The unsteady flow past a bluff body (like a high speed train) can be significantly altered using small rotating cylinders. It is observed that when the control cylinders rotate at high speed, such that the tip speed is five times the free-stream speed, the flow at Re =100 achieves a steady state. At higher Reynolds numbers, even though the flow remains unsteady, the wake is highly organized and narrower compared to the one without control. In all the cases, a significant reduction in the overall drag coefficient and the unsteady aerodynamic forces acting on the main cylinder is observed. This study brings out the relevance of the gap, between the main and control cylinders, as a design parameter for such flow control devices. Compared to the drag coefficient of 1.4 for a single cylinder, one of the cases with close to optimal gap results in a drag coefficient of 0.25, approximately. Figure 8 shows the vorticity field for flow past a cylinder with and without control.

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