ACTIVE FLOW CONTROL OF FLOW AROUND A CIRCULAR CYLINDER AT A SUBCRITICAL REYNOLDS NUMBER

Several complicated flow features such as boundary-layer separation, shear layer, wake region, and vortex shedding involve in bluff-body flows causing aerodynamic drag. Active flow control is widely implemented to modify flow characteristics around a bluff body in which form drag plays an essential role due to boundary-layer separation. In the present study, the flow around a circular cylinder, a canonical flow, was investigated at Reynolds number (Re) of 3900, a sub-critical Reynolds number, to understand better the underlying physics related to drag reduction using active flow control. Synthetic jet actuators were installed at the angles of 90 and 270 degrees from the stagnation point, corresponding to the boundary-layer separation points of such flow. Synthetic jet actuators were assigned to operate at a constant frequency since the vortex shedding frequency in a sub-critical range is constant at Strouhal number (St) around 0.2. Computation fluid dynamics simulations were employed. The OpenFOAM, the open-source computational fluid dynamics software, was utilized. Unsteady Reynolds-averaged Navier–Stokes (URANS) simulations with the SST k-ɷ turbulence model were performed. Flows with and without synthetic jet actuators were observed. The drag coefficient (Cd) and the Strouhal number for uncontrolled flow are 0.996 and 0.207, respectively. These results agreed well with Norberg's wind tunnel experimental data (1987). For controlled flow, the drag coefficient was reduced to 0.92, equal to a 7.46% reduction. The vortex shedding frequency is also reduced to St ≈ 0.16. It is suggested that the synthetic jet actuators cause a reduction in aerodynamic drag by suppressing the occurrence of vortex shedding at the rear of a cylinder.