A flexible sheet in the wake of a cylinder forced to rotate

Abstract

We discuss the behavior of a flexible sheet placed in the wake of a cylinder that is forced to rotate periodically. This is done through a series of experiments conducted in a water tunnel and by simultaneously visualizing the flow behavior and tracking the motion of the flexible sheet in the wake of the cylinder. We show how the response of a flexible sheet in the wake of a fixed cylinder, which is the result of the sheet’s interaction with the vortices that are shed at a frequency predicted by the Strouhal law can be changed to a “desired” response by forcing the upstream cylinder to rotate periodically. Large-amplitude oscillations at a frequency different from the Strouhal frequency can be imposed on the flexible sheet, and the sheet’s oscillations can be suppressed if the cylinder is forced to rotate at a higher frequency. The flexible sheet finds its way in between the vortices that are shed in the wake of the cylinder, and by controlling the frequency and location of the shed vortices in the wake of the cylinder, one can impose a desired motion on the sheet. Besides imposing a symmetric oscillatory response on the sheet, we show that asymmetric responses can be imposed on the sheet when an asymmetric waveform is used to force the upstream cylinder.