Experimental Investigation Into Shock Waves Formation and Development Process in Transonic Ow

Scientia Iranica B (2017) 24(5), 2457{2465 Sharif University of Technology Scientia Iranica Transactions B: Mechanical Engineering www.scientiairanica.com Experimental investigation into shock waves formation and development process in transonic ow M. Farahani and A. Jaberi Department of Aerospace Engineering, Sharif University of Technology, Tehran, Iran. Received 11 May 2016; received in revised form 12 August 2016; accepted 8 October 2016 KEYWORDS Abstract. An extensive experimental investigation was performed to explore the shock waves formation and development process in transonic ow. Shadowgraph visualization Shock waves technique was employed to provide visual description of the ow eld features. Based on formation process; the visualization, the formation process was categorized into two intrinsically di erent Transonic ow; phases, namely, subsonic and supersonic. The characteristics of subsonic phase are well Shadowgraphy known; however, those of the supersonic one are far less studied. The supersonic phase visualization; itself is made up of two consecutive phases, namely, approaching and sweeping. The e ects Splitting of shock of each phase on the ow eld characteristics and on shaping the supersonic regime were waves; studied in details. In order to generalize the results, three di erent models were tested. A Zeh shock. special terminology is suggested by authors to ease the process description and pave a way for future studies. Above all, as the transition from transonic regime to the supersonic one is a vague concept in terms of physical reasoning, a new explanation is proposed that can be used as a criterion for distinguishing between transonic and supersonic regimes. © 2017 Sharif University of Technology. All rights reserved. 1. Introduction streamlines tangent along the non-penetrating surface or/and, second, to compensate for the pressure di er- Knowledge of the ow conditions over a body in ence between upstream and downstream and satisfy transonic and supersonic speeds is of great importance pressure boundary conditions. The former is usually in various aspects, particularly the appearance of shock seen in external ows and the latter mostly happens in waves. These established shocks have profound e ects nozzles and di users. In some supersonic wind tunnels on the aerodynamic characteristics of a vehicle and, such as the one used here, at rst, the nozzle exit therefore, a detailed investigation is necessary. A to throat ratio is set; then, during the startup and principal phenomenon related to shock waves is their after the ow reaches sonic conditions at the throat, formation and development process that takes place a primary normal shock appears, which separates within transonic regime. This process has been studied supersonic and subsonic regions. In order to reach the by many research studies; however, there are still many determined Mach number at test section by increasing aspects that require further investigation. blowing power of wind tunnel engine, the normal shock Broadly speaking, a shock wave occurs in super- moves downstream and becomes stronger. In this sonic ow because of two reasons: rst, to turn the phase, acceleration of the ow results in the variable strength of the normal shock wave, which is the main source of unsteadiness. On the other hand, in the *. Corresponding author. Tel.: +98 21 66022731; Fax: +98 21 66022731 presence of a model in test section, some shock or E-mail address: [email protected] (M. Farahani) expansion waves would be established over it to ensure that the ow remains in the direction of the body doi: 10.24200/sci.2017.4309 surface everywhere. Due to this, the strengths of 2458 M. Farahani and A. Jaberi/Scientia Iranica, Transactions B: Mechanical Engineering 24 (2017) 2457{2465 these waves vary by acceleration of ow as they are development, qualitatively, in an attempt to illustrate dependent upon the upcoming normal shock. This its essential features and, further, to provide a tentative introduces the second source of unsteadiness in this basis for more rigorous investigations in future. In case. Therefore, the current investigation deals with the following, in Section 2, wind tunnel and testing the transient case of transonic regime. systems will be described. The results will be discussed The formation process can consist two consecu- and interpreted in Section 3, and the corresponding tive phases; the rst begins as the free-stream Mach conclusions are summarized in Section 4. number reaches its critical value and the second starts when M1 passes the unity. Numerous studies have 2. Experimental details investigated the rst phase of the formation process (e.g., [1-6]); establishment of lambda shock waves, All tests are conducted in a suction type Trisonic wind interaction with boundary layer, and separation under tunnel with a Mach number capability of 0.4-2.2. It di erent circumstances are the main focus of such has a test section of 60 cm 60 cm 120 cm and is studies. In contrast, there are only a few works equipped with various internal strain gauge balances, regarding the case when the free-stream ow passes pressure transducers, high-speed camera, shadowgraph the sound speed. Patz [7] investigated the formation system, etc. A detailed description of the wind tunnel of bow waves around blunt bodies in the ow behind can be found in [10]. a mowing shock and derived stand-o distances and Three models with di erent con gurations and formation times for various blunt bodies. Davies [8] di erent nose shapes are provided for tests. The rst also experimentally explored the shock waves structure model is an axisymmetric body with a blunt nose and a when a primary normal shock passed over a blunt body; maximum diameter of 64.3 mm. The detailed descrip- further, he suggested a schematic description of the tion of the model geometry and its nomenclature is formation process, which was reproduced with very given in Figure 1(a). This geometrical nomenclature is more details in this work. used to ease the interpretation of ow characteristics. In this survey, both phases are investigated and The second model, the ogive one, is a long axisym- organized in details; but, since less attention has been metric body and has a neness ratio of 2.5 equipped paid to the supersonic phase, an in-depth e ort is with a circular-arc, ogival nose tangent to a cylindrical put to identify its unique characteristics. The process after-body with L=d = 15 (Figure 1(b)). Moreover, continues until the transonic regime ends and the in order to study the e ects of cross section changes supersonic regime begins. To authors' knowledge, there on the formation process, a belt with inclination angle is no approved physical explanation to determine this of 5 degrees is installed on its cylindrical section. In transition, even though Mach number of 1.2, which is addition, a guitar wire with a diameter of 0.9 mm is strictly a rule of thumb, is usually implemented. In this installed at x=d = 6:25 to induce disturbances into the study, a physical de nition is proposed to di erentiate ow eld. These two cases are very helpful to survey between transonic and supersonic regimes. the supersonic phase of the shock formation process. To follow the formation and development pro- The third model, namely, the inlet model, is a cesses of shock waves in details, visualization meth- supersonic axisymmetric external compression model, ods seem to be very useful tools. In this research, as shown in Figure 1(c), of which the main model parts shadowgraph technique is employed. Shadowgraphy are speci ed. The main parts include an external cowl is the most ubiquitous and simplest of all the optical that produces ow compression and is connected to an ow diagnostics, which is also the best for imaging axisymmetric subsonic di user, consisting of a cowl and shock waves. It reveals shock waves clearly while de- a spike. The selection of these models is based on their emphasizing other less abrupt ow features [9]. di erent nose shapes, which mostly a ect the char- The supersonic phase of shock wave formation acteristics of supersonic phase. In e ect, blunt nose, and development process is described thoroughly for sharp nose, and inlet nose models cover nearly all possi- the model and new phenomena and characteristics bilities that can happen in the shock formation process. are introduced. Obviously, these results cannot be Tests are performed at free-stream Mach numbers con rmed and generalized if only one case is taken ranging from 0.6 to 2.0. For recording the shadow- under consideration. For this sake, shadowgraphs of graphs, a CASIO EX-F1 digital video camera is used, three models are compared. Further, the e ect of nose which provides high-speed imaging. It can record 300 shape on the process is investigated. frames per second with a resolution of 512 384 pixels Experimental studies are necessary in order to and is capable of 1200 fps at further reduced image size. provide \benchmark" data for CFD code validation and, primarily, to understand the uid dynamics of a 3. Results and discussion phenomenon. The present study falls within the latter category. It describes the shock system formation and Based on the free-stream Mach number, shock wave M. Farahani and A. Jaberi/Scientia Iranica, Transactions B: Mechanical Engineering 24 (2017) 2457{2465 2459 Figure 2. Shadowgraphic visualization of the subsonic phase around model A. begins with the rst appearance of a shock wave over the model (critical free-stream Mach number) in Step 1 and ends with the rst appearance of the shock in front of the model (supersonic free-stream Mach number) in Figure 1. Schematic of di erent models used (all linear Step 12. dimensions in calibers): (a) Model A: its geometry, and At the rst step, the ow expansion after the nomenclature, (b) model B: Ogive model Perturbed wire parabola-cylinder junction produces a locally super- (up), belt-installed (down), and (c) model C: inlet model.

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