The flow on the upper surface airfoil is subject to an adverse pressure gradient when the incidence increases. This leads to the boundary layer separation which causes looses in the aerodynamic performances (lift decrease and drag increase). Passive control using vortex generators (VGs) is the simplest solution to delay or eliminate the flow separation from the wall. Vortex generators enhance the aerodynamic performances and the most efficiency ones are the LIN's V-shaped when their height is less than the boundary layer thickness [1].

The present study concerns the flow control using a new vortex generators shape with counter-rotating vortices, obtained by adding a small element to the LIN's VGs configuration. The experiments were performed in order to determine the VGs answer when they were placed at 10% of chord from the leading edge on the suction face of an airfoil NACA 0015.

An optimized geometry form is given in this work by using the experimental designs method [2-4]. The aerodynamic measurements were accomplished in wind tunnel for several Reynolds numbers. The obtained results are analyzed according to several parameters such as the VG height, the aperture angle, the space between the same VG pair and the additional factor. A three-dimensional controlled flow pressure field was also displayed at different velocities, attack angles and taking into account the additional element effect.

The results show a profit brought by the passive devices estimated at about 22.2% in relative lift increase and 16% of maximum drag decrease. The experimental results were also compared to those obtained by a three-dimensional numerical simulation (3D-RANS) and showed a good agreement. The CFD study gives interesting qualitative and quantitative results associated with the different experimental tests and highlights the generation of vortex structures along the profile, which allow the supply of external energy to the boundary layer by forcing the flow towards the wall. The results highlighted a clear improvement in the momentum thickness along the airfoil's upper face, particularly a rate of 44.44% at 40% of the chord length.

Références

[1] Lin John C. Review of research on low-profile vortex generators to control boundary-layer separation. Prog Aerosp Sci 2002;38(4-5):389-420. Doi: 10.1016/S0376-0421(02)00010-6.

[2] Montgomery Douglas C. Design and Analysis of Experiments. 3rd ed. New York: John wiley & sons; 1991.

[3] Zeng M, Tang LH, Lin M, Wang QW. Optimization of heat exchangers with vortex-generator fin by Taguchi method. Appl Therm Eng 2010;30(13):1775-1783. Doi: 10.1016/j.applthermaleng.2010.04.009.

[4] Lundstedt Torbjörn, Seifert Elisabeth, Abramo Lisbeth, Thelin Bernt, Nyström Åsa, Pettersen Jarle, et al. Experimental design and optimization. Chemom Intell Lab Syst 1998;42(1-2):3-40. Doi: 10.1016/S0169-7439(98)00065-3.