Friction drag reduction achievable by near-wall turbulence manipulation at high Reynolds numbers

The Reynolds-number dependence of the drag reduction achievable by diminishing to zero the near-wall turbulent velocity fluctuations is clarified. This reduction could be obtained by a virtual active feedback control system. The formula derived suggests that large drag reduction can be attained even at high Reynolds numbers if turbulence fluctuations adjacent to the wall are completely damped. For example, 35% drag reduction rate can be obtained at Re_τ=10⁵ if the turbulence only below y⁺=10 vanishes. Thus, the active feedback control strategy, which has been studied mostly at low Reynolds numbers, would be much promising even in high Reynolds number flows of real applications. Results from the direct numerical simulation of turbulent channel flow at a Reynolds number of Re_τ=642 are also presented to clarify the phenomena in the controlled flow.