Flow Control on Wing by Means of Trapped Vortex Cell

European Drag Reduction and Flow Control Meeting – EDRFCM 2010 September 2–4, 2010, Kyiv, Ukraine FLOW CONTROL ON WING BY MEANS OF TRAPPED VORTEX CELL D. Lasagna, G. Iuso Dipartimento di Ingegneria Aeronautica e Spaziale, Politecnico di Torino, Corso Duca degli Abruzzi, 10129 Torino, Italy [email protected] INTRODUCTION A flow control strategy which has not received a great attention is the so called ”trapped vortex cavity”, (TVC). The proposed technique consists in a cavity accommodated on the upper surface of the airfoil, with a shape optimized for trapping a vortex. Under appropriate conditions, a steady, large vor- tical structure forms in the cavity creating a ”moving interface”. The vortex cell causes the boundary layer flowing past it to remain more filled, moving the separation point dowstream. The goal is to increase the aerodynamic performances of the wing, both at low and high angles of attack. Most of the work on this subject, see [1], has been done in the framework of the VortexCell2050 project, even if pre- vious implementation of this technique are known, such as the EKIP aircraft [2] and the Kasper wing [3]. Both numerical and experimental investigations have shown that the success of this flow control technique tightly depends on the stabilization of the vortex. In fact theoretical investigations [4] have shown that a trapped vortex can have a limited stability region, i.e. it is unstable and cannot be kept trapped if some control in the cavity region is not exerted. Several authors have demonstrated experimentally [5] and numerically [6] the positive effects of suction for controlling the stability of the vortex in the cavity. In this paper we provide experimental data that assess the effectiveness of a TVC control system in increasing the lift and re- ducing the drag of a wing profile. The ex- periments were conducted on a NACA0024 airfoil. The modularity of the model allowed to test both the clean airfoil and the trapped vortex cell configurations. The TVC module houses the cavity, reported in Figure 1: NACA0024 airfoil with trapped vortex cavity. figure 1, with a shape suited for trapping a vortex. Suction was operated through the upstream perforated surface of the cavity, in order to stabilize the vortex flow. Lift coefficient were measured by integration of pressure distribution on the model’s surface. The wake investigations allowed the evaluation of the drag coefficient by momentum balance. RESULTS Some of the most representative results concerning pressure distributions and wake velocity profiles are presented. Results at α = 2◦ for the baseline, (B), and the TVC with suction cases, (TVCS) are reported ∗ in figure 2. The TVCS configuration is characterized by a suction parameter S = Q /(c/Ue)=0.0037, ∗ where Q is the volumetric suction per unit cavity span, c is the chord and Ue is the reference speed. At such low incidence the flow is attached over most of the airfoil’s surface, for both the controlled and baseline configurations. The slight difference in the Cp value is probably due to a potential effect of the suction. Nevertheless, the wake velocity profiles are significantly different, because the wake of the TVCS configuration is narrower and with a lower momentum deficit. As a consequence, the drag coefficient is dramatically reduced from 0.010 to 0.0052, while the lift coefficient increases only slightly from 0.173 to 0.225. The results obtained at α = 14◦ are reported in figure 3. The pressure distribution of the baseline configuration shows a constant pressure region starting from x/c ≈ 0.8, indicating the separation of the flow. On the other hand, the pressure distribution of the TVCS configuration highlights a continuous pressure recovery up to the trailing edge and a larger upper-lower pressure difference over all the airfoil’s chord. The wake velocity profiles indicate that the TVCS system effectively controls the flow, because the wake is 82 once again substantially narrower and with a lower momentum deficit for the controlled configuration, with respect to the baseline case. This leads to a reduction of the drag coefficient from 0.022 to 0.010, associated with a more substantial lift coefficient increase, from 0.93 to 1.05. a) b) Figure 2: a) Pressure coefficient distributions; b) Non-dimensional wake velocity profiles. α =2◦. a) b) Figure 3: a) Pressure coefficient distributions; b) Non-dimensional wake velocity profiles. α = 14◦. CONCLUSIONS The flow control effectiveness of a trapped vortex cell system has been demonstrated also experimentally in the case of an airfoil at incidence. The results indicate a significant drag reduction with respect to the clean configuration at both low and high angles of attack, accompanied with a lift increase especially near the stall incidences. REFERENCES [1] Donelli, R. Chernyshenko, S., Iannelli, P., Iollo, A. and Zannetti, L. (2009), Flow models for a vortex cell, AIAA Paper, 2-47, 451-467 [2] http://www.ekip-aviation-concern.com/eng-b/4.html [3] Kasper, W.A, 1974, Aircraft wing with vortex generation, U.S Patent No.3831885 [4] Zannetti, L., Iollo, A.”, Trapped vortex optimal control by suction and blowing at the wall, (2001), European J. of Mech. B-Fluids” 20, 7-24 [5] De Gregorio, F., Fraioli, G., Flow control on a high thickness airfoil by a trapped vortex cavity, (2008) International Symposium on Applications of Laser Techniques to Fluid Mechanics, 7-10 [6] Andronov, P.R., Buchin, V.A., Gircha, A.I. Guvernyuk, S.V., (2009), Report on the numerical experiment of vortex capturing using suction and feedback active control, Vortexcell2050 Deliverable D10.4a 83.

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