Direct numerical simulations of unsteady turbulent channel flow are performed to study the evolution of streamwise vortices in the wall vicinity. In the first stage of those numerical simulations, turbulence decays temporarily after one of the walls bounding the flow becomes free-slip, and streamwise vortices in the vicinity of that wall almost disappear. In the second stage, the boundary condition is restored to a non-slip wall, and vortices in the near-wall region are excited again by the imposed mean shear there. In this second stage, the increase in skin friction coefficient is found to be triggered by the enhanced sweep motion (inrush motion of high-speed fluids), and the increase in the number of the vortices related to the sweep in the wall vicinity. By studying evolution of streamwise vortices in the wall vicinity, it is found that the persistent streamwise vortices in the wall vicinity are connected to the vortices away from the wall, and they shift downward when accompanied by intense inrush motion toward the wall. In contrast, newly generated vortices in the wall vicinity, disconnected to the vortices away from the wall, tend to soon be attenuated.
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