Wake topology and hydrodynamic performance of low-aspect-ratio ...

330 H. Dong, R. Mittal and F. M. Najjar2y0–20 2 4xFigure 13. Contours of mean streamwise velocity for the AR =1.27 case along the spanwisesymmetry plan. Contour levels in this plot are the same as those in figure 7.(a)2(b)2(c)2y000–2–2–2–2 0 2z–2 0 2z–2 0 2zFigure 14. Contours of mean streamwise velocity for the AR =1.27 case on three streamwiseplanes in the near wake (a) x/a x =1. (b) x/a x =2. (c) x/a x = 4. Contour levels in this plot arethe same as the ones in figure 7.absence of two sets of obliquely convecting vortex structures. Instead, the two setsof vortex loops stay intertwined with each other. However, the wake itself expandsrapidly in the transverse direction and this is clearly due to the self-induced velocityof the vortex loops. The third difference is the presence of distinctly spanwise orientedvortices in the very near wake such as vortex V 2 identified in figures 15(a) and 15(c).In fact, vortices V 1 ,V 2 and V 3 in figure 15(d) clearly signal the re-emergence ofthe conventional inverse Kármán vortex street seen in the wake of two-dimensionalthrust-producing flapping foils. It should, however, be noted that further downstream,the spanwise vorticity diminishes in strength and the wake is dominated by the tipvortices.This is primarily because while the spanwise vortices induce a stretching andtherefore intensification of the tip-vortices, the tip-vortices induce a compression onthe associated spanwise vortices which tends to diminish their strength. The spanwise