Abstract
Direct numerical simulations of a turbulent channel flow are performed to investigate the transition period of turbulence near the free-slip wall just after the boundary condition is changed from the non-slip to the free-slip one. In the side of a free-slip wall, the mean shear is attenuated with time development; the dissipation rate of the turbulent kinetic energy surpasses the production rate, and hence turbulence decays. It is found that turbulence becomes vertically homogeneous over the long distance away from the free-slip wall; the turbulence kinetic energy, the dissipation rate and the pressure-strain rate show the plateau over almost entire vicinity of the free-slip side. The horizontal energy spectrum show the definite k_H^<-3> slope as predicted by turbulence theory in decaying two-dimensional turbulence, and it decays in the self-similar form. The effect of the bottom wall, which makes turbulence three-dimensional, does not appear until the mean velocity profile becomes even in the free-slip side.
