Flow control over tandem cylinders using plasma actuators

The flow over two circular cylinders, arranged in a tandem setup, is controlled with the help of dielectric-barrier-discharge (DBD) plasma actuators mounted on the upstream cylinder at a Reynolds number (Re) of 4700. The plasma actuators are mounted at $\pm 80^{\circ }$ from the forward stagnation point of the upstream cylinder. Three tandem configurations are tested, where the distance, $L$, by which the cylinder centers are separated are fixed at 3, 4, and 5 cylinder diameters (D). For each configuration, the plasma actuators are operated at two distinct blowing ratios (BR) of 0.8 and 1.4, which are named as the low-power and high-power forcing cases, respectively. Results include static-pressure measurements on the downstream cylinder and wake surveys using Particle Image Velocimetry (PIV). High-power forcing changes the flow pattern in the $L=3D$ upstream wake from reattached to co-shedding flow, enabling alternating vortex shedding to occur between the tandem cylinders. High-power forcing also significantly weakens vortex shedding from the upstream cylinder for $L=4D$ and $L=5D$. This weakening is manifested through 39.27% and 35.32% reductions in the total area of vorticity contours for $L=4D$ and $L=5D$, respectively. However, the effect of this cancellation is most prominent on the downstream cylinder when the separation distance is $L/D=3$. During forcing with BR = 1.4, the static pressure on the downstream cylinder resembles that of a flow over a regular cylinder for all the cases tested. Hence, at this blowing ratio, the wake signature of the upstream cylinder is severely diminished, by delaying the shear-layer separation point. During forcing with BR = 0.8, no significant effect on the downstream cylinder is observed.