Turbulent drag reduction by constant near-wall forcing

Computational experiments based on the direct numerical simulation of turbulent channel flow reveal that the skin friction can be reduced as much as 70% by the action of a localized steady force acting against the flow close to the wall. In addition, the excessive shear stresses observed during the laminar-to-turbulence transition can be substantially reduced. For a sustained reduction in the skin friction, the control force has to act within a distance of 20 wall units (scaling with the location of the maximum Reynolds stress gradient); otherwise a transient drag reduction is observed or even an increase in drag. The forcing leads to the formation of a shear layer close to the wall that reduces the skin friction and limits the development of the Reynolds shear stresses. As the amplitude of the forcing is increased, the shear layer breaks down and generates its own turbulence, setting an upper limit to the level of drag reduction. This transition of the shear layer is correlated with a Reynolds number based on the forcing amplitude and length scale.