NUMERICAL EXPERIMENTS ON THE EFFECT OF SURFACE SHEAR STRESS ACTING ON TURBULENT OPEN-CHANNEL FLOW

In the case which shear stress is added to the free surface of turbulent open-channel flow, we perform numerical experiments by using DNS and the standard k-ε model to investigate how the turbulent field responds to the surface shear stress. The velocity-shear profile of the main flow varies with the positive or negative magnitude of the shear stress, so that the vertical distributions of turbulent quantities change obviously depending on the positive or negative sign of the stress. The results of DNS in the case of the negative shear stress show that the main flow velocity near the half water depth increases compared to the case without the shear stress though the standard k-ε model cannot simulate such a behavior. The scaling relation of the surface divergence based on Taylor microscale are found to be universally established regardless of the positive or negative sign of the stress, whereas the scaling relation with the turbulence energy dissipation rate breaks down. This means that the mechanism of the scalar transport changes by the sign of the surface shear stress.