Numerical simulations of flows around a dual step cylinder with different diameter ratios at low Reynolds number

Flows around a dual step cylinder are numerically investigated at a Reynolds number of 200 for three diameter ratios (Did) of 2, 1.43 and 1.19. Simulation results reveal three distinctive vortex shedding modes: (i) the N-S mode for Did = 2, (ii) the transition mode for Did = 1.43, and (iii) the L-S mode for Did = 1.19. The vortex shedding frequency transition from the smaller-diameter cylinder to the larger-diameter one, the dominant shedding frequency downstream of the larger cylinder, and the vortex interactions exhibit significant differences in the three vortex shedding modes. As Did = 2, the direct cross-boundary and half-loop connections take place downstream of the step where vortex dislocations are essentially fixed. When decreasing Did, two new vortex interactions, the double-halfloop and three-half-loop connections, appear with a larger spanwise vortex dislocation occurrence range. In addition, flows in different Did cases are governed by a distinctive vortex shedding law. A pair of streamwise vortices occur downstream the centroid of the larger cylinder. This reduces the cylinder step effect on the local vortex shedding, absorbing the shear layer of the spanwise N-cell vortices, and making the hydrodynamic force coefficients smaller. Interestingly, these fluid force coefficients further decrease as Did increases. (C) 2019 Elsevier Masson SAS. All rights reserved.