Chinese Journal of Aeronautics Chinese Journal of Aeronautics 23(2010) 312-319 www.elsevier.com/locate/cja

Investigation of Vortex Interaction in -FSW Configurations Based on the Numerical Wind Tunnel Method Zhang Guoqing, Yang Shuxing*, Xu Yong Institute of Aerospace Science and Technology, Beijing Institute of Technology, Beijing 100081, China

Received 31 May 2009; accepted 12 October 2009

Abstract The vortex interference mechanism on low Reynolds number between the canard and main of the canard-forward sweep wing (Canard-FSW) configurations is simulated numerically by employing the numerical wind tunnel method. The variations of aerodynamic characteristics of Canard-FSW configurations with different positions of the canard are investigated, finding that the aerodynamic interference and mutual coupling effect between the canard and main wing have made great contributions to the lift and stability characteristics of the whole . Canard can radically improve the surface flow pattern of the main wing. And its own vortex can have a favorable interference on the main wing and can effectively control the airflow separation. At small angles of attack, the aerodynamic characteristics are sensitive to the positions of the canard and the main wing, but at high angles of attack, the aerodynamic performances of the configuration are not only related to the shape of the canard (forward or backward), but also with the size of control force as well as the features of the vortices generated above the main wing and the canard. The different configurations and vortices are illustrated using the velocity vector, streamlines and pressure contours.

Keywords: cluster system; Canard-FSW; backward sweep canard; forward sweep canard; vortex; interaction; mesh

1. Introduction1 that the canard can reduce shock waves between the leading-edge vortex and the upper surface of the main With the development of new aviation technology, wing as well as strengthen the leading-edge vortexes the advanced aerodynamic performance of modern air and delay the vortex breakdown. fighters has received more and more attention. Even The canard plays many important roles in the FSW the overload mobility has become one of the basic configuration, and the interference of vortices gener- features of the new generation of fighter planes. ated by the canard and the main wing leads to better Since the mid-1960s when H.Behrbohm, a Swedish lift characteristics[5]. The canard vortex can effectively professor in near decoupling, discovered the advan- control separation of the boundary layer on the lead- tages of the close-coupled canard/main wing configu- ing-edge of the main wing and develop an effective [1] ration , it has found wide application in de- control on the vortex over the main wing[6-10]. How- signs. The aircraft equipped with forward sweep wing ever, at present, the aerodynamic characteristics and (FSW) have many advantages, such as better stall the interferences between the canard and the FSW characteristics at high angles of attack, better have not been fully understood. low-speed maneuverability, lower wave resistance, This article applied the three-dimensional computa- [2] etc . J. Er-El, et al. studied the vortex interference tional fluid dynamics(CFD) technique to studying the mechanism of the canard/main wing configuration at characteristics of geometry configura- [3] high angles of attack . G. E. Erickson, et al. used oil tions with various angles of attack. The results are [4] and steam laser-screen methods to investigate the analyzed by means of diagrams of velocity vectors and surface and space flow patterns of FSW with and pressure contours rather than wind tunnel tests, which without canard to be equipped. The results displayed are deficient in more consumed time and less cost effi- ciency. *Corresponding author. Tel.: +86-10-68915842. E-mail address: [email protected] 2. Setup for Numerical Simulation Foundation item: National Defense Basic Research Program (B222006060) 2.1. Size function and grid technology

1000-9361/$ - see front matter © 2010 Elsevier Ltd. All rights reserved. In the numerical simulation of the flow field over doi: 10.1016/S1000-9361(09)60221-1

No.3 Zhang Guoqing et al. / Chinese Journal of Aeronautics 23(2010) 312-319 · 313 · the whole aircraft, the size of the mesh reaches over Both the main wing and the canard are FX63-137 2.5 million. An ordinary computer and a single server with length 6 in (1 in=25.4 mm) and memory are unable to produce reliable accuracy and aspect ratio 2.67. The dynamic deflection of the feasibility. So this numerical experiment is operated FX63-137 has been simulated numerically, and using a high-performance cluster system structured the impacts on the lift and drag characteristics of the with parallel cluster architecture. This system consists made by different angles of the canard of 36 Itanium 2 processors and a switchboard with 24 and main wing δ (αc, αw) have been analyzed. The interface infiniband. Network management is equipped model attached to the balance is rotated (δ = −15°-20°) with 3Com 100 MB 24 interface infiniband of one in order to record any aerodynamic hysteretic effects. switchboard. Theoretical peak of calculation can reach As shown in Fig.2，it can be seen that the numerical 230.4 Glops (Glops is 10 billion calculations per sec- results are in good agreement with the wind tunnel ond). experimental data. After the reliability of the numerical Firstly, the simulation is run at Reynolds number method has been verified, the simulation of the whole Re = 85 000, as shown in Fig.1(a), and all the parame- aircraft can get started. ters of the model are the same as the wind tunnel test performed by D. F. Scharpf, et al[11]. Fig.1(b) is the schematic of the computational domain. And the mesh around in configuration on a cross section has been given in Fig.1(c).

Fig.2 Variation of lift characteristics of main wing.

When modeling the whole aircraft, the results are significantly affected by the quality of the mesh. As shown in Fig.3, in mesh generation around the main wing and the canard, the method of size function con- trolling grid size is adopted to improve the mesh[12-13]

[11] Fig.1 Wind tunnel test of D. F. Scharpf, et al and network Fig.3 Three dimensional view of computational domain diagram of calculated pattern. and mesh.

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(It does not change the entire structure of the network responds to the no-canard case. Then in the BSC topology). The radial length of the computational do- configurations, 2# stands for the length from the lead- main is 30 times as large as the cross section of the ing-edge of the canard to the nose accounting for 42% . The computational domain is divided into 24 of the length of the fuselage and 3# for 17%. And in sub-regions to generate structural mesh, concentrating the FSC configuration, 4#, 5# and 6# represent the on the regions close to the body surface. And the same lengths of 54%, 42% and 17% respectively. The whole mesh has been smoothed and swapped, which high canard is reckoned to be located at +1/6 of the can rebuild the nodes, modify the connectivity of units canard chord from the axis of fuselage and the low and improve the accuracy of the calculation. canard at −1/6. The velocity of inlet, Reynolds number Re and temperature are set to be 20 m/s, Re=1.6×105 2.2. Experimental model and boundary conditions and 300 K respectively. In the numerical simulation, the relative pressure is adopted instead of absolute In order to show the process of the simulation, Table 1 pressure, which seems too large in value. By using the has listed all the parameters of aerodynamic experi- couple-implicit solver, Simple iteration is adopted with mental model, which include the main wing and for- k-ε turbulence model and wall functions. ward sweep canard (FSC)/backward sweep canard (BSC). 3. Results and Analysis Table 1 Parameters of aerodynamic experimental model 3.1. Effects of longitudinal positions of BSC Geometric parameters FSW FSC/BSC Reference areas /m2 0.056 25 0.006 80 The vortex interferences around the canard and the Span /m 0.437 68 0.189 20 main wing fall into two broad categories: swirling and inducing[6]. The swirling is a direct interference, which Root chord /m 0.115 0.060 occurs between two relatively close vortices, and is Tip chord /m 0.056 0.020 generated through the shear layer. The other interfer- Aspect ratio 5.12 4.73 ence is the indirect inducing, which occurs between Root-tip ratio 0.487 0.333 two relatively far vortices and is irrelevant to the shear Forward/backward swept layer. And Fig.4 shows the lift, drag and pitching mo- −21, −34 46, 26 angle/(°) ment characteristics of the longitudinal positions of BSC. Based on this, we use the streamlines to pursue After giving the parameters of model, we will de- the airflow, and Fig.5 gives the vortex constructions of scribe the meaning of various symbols. Firstly 1# cor- all the positions.

Fig.4 Comparison of lift, drag and pitching moment characteristics in different longitudinal positions of BSC.

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Fig.5 Streamlines on a cross section perpendicular to body axis in different longitudinal positions of BSC, at α = 20°.

From Fig.4(a) we can see that compared with 1# and main wing have not generated, if the canard moves (non-canard), the longitudinal aerodynamic character- closer to the main wing, the downwash would become istics of 2# and 3# have been improved significantly. stronger, thereby reducing the effective angles of at-

The CLα (α = 40°) has increased about 47.27% and tack of the main wing and postpone the generation of 37.37%, while CLmax about 44.69% and 35.45%. We vortexes over the leading-edge of the main wing. But can conclude that the aerodynamic interference and the upwash effect that the main wing exerts on the mutual coupling between the canard and main wing canard becomes stronger. The net effect results in al- have made great contribution to the lift characteristics most absence of obvious changes in the longitudinal and stability of the whole aircraft. aerodynamic characteristics. But when α = 20°, as In contrast, as shown in Fig.4(b), the drag charac- shown in Fig.5, we can see that a strong concentrated teristics do not show clear changes when α ≤ 10°. On vortex is generated above the surface of closed-couple the one hand, the Canard-FSW configuration can main wing. This vortex is induced by the pressure dif- enlarge the surface area of the entire aircraft so as to ference between the upper surface and lower surface, increase friction resistance, on the other hand, the dy- which generates a vertical velocity component and namic pressure of flow around the main wing after drives the flow to separate and wrap up from the lead- passing the canard is lower than that without passing a ing edge. The vortex induces negative pressure on the canard, and the effective of the main upper surface of the main wing, which can provide wing is reduced owing to the downwash effect so as to substantial vortex lift. These contributions account for a decrease the drag coefficient. The two effects cancel very large proportion in the total lift force and the lift each other and make the drag to be almost unchanged. curve shows nonlinear effects. With the BSC moves But with the increase of angles of attack, the rise of closer to the main wing, the upwash on the canard cre- lift-related drag becomes more and more dominating ated by the main wing would become stronger, and it and finally produces larger net drag coefficient. With would form a secondary vortex inside the canard, which the development of the laminar flow technology makes the lift characteristics benefit from the canard (Mixed-Flow Control, HLFC) and the turbulent drag interference and proves that the close coupling configu- reduction technologies[14-15], the friction of Ca- ration has better longitudinal dynamic characteristics. nard-FSW configuration is expected to be calculated more accurately. 3.2. Effects of vertical positions of BSC As can be seen in the Fig.5, when α < 10°, the change of vertical canard positions for 2# and 3# has Fig.6 shows the lift, drag and pitching moment little connection with the longitudinal aerodynamic characteristics of the vertical positions of BSC, and characteristics of the whole aircraft. That is because at Fig.7 gives the comparison of velocity v fields by small angles of attack, the vortices above the canard means of the velocity vector.

Fig.6 Comparison of lift, drag and pitching moment characteristics in different vertical positions of BSC.

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Fig.7 Comparison of velocity fields of BSC (tip of canard position) at α = 40°.

From Figs.6-7 we can see that throughout the an- the main wing, the downwash created by the low gles of attack considered, the longitudinal aerody- canard becomes stronger; the effective angles of at- namic characteristics of high canard are better than tack of the main wing decrease and the extra lift as- those of the low canard combination. When α < 10°, sociated with the vortex will decrease. Consequently, we can see that there is no vortex breakdown above at all angles of attack, the lift characteristics with the the main wing and the canard. This “conventional” high canard are better than those with the low canard. flow pattern makes the lift and drag characteristics Although the drag characteristics keep the same trend insensitive to the vertical position of the canard. But as the lift, they show insignificant changes because when α < 10°, the vortex created by the low canard the friction plays a dominant part at high angles of is obviously much weaker than that of the high ca- attack. nard created ones and breaks down earlier, which will reduce the vortex lift force created by the canard. 3.3. Effects of longitudinal positions of FSC In addition, the influence of backward-high positions on the main wing is weakening. The control force on In FSC configuration, the lift, drag and pitching the main wing vortex created by the low canard is moment characteristics have been given in Fig.8, and weak and the main wing vortex is unstable, which based on this, the streamlines have shown the vortex makes the flow separate earlier and the lift created by constructions of all the positions in Fig.9. the vortex drop sharply. If the canard moves closer to

Fig.8 Comparison of lift, drag and pitching moment characteristics in different longitudinal positions of FSC.

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Fig.9 Streamlines on a cross section perpendicular to body axis in different longitudinal positions of BSC at α = 20°.

From Figs.8-9, we can see that the aerodynamic and the vortex center is closer to the surface of main characteristics of the FSC configuration are signifi- wing. This vortex almost covers more than 60% span cantly different from the BSC configuration. At all of length of main wing. The control over the flow of the angles of attack considered, when the canard main wing has been strengthened, and the vortex in- moves closer to the main wing, the aerodynamic char- tensity of 5# is also very strong. But its vortex center is acteristics do not become better and alternatively, there further away from the surface of the main wing, and is a neutral position (42% of the fuselage), just as 5#, the ability to control the airflow of the main wing is after which both of the close-coupled and far- weaker than 4#. The closer is the canard to the main ther-lower positions have better aerodynamic wing, the stronger the upwash created by the wing will characteristics such as 4# (54% of the fuselage) and 6# be. So the vortex intensity created by the canard is (17% of the fuselage). quite strong and makes a great contribution to the vor- This is mainly due to the opposite trend of the sur- tex lift to produce good lift characteristics. face flow pattern of the FSC and BSC. The flow of the FSC is always deflected to the symmetric plane of the 3.4. Effects of vertical positions of FSC wing, and the air coming from the head canard would mainly focus on the inside of the main wing, which is In FSC configuration, Fig.10 shows the lift, drag just the worst root separation zone of the main wing. and pitching moment characteristics of the vertical In the FSW configuration, the vortex created by FSC positions and the velocity fields have also been given appears earlier, which also breaks down earlier. The in Fig.11. vortex lift force will decrease, speeding up the flow From Figs.10-11 we can see that the aerodynamic separation of the root main wing. So in the FSC con- characteristics of the FSC are also very different with figuration the downwash on the main wing imposed by those of the BSC. After passing a certain angle of the canard will occupy a dominating position. So the attack, the low canard will overrun the high canard, closer is the canard to the main wing, the stronger the which can be mainly attributed to the difference in downwash effect will be, and the worse the character- the flow pattern. In the FSC configuration, the canard istics will be. But when the canard moves over the vortex breaks down earlier and the force to control neutral position, the upwash on the canard created by the main wing vortex will be weaker, which means a the main wing will rebound, making the root wing large proportion of the vortex-induced lift will be much cleaner and improving the flow pattern of the lost, but the downwash on the main wing made by root main wing, so better aerodynamic characteristics the canard would convert into the favorable interfer- such as 4# (54% of the fuselage) can be obtained in the ence. The downwash on the main wing made by the close-coupled positions. low canard is stronger than the high canard. Al- From Fig.9 we can see, in 4# (54% of the fuselage), though the effective angles of attack of the main the flow has formed its own vortex construction and wing decrease, the root of the main wing has become vortex center, the vortex intensity of 4# is stronger, cleaner, the flow pattern of the root wing has been

Fig.10 Comparison of lift, drag and pitching moment characteristics in different vertical positions of FSC.

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Fig.11 Comparison of velocity vector of FSC (root of canard position) at α = 40°. improved, and the upwash on the canard made by the main wing becomes stronger. The net effects will produce better aerodynamic characteristics for the low canard than the high canard. Finally Fig.12 shows the pressure p contour on upper surface of different FSW configurations.

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over delta . Chinese Journal of Theoretical and Applied Mechanics, 2006; 38(1)：113-118. [in Chinese] [7] Xu Y, Wang J J, Guo H. Experimental investigations on the unsteady characteristics of vortex breakdown over . Acta Aerodynamica Sinica, 2005; 23(2)： 200-216. [in Chinese] [8] Liu P Q, Wei Y. Vortex structures on close-coupled canard configurations. Journal of Experiments in Fluid Mechanics, 2005; 19(3): 85-89. [in Chinese] [9] Song S H, Zhu G L, Zhang S H. Analysis of wingtip vortex characteristics in flows around low-aspect-ratio thin wings for micro air vehicles. Acta Aerodynamica Sinica, 2008; 26(3): 310-316. [in Chinese] [10] Patrick R S, Douglas R S. Aerodynamic control of a wing section at low angles of attack using synthetic Fig.12 Pressure contour on upper surface of different FSW jets and a gurney . AIAA-2008-601, 2008. configurations (upper surfaces) at α = 40°. [11] Scharpf D F, Muellert T J. Experimental study of a low Reynolds number tandem airfoil configuration. Journal of Aircraft, 1992; 29(2): 231-236. 4. Conclusions [12] Zhang J F, Liu G Z. Automatic generation of finite element planar triangular meshes. Journal of Shandong (1) At small angles of attack, the aerodynamic char- Institute of Mining and Technology(Natural Sci- acteristics of Canard-FSW configuration mainly de- ence)1999; 18(4):48-52.[in Chinese] pend on the relative positions of the canard and main [13] Snyder D.O, Koutsavdis E K, Anttonen J S R. Tran- wing, which affect the up-down wash and the aerody- sonic store separation using unstructured CFD with namic interferences. dynamic meshing.AIAA-2003-3913, 2003. (2) At high angles of attack, the aerodynamic per- [14] Tang W, Gui Y W. Aerodynamic design and optimiza- formance of the configuration is related not only to the tion for a wing-body vehicle. Journal of Astronautics, 2007; .28 (1):198-202. [in Chinese] shape of the canard (forward or backward), but also [15] Daniel A S, Raymond C M. CFD analysis of a T-38 features of the vortices above the main wing and the . AIAA-2008-331-165, 2008. canard, including vortex intensity, position and dura- tion as well as the size of control force and so on. Biographies: Reference Zhang Guoqing Born in 1986, he is now pursuing a doc- [1] Behrbohm H. Basic low speed aerodynamic of torate in Beijing Institute of Technology. His main research short-coupled canard configuration of small aspect ra- interests lie in aircraft designing and numerical wind tunnel tio. SAABTN-60 Linkoping, Sweden, 1965. method. [2] Putnam T W. The X-29 flight-research program. AIAA E-mail: [email protected] Student Journal 1984; 39: 2-12. [3] Er-El J, Seginer A. Vortex trajectories and break down Yang Shuxing Born in 1962, professor of Beijing Institute on wing-canard configurations. Journal of Aircraft of Technology. He is now engaged in guided rocket system 1985; 22(8): 641-648. design, guidance and control theory, flight mechanics, and [4] Erickson G E, Schreiner J A, Rogers L W. Canard-wing computational fluid dynamics research. vortex interaction at subsonic through supersonic E-mail: [email protected] speeds. AIAA-1990-2814, 1990. [5] Wang J J, Zhao X. Experimental investigation on lon- Xu Yong Born in 1969, vice professor of Beijing Institute gitudinal aerodynamic characteristics of ca- of Technology. His main research interests lie in aerody- nard-forward- configuration. Acta Aerody- namics of the aircraft and numerical wind tunnel method. namica Sinica, 2004; 22: 237-240. [in Chinese] E-mail: [email protected] [6] Lu Z Y, Zhu L G, Zhang M L. Study on some break- down forms and characteristics of leading edge vortex