Active Control of Wall Turbulence with Wall Deformation

The performance of an active feedback control with wall deformation was tested by direct numerical simulation of a fully developed turbulent channel flow. The local wall movement is determined based on the flow condition detected by virtual sensors distributed in the vicinity of the wall, i.e., the wall velocity is made opposite to the wall-normal velocity at y/δ=0.1. The turbulent friction drag is reduced by 10% on average. The gain in the pumping power reaches about 28 times the energy consumption for actuating the wall. The primary mode of the resultant wall velocity distribution, which should be most effective in the present drag reduction scheme, is found to have wavelengths of about 300 and 50 viscous wall units in the streamwise and spanwise directions, respectively, while the time period is of the order of the time scale of the quasi-coherent vortical structure of near-wall turbulence. The effects of active wall deformation on quasi-coherent structures are investigated by a conditional averaging technique. In the present control scheme, the Q2 vortex is displaced away from the wall and the wall-normal and spanwise velocity fluctuations associated with the Q2 event are decreased. On the other hand, the location of the Q4 vortex from the wall remains unchanged while the vorticity of the Q4 vortex is substantially decreased. In the region downstream of the Q2 event, the wall is deformed in the shape of a shallow groove, which stabilizes the near-wall streaky structures along the groove.