High Swept-back Delta Wing Flow

Transcription

1 Advanced Materials Research Submitted: ISSN: , Vol. 1016, pp Accepted: doi: / Online: Trans Tech Publications, Switzerland High Swept-back Delta Wing Flow HOANG Thi Kim Dung 1, a, NGUYEN Phu Khanh 2,b and NAKAMURA Yoshiaki 3,c 1, 2 Department of Aeronautical and Space Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam 3 Department of Aerospace Engineering, Nagoya University, Nagoya, Japan a dung.hoangthikim@hust.edu.vn, b khanh.nguyenphu@hust.edu.vn and c nakamura@nuae.nagoya-u.ac.jp Keywords: Delta Wing; Low Speed; Vortex; High Swept-back Angle; Rolling Angle. Abstract. In this study, an experimentally and numerically investigation was carried out to obtain characteristics (lift force, drag force ) on 74.5 degree Delta wing. The experiment tests were conducted at Hanoi University of Science and Technology low-speed wind tunnel facility, whereas the numerical tests were performed using the commercial computational fluid dynamics software ANSYS/FLUENT. The apparition of the vortices upon the Delta wing caused the negative pressure distribution on the wing which reached a maximum absolute value at the vortex core. The characteristics of high swept-back Delta wing were investigated at air velocity of 10 m/s and attack angle of 20 degree in changing the rolling angle of the wing from 0 to 20 degree. Introduction Many supersonic aircrafts used delta wing and they often flied at high angles of attack [1-7]. For example, in landing or taking off phase, they needed to fly at very high angle of attack due to their poor aerodynamic performance at low speeds. When an aircraft with delta wing flied at high attack angle in low speeds, there appeared two large counter-rotating leading edge vortices which induced an important lift force [1, 4, 5, 6, 7]. However, when an aircraft with delta wing flies at much higher speeds, the flow became complicated because there appeared shock waves which interacted with vortices. Oyama et al [2] remarked that the change of flow type did not significantly change the aerodynamic characteristics of the delta wing such as normal force, pitching and rolling moments [2]. Recently, Al-Garni et al [3] computed flow fields of a 65 degree delta wing and 65/40 degree double-delta wings using low-speed wind tunnel and commercial computational fluid dynamics software FLUENT to obtain the aerodynamics characteristics of these delta wings. The numerical results were in good agreement with experiments. Jones et al [4] observed the vortex breakdown over a highly swept delta wing. The existence of breakdown was associated with the presence of a change in sign of the pressure. Sideslip angle, attack angle and rolling angle had an important role to the characteristics of delta wing [5-7]. The stall angle of the delta wing was higher than that of high aspect ratio wing. The stall angle of delta wing was above 30 degree [6, 7]. The aim of present study is to examine fundamentals of high sweep-back angle delta wing flow at different rolling angle. To make the analysis simple, only a main wing was chosen as a target (delta wing had swept-back angle of 74.5, span length of 200 mm, root chord length of 360 mm and thickness of 5 mm). To understand the basic characteristics of delta wing flow, static aerodynamic force measurement and numerically flow visualization had been carried out by using Hanoi University of Science and Technology low-speed wind tunnel facility and ANSYS/FLUENT software respectively. The considered parameters were: air velocity of 10m/s, attack angle of 20 degree and rolling angle varied from 0 to 20 degree. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-05/03/16,05:53:48)

2 378 Mechanical and Aerospace Engineering V Delta wing model The delta wing had a swept-back angle, Λ, of 74.5 deg, a span length, b, of 200 m, a root chord length, c r, of 360 mm, and a thickness ratio, τ, of 1.4% (Fig. 1b). For experimental study, the wing model had 45 pressure taps, which were located on the port side (Fig. 1 a). These pressure taps were connected to an external digital manometer via stainless and silicon tubes. Each pressure tap was measured, using Keygence pressure measurement, one time with waiting time of 5 seconds (average of about 1000 instant values). The standard deviation of measurement errors was within ±0.001Pa. Experiments were conducted at the low speed blow-down wind tunnel, which belongs to Department of Aeronautic and Space Engineering at Hanoi University of Science and Technology (HUST), Vietnam. The maximum free stream velocity in the empty test section was 30m/s corresponding to Reynolds numbers 10 6 and the turbulence level was slightly less than 1%. The wind tunnel was operated continuously and a centrifugal blower was driven by an 8kW electric motor. The free-stream velocity was kept constant within ±2%. The free-stream total and dynamic pressures were measured by Pitot tube within ±2%. The air temperature was measured within ±1%. Positions of pressure taps Delta wing model in wind tunnel a. Experimental model Delta wing Calculation domain b. Meshing of numerical model Fig. 1 Delta wing model For numerical study, the Delta wing was simulated using ANSYS/FLUENT software. The computational mesh was composed of 10 6 elements (Fig. 1b). The turbulence model was SST (Shear Stress Transport) k-ω model. This model was two-equation eddy-viscosity model. The use of a k-ω formulation in the inner parts of the boundary layer made the model directly usable from the wall through the viscous sub-layer. Hence, the SST k-ω model could be used as a Low-Reynolds turbulence model without any extra damping functions. The SST formulation also switched to a k-ε behavior in the free-stream and thereby avoided the common k-ω

3 Advanced Materials Research Vol problem that the model was too sensitive to the inlet free-stream turbulence properties. The SST k-ω model was often merited for its good behavior in adverse pressure gradients and separating flow.

4 380 Mechanical and Aerospace Engineering V Windward -1 Leeward Windward -1 Leeward Cp y/(b/2) CFD EXP Cp -0.6 Cp,W < Cp,L Cp,W -0.4 Cp,L y/(b/2) EXP_R0 ϕ = 0 EXP_R5 ϕ = 5 a. Rolling angle ϕ = 0 0 b. Rolling angle ϕ = 5 0 Fig. 5 Spanwise pressure at x/c r = 0.9 Results Vortex breakdown. There was no remarkable phenomenon on the lower surface while on the upper surface, there were two different parts. One was the vortices which were shed from the leading edges of the wing. This part created a large pressure difference that induced a lift for delta wing. Other was attached flow on the wing. This attached flow caused another lift for delta wing. These remarks due to the lift force theory of delta wing that lift force of delta wing included potential lift force and vortices lift force [4]. This numerical study captured well appearance of vortices upon delta wing [6, 7]. At zero rolling angle, these vortices were symmetric (Fig. 2a and Fig. 3a). But at rolling angle of 20 degree (the wing was rotated from windward wing-haft to leeward wing-haft), the vortices were asymmetric (Fig. 2b and Fig. 3b). The intensities of vortices at symmetric case were stronger than those of the windward vortex of asymmetric case, but weaker than those of leeward vortex of the asymmetric case (Fig. 2 and Fig. 3). At rolling angle of 20 degree, the core of windward vortex was displaced far from the upper surface of delta wing, while the core of leeward vortex seemed close to the upper surface of delta wing (Fig. 3a, b). The vortices affected strongly to the surface pressure on the wing [4-7], that would predict a stronger effect of vortex at leeward side than at windward side. Pressure distribution In regard Fig. 4, the pressure on the upper surface of delta wing was negative and had a maximum near the leading edge. The maximum Cp presented the center of vortex. It seemed that the maximum value of Cp could be the origin of apparition of vortex [4-7]. At zero rolling angle, symmetry case, the pressure on the upper surface was symmetric (Fig. 4a), while at rolling angle of 20 degree, the pressure was asymmetric (Fig. 4b). At rolling angle of 20 degree, the maximum value of Cp at windward side was lower than that at leeward side and lower than that of symmetric case. While the maximum Cp at leeward side was stronger than that of symmetric case. This remark confirmed the prediction that effect of vortex at leeward side was stronger than at the windward side when the wing rotated from windward side to leeward side (cf. precedent paragraph). There were a good accord between experimental results and numerical results. We found the same tendency of pressure distribution above and on the wing. The vortices affected pressure on upper surface and caused two negative pressure zones. The aerodynamic characteristics of Delta wing had a good accord between numerical and experimental results within ±20% (Fig. 5a). The main reasons of the error were the quality of mesh, accuracy of used model, experimental set-up In comparing with

5 Advanced Materials Research Vol the experimental and numerical results of ref. [6] and [7], the same tendency of spanwise pressure upon the wing was found. In the region near the center line of the wing, the pressure difference between upper and lower surfaces, Cp, was low, while the values of Cp near the leading edge were much different. There were maximum values of negative pressures on the upper surface near the leading edge (Fig. 5b). On the other hand, by rotating (Fig. 5b), the pressures on the upper surface of windward wing-haft became lower than those of the leeward wing-haft. More specifically, compared with the case with no rotating, the pressures on the leeward side increased about 5%, while those on the windward side decreased about 25%. At rolling angle of 20 degree, the values of pressure difference between upper and lower surfaces, Cp, on the leeward side became larger than those on the windward side. This asymmetric aerodynamic force distribution caused from a counter clock-wise rotation of the wing, when viewed from upward. Aerodynamic characteristics. The lift and drag coefficient force of zero rolling angle were 0.83 and 0.32 respectively. These values slightly decreased with the increasing of rolling angle (Fig. 6). The decrease of aerodynamic characteristics was explained by the more complex of vortices when the wing was rotated. [2] CL CD Rolling angle (Deg) CL CD Fig. 6 Aerodynamic characteristics Summary Both experimentally and numerically studies capture well the delta wing flow field within ±20%. The main results are summarized as follows: Vortex breakdown appeared upon the delta wing. That caused two negative pressure zones on upper surface of delta wing. The center of vortex could be determined by the maximum of negative pressure. The lower surface had no remarkable change. The aerodynamic characteristics slightly decreased with increasing of rolling angle. With rotating from windward to leeward, the intensities of vortex at windward side decreased, while those at leeward side increased. The effect of leeward vortex to the distribution of pressure on the wing was stronger than that of windward vortex. References [1] Gursul M. Allan and K. Badcock, in: Delta wing aerodynamics, Requirements from CFD and experiments (September 2003). [2] A. Oyama, G. Imai, A. Ogawa and K. Fujii, in: Characteristics of a Delta wing at Hight angles of Attack, 15 th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, Dayton, Ohio, (2008). [3] A. Z. Al-Garni, F. Saeed and A. M. Al-Garni, in: Experiment and Numerical Investigation of 65 degree Delta and 65/40 degree Double Delta wings, Journal of Aircraft, Vol.45, No.1, (2008).

6 382 Mechanical and Aerospace Engineering V [4] M. Jones, A. Hashimoto and Y. Nakamura, in: Criteria for vortex breakdown above high-sweep delta wings, AIAA Journal, Vol.47, No.10, (2009). [5] Y. Nakamura and T. Yamada, in: Aerodynamic characteristics of spin phenomenon for Delta wing, ICAS , (2002). [6] T. K. D. Hoang and Y. Nakamura, in: Characteristics of Delta wing flow, The 4 th AUN/SEED-Net RC MeAe 2012, HCMUT - Vietnam, (2012). [7] D. T. Tran, T. K. D. Hoang and P. K. Nguyen, in: Numerical study of high swept-back angle Delta wing, Conference on Mechanical and Manufacturing Engineering, Kuala Lumpur, Malaysia, (2013).

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