Dynamics and control of near-wall cohernet structures

摘要

Using direct numerical simulations of turbulent channel flow,we present a new method for skin friction reduction by prevention of streamwise vortex formation near the wall.Instability of lifted,vortes-free low-speed streaks is shown to generate new streamwise vortices,which dominate near-wall turbulence phenomena.Significantly,the 3D features of the (instantaneous) instability-generated vortices agree well with the coherent structues educed (i.e. ensemble-averaged) from fully turbulent flow.Based on this crucial role of streak instability in vortes generation,we develop a new technique for drag reduction,enabling large-scale flow forcing without requiring instantaneous flow information.As proof-of-principle,x-independent forcing,with a wavelength of 400 wall units and an amplitude of only 6