On the limits of wavy cylinder wavelength and amplitude for effective wake and vortex-shedding control

An experimental time-resolved particle-image velocimetry study was conducted on wavy cylinders possessing wavelength (λ) and amplitude (a) combinations that are significantly different from earlier studies at Re_Dm = 2700. Results show that vortex formation length increases as the wavelength decreases from λ/D_m = 2.4 to 1.2, but decreases when the latter decreases to λ/D_m= 0.6. Amplitude increments lead to significant vortex formation length growths and reductions at the saddles/nodes of λ/D_m= 2.4 and 1.2 wavy cylinders, respectively. In contrast, λ/D_m= 0.6 wavy cylinders produce significantly shorter vortex formation lengths like a baseline cylinder, regardless of amplitude. Regular reversed flow “lobes” are observed for λ/D_m= 2.4 and 1.2 wavy cylinders, but not λ/D_m= 0.6 ones, which lead to variations in the spanwise vortex formation lengths. Proper orthogonal decomposition (POD) analysis shows that only a/D_m= 0.4, λ/D_m= 0.6 wavy cylinder has the same vortex-shedding frequency as the baseline cylinder. Other POD results also demonstrate that the vortex-shedding behaviour between λ/D_m= 0.6 wavy and baseline cylinder is very similar. The present study shows that there exists a minimal wavelength below which that coherent streamwise vortices will not be produced and wake control benefits of wavy cylinders will be considerably reduced.