a third-order, low-storage Runge-Kutta method for the convective terms, and a second-order Crank-Nicolson scheme for the viscous terms. The mixed spatial discretization is advantageous from the viewpoint of parallel computing [@luchini-quadrio-2006], and allows employing computing machines connected by standard networking hardware to achieve a large computational throughput.

The simulations described in this paper have been carried out on a computing system available in dedicated mode at the University of Salerno, made by 64 machines, each with two Opteron CPUs. We have performed the calculations at two values of the Reynolds number: $Re_\tau=200$, 400 based on $h$, half the distance between the channel walls, and on $u_\tau$, the friction velocity of the uncontrolled case. The computational parameters for $Re_\tau=200$ have been chosen to replicate those in QR, if exception is made for a slightly finer wall-normal discretization. We have employed a computational box with the following dimensions: $L_y = 2 h$, $L_x = 21 h$, $L_z = 4.2 h$ in the wall-normal, streamwise and spanwise directions, respectively. We have used 160 grid points in the wall-normal direction, and $321$ and $129$ Fourier modes in the streamwise and spanwise directions. For the $Re_\tau=400$ case, we have employed 256 grid points in the 