## PHCK_JFM_2008

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

Poncet Ph., Hildebrand R., Cottet G.H., Koumoutsakos P., Journal of Fluid Mechanics, 599, 111-120, 2008

**ABSTRACT**

We report high drag reduction in direct numerical simulations of controlled flows past ircular cylinders at Reynolds numbers of 300 and 1000. The flow is controlled by the zimuthal component of the tangential velocity of the cylinder surface. Starting from spanwise-uniform velocity profile that leads to high drag reduction, the optimization rocedure identifies, for the same energy input, spanwise-varying velocity profiles that ead to higher drag reduction. The three-dimensional variations of the velocity field, orresponding to modes A and B of three-dimensional wake instabilities, are largely esponsible for this drag reduction. The spanwise wall velocity variations introduce treamwise vortex braids in the wake that are responsible for reducing the drag nduced by the primary spanwise vortices shed by the cylinder. The results demonstrate hat extending two-dimensional controllers to three-dimensional flows is not optimal s three-dimensional control strategies can lead efficiently to higher drag reduction.