How to increase the MHD drag reduction efficiency for turbulent flows

Recently, permanent magnets and high electric current densities are often used to achieve reasonable Lorentz forces for a magnetohydrodynamic (MHD) flow control. This choice, however, usually leads to a low energetic efficiency for the flow control of seawater. We present results of direct numerical simulations of turbulent channel flow drag reduction using electromagnetic forces. The Lorentz force is created by a permanent magnetic field and an electric current from electrodes placed at the bottom wall. We consider the two cases of a spanwise oscillating force and a streamwise steady force. The main result is that a significant efficiency improvement is possible if load factors k ~ 1 are used. The application of the streamwise Lorentz force leads to a much more effective drag reduction if we consider the drag as a full force applied to the body. The skin-friction drag increases but the full drag may be strongly reduced with a good efficiency.