Drag reduction in turbulent channel flow by large-scale optimal control input

Abstract

Aiming at the development of a better open-loop turbulence control strategy, large-scale wall transpiration is optimized so as to suppresses turbulent kinetic energy in a fully developed channel flow. High-wavenumber control inputs are constrained by adding a penalty term in the cost functional. It is shown that more than 17% drag reduction can be achieved even under the strong constraint of uniformity in the streamwise or spanwise direction.

Further analyses suggest different mechanisms of drag reduction for each controlled case.

Finally, we apply the obtained large-scale optimal input as an open-loop control, and confirm the temporary reduction of the turbulent kinetic energy after the onset of the control.