抄録
Direct numerical simulations of unsteady turbulent channel flow are performed to study evolution of small-scale vortices in the wall vicinity. In the first stage of those numerical simulations, turbulence decays temporarily after one of the walls bounding flow becomes free-slip, and streamwise vortices in the very vicinity of that wall almost disappear. In the second stage, the boundary condition is again put back to non-slip wall, and vortices in near-wall region are excited again by the imposed mean shear there. In this second stage, the increase in $c_f$ is found to be triggered by enhanced sweep motion. Moreover, evolution of small-scale vortices emerging in the wall vicinity is investigated, to find that they are also triggered by the sweep motion, to later generate the ejection motion pumping the low-speed fluids away from the wall. The intensive small-scale vortices in the wall vicinity are mostly connected to the vortices away from the wall, which are stretched downward when they are related to intense sweep motion. In contrast, newly generated vortices disconnected to them are soon attenuated without becoming the elongated longitudinal vortical structures.
