Stabilisation of the Absolute Instability of a Flow Past a Cylinder via Spanwise Forcing at Re = 180

Abstract

Vortex shedding in the wake of bluff bodies is often an undesired phenomenon which generates unsteady loads, vibrations and fluctuations of the aerodynamic forces. Consequently, the attenuation or suppression of the self-sustained oscillations associated to the vortex shedding is a fundamental problem in a wide range of engineering applications. Three-dimensional control techniques to control the vortex shedding are characterised by a variation of the control input along the spanwise direction and offer a promising methodology due to their versatility and high potential efficiency 1. In the present paper, the control of vortex dynamics of the wake of a flow past a cylinder at Reynolds number Re = 180 is performed by means of spanwise distributed forcing 1 2; starting from a fully developed shedding, a sufficiently high spanwise forcing is introduced on the surface of the cylinder, close to the separation regions, to stabilise the near-wake in a time-independent state, similarly to the effect of a sinusoidal stagnation surface 3,4. The effects of the forcing on the drag reduction and the dynamics of the vorticity have been investigated using a spanwise gaussian forcing, which generates a significant redistribution of the spanwise vorticity into streamwise and vertical components was ob- served, leading to a minor susceptibility of the three-dimensional shear layers to roll-up into the vortex street 4? . An insight into the main physical mechanisms underlying the suppression is provided by the hydrodynamic stability theory. Three different regimes were found for different forcing amplitude and the computation of the leading modes helps to shed light on some mechanisms responsible for the suppression of the Von-Karman shedding.