730 1216 1216 FS temperature 221 221 224 224 224 224 FS Mach number 5 5 6.5 6.5 8 8

2.4. Computational and Experimental Validation of Numerical Methodology The numerical methodology adopted in this study was first validated against the SERN results of Huang et al. [18] and Monta [29]. Huang et al. [18] explored the SERN design through parametric studies which was then validated against experimental results by Monta [29]. The major differences in the numerical methodology of Huang et al. [18] from the present study was the application of the Re-Normalisation Group (RNG) k-ε turbulent model with non-equilibrium wall functions, the advection upstream splitting method (AUSM), adiabatic walls, and a constant courant number within the solver. Figures6 and7 compare the flow field obtained in the present analysis with that of Huang et al. [18]. The flow field contours of the expansion fan region between the inner nozzle cowl transition points were compared. It can be observed that the upper and lower shear layers are accurately replicated in the present analysis. The weak oblique shock wave forms at the same location and deflects at the same angle from the upper shear layer down to the lower shear layer. As expected, the velocity behind the oblique shock wave decreases and causes the direction of the lower shear layer to vary from being parallel to slightly inclined towards the upper wall of the afterbody. Similar to Huang et al. [18] findings, the lower shear layer was near parallel to the upper shear layer and the external flow round the nozzle was completely developed. The flow features illustrated in each case have a near perfect resemblance to one another except for the differences in the scale values, varying slightly due to the difference in the range specified. It should be noted that, the scales in Huang et al.’s [18] contour plots did not show the maximum value, making complete quantitative validation difficult from the contour plots. Figure8 shows the comparison of the pressure distribution behind the normal shock wave with the experimental data. The pressure distribution at a horizontal distance of 0.1143 m was obtained by calculating the pitot pressure. The results provided good agreement with the computational and experimental results of Huang et al. [18] and Monta [29]. Energies 2018, 11, x FOR PEER REVIEW 8 of 29 Energies 2018, 11, 3449 8 of 29 Energies 2018, 11, x FOR PEER REVIEW 8 of 29

Figure 6. Comparison of static pressure contours between Huang et al. [10] (left) and the present Figure 6. Comparison of static pressure contours between Huang et al. [10] (left) and the present analysisFigure 6. (rightComparison). of static pressure contours between Huang et al. [10](left) and the present analysisanalysis (right (right). ).

Figure 7. Comparison of Mach number contours between Huang et al. [10] (left) and the present FigureFigure 7. 7.ComparisonComparison Comparison of of of Mach Mach Mach number numbernumber contours contourscontours betweenbetween Huang Huang etet et al.al. al. [10][10] [10 ](( left(leftleft) ))and and the the present present analysis (right). analysis ( right).

Figure 8. Comparison of pressure distribution in the nozzle. Figure 8 8.. ComparisonComparison of pressure distribution in the nozzle nozzle..

Energies 2018, 11, 3449 9 of 29

3. Results and Discussion A 2D parametric study of the SERN was performed at three different operating conditions; 80,000 ft., 90,000 ft., and 100,000 ft. This sensitivity analysis will determine which geometrical parameters of the SERN have the most significant impact on the performance parameters. In total, 470 different SERN geometries were successfully tested for the three operating conditions and two nozzle inlet conditions, totaling 2820 cases overall.

3.1. SERN Selection Justifications The corrected axial thrust was the focal performance parameter on which the SERN design was selected. Maximum thrust was identified as the most important performance parameter because the required thrust for the final aircraft and integrated combined cycle engine design was unknown for Mach 8 cruise conditions. Axial thrust was calculated with a correction factor applied relating to the expansion ramp divergence angle. The nozzle axial thrust (Fa), ideal thrust (Fi), correction factor (λ), nozzle divergence coefficient (Cθ), and net thrust (Fn) were calculated using the following equations:

2 Fe = ρeve w[lsin(θe)] (1)

. Fi = meve (2)

γ = 0.5[1 + cos(θe)] (3)

Faxial Cθ = (4) Fideal .
. Fn = λ meve − m0v0 + (pe − p0)Ae (5)

3.1.1. Effect of Operating Conditions and Nozzle Inlet Conditions on SERN Expansion Table6 shows the SERN geometries tested at BCC 1.1. The SERN geometries were analyzed for different conditions with the objective to achieve perfect expansion and to maximize the nozzle efficiency. Only in BCC 1.1 configuration does the nozzle achieve near to perfect expansion. However, the axial thrust production in these SERNs are in the lowest 10%. The pressure contours in Figure9 shows the DP351 nozzle performance at 80,000 and 100,000 feet. The higher nozzle pressures are associated with the lower ambient pressures which is the character of an air-breathing engine. The comparatively similar pressure distribution proves its adaptability to altitude. This can be attributed to the shear layers downstream of the SERN making it insensitive to the pressure change with altitude. Table6 suggests that the expansion ramp radius (P3) is an important geometric factor in achieving perfect expansion. With the radius at 5000 m, the expansion ramp becomes a conical nozzle. An initial expansion ramp angle of 10◦ and an initial conical length of 3.2 m establishes conditions in which the expansion is favorable to achieve perfect expansion. The expansion ramp length does not have a significant effect for the chosen geometric values. Any length, shorter than 30 m may encourage significant under-expansion. The lack of a lower surface beneath the expansion ramp length has an effect on the type of expansion. The lower surface expansion takes place much earlier compared to the expansion ramp. In the boundary conditions investigated, under-expansion is present from the cowl surface until the freestream directs the flow back towards the nozzle exhaust plume, occurring in cases of higher ambient pressure and lower nozzle inlet speeds and pressure. The near perfect expansion was observed for a nozzle area ratio of 9.3. These nozzle area ratios are large in comparison, and therefore, the SERNs tend to be larger and heavier. Considering the low axial thrust produced and the larger size and weight, these nozzles were not favored for optimum SERN geometries. Energies 2018, 11, 3449 10 of 29 Energies 2018, 11, x FOR PEER REVIEW 10 of 29

FigureFigure 9.9.Static Static pressurepressure distributiondistribution ofof SERNSERN DP351 DP351 for for 80,000 80,000 ft. ft. (top(top)) and and 100,000 100,000 ft. ft. ((bottombottom).).

TableTable 6. 6.SERN SERN geometriesgeometries forfor differentdifferent design design points points (DPs). (DPs).

DPDP P1 P1 P2P2 P3P3 P4P4 P5P5 P6P6 P7 P8 P8 P9 P9Pexit Pexit Expansion Expansion 351351 1 m1 m 9.3 m9.3 m 5000 5000 m m 3.23.2 m m 1010◦ ° 3030 m m 3 3 m m 0.3 0.3 m m 5.72° 5.72 ◦ 26952695 Over Over 470470 1.5 m1.5 9.3m m9.3 m 5000 5000 m m 3.23.2 m m 1010°◦ 4040 m m 3 3 m m 0.3 0.3 m m 33° 33 ◦ 28352835 Under Under 1524 15241.5 m1.5 9.3m m9.3 m 5000 5000 m m 3.23.2 m m 1010°◦ 3030 m m 3 3 m m 0.3 0.3 m m 5.72° 5.72 ◦ 28462846 Under Under

Single expansion ramp nozzle DP351 Mach contour flow field for BCC 1.1 and 3.2 in Figure 10 Single expansion ramp nozzle DP351 Mach contour flow field for BCC 1.1 and 3.2 in Figure 10 demonstrates the near perfect expansion on the upper shear layer of the exhaust plume. A weak demonstrates the near perfect expansion on the upper shear layer of the exhaust plume. A weak oblique shock wave is observed just below the upper shear layer where the Mach number suddenly oblique shock wave is observed just below the upper shear layer where the Mach number suddenly decreases. The internal cowl expansion has slight under-expansion initially, but due to the external decreases. The internal cowl expansion has slight under-expansion initially, but due to the external flow effects, it is made to over-expand as it travels further downstream. Boundary condition BCC 3.2 flow effects, it is made to over-expand as it travels further downstream. Boundary condition BCC 3.2 shows that the lower ambient pressure, the high Mach number, and pressure at the nozzle inlet shows that the lower ambient pressure, the high Mach number, and pressure at the nozzle inlet induce induce significant under-expansion from the internal cowl and external expansion ramp nozzle, significant under-expansion from the internal cowl and external expansion ramp nozzle, which both which both cause shock waves due to the outward turning jet flow interacting with the external flow. cause shock waves due to the outward turning jet flow interacting with the external flow. Although, Although, achieving near perfect expansion would result in reduction of thrust performance, it is achieving near perfect expansion would result in reduction of thrust performance, it is unlikely to unlikely to achieve anywhere near perfect expansion at 100,000 ft. because of the extremely low achieve anywhere near perfect expansion at 100,000 ft. because of the extremely low ambient pressure. ambient pressure. Increasing exit height, initial expansion angle, and cowl angles for both operating Increasing exit height, initial expansion angle, and cowl angles for both operating conditions would conditions would increase the likelihood of achieving perfect expansion, but this would encourage increase the likelihood of achieving perfect expansion, but this would encourage more divergence more divergence losses and a further decrease in thrust performance. losses and a further decrease in thrust performance. The operating conditions do not have significant effects on the majority of performance The operating conditions do not have significant effects on the majority of performance parameters parameters at higher altitudes and different Mach, which is confirmed by Ogawa and Boyce [30]. The at higher altitudes and different Mach, which is confirmed by Ogawa and Boyce [30]. The adaptability adaptability of the nozzle with changing ambient conditions make it robust throughout the of the nozzle with changing ambient conditions make it robust throughout the hypersonic flight profile. hypersonic flight profile.

Energies 2018, 11, 3449 11 of 29 Energies 2018, 11, x FOR PEER REVIEW 11 of 29

FigureFigure 10. ContoursContours of of Mach Mach number number for for DP DP 351 at BCC 1.1 ( toptop) and BCC 3.2 ( bottom))..

3.1.23.1.2.. Effect Effect of SERN Geometry on Axial Thrust The effects of varying varying upper ramp ramp length length (P6) (P6) on on the the nozzle nozzle internal internal and and propulsive propulsive performance performance is significant.significant. IncreasingIncreasing the the length length of of the the upper upper expansion expansion ramp ramp has ahas tendency a tendency to decrease to decrease the thrust. the thrust.The optimal The optimal expansion expansion ramp ramp length length in regard in regard to achieving to achievin maximumg maximum thrust, thrust, in this in this case, case, is 30is 30 m. m.Comparing Comparing the the maximum maximum axial axial thrust thrust at expansion at expansion ramp ramp length length of 30 of m 30 and m 40 and m shows40 m shows that the that SERN the SERNgeometries geometries differ extensivelydiffer extensively overall. overall. Therefore, Therefore, as the length as the oflength the upper of the expansion upper expansion ramp changes, ramp changes,other geometrical other geometrical parameters parameters of the SERN of the differ SERN when differ trying when to achieve trying to maximum achieve maximum thrust. The thrust. SERN Thegeometries SERN geometries in Table7 are in for Table 30 m 7 andare 40for m 30 expansion m and 40 ramp m expansion length, which ramp yield length, the maximumwhich yield axial the maximumthrust for allaxial BCCs. thrust In for the all top BCCs. 10% ofIn axialthe top thrust, 10% of 98% axial of thethrust, SERN 98% geometries of the SERN have geometries an expansion have anramp expansion length of ramp 30 m, length and hence, of 30 m is, consideredand hence, asis co thensidered optimum as length.the optimum length.

Table 7. SERN geometries which yield maximum thrust at 30 m and 40 m expansion ramp length for Table 7. SERN geometries which yield maximum thrust at 30 m and 40 m expansion ramp length for BCC 3.2. BCC 3.2. DP P1 P2 P3 P4 P5 P6 P7 P8 P9 Axial Thrust DP102 P1 2 m P2 9.3 m P365 m 1.6 P4 m 10° P5 30 P6 3 P70.3 m P833° P95,668,441 Axial N Thrust 102 2544 m 2 m 9.3 m9.3 m 65125 m m 1.61.6 m m 10° 10◦ 4030 3 30.3 m 0.3 5.72° m 335,063,079◦ 5,668,441N N 544 2 m 9.3 m 125 m 1.6 m 10◦ 40 3 0.3 m 5.72◦ 5,063,079 N The expansion ramp radius confirms SERN geometries with larger contoured expansion ramp and aThe smaller expansion initial rampexpansion radius angle confirms after SERNa short geometries conical section with produces larger contoured the greatest expansion axial thrust. ramp Theand more a smaller extreme initial contoured expansion expansion angle after ramps a short allow conical for the section flow to produces expand faster the greatest within th axiale external thrust. sectionThe more of extremethe SERN. contoured expansion ramps allow for the flow to expand faster within the external sectionHowever, of the SERN. it is not so extreme that flow separation occurs as the pressure expands into this vacant area ofHowever, air. The itlarge is not initial so extreme expansion that flowangle separation of the smaller occurs radius as the contours pressure mean expands nearer into the this trailing vacant edgearea ofof air.the The expansion large initial ramp, expansion and the angledivergence of the angle smaller is radiussmaller, contours levelling mean off horizontally nearer the trailing more graduallyedge of the allowing expansion for less ramp, flow and divergence, the divergence maximizing angle isaxial smaller, thrust. levelling The conical off horizontallyand larger radius more contouredgradually allowingexpansion for ramps less flow have divergence, a larger divergence maximizing angle axial at thrust.the end The of conicalthe nozzle and subsequently larger radius inheritingcontoured more expansion losses. ramps In thehave top 10% a larger of the divergence axial thrust, angle 77% at of the the end initial of the expansion nozzle subsequentlyramp angles were 10°, indicating that the shallower initial expansion ramp angle or angles below 30° produce better axial thrust results. An ideal contour radius value for axial thrust is not reasonable, as a contour

Energies 2018, 11, 3449 12 of 29 inheriting more losses. In the top 10% of the axial thrust, 77% of the initial expansion ramp angles were 10◦, indicating that the shallower initial expansion ramp angle or angles below 30◦ produce Energies 2018, 11, x FOR PEER REVIEW 12 of 29 better axial thrust results. An ideal contour radius value for axial thrust is not reasonable, as a contour radiusradius ofof thethe samesame valuevalue dodo notnot necessarilynecessarily adoptadopt thethe samesame shapeshape duedue toto dependenciesdependencies onon otherother geometrical parametersparameters such such as as nozzle nozzle length, length, initial initial conical conical length, length, initial initial expansion expansion ramp ramp angle, angle and, exitand area.exit area. However, However, amplified amplified contoured contoured expansion expansion ramp shapesramp shapes with small with initialsmall expansioninitial expansion angles (P5)angles generate (P5) generate greater axialgreate thrust.r axial The thrust. contoured The contoured expansion expansion ramp surfaces ramp thatsurfaces were that exposed were to exposed higher wallto higher pressures wall pressures assisted in assisted achieving in achieving higher axial higher thrust. axial thrust. Again, anan optimum optimum value value or or range range for for the the initial initial conical conical length length (P4) of(P4) the of expansion the expansion ramp cannot ramp becannot determined. be determined. The initial The conical initial expansionconical expansion ramp length ramp of length 1.6 m of is part1.6 m of is the part DP102 of the geometry DP102 conversely,geometry conversely, only 53% of only the SERN53% of geometries the SERN geometries in the top 10% in the incorporated top 10% incorporate this dimension,d this showingdimension an, absenceshowing of an an absence optimum of an value optimum between value 1.6 mbetween and 3.2 1.6 m. m and 3.2 m. The effects of the cowl di divergencevergence angle were momentous in the expansion of the exhaust plume plume,, and in turn effectedeffected the losses experienced in the internal section of the nozzle. However, axial thrust ◦ ◦ did notnot havehave aa trendtrend forfor thethe twotwo anglesangles ofof 5.725.72° andand 3333°.. The shape of the cowl waswas diverse betweenbetween ◦ thesethese anglesangles andand hadhad aa significantsignificant effecteffect onon thethe aero-propulsiveaero-propulsive performance.performance. TheThe 5.725.72° cowlcowl angleangle ◦ began from thethe nozzlenozzle inlet.inlet. Similarly, thethe internalinternal expansionexpansion withwith thethe cowlcowl angleangle atat 3333° was slower ◦ due toto the cowlcowl beingbeing horizontalhorizontal forfor aa largelarge portionportion ofof itsits length,length, withwith thethe 3333° convex corner near the end of the cowl. Figure 11 displaydisplayss DP102 and DP464 which have near identical geometry, excluding cowlcowl angles.angles.

Figure 11. Contours of velocity for DP 464 (toptop)) andand DPDP 102102 ((bottombottom).).

Cowl angleangle 5.72 5.72°◦ subsequently subsequently has has a lessa less under-expanded under-expanded shear shear layer layer compared compared to the to cowl the anglecowl atangle 33◦ .at The 33°. nozzle The inletnozzle profile inlet presents profile slight presents differences slight indifferences the downstream in the flow,downstream and therefore, flow, axialand thrusttherefore which, axial slightly thrust benefits which theslightly downstream benefits velocitythe downstream for cowl anglesvelocity of for 33◦ cowl. The angles horizontal of 33°. section The ofhorizont the cowlal section helps maximize of the cowl the helps axial maximize velocity and thelimits axial velocity the divergence and limit lossess the of divergence the 33◦ cowl losses angle. of Tablethe 33°8 presents cowl angle the. maximumTable 8 presents thrust the for maximum different nozzle thrust area for different ratios tested nozzle in thearea parametric ratios tested study. in the parametric study. The SERN exit height proved to be significant in achieving maximum thrust. However, determining the axial thrust through area ratio for the SERN is misleading as it does not establish a consistent relationship. The area ratio for the SERN does not behave as experienced in axisymmetric nozzles. A combination of inlet and exit height at 2 m and 9.3 m, respectively, were the dominant geometries in producing higher axial thrust as they were present in 93% of the geometries which

Energies 2018, 11, 3449 13 of 29

Table 8. Maximum axial thrust for varying nozzle area ratios of SERN.

DP P1 P2 P3 P4 P5 P6 P7 P8 P9 Area Ratio Axial Thrust (N) 102 2 m 9.3 m 65 m 1.6 m 10◦ 30 3 0.3 33 4.65 5,668,441 456 2 m 7.3 m 90 m 1.6 m 10◦ 30 3 0.3 5.72 3.65 5,144,993 623 1.5 m 7.3 m 90 m 1.6 m 10◦ 30 3 0.3 5.72 4.87 4,333,759 86 1.5 m 9.3 m 60 m 1.6 m 10◦ 30 3 0.3 33 6.2 3,990,316 68 1 m 9.3 m 57.5 m 1.6 m 10◦ 30 3 0.3 33 9.3 2,833,642

The SERN exit height proved to be significant in achieving maximum thrust. However, determining the axial thrust through area ratio for the SERN is misleading as it does not establish a consistent relationship. The area ratio for the SERN does not behave as experienced in axisymmetric nozzles. A combination of inlet and exit height at 2 m and 9.3 m, respectively, were the dominant geometries in producing higher axial thrust as they were present in 93% of the geometries which were in the top 10% of axial thrust. The 9.3 m exit height allowed for more expansion of the exhaust flow to take place within the internal and external section of the SERN, maximizing the nozzle axial thrust capability. The difference in axial thrust was substantial with decreasing nozzle inlet height but the geometrical relationships behaved the same as the geometries were incredibly similar shapes. In comparison to findings in Huang et al.’s [18] SERN geometry study, this challenges the conclusion that increasing the expansion ramp length improves thrust. The contoured 30-m expansion ramps can produce more thrust than the conical 40-m expansion ramps, prompting that increasing the expansion ramp length in conical SERNs greatly improves thrust but not necessarily for contoured SERNs. Ogawa and Boyce [30] observed parametrically optimum conical nozzles produced 0.2% and 2% less in thrust levels than optimum contoured nozzles in fuel-on and -off conditions; however, the DP102 contoured nozzle produced 10.68% more axial thrust than the parametrically optimum DP544 conical nozzle. The uniform inlet conditions exaggerate the performance compared to fuel-on and -off conditions. DP102 and DP544 thrust difference would be expected to be less in non-uniform flow. Despite this, thrust relationships observed between conical and contoured nozzles match Ogawa and Boyce’s [30] findings.

3.1.3. Effect of SERN Geometry on Drag The effect of SERNs’ geometry on drag was analyzed by monitoring the coefficient of drag (Cd) at the SERNs’ inner walls. The drag was analyzed for BCC 3.1 considering drag will be paramount at the cruise speed of Mach 8. The elevated wall temperatures will amplify skin friction and thereby in drag. The expansion ramp had negligible impact on the drag performance. Contour radii that closely or fully resemble conical expansion ramps tend to increase drag. However, it does not necessarily mean they are the main contributor of drag. The initial expansion ramp length and angle govern the effect of drag produced by the SERN, with the cowl angle having a minor influence. The top 10% of SERN geometries producing the largest Cd all had an initial expansion ramp length of 3.2 m at an angle of 30◦ from the inlet. Figure 12 displays the DP279, DP690, and DP487 cases which had Cd values of 0.648, 0.328, and 0.065, respectively. DP279 and DP690 confirm that the initial expansion ramp angle and length generated an expansion fan from the inlet and a weak oblique shock at the concave corner of the expansion ramp. The oblique shock introduced wave drag and decreased total pressure, which was detrimental to the scramjet performance. The expansion fan at the nozzle inlet and cowl angle increased the Mach number but this was thwarted by the oblique shock. The 33◦ cowl angle, compared to 5.72◦, produced a thicker turbulent shear layer resulting in increased Cd. Compared to DP690, which had an initial expansion ramp of 1.6 m and initial expansion angle of 30◦, the shorter initial expansion length generated the oblique shock earlier and the shock was deflected towards and reached the lower shear layer in a shorter distance, because of the larger concave angle, which destroyed the shock. This allowed for the flow behind the weak oblique shock to recover faster while producing less wave drag. The SERN Energies 2018, 11, 3449 14 of 29 geometries that yield the lowest Cd have common geometrical factors of a 10◦ initial expansion ramp, 5.72◦ cowl angle, and a convex corner before the conical or contoured ramp, producing expansion fans rather than shock waves. Energies 2018, 11, x FOR PEER REVIEW 14 of 29

Figure 12. Mach number (left) and pressure contours (right) for DP279 (top), DP690 (middle), and Figure 12. Mach number (left) and pressure contours (right) for DP279 (top), DP690 (middle), and DP487 (bottom). DP487 (bottom). 3.1.4. Effect of SERN Geometry on Lift and Moments 3.1.4. Effect of SERN Geometry on Lift and Moments As expected, the coefficient of lift (CL) and coefficient of moment (Cm) values are much greater As expected,when simulating the coefficient the SERN at of 100,000 lift (CL) feet andand coefficientMach 8. These of performance moment (Cm) parameters values are are coupled much greater when simulatingand the natural the SERN moments at of 100,000 SERN’s feetare to and pitch Mach downward 8. These when performance increasing in speed. parameters At maximum are coupled and the naturalCL, Cm is moments maximum ofbut SERN’s when lift arefluctuates to pitch above downward and below zero, when the increasing moment becomes in speed. the inverse. At maximum The range of the CL and Cm when they become inversely proportional is from 0.05325 to −0.2004 for CL, Cm isDP640 maximum and −0.1803 but whento 0.0524 lift for fluctuates DP52, respectively. above and Below below 0.2004 zero, for theCL, momentthe Cm yields becomes negative the inverse. The rangemoment of the values. CL and In Cmthis region, when theyfor positive become lift inverselyand negative proportional moment, the iscommon from 0.05325geometrical to −0.2004 for DP640parameters and −0.1803 are an toinitial 0.0524 expansion for DP52, ramp respectively.angle of 30° and Below cowl angle 0.2004 of 5.72°. for The CL, opposite the Cm coupling yields negative momentobs values.erves the In initial this expansion region, forramp positive angle being lift 10° and and negative the cowl angle moment, is 33°. The the initial common expansion geometrical ramp length of 3.2 m, initial expansion ramp angle of 10°, and the cowl angle at 5.72° are present in parameters are an initial expansion ramp angle of 30◦ and cowl angle of 5.72◦. The opposite coupling observes the initial expansion ramp angle being 10◦ and the cowl angle is 33◦. The initial expansion ramp length of 3.2 m, initial expansion ramp angle of 10◦, and the cowl angle at 5.72◦ are present in all the geometries in the top 10% for CL and Cm. Figure 13 illustrates the pressure distribution for DP471, DP640, DP52, and DP205 (Minimum CL = −0.586). Energies 2018, 11, x FOR PEER REVIEW 15 of 29

Energiesall2018 the, 11geometries, 3449 in the top 10% for CL and Cm. Figure 13 illustrates the pressure distribution for15 of 29 DP471, DP640, DP52, and DP205 (Minimum CL = −0.586).

FigureFigure 13. Pressure13. Pressure contours contours for for DP471, DP471, DP640,DP640, DP52 DP52,, and and DP205 DP205 (clockwise (clockwise from from top topleft). left ).

DP471DP471 and and DP640 DP640 produced produced positive positive liftlift duedue to the axial axial flow flow profile profile at atthe the nozzle nozzle inlet inlet which which providedprovide higherd higher pressures pressures on theon the upper upper expansion expansion ramp ramp surface. surface. DP640DP640 induce inducedd a a shock shock wave wave but but its deflectionits deflection was gradual was gradual compared compared to DP205. to DP205. In relation In relation to the to upperthe upper surface, surface, as itas maintained it maintained higher higher pressure along the length of the expansion ramp a negative moment despite positive lift was pressure along the length of the expansion ramp a negative moment despite positive lift was instigated. instigated. DP52 and DP205 inlet flow profile exposed the cowl surface to higher pressures than the DP52expansion and DP205 ramp inlet surface flow generating profile exposed negative the lift. cowl DP52 surface and DP471 to higher had a pressureshigh-pressure than region the expansionat the rampbeginning surface generating of the expansion negative ramp lift. but DP52 this reduce and DP471d as it progresses had a high-pressure towards the regiontrailing at edge, the granting beginning of the expansiona positive moment. ramp but DP205 this reducedhad an ext asremely it progresses high pressure towards at the the expansion trailing ramp edge, surface granting due to a positivethe moment.shock DP205which help haded an maintain extremely a high high pressure pressure along at the the entire expansion expansion ramp ramp surface length due producing to the a shock whichmaximum helped maintain negative aCm high of − pressure2.12. These along geometry the entire components expansion of the ramp SERN length significantly producing impact aed maximum the negativeCL a Cmnd Cm. of − The2.12. expansion These geometry ramp length components and contour of radius the SERN had a significantly minimal effect impacted in regard the to the CL and Cm. Thecoefficient expansion of lift rampas in the length top 10% and of contour lift coefficient radius values, had a minimal60% were effect 30 m. inMaximum regard to CL the and coefficient Cm of liftwere as in 0.4851 the top and 10% 0.6797 of lift(DP471), coefficient achieved values, when 60%the expansion were 30 m.ramp Maximum was 40 m but CL the and maximum Cm were CL 0.4851 and Cm at expansion ramp length 30 m were 0.0037% and 0.0073% less, respectively. and 0.6797 (DP471), achieved when the expansion ramp was 40 m but the maximum CL and Cm at expansion3.1.5. SERN ramp Selection length 30 m were 0.0037% and 0.0073% less, respectively.

3.1.5. SERNFigure Selection 14 presents the SERN geometries which attained the optimal results in regard to Fa, Cd, CL, Cm, Cθ, and FN compared against DP102. These results were taken at BCC 3.1, the closest Figurerepresentation 14 presents of cruise the condition. SERN geometries Table 9 shows which the attainedSERN dimensions the optimal for each results geometry in regard analyzed to Fa,. Cd, CL, Cm,DP102 C θis, superior and FN for compared axial thrust against while producing DP102. Thesevery little results mom wereent but taken at a higher at BCC drag 3.1, and the lower closest representationlift penalty. of DP471 cruise provided condition. a considerable Table9 shows amount the of SERN lift at dimensionslow drag; however, for each the geometry lift and moment analyzed. DP102coupling is superior meant for an axial extremely thrust large while moment producing would very be subject little moment to the aircraft but at from a higher the nozzle drag andwhile lower lift penalty.generating DP471 a small provided amount a considerableof thrust. DP633 amount had the of lowest lift at lowmoment drag; with however, a positive the lift lift forc ande momentand coupling meant an extremely large moment would be subject to the aircraft from the nozzle while generating a small amount of thrust. DP633 had the lowest moment with a positive lift force and sufficient thrust. Despite the promising performance in other areas, it created the most amount of drag of the selection. DP487 accomplished minimum drag and generated considerable amounts of lift. Consequently, a copious moment was generated, and a lack of axial thrust makes it unappealing. DP456 establishes high amounts of axial thrust with low drag and respectable lift. It is considered Energies 2018, 11, x FOR PEER REVIEW 16 of 29 sufficient thrust. Despite the promising performance in other areas, it created the most amount of drag of the selection. DP487 accomplished minimum drag and generated considerable amounts of lift. Consequently,Energies 2018, 11a, copious 3449 moment was generated, and a lack of axial thrust makes 16it ofunappealing. 29 DP456 establishes high amounts of axial thrust with low drag and respectable lift. It is considered a good overall design, despite the large moment coefficient, which can be countered by the aircraft. All a good overall design, despite the large moment coefficient, which can be countered by the aircraft. the SERNAll geometries the SERN geometries have a haverelatively a relatively high high divergence divergence coefficient coefficient which which indicates indicates good nozzle good nozzle designs. However,designs. However, they are they all are below all below 0.96 0.96 meaning meaning they they may may encounter encounter issues withissues separation. with separation.

Figure 14.Figure SERN 14. SERN geometries geometries achieving achieving bestbest performance performance parameters parameters for BCC for 3.1. BCC 3.1. Table 9. SERN geometries for best performance. Table 9. SERN geometries for best performance. DP P1 P2 P3 P4 P5 P6 P7 P8 P9 102DP 2 mP1 9.3 mP2 65 mP3 1.6 mP4 10◦ P5 30P6 P7 3 P8 0.3 P9 33 ◦ 471102 1 m2 m 9.3 m9.3 m5000 65 m m 3.2 m1.6 m 10 10° 4030 3 0.3 0.3 33 5.72 633 1.5 m 9.3 m 75 m 1.6 m 30◦ 30 3 0.3 5.72 487471 1.5 m1 m 7.3 m9.3 m 2005000 m m 3.2 m3.2 m 10◦10° 4040 3 0.3 0.3 5.72 5.72 456633 2 m1.5 m 7.3 m9.3 m 90 m75 m 1.6 m1.6 m 10◦30° 3030 3 0.3 0.3 5.72 5.72 487 1.5 m 7.3 m 200 m 3.2 m 10° 40 3 0.3 5.72 The SERN456 DP102 2 m (Table 7.3 10 )m produced 90 m the highest1.6 m axial10° thrust30 compared3 0.3 to5.72 allother SERN geometries at each BCC showing the robustness and adaptability of DP102. Therefore, the complication of variable nozzle geometry can be excluded from the SERN design. The axial thrust of SERN DP102 The SERN DP102 (Table 10) produced the highest axial thrust compared to all other SERN significantly increases as the nozzle inlet Mach number increases and pressure decreases, showing geometriesthe substantialat each BCC increase showing in Mach at the inletrobustness is enough toand compensate adaptability for the decreased of DP102. pressure Therefore, for the complicationuniform of variable inlet conditions. nozzle The geometry best SERN can geometry be excluded is DP102 from for achieving the SERN maximum design. axial The thrust axial thrust of SERN DP102in each significantly BCC apart from increases the erroneous as BCC the 1.1 nozzle results, whereinlet itMach was ranked number third. increases However, initial and pressure integration evaluations judged that the 9.3-m exit height of DP102 would degrade performance of the decreases, showing the substantial increase in Mach at the inlet is enough to compensate for the entire aircraft due to the SERN upper surface being approximately 2 m above the airframe. Therefore, decreasedthe pressure SERN which for obtaineduniform the inlet best axialconditions. thrust and The overall best performance SERN geometry with an exit is height DP102 of 7.3 for m achieving maximumwas axial selected. thrust Consequently, in each BCC DP456 apart was chosen from tothe be analyzederroneous further. BCC 1.1 results, where it was ranked third. However, initial integration evaluations judged that the 9.3-m exit height of DP102 would Table 10. SERN DP102 overall performance at each BCC. degrade performance of the entire aircraft due to the SERN upper surface being approximately 2 m above the airframe.Nozzle InletTherefore, Conditions the SERN BCC which 1.1 obtained BCC 1.2 and the 2.1 best BCC axial 2.2 thrust and 3.1 and BCCoverall 3.2 performance Axial thrust (N) 4,084,299 4,918,431 5,144,993 5,668,440 with an exit heightEstimated of 7.3 net m thrust was selected. N/A Consequently, N/A DP456 was 4,874,588 chosen to be 5,263,095 analyzed further. Nozzle divergence coeff. 0.934 0.934 0.934 0.934 Lift coeff.Table 10. SERN−0.107 DP102 overall− 0.153performance at each−0.200 BCC. −0.228 Drag coefficient 0.139 0.155 0.166 0.173 Moment coefficient 0.112 0.069 0.021 −0.0102 NozzleExit Inlet velocity Conditions BCC 2929 1.1 BCC 32451.2 and 2.1 BCC 3519 2.2 and 3.1 3809BCC 3.2 AxialExit thrust pressure (N) 4,084,299 21,061 4,918,431 22,069 20,7235,144,993 20,5095,668,440 EstimatedNozzle expansion net thrust underN/A underN/A 4,874,588 under Under5,263,095 Nozzle divergence coeff. 0.934 0.934 0.934 0.934 Lift coeff. −0.107 −0.153 −0.200 −0.228 Drag coefficient 0.139 0.155 0.166 0.173 Moment coefficient 0.112 0.069 0.021 −0.0102 Exit velocity 2929 3245 3519 3809 Exit pressure 21,061 22,069 20,723 20,509 Nozzle expansion under under under Under

Energies 2018, 11, 3449 17 of 29 Energies 2018,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 17 of 29

3.2.3.2.. SERNSERN ParametricParametric Studies—SERNStudies—SERN DP102DP102 andand DP456DP456 ComparisonComparison DP102DP102 yieldsyields moremore axialaxial andand netnet thrustthrust butbut DP456DP456 producesproduces lessless drag,drag, moremore lift,lift,, andand hashas aa higherhigher divergencedivergence angleangle making making it it less less prone prone to to axial axial losses losses and and less less likely likely to to encounter encounter separation. separation. Figure Figure 15 shows15 shows the the static static pressure pressure along along the SERNthe SERN inlet inlet centerline centerline for DP102 forfor DP102DP102 and and DP456.and DP456.DP456. For 3 ForFor m, 3 DP456 m, DP456 was constantwas constant due due to the to morethe more stable stable parabolic parabolic flow flow profile profile entering entering the the nozzle nozzle as as a a result result of of thethe smallsmall divergencedivergence anglesangles ofof thethe expansionexpansion rampramp andand thethe cowlcowl (Figure(Figure 1616).). TheThe densitydensity contourscontours displaydisplay thethethe shocksshocks generatedgenerated inin bothboth SERNsSERNs andand thethe increaseincrease inin pressurepressure confirmconfirm it.it. DP102DP102 hadhad aa slightslight increaseincrease ininin pressurepressurepressure justjustjust beforebeforebefore 333 mmm asasas thethethe cowlcowlcowl waswas exposedexposed toto higherhigher pressures.pressures. ThisThis isis becausebecause ofof thethe initialinitial horizontalhorizontal sectionsection causingcausing lowerlower velocityvelocity flowflow aboveabove thethe cowl.cowl.

FigureFigure 15.15. StaticStatic pressurepressure ofof DP102DP102 andand DP456DP456 alongalong thethe nozzlenozzle inletinlet centerline.centerline.

FigureFigure 16.16. Density contourscontours ofof DP102DP102 ((toptop))) andandand DP456DP456DP456 (((bottombottom))) illustratingillustratingillustrating shockshockshock structures.structures.structures.

AA faintfaint shock shock created created from from where where the expansionthe expansion ramp ramp diverges diverges significantly significantly assisted assist in theed gradual inin thethe pressuregradual pressure decrease. decrease. The larger The divergence larger divergence angle of theangle expansion of the expansion ramp and ramp cowl and surface cowl created surface a gradualcreated a decline gradual in decline pressure in pressure along the along centerline the centerline as the cowlas the expansion cowl expansion fans didfans not did intersect not intersect the centerlinethethe centerline beyond beyond the cowl. the cowl. The rapid The rapid decrease decrease in pressure in pressure was because was because of the expansion of the expansion fans induced fans atinducedinduced the nozzle atat thethe inlet nozzlenozzle from inletinlet the diverging fromfrom thethe cowl.divergingdiverging DP456 cowl.cowl. experienced DP456DP456 experience another pressured another increase pressure before increase 25 m indicatingbefore 25 m an indicating encounter an with enc anotherounter shock.with another The density shock. contours The density portray contours these shock portray interactions, these shock as interactions,interactions, asas thethe boundariesboundaries wherewhere thethe densitydensity increases.increases. DP102DP102 onlyonly hashas oneone shockshock producedproduced byby

Energies 2018, 11, 3449 18 of 29 theEnergies boundaries 2018, 11, x FOR where PEER the REVIEW density increases. DP102 only has one shock produced by the expansion18 of 29 ramp but this did not interact with the centerline within the 40 m domain. SERN DP102 clearly yields the expansion ramp but this did not interact with the centerline within the 40 m domain. SERN DP102 better pressures and densities within the internal and external sections of the nozzle. clearly yields better pressures and densities within the internal and external sections of the nozzle. The higher exit area produced less shock as the expansion ramp was less concaved. The larger The higher exit area produced less shock as the expansion ramp was less concaved. The larger region of high momentum flow achieved higher velocities at the exit area. Mach contours in Figure 17 region of high momentum flow achieved higher velocities at the exit area. Mach contours in Figure show the development of the kernel region in both the geometries. SERN DP456 holds advantages 17 show the development of the kernel region in both the geometries. SERN DP456 holds advantages over DP102 as a smaller divergence coefficient entailing less losses and the additional lift which will over DP102 as a smaller divergence coefficient entailing less losses and the additional lift which will support the aircraft at a cost of lower drag. support the aircraft at a cost of lower drag.

Figure 17.17. Mach number contours for DP102DP102 ((toptop)) andand DP456DP456 ((bottombottom)) depictingdepicting thethe kernelkernel region.region.

3.3.3.3. 2D2D ScramjetScramjet EngineEngine SERNSERN AnalysisAnalysis

UniformUniform andand Non-UniformNon-Uniform FlowFlow ConditionsConditions TheThe MachMach profilesprofiles ofof DP102DP102 andand DP456DP456 forfor thethe uniformuniform flow,flow, scramjetscramjet coldcold flow,flow, andand scramjetscramjet combustioncombustion flowflow areare shownshown inin FigureFigure 1818.. The Mach profileprofile were takentaken atat thethe nozzlenozzle inletinlet andand atat aa horizontalhorizontal distancedistance of of 1 1 m, m, 2 2 m, m and, and 3 3 m m from from the the nozzle nozzle inlet inlet to to demonstrate demonstrate the the initial initial development development of theof the flow flow from from the the nozzle nozzle inlet. inlet. The The uniform uniform flow flow for DP102for DP102 and and DP456 DP456 is near is near identical identical at 0 mat and0 m 1and m. However,1 m. However, DP456 DP456 has ahigher has a Machhigher at Mach the cowl at the in bothcowl instances in both instances due to the due cowl to divergence the cowl d beginningivergence frombeginning the inlet. from At the the inlet. cowl, At DP102’s the cowl, Mach DP102’s exceeds Mach DP456 exceeds at 3 DP456 m distance at 3 m because distance of thebecause expansion of the fansexpansion created fans by thecreated larger by cowl the larger angle. cowl The Machangle. atThe the Mach expansion at the rampexpansion surface ramp is higher surface for is DP102higher fromfor DP102 axial distancefrom axial of distance 2 m and of onwards 2 m and due onwards to the du rampe to contourthe ramp radius contour being radius smaller. being This smaller. results This in largerresults divergence in larger divergence angles and angles thereby and leads thereby to expansions leads to expansions fans with afans greater with Prandtl–Meyer a greater Prandtl angle–Meyer and increaseangle and in increase Mach number. in Mach number. TheThe cold flowflow of DP102 and and DP456 DP456 prove prove the the inaccuracy inaccuracy of of uniform uniform nozzl nozzlee inflow. inflow. The The Mach Mach of ofthe the cold cold flow flow at atthe the walls walls and and axial axial positions positions are are lower lower and and higher higher,, respectively respectively,, compared to thethe uniformuniform flowflow overover allall horizontalhorizontal distances,distances, untiluntil thethe expansionexpansion fanfan effectseffects becomebecome tootoo great.great. AA developeddeveloped parabolicparabolic flowflow waswas observedobserved atat 00 mm forfor coldcold flowflow tests,tests, whichwhich waswas duedue toto thethe expansionexpansion fansfans fromfrom thethe divergingdiverging combustorcombustor geometry.geometry. TheThe uniformuniform flowflow startsstarts toto taketake onon aa similarsimilar MachMach profileprofile near near the the expansion expansion ramp ramp wall, wall, but but the the combustor combustor and and nozzle nozzle geometry geometry impacts impacts the flow the flowresulting resulting in considerably in considerably different different inflow inflow Mach Mach profiles profiles across across the remainder the remainder of the ofinlet the height inlet height down downto the todifferent the different cowl shapes. cowl shapes. The boundary The boundary inflow inflowconditions conditions have a havesignificant a significant impact impacton the results. on the Contour plots of the SERN only investigations have shown that setting high uniform boundary conditions provides desirable qualitative and quantitative results. Conversely, when the SERN is combined with the inlet, isolator, and combustion chamber for full engine analysis, the results can be

Energies 2018, 11, 3449 19 of 29 results. Contour plots of the SERN only investigations have shown that setting high uniform boundary conditions provides desirable qualitative and quantitative results. Conversely, when the SERN is Energies 2018, 11, x FOR PEER REVIEW 19 of 29 combined with the inlet, isolator, and combustion chamber for full engine analysis, the results can be regardedregarded as as more more realistic. realisti Figuresc. Figures 19 and 19 20and compares 20 compares DP102 DP102 cold flow cold and flow combustion and combustion flow contours flow tocontours distinguish to distinguish the differences. the differences. The contours The show contours a large show contrast a large in contrast the resulting in the solution resulting of thesolution cold andof the hot cold flow and through hot flow the through nozzle. the nozzle.

FigureFigure 18.18. MachMachnumber number profiles profiles for for different different cases cases at at distances distances (from (fromtop topto bottomto bottom) of:) of: 0 m, 0 m, 1 m, 1 2m, m, 2 andm, and 3 m 3 from m from the nozzlethe nozzle inlet. inlet.

TheThe MachMach withinwithin thethe internalinternal andand externalexternal sectionssections ofof thethe nozzlenozzle demonstratedemonstrate aa vastvast variancevariance inin thethe rangerange ofof MachMach achieved.achieved. ColdCold flowflow expansionexpansion fromfrom thethe combustioncombustion chamberchamber hashas notnot beenbeen affectedaffected byby thethe fuelfuel mixingmixing andand chemicalchemical reaction,reaction, unlikeunlike thethe hothot flowflow test,test, resultingresulting inin undisturbedundisturbed andand higherhigher MachMach numbernumber flowflow expandingexpanding throughthrough thethe nozzle.nozzle. Whereas,Whereas, thethe combustedcombusted flowflow MachMach isis slowedslowed causingcausing lowerlower Mach number number distribution distribution throughout throughout the the nozzle. nozzle. The The higher higher pressure pressure throughout throughout the theinternal internal and and external external nozzle nozzle in combusted in combusted flow, flow, causes causes greater greater under under-expansion-expansion in contrast in contrast to cold to coldflow flow because because of combustion of combustion increasing increasing the the pressure pressure signif significantly.icantly. The The temperature temperature distribution isis mainlymainly inin thethe streamlinestreamline ofof thethe combustedcombusted flowflow withwith higherhigher temperaturestemperatures experiencedexperienced throughoutthroughout thethe areaarea ofof thethe nozzle.nozzle. ComparedCompared toto coldcold flow,flow, thethe maximummaximum temperaturestemperatures occuroccur atat thethe wallswalls andand shearshear layers.layers. IncreasedIncreased temperaturetemperature inin thethe corecore ofof thethe flowflow consequentlyconsequently reducedreduced thethe MachMach numbernumber in combusted flow. With combusted flow, the maximum velocity occurs within the plumes, but the velocity distributed throughout the nozzle is significantly higher in comparison to cold flow.

Energies 2018, 11, 3449 20 of 29 in combusted flow. With combusted flow, the maximum velocity occurs within the plumes, but the velocityEnergiesEnergies 2018 2018 distributed, ,11 11, ,x x FOR FOR PEER throughoutPEER REVIEW REVIEW the nozzle is significantly higher in comparison to cold flow. 2020 ofof 2929

FigureFigure 19.19. Mach Mach numbernumber ((toptop)) andand pressurepressure contourscontours ((bottombottom)) ofof DP102DP102 forfor coldcold flowflowflow ((leftleft)) andand hothot flowflowflow ((rightright).).

FigureFigure 20. 20. TemperatureTemperatureTemperature (top)(top) (top) andand and velocityvelocity velocity contourscontours contours (bottom)(bottom) (bottom) ofof ofDP102DP102 DP102 forfor for coldcold cold flowflow flow ((leftleft ()left) andand) and hothot hotflowflow flow ( (rightright (right).). ).

3.43.4.. 2D2D SScramjetcramjet TwinTwin StrutStrut CombustionCombustion——SERNSERN DP102DP102 andand DP456DP456 ComparisonComparison DP102DP102 andand DP456DP456 exhibitexhibiteded differentdifferent flowflow structuresstructures whenwhen incorporatedincorporated withwith twintwin strutstrut combustion.combustion. Figure Figure 21 21 shows shows the the velocity velocity and and pressure pressure contours contours of of SERN SERN DP456. DP456. Compared Compared to to SERN SERN DP102,DP102, DP456DP456 hashas largerlarger underunder expansionexpansion mainlymainly duedue toto thethe higherhigher--pressurepressure regionsregions beingbeing

Energies 2018, 11, 3449 21 of 29

3.4. 2D Scramjet Twin Strut Combustion—SERN DP102 and DP456 Comparison DP102 and DP456 exhibited different flow structures when incorporated with twin strut combustion. Figure 21 shows the velocity and pressure contours of SERN DP456. Compared to Energies 2018, 11, x FOR PEER REVIEW 21 of 29 SERNEnergies DP102,2018, 11, x DP456 FOR PEER has REVIEW larger under expansion mainly due to the higher-pressure regions21 being of 29 distributed throughout a larger area of the nozzle (Figure 20). The cowl induced a shock passing distributed throughout a larger area of the nozzle (Figure 20). The cowl induced a shock passing throughdistributed the throughout exhaust flow a oflarger SERN area DP456. of the nozzle (Figure 20). The cowl induced a shock passing through the exhaust flow of SERN DP456. through the exhaust flow of SERN DP456.

Figure 21. Pressure and velocity contours for SERN DP456. Figure 21.21. Pressure andand velocityvelocity contourscontours forfor SERNSERN DP456.DP456. Effect3.4.1 Effect of Flow of TypesFlow Types on Performance on Performance 3.4.1 Effect of Flow Types on Performance TheThe differentdifferent uniformuniform andand non-uniformnon-uniform coldcold andand hothot (combustion)(combustion) flowflow conditionsconditions affectaffect thethe The different uniform and non-uniform cold and hot (combustion) flow conditions affect the parametersparameters entering entering the the SERN. SERN. Therefore, Therefore, the flowthe behaviorflow behavior and performance and performance parameters parameters will contrast will parameters entering the SERN. Therefore, the flow behavior and performance parameters will throughoutcontrast throughout the SERN. the Single SERN. expansion Single expansion ramp nozzle ramp DP456nozzle withDP456 combustion with combustion and an and outlet an outlet 10 m contrast throughout the SERN. Single expansion ramp nozzle DP456 with combustion and an outlet downstream10 m downstream of the of nozzle the nozzle exit isexit seen is seen in Figure in Figure 22 and22 and will will be be referred referred to to as as “Hot “Hot DP456-2” DP456-2” forfor 10 m downstream of the nozzle exit is seen in Figure 22 and will be referred to as “Hot DP456-2” for convenience.convenience. FigureFigure 2222 illustratesillustrates similarsimilar characteristicscharacteristics toto thethe hothot flowflow withwith thethe downstreamdownstream outlet;outlet; convenience. Figure 22 illustrates similar characteristics to the hot flow with the downstream outlet; however,however, thethe higherhigher velocityvelocity regionregion occursoccurs muchmuch furtherfurther downstreamdownstream than compared to Figure 2121.. however, the higher velocity region occurs much further downstream than compared to Figure 21. TheThe shockshock reflectsreflects offoff thethe expansionexpansion rampramp surfacesurface backback intointo thethe higherhigher velocityvelocity exhaustexhaust regionregion beyondbeyond The shock reflects off the expansion ramp surface back into the higher velocity exhaust region beyond thethe nozzlenozzle exitexit causingcausing aa minorminor reductionreduction inin velocity.velocity. the nozzle exit causing a minor reduction in velocity.

Figure 22. Velocity contour of SERN DP456 with combustion at 10 m downstream of the SERN exit. Figure 22. Velocity contourcontour ofof SERNSERN DP456DP456 withwith combustioncombustion atat 1010 mm downstreamdownstream ofof thethe SERNSERN exit.exit. The flow types and their performance in regard to static pressure can be seen in Figure 23. The The flow types and their performance in regard to static pressure can be seen in Figure 23. The combusted flow produces a substantial increase in static pressure in the nozzle. Therefore, the final combusted flow produces a substantial increase in static pressure in the nozzle. Therefore, the final performance characteristics will be substantially altered. Comparison of the axial and net thrust performance characteristics will be substantially altered. Comparison of the axial and net thrust calculated for both DP102 and DP456 for the uniform, cold non-uniform, and combusted non- calculated for both DP102 and DP456 for the uniform, cold non-uniform, and combusted non- uniform flow is provided in Table 11. These were calculated using the Equations (1)–(5) and using uniform flow is provided in Table 11. These were calculated using the Equations (1)–(5) and using the same method applied in the SERN parametric studies for consistency. Accordingly, “Hot DP456- the same method applied in the SERN parametric studies for consistency. Accordingly, “Hot DP456- 2” is the most realistic setup tested. 2” is the most realistic setup tested.

Energies 2018, 11, 3449 22 of 29

The flow types and their performance in regard to static pressure can be seen in Figure 23. The combusted flow produces a substantial increase in static pressure in the nozzle. Therefore, the final performance characteristics will be substantially altered. Comparison of the axial and net thrust calculated for both DP102 and DP456 for the uniform, cold non-uniform, and combusted non-uniform flow is provided in Table 11. These were calculated using the Equations (1)–(5) and using the same Energies 2018, 11, x FOR PEER REVIEW 22 of 29 method applied in the SERN parametric studies for consistency. Accordingly, “Hot DP456-2” is the most realistic setup tested. The uniform flow outputs extremely high pressures and velocity in comparison to the cold flow The uniform flow outputs extremely high pressures and velocity in comparison to the cold and flowcombustion and combustion flow tests. flow The tests. nature The natureof the ofuniform the uniform flow flowmeans means the theperformance performance will will be be over- estimatedover-estimated and limitations and limitations have prevented have prevented accurate accurate domain domain setup setup to produce to produce accurate accurate uniform uniform flow. Regardlessflow. Regardless of the considerably of the considerably lower lower thrust thrust values, values, both both DP102 DP102 andand DP456 DP456 combustion combustio flowsn flows achievedachieved higher higher thrust thrust and and pressures, pressures, for forthe therespective respective geometries, geometries, compared compared to to the the cold cold flow flow tests. Coldtests. uniform Cold uniformand cold and non cold-uniform non-uniform flow flow tests tests provided provided good good insightinsight into into SERN SERN performance performance characteristicscharacteristics and and flow flow structures.structures. Nevertheless, Nevertheless, they lackthey realism lack comparedrealism tocompared modelling combustedto modelling combustedflow through flow through the SERN. the SERN.

FigureFigure 23. 23. PressurePressure variation variation along along the scramjetscramjet for for different different flow flow types. types.

Table 11. Performance comparison of different nozzle inlet conditions at 10,000 ft. and Mach 8. Table 11. Performance comparison of different nozzle inlet conditions at 10,000 ft. and Mach 8. Nozzle Inlet Uniform Uniform Cold Cold Hot Hot Hot Nozzle ConditionsInlet UniformDP102 UniformDP456 DP102Cold DP456Cold DP102Hot DP456Hot DP456-2Hot Conditions DP102 DP456 DP102 DP456 DP102 DP456 DP456-2 Axial thrust (N) 5,144,993 4,829,270 2,117,356 1,992,168 2,316,858 2,085,313 2,098,602 AxialEstimated thrust (N) Net Thrust5,144,993 4,874,588 4,829,270 4,467,074 1,826,5592,117,356 1,732,0031,992,168 2,067,1732,316,858 1,836,5642,085,3131,873,415 2,098,602 EstimatedExit velocity Net (m/s) 3518 3474 2223 2423 2523 2482 2500 4,874,588 4,467,074 1,826,559 1,732,003 2,067,173 1,836,564 1,873,415 ThrustExit Pressure 20,722 20,098 838 558 1826 2122 2083 Exit velocity (m/s) 3518 3474 2223 2423 2523 2482 2500 Exit3.5. Pressure 3D SERN Studies20,722 20,098 838 558 1826 2122 2083 Based on the evaluations on the initial aircraft and engine SERN DP456 was selected. The smaller 3.5. 3D SERN Studies SERN exit height was beneficial towards drag and flow structures over the aircraft body. Therefore, theBased 3D SERNon the studies evaluations were conducted on the initial on DP456 aircraft only. and The engine purpose SERN was DP456 to compare was theselected. 2D results The and smaller SERNdesign exit height the side was fences beneficial for the SERN. towards drag and flow structures over the aircraft body. Therefore, the 3D SERN studies were conducted on DP456 only. The purpose was to compare the 2D results and 3.5.1. Effect of SERN Side Fence Length design the side fences for the SERN. Side fences contain the flow, preventing spillage from the sides of the SERN. Abundant losses 3.5.1in. Effect thrust, of lift, SERN and Side longitudinal Fence Length stability can result from the 3D flow. Two side fence configurations were analyzed, including a small side fence configuration which ended at the cowl while matching the heightSide fences between contain the expansion the flow, ramp preventing and the cowl, spillage and a largefrom side the fence sides which of the covered SERN. the Abundant height up tolosses in thrust,the cowl lift and, and linearly longitudinal extended stability from the cowlcan result to the nozzlefrom the exit. 3D Figure flow. 24 Twoshows side the Machfence distributionconfigurations wereof analyzed the SERNs, inclu withding the small a small and side large fence side fence configuration across the walls,which symmetry, ended at and the different cowl while planes. matching the height between the expansion ramp and the cowl, and a large side fence which covered the height up to the cowl and linearly extended from the cowl to the nozzle exit. Figure 24 shows the Mach distribution of the SERNs with the small and large side fence across the walls, symmetry, and different planes.

Energies 2018, 11, 3449 23 of 29 Energies 2018, 11, x FOR PEER REVIEW 23 of 29

Energies 2018, 11, x FOR PEER REVIEW 23 of 29

FigureFigure 24. 24.Mach Mach number number contours contours for for SERN SERN DP456 with with small small ( top (top) and) and large large (bottom (bottom) side) side fence. fence.

The small side fence allows span-wise expansion to take place immediately after the internal TheThe small small side side fence fence a llowsallows span span--wisewise expansion to to take take place place immediately immediately after after the internalthe internal nozzle section. This degrades the axial flow and induces performance losses throughout the nozzle. nozzlenozzle section. section. This This degrades degrades the the axial axial flow flow and inducesinduces performance performance losses losses throughout throughout the nozzle.the nozzle. At 10 m along the SERN (i.e., third plane), the span-wise expansion Mach increases as it expands into At 10At m 10 along m along the the SERN SERN (i.e. (i.e., third, third plane), plane), thethe span--wisewise expansion expansion Mach Mach increases increases as it as expands it expands into into the freestream and beyond 20 m (i.e., fourth plane) the kernel region seems to form where significant the freestream and beyond 20 m (i.e., fourth plane) the kernel region seems to form where significant the freestreammomentum and is concentrated. beyond 20 Atm (i.e. 20 m,, fourth the flow plane) begins the to ke spillrnel over region to the seems top surface to form which where will significant create momentum is concentrated. At 20 m, the flow begins to spill over to the top surface which will create momentummore losses is concentrated. through turbulence At 20 andm, the high flow vorticity begins flow. to spill However, over to the the large top side surface fence which contains will the create more losses through turbulence and high vorticity flow. However, the large side fence contains the moremajority losses through of the flow turbulence until the endand ofhigh the vorticity expansion flow. ramp. However, The span-wise the large expansion side fence of the contains Mach the majority of the flow until the end of the expansion ramp. The span-wise expansion of the Mach majoritytransition of the is slowerflow until compared the end to the of small the fenceexpansion and it preventsramp. The significant span-wise spillage expansion and flow of turning the Mach transition is slower compared to the small fence and it prevents significant spillage and flow turning upward and over the SERN. As a result, the reduced spillage will prove better thrust performance of transitionupward is andslower over compared the SERN. Asto thea result, small the fence reduced and spillageit prevents will provesignificant better spillage thrust performance and flow turn of ing the nozzle, but the larger wall surface means drag will be increased. upwardthe nozzle and over, but the largerSERN. wall As surfaca result,e means the reduced drag will spillage be increased. will prove better thrust performance of The short fence streamlines the display, and the flow instantly begins expanding span wise at the nozzleThe, but short the fence larger streamlines wall surfac thee displaymeans, dragand the will flow be increased.instantly begins expanding span wise at large angles of divergence (Figure 25). The flow near the symmetry spills out over a short length largeThe shortangles fenceof divergence streamlines (Figure the 25). display The flow, and near the the flow symmetry instantly spills begins out overexpanding a short lengthspan wiseof at of the expansion ramp. The flow turning outward and upward as the expansion ramp gets thinner largethe angles expansion of divergence ramp. The (Figure flow turning 25). The outward flow andnear upward the symmetry as the expansion spills out ramp over gets a short thinner length is of is illustrated which would introduce unpredictable surface pressures on the SERN. A curved shock the expansionillustrated whichramp. would The flow introduce turning unpredictable outward and surface upward pressures as the on expansion the SERN. rampA curved gets shockthinner is formation can be seen attached to the ramp surface which results in separation. The larger fence illustratedformation which can bewould seen introduceattached to unpredictablethe ramp surface surface which pressures results in separation.on the SERN. The Alarger curved fence shock streamlines control the spillage much better compared to the shorter fence. Spillage is present where streamlines control the spillage much better compared to the shorter fence. Spillage is present where formationthe wall can begins be toseen incline attached away from to the the ramp cowl; however,surface thewhich majority results of spillage in separation. is delayed The significantly. larger fence the wall begins to incline away from the cowl; however, the majority of spillage is delayed streamlinesThe divergence control of the the spillage spillage getsmuch greater better as compared the wall gets to smaller the shorter when fence. reaching Spillage the expansion is present ramp where significantly. The divergence of the spillage gets greater as the wall gets smaller when reaching the the wall begins to incline away from the cowl; however, the majority of spillage is delayed trailingexpansion edge. ramp The trailing wall prevents edge. The the wa flowll prevents from circulating the flow upfrom and circulating over the nozzle,up and keeping over the the nozzle, flow significantly.axialkeeping in athe concentratedThe flow divergence axial in region. a concentrated of the spillage region. gets greater as the wall gets smaller when reaching the expansion ramp trailing edge. The wall prevents the flow from circulating up and over the nozzle, keeping the flow axial in a concentrated region.

Figure 25. Mach streamlines for SERN DP456 with small ( left) and large (right) side fence.

3.5.2. Effect of SERN Side Fence Angle FigureFigure 26 25. ill Machustrates streamlines the Mach for contour SERN andDP456 streamlines with small of ( leftDP456) and with large a (largeright )wall side andfence. a 4.29° fence angle. The Mach contour displays similar traits of the large straight wall. The flow spillage is 3.5.2 . Effect of SERN Side Fence Angle Figure 26 illustrates the Mach contour and streamlines of DP456 with a large wall and a 4.29° fence angle. The Mach contour displays similar traits of the large straight wall. The flow spillage is

Energies 2018, 11, 3449 24 of 29

3.5.2. Effect of SERN Side Fence Angle Energies 2018, 11, x FOR PEER REVIEW 24 of 29 Figure 26 illustrates the Mach contour and streamlines of DP456 with a large wall and a 4.29◦ fence angle. The Mach contour displays similar traits of the large straight wall. The flow spillage is restricted along most of the length of the side fence. However, the outer face of the side fence affects restricted along most of the length of the side fence. However, the outer face of the side fence affects the Mach distribution. It expands the flow downward rather as well as span wise. The side fence the Mach distribution. It expands the flow downward rather as well as span wise. The side fence angle delays the rapid span-wise expansion producing less losses from spillage. The streamlines angle delays the rapid span-wise expansion producing less losses from spillage. The streamlines prove prove the span-wise expansion transitions at a much slower rate compared to the large straight side the span-wise expansion transitions at a much slower rate compared to the large straight side fence. fence. The controlled expansion is due to the span-wise divergence of the side fence. The controlled expansion is due to the span-wise divergence of the side fence. The thrust of the SERN DP456 with the different nozzle configurations is shown in Table 12. In The thrust of the SERN DP456 with the different nozzle configurations is shown in Table 12. 2D DP456, the overestimated nozzle inlet boundary conditions produced larger thrust. In addition, In 2D DP456, the overestimated nozzle inlet boundary conditions produced larger thrust. In addition, the span wise expansion was not accounted for reducing the exit velocity and pressure. However, the span wise expansion was not accounted for reducing the exit velocity and pressure. However, with the actual exit area put into the thrust calculation, the final values were significantly increased. with the actual exit area put into the thrust calculation, the final values were significantly increased. The straight large fence approximately doubled both axial and net thrust due to the increased density The straight large fence approximately doubled both axial and net thrust due to the increased density and pressure. The 4.29° fence angle with the large wall saw axial and net thrust increase by 9.63% and pressure. The 4.29◦ fence angle with the large wall saw axial and net thrust increase by 9.63% and and 10.93%, respectively, compared to the large side fence with a 0° fence angle. However, as 10.93%, respectively, compared to the large side fence with a 0◦ fence angle. However, as previously previously established, uniform nozzle inlet flows produce substantial overestimations in established, uniform nozzle inlet flows produce substantial overestimations in performance. performance.

FigureFigure 26.26. Mach contourscontours andand streamlinesstreamlines for for SERN SERN DP456 DP456 with with the the large large side side fence fence configuration configuration at at a 4.29a 4.29°◦ angle. angle.

Table 12. Performance comparison of different nozzle conditions at 100,00 ft. and Mach 8. Table 12. Performance comparison of different nozzle conditions at 100,00 ft. and Mach 8.

Nozzle Inlet 3D3D Small Small Fence Fence 3D3D Large Large Fence Fence 3D Large3D Large Fence Fence Nozzle Inlet Conditions2D DP4562D DP456 ◦ ◦ ◦ Conditions 0 0°Fence Fence Angle Angle 0°0 FenceFence Angle Angle 4.29°4.29 FenceFence Angle Angle AxialAxial thrust thrust (N) (N) 4,829,2704,829,270 7,533,1457,533,145 13,321,487 13,321,487 14,741,256 14,741,256 EstimatedEstimated net Thrust net Thrust (N) (N)4,467,074 4,467,074 6,179,9266,179,926 12,269,893 12,269,893 13,775,863 13,775,863 Exit velocityExit velocity (m/s) (m/s) 34743474 25872587 2502 2502 24682468 Exit pressureExit pressure (Pa) (Pa) 20,09820,098 47324732 11,848 11,848 13,893 13,893

3.6.3.6. 3D Scramjet Engine—SERNEngine—SERN AnalysisAnalysis PostPost individualindividual 3D studies on the scramjet components were completed, the inlet and isolator, twintwin strutstrut combustor,combustor, andand thethe SERNSERN werewere assembled,assembled, withoutwithout anyany modifications,modifications, forfor fullfull scramjetscramjet flowflow fieldfield testingtesting withwith combustion.combustion. The scramjet engine was simulatedsimulated atat operatingoperating conditionsconditions representingrepresenting MachMach 88 cruisecruise atat 100,000100,000 feet.feet. FigureFigure 2727 demonstratesdemonstrates thethe streamlinestreamline flowflow throughthrough andand aroundaround thethe scramjet scramjet engine. engine. The The 3D flow3D behavedflow behaved analogously analogously regarding regarding the 2D twin the strut 2D combustiontwin strut combustion through the SERN with the stream of exhausted combustion containing regions of high velocity and temperature with lower values around these streamlines of the combusted flow. The increased velocity distributed throughout the nozzle results in lower pressure regions reducing the rate of axial expansion downstream of the SERN. The cowl lip expansion is still the region which

Energies 2018, 11, 3449 25 of 29 through the SERN with the stream of exhausted combustion containing regions of high velocity and temperature with lower values around these streamlines of the combusted flow. The increased velocity distributed throughout the nozzle results in lower pressure regions reducing the rate of axial expansion downstream of the SERN. The cowl lip expansion is still the region which observes the fastest span-wise and vertical expansion from the absence of the extended cowl length and side fence height. The density streamline plot at the symmetry shows the complex waves belonging to the families of the expansion fans from the upper expansion ramp surface and the lower cowl surface at the nozzle inlet. The complex waves make up the Mach region in which the Mach increases rapidly before the flow expands into the external flow and beyond the side fence. The complex flow is reflected from the expansion ramp surface and cowl surface as well as the shear layer produced from the cowl. The Mach contours of the SERN flow field from the resultant combusted flow is shown in Energies 2018, 11, x FOR PEER REVIEW 25 of 29 Figure 28. The combusted flow through the SERN does not intensify the spillage beyond the side fence.observes The the distribution fastest span of-wise the exhausted and vertical flow expansion expands fro overm the a small absence area of in the comparison extended cowl to the length flow ◦ seenand inside the fence previous height. side The fence density study withstreamline an angle plot of 4.29at the. The symmetry Mach is reducedshows the significantly complex waves in the 3Dbelonging full scramjet to the combustionfamilies of the simulation expansion for fans the from same the reasons upper asexpansion the 2D cases, ramp due surface to the and increase the lower in temperature.cowl surface Theat the axial nozzle flow inlet. exhausted The complex from the waves SERN make is better up the as theMach expansion region in transitions which the muchMach slowerincreases span-wise rapidly andbefore smaller the flow divergence expands angles into resultthe external in less3D flow losses. and beyond the side fence. The complexThe thrustflow is was reflected calculated from forthe theexpansion 3D scramjet ramp with surface twin and strut cowl combustion surface as utilizing well as the SERN shear DP456 layer ◦ withproduced a 4.29 fromside the fence cowl. angle to ensure the solution accounts for the 3D losses to procure an accurate evaluationThe Mach of the contours overall of engine the SERN performance. flow field The from final the axialresultant and combusted net thrust generated flow is shown by the in SERNFigure ◦ DP45628. The with combusted a side fenceflow through angle of the 4.29 SERNare does 8,362,313 not intensify N and 6,854,746 the spillage N, respectively.beyond the side A reductionfence. The ofdistribution 43.27% and of 50.24%the exhausted occurred; flow however, expands the over full a scramjetsmall area simulation in comparison encompasses to the flow more seen realistic in the conditions,previous side taking fence into study account with thean effectangle ofof the4.29°. prior The components Mach is reduced before significantly the flow field in reached the 3D thefull SERN,scramjet whereas combustion the previous simulation 3D nozzle for simulationsthe same reasons incorporated as the uniform 2D cases, flow conditionsdue to the at increase the nozzle in inlet.temperature. Therefore, The the axial scramjet flow SERN exhausted operates from successfully the SERN when is better integrated as the intoexpansion the scramjet transitions engine much and producesslower span better-wise performance and smaller than divergence anticipated angles from result the previousin less 3D studies. losses.

FigureFigure 27.27. VelocityVelocity andand densitydensitystreamlines streamlines of of the the scramjet scramjet engine. engine.

Figure 28. Mach number contours of the scramjet engine with planes positioned from the nozzle inlet onwards.

Energies 2018, 11, x FOR PEER REVIEW 25 of 29

observes the fastest span-wise and vertical expansion from the absence of the extended cowl length and side fence height. The density streamline plot at the symmetry shows the complex waves belonging to the families of the expansion fans from the upper expansion ramp surface and the lower cowl surface at the nozzle inlet. The complex waves make up the Mach region in which the Mach increases rapidly before the flow expands into the external flow and beyond the side fence. The complex flow is reflected from the expansion ramp surface and cowl surface as well as the shear layer produced from the cowl. The Mach contours of the SERN flow field from the resultant combusted flow is shown in Figure 28. The combusted flow through the SERN does not intensify the spillage beyond the side fence. The distribution of the exhausted flow expands over a small area in comparison to the flow seen in the previous side fence study with an angle of 4.29°. The Mach is reduced significantly in the 3D full scramjet combustion simulation for the same reasons as the 2D cases, due to the increase in temperature. The axial flow exhausted from the SERN is better as the expansion transitions much slower span-wise and smaller divergence angles result in less 3D losses.

Energies 2018, 11, 3449 26 of 29 Figure 27. Velocity and density streamlines of the scramjet engine.

FigureFigure 28. Mach 28. Machnumber number contours contours of the scramjet of the scramjet engine with engine planes with positioned planes positioned from the fromnozzle the inlet nozzle onwards. inlet onwards.

3D Integrated Aircraft and Combined Cycle Engine—SERN Analysis The integration of an engine into aircraft is highly complex. The entire scramjet engine requires careful integration into the airframe to be utilized effectively as it needs to capture the large airflow requirements at high speed. Both the airframe and engine affect each other, therefore they need to be designed conscious of the consequences they have [31]. The integrated aircraft and combined cycle engine CFD were conducted through cold flow tests for operating conditions at Mach 4.5 and 8. Design changes to the cowl were implemented to allow the model to mesh for CFD calculations to take place. Therefore, thrust will not be evaluated but rather the altered flow characteristics of the SERN from the aircraft and combined cycle engine design. At Mach 4.5, the ramjet will begin to reach its limit on maximum performance and the scramjet will be engaged to continue the aircrafts acceleration and climb. The ramjet will be producing the thrust and because of the modified engine designs, the ramjet exhaust flow will spill into the SERN exhaust flow. Figure 29 displays the velocity streamlines, Mach contours, and volume flowing downstream over the aircraft, from the ramjet nozzle and the SERN. The streamlines show the flow over the wing and ramjet nozzle cause vortices to form. Through the spiraling flow, the velocity decreases and flow is increasingly turbulent. Combusted flow through the ramjet nozzle would likely reduce the spillage from the exhaust plume into the SERN jet stream. Currently, the ramjet exhaust flow channels through the downstream expansion of the SERN which may reduce the axial thrust performance for both the ramjet nozzle and the SERN. However, the vortices generated from the upper surface of the wing and after body will most likely be present throughout the duration of the flight. Subsequently, part of the flow through the SERN separates near the upper expansion ramp trailing edge, contributing to the vortex generation. When the ramjet is disengaged, the engine will be shut off by a flap directing flow around the side of the aircraft. Figure 30 shows the streamline flow, Mach contours, and volume to visualize the SERN exhaust flow at Mach 8. The SERN still expands the flow efficiently as it increases the Mach flow from approximately Mach 2 inside the scramjet, to approximately Mach 5 through the SERN. The Mach volume and streamlines still exhibit the vortices being formed from the ramjet nozzle with the blended wall and the upper surface of the proposed aircraft. The extreme vorticity may have a considerable effect on the combusted exhaust plume from the upper strut injector. The axial thrust will be worsened as these vortices will promote separation from at the upper surface of the SERN. In this vortex region it can be seen that the Mach is reduced significantly and will add to the drag. The changed cowl design deteriorates the lower expansion region which produces misleading results of the SERN integrated into the proposed aircraft. Cold flow contributes to the considerable decrease in performance expectations. Energies 2018, 11, 3449 27 of 29 Energies 2018,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 27 of 29

FigureFigure 29. Velocity 29. Velocity streamlines streamlines and and displaying displaying the effecteffect on on the the SERN SERN performance performance at Mach at Mach 4.5. 4.5.

FigureFigure 30. 30.MachMach streamlines, streamlines, contours contours,,, and and volume of of the the proposed proposed aircraft aircraft at Mach at Mach 8. 8. 4. Conclusions 4. Conclusions The study investigated SERNs for a scramjet required to propel an aircraft for sustained hypersonic flightThe study at Mach investigated 8. An efficient SERNs SERN geometryfor a scramjet was obtained required through to propel 2D and 3Dan parametricaircraft for analysis sustained hypersonicand through flight 3D at Mach combustion 8. An efficient modelling SERN of the geometry scramjet. was It provides obtained further through detailed 2D and analysis 3D parametric to the analysisoverall and aircraft through configuration 3D combustion published modelling previously of asthe part scramjet. of this projectIt provides by References further detailed [2,4,32–34 analysis]. toto thethe overalloverallThemain aircraftaircraft conclusions configurationconfiguration of this studypublished are: previously as part of this project by References [2,,4,,32– 34]. Numerous geometries were produced with different performance benefits. The chosen SERN designThe main exerts conclusion equivalents of performance this study overare: a range of altitudes and Mach numbers demonstrating adaptabilityNumerous andgeometries robustness were in off-design produced conditions. with different performance benefits. The chosen SERN The sensitivity analysis revealed that thrust can be improved through contouring the SERN. design exerts equivalent performance over a range of altitudes and Mach numbers demonstrating The best contoured SERN configuration produced 10.68% more thrust than the best conical SERN with adaptability and robustnesstness inin offoff-design conditions. uniform inlet flow, including larger exit areas, and smaller initial expansion ramp angles due to their expansionThe sensitivity capabilities. analysis revealed that thrust can be improved through contouring the SERN. The best contouredDrag was SERN found configuration to increase with produced increase 10.68% in SERN more length. thrust Lift than and thethe moments bestbest conicalconical are influenced SERNSERN withwith uniformthrough inlet the flow, geometries’ including effect larger on theexit incoming areas,, and flow smaller profile initial and pressure expansion distribution ramp angles between due theto their expansionexpansion capabilities. ramp and cowl. DragThrough was found 3D combustion to increase modelling with increase analysis, in side SERN fences length. extending Lift toand the moments SERN trailing are edgeinfluenced at ◦ throughthroughan angle thethe ofgeometries 4.29 , was’’ foundeffect toon enhance the incoming thrust by flow approximately profile and 10.93% pressure compared distribution to a straight betwe fence.en the expansionDespite ramp theand in-depth cowl. analysis conducted within this report, there is still further refinements to be doneThrough which 3D could combustion enhance the modelling final outcome analysis, of the side SERN fences and engineextending integration. to the SERN This includes, trailing edge at an angle(a) ofSERN 4.29°, parametric was found studies to enhance involving thrust the combustion by approximately exit conditions 10.93% to identifycompared the to optimal a straight design fence. Despitein more the realisticinin-depth operating analysis and conducted engine design within conditions. this report, there is still further refinements to be done(b) whichCFD analysis could enhance of the scramjet the final engine outcome through of each the stageSERN of and the flight engineine profile, integration.integration. i.e., from ThisThis subsonic includes,includes, to hypersonic, with altitude, cold, and combusted flow included, to gather definite performance (a) SERN parametrictric studiesstudies involvinginvolving thethe combustioncombustion exitexit conditionsconditions toto identifyidentify thethe optimaloptimal designdesign and effects of the scramjet engine through the entire mission profile. inin moremore realisticrealistic operatinoperating and engine design conditions. (b) CFD analysis of the scramjet engine through each stage of the flight profile, i.e.,, from subsonic toto hypersonic,hypersonic, with altitude, cold,, and combusted flow included, to gather definite performance and effects of the scramjet engine through the entire mission profile. (c) Perform scramjet combustion tests on the final aircraft for cruise conditions to evaluate the performance from a fully-integratedintegrated aircraft and engine standpoint. (d) Thermochemical non-equilibrium effects on the location of shock waves in the intake and in the expansion of high enthalpy flows for combustion [35].

Energies 2018, 11, 3449 28 of 29

(c) Perform scramjet combustion tests on the final aircraft for cruise conditions to evaluate the performance from a fully-integrated aircraft and engine standpoint. (d) Thermochemical non-equilibrium effects on the location of shock waves in the intake and in the expansion of high enthalpy flows for combustion [35].

Author Contributions: A.R. was the research student that conducted the detailed study and A.A.S. wrote the paper and checked the validity of the methodology and results. A.P. conceived of the project, created the layout of the investigations, and checked the computational outcome of the resultant modelling effort and subsequent discussion. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Anderson, J.D., Jr. Hypersonic and High Temperature Gas Dynamics, 2nd ed.; AIAA: Reston, VA, USA, 2006. 2. Baidya, R.; Pesyridis, A.; Cooper, M. Ramjet Nozzle Analysis for Transport Aircraft Configuration for Sustained Hypersonic Flight. Appl. Sci. 2018, 8, 574. [CrossRef] 3. Borgon, J. Limitations in the operational range of turbojet engines. Technica Lotnicza Astronautyczna 1977, 32, 27–29. 4. Neill, S.M.; Pesyridis, A. Modeling of Supersonic Combustion Systems for Sustained Hypersonic Flight. Energies 2017, 10, 1900. [CrossRef] 5. Fry, R.S. A Century of Ramjet Propulsion Technology Evolution. J. Propuls. Power 2004, 20, 27–58. [CrossRef] 6. Curran, E.T. Scramjet Engines: The First Forty Years. J. Propuls. Power 2001, 17, 1138–1148. [CrossRef] 7. Heiser, W.; Daley, D.; Pratt, D. Hypersonic Airbreathing Propulsion; American Institute of Aeronautics and Astronautics: Washington, DC, USA, 1994; ISBN 978-1-56347-035-6. 8. Curran, E.T.; Murthy, S. Scramjet Propulsion, 1st ed.; Murthy, S.N., Ed.; Progress in Astronautics and Aeronautics; AIAA: Reston, VA, USA, 2001. 9. Khandai, S.C.; Parammasivam, K.M. Experimental Study of Single Expansion Ramp Nozzle Flows (SERN) at Low Supersonic Speeds. Int. J. Mech. Mechatron. Eng. 2014, 14, 84–89. 10. Bowers, D.; Laughrey, J. Survey on Techniques Used in Aerodynamic Nozzle Airframe Integration; Advisory Group for Aerospace Research and Development: Neuilly sur Seine, France, 1992. 11. Perrier, P.; Rapuc, M.; Rostand, P.; Hallard, R.; Regard, D.; DuFour, A.; Pennanhoat, O. Nozzle and Afterbody Design for Hypersonic Airbreathing Vehicles. In Proceedings of the Space Plane and Hypersonic Systems and Technology Conference, Norfolk, VA, USA, 18–22 November 1996. 12. Zellner, B.; Sterr, W.; Herrmann, O. Integration of Turbo-Expander and Turbo-Ramjet Engines in Hypersonic Vehicles. J. Eng. Gas Turb. Power 1994, 116, 90. [CrossRef] 13. Hirschen, C.; Gulhan, A. Influence of Heat Capacity Ratio on Pressure and Nozzle Flow of Scramjets. J. Propuls. Power 2009, 25, 303–311. [CrossRef] 14. Spaid, F.W.; Keener, E.R. Experimental Results for a Hypersonic Nozzle/Afterbody Flow Field. In Proceedings of the 28th Joint Propulsion Conference and Exhibit, Nashville, TN, USA, 6–8 July 1992. 15. Dalle, D.J.; Torrez, S.M.; Driscoll, J.F. Reduced-Order Modeling of Reacting Supersonic Flows in Scramjet Nozzles. In Proceedings of the 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Nashville, TN, USA, 25–28 July 2010. 16. Bonelli, F.; Cutrone, L.; Votta, R.; Viggiano, A.; Magi, V. Preliminary design of a hypersonic air-breathing vehicle. In Proceedings of the 17th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, San Franciso, CA, USA, 11–14 April 2011. 17. Thiagaraian, V.; Panneerselvam, S.; Rathakrishnan, E. Numerical Flow Visualization of Single Expansion Ramp Nozzle with Hypersonic External Flow. J. Visual. 2006, 9, 91–99. [CrossRef] 18. Huang, W.; Wang, Z.; Ingham, D.B.; Ma, L.; Pourkashanian, M. Design Exploration for a Single Expansion Ramp Nozzle (SERN) Using Data Mining. Acta Astron. 2013, 83, 10–17. [CrossRef] 19. Jianping, L.; Wenyan, S.; Ying, X.; Feiteng, L. Influences of Geometric Parameters upon Nozzle Performances in Scramjets. Chin. J. Aeronaut. 2008, 21, 506–511. [CrossRef] Energies 2018, 11, 3449 29 of 29

20. Marathe, A.G.; Thiagaraian, V. Effect of Geometric Parameters on the Performance of Single Expansion Ramp Nozzle. In Proceedings of the 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Tucson, AZ, USA, 10–13 July 2005. 21. Candon, M.J.; Ogawa, H.; Dorrington, G.E. Thrust Augmentation through After-Burning in Scramjet Nozzles. Adv. Aircr. Spacecr. Sci. 2015, 2, 183–198. [CrossRef] 22. Gülçat, Ü. Fundamentals of Modern Unsteady Aerodynamics, 2nd ed.; Springer: Berlin, Germany, 2006. 23. Viviani, A.; Pezzella, G. Aerodynamic and Aerothermodynamic Analysis of Space Mission Vehicles; Springer: Berlin, Germany, 2015. 24. Slater, J.W.; Saunders, J.D. Computational Fluid Dynamics (CFD) Simulation of Hypersonic Turbine-Based Combined-Cycle (TBCC) Inlet Mode Transition. In Proceedings of the 16th AIAA/DLR/DGLR International Space Planes and Hypersonic Systems and Technologies Conference, Bremen, Germany, 19–22 October 2009. 25. Candon, M.J.; Ogawa, H. Thrust Augmentation Optimization through Supersonic After-burning in Scramjet Engine Nozzles via Surrogate-assisted Evolutionary Algorithms. Acta Astron. 2015, 116, 132–147. [CrossRef] 26. Murugan, T.; De, S.; Thiagarajan, V. Validation of Three-Dimensional Simulation of Flow through Hypersonic Air-breathing Engine. Defense Sci. J. 2015, 65, 272–278. [CrossRef] 27. Zhang, S.; Li, J.; Qin, F.; Huang, Z.; Xue, R. Numerical Investigation of Combustion Field of Hypervelocity Scramjet Engine. Acta Astron. 2016, 129, 357–366. [CrossRef] 28. Savino, R.; Russo, G.; D’Oriano, V.; Visone, M.; Battipede, M.; Gili, P. Performances of a Small Hypersonic Airplane (HyPlane). Acta Astron. 2015, 115, 338–348. [CrossRef] 29. Monta, W. Pitot Survey of Exhaust Flow Field of a 2-D Scramjet Nozzle at Mach 6 with Air or Freon and Argon Used for Exhaust Simulation; NASA-Technical Report, NASA-TM-4361; NASA Langley Research Center: Hampton, VA, USA, 1992. 30. Ogawa, H.; Boyce, R. Nozzle Design Optimization for Axisymmetric Scramjets by Using Surrogate-Assisted Evolutionary Algorithms. J. Propuls. Power 2012, 28, 1324–1338. [CrossRef] 31. Mcclinton, C.R. High Speed/Hypersonic Aircraft Propulsion Technology Development. In Advances on Propulsion Technology for High-Speed Aircraft; Educational Notes RTO-EN-AVT-150; RTO: Neuilly-sur-Seine, France, 2008. 32. Alkaya, C.; Sam, A.A.; Pesyridis, A. Conceptual Advanced Transport Aircraft Design Configuration for Sustained Hypersonic Flight. Aerospace 2018, 5, 91. [CrossRef] 33. Veeran, S.; Pesyridis, A.; Ganippa, L. Ramjet Compression System for a Hypersonic Air Transportation Vehicle Combined Cycle Engine. Energies 2018, 11, 2558. [CrossRef] 34. Sen, D.; Pesyridis, A.; Lenton, A. A Scramjet Compression System for Hypersonic Air Transportation Vehicle Combined Cycle Engines. Energies 2018, 11, 1568. [CrossRef] 35. Bonelli, F.; Tuttafesta, M.; Colonna, G.; Pascazio, G. An MPI-CUDA approach for hypersonic flows with detailed state-to-state air kinetics using a GPU cluster. Comput. Phys. Commun. 2017, 219, 178–195. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).