Numerical investigation of flow past a rotating dual-step cylinder

Flow past a rotating dual-step cylinder is investigated numerically for a Reynolds number of Re=150 and a smaller to larger diameter ratio d=0.5. The dual-step cylinder comprises one larger diameter cylinder with two smaller diameters on its two ends, arranged coaxially. Simulations are conducted for L=2 to 40, where L is the larger cylinder length to diameter ratio. The length of the larger cylinder affects the wake. For a non-rotating cylinder, the two N-cells near the two steps combine, forming a mode N-S with one N-cells and two S-cells when L = 2 to 12. When L =15 to 40, L-cell coexist with N-cells, and the wake is in mode N-L-S with two N-cells, one L-cell and two S-cells. For a rotating cylinder, mode N-L-S doest not exist when the rotation rate α=0.5 because the wake is in the direct mode. The boundary length L of the SC between mode N-S and mode N-L-S for α=1 is significtly reduced compared with a non-rotating step cylinder. The distinct feature of the wake for larger rotation rates between 2 and 4 is the existence of two streamwise hub vortices generated from the two steps, respectively. The attraction and cancellation effects between the two hub vortices affects the wake. An increase in α enhances the two hub vortices and attraction between them, howewer the stronger attraction effect between the two hub vortices reduces their strength. At L=2, the two hub vortices are so close to each other that they comibine, resulting in a very chaortic wake.