Spatially distributed control for optimal drag reduction of the flow past a circular cylinder

International audienceWe report high drag reduction in Direct Numerical Simulations of controled flows past circular cylinders at Reynolds numbers of 300 and 1000. The flow is controlled by the azimuthal component of the tangential velocity of the cylinder surface. Starting from a spanwise uniform velocity profile that leads to high drag reduction, the optimization procedure identifies, for the same energy input, spanwise varying velocity profiles that lead to higher drag reduction. The three dimensional variations of the velocity field, corresponding to the modes A and B of three dimensional wake instabilities, are largely responsible for this drag reduction. The spanwise, wall velocity variations introduce streamwise vortex braids in the wake that are responsible for reducing the drag induced by the primary spanwise vortices shed by the cylinder. The results demonstrate that extending two dimensional controllers to three-dimensional flows is not optimal as three dimensional control strategies can lead efficiently to higher drag reduction