Experimental observations of the effects of spanwise blowing on the wingtip vortex evolution at low Reynolds numbers

Abstract

We experimentally investigate the effect of a spanwise continuous jet on the trailing vortex behind a wing model NACA0012 airfoil (AR = 2) using a local injection at a fixed angle of attack, α = 9◦ . Three different chord-based Reynolds numbers, Re, and different blowing continuous jets characterized by the momentum coefficient, Cμ , have been analyzed by means of 2D-PIV measurements. We show how these jets are good candidates to reduce the strength of the wingtip vortices at the lowest Reynolds number considered, e.g., Re = 7 × 103. Conversely, the blowing has a weak influence on the vortex strength at Re = 15 × 103 and 20 × 103. Making use of classical vortex models we provide a set of theoretical parameters which offer a general, and quantitative characterization of the effect of continuous blowing jets on the wingtip vortex as we vary the Reynolds number. Besides, the mode decomposition analysis from 2D velocity fields concludes that the perturbed flow structure has a predominant axisymmetric mode at Re = 7 × 103. However, the predominant mode is not only m = 0, but also |m|=1 for greater Reynolds numbers. Two different perspectives are discussed to support the mixing process between the spanwise blowing and the wake behind the wing model. Firstly, the theoretical parameter that corresponds to the exponent of the vortex decay in Moore & Saffman model is n = 1, so the vortex formed downstream is inviscid, and consequently, the turbulent diffusion is acting rapidly at Re = 7 × 103 . Secondly, we pay our attention to the location of the injection relative to the streamwise vorticity formed downstream. We observe that the lower the vorticity level downstream the injection area, the higher the influence to break the wingtip vortex.