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

Low-aspect-ratio flapping foils 333(a)yxx(b)Circular vortices(c)Figure 18. Vortex topology for AR =2.55 foil for different Strouhal numbers. The Reynoldsnumber is 200 for all these cases. (a) St =0.4. (b) St =0.8. (c) St =1.2.the St =0.6 case in figure 9. Figure 18(a) shows the vortex topology for the Strouhalnumber of 0.4. The salient feature that must be pointed out here is the absence of anylinkage between the tip vortices as seen for St =0.6. This is because the tip vorticesformed at this lower Strouhal number have lower strength and this weakens themutual induction mechanism that is responsible for the linking of the tip vortices asdescribed in § 3.1.2. This observation regarding increased interaction between adjacentvortex loops is also consistent with the simulations of Blondeaux et al. (2005a,b) whofind that increase in Strouhal number from 0.175 to 0.35 leads to similar behaviour.The current plot also shows general agreement with figure 5(b) in von Ellenriederet al. (2003) where the authors have shown a top-view of the vortex topology for arectangular AR =3.0 foil oscillating at a Strouhal number of 0.4 and Reynolds numberof 163. Their plot clearly shows that the wake is dominated by circular vortex loopssimilar to those seen here.For the higher Strouhal numbers of 0.8 and 1.2, we observe that as with St =0.6,the tip vortices link together to form spanwise oriented vortex filaments. In fact,at these higher Strouhal numbers, the spanwise spacing of the tip vortices furtherdownstream reduces with increasing Strouhal number as indicated by the arrows