Session3: Environmental Hydraulics

3-13

Title: SIMULATING THE EFFECT OF WIND-DRIVEN SHEAR STRESS ON TURBULENT OPEN-CHANNEL FLOW

Shoko Sugimoto, Ko Shirakawa, Yuji Sugihara, Michio Sanjou, Takaaki Okamoto

In order to investigate characteristics of open-channel flow under wind-driven shear stress on the water surface, we carry out numerical simulations by using the direct numerical simulation, i.e., DNS, and RANS with the standard 𝑘‐ ε model. The vertical distributions of the streamwise velocity and the Reynolds stress vary depending on the sign and magnitude of the surface shear stress. The numerical results from DNS demonstrate that under the condition of negative shear stress, the streamwise velocity around the half-water depth is increased than that in the case of no shear stress. The surface divergence calculated from DNS is confirmed to be universally scaled with the Taylor microscale regardless of the positive or negative sign of the shear stress. Though the volume flow rate doesn’t change regardless of the variation of the surface shear stress, the scalar flux at the water surface is found to be decreased in the case of negative shear stress. This suggests that the direction of wind-driven shear stress relative to the main flow becomes important for the scalar transport. In addition, the cross-correlation coefficients between the scalar flux and physical quantities such as the surface divergence and the vorticities close to the water surface are investigated to identify turbulent vortex structure controlling the air-water scalar transport. It has been concluded that the scalar flux increases considerably when the vortex structure is arranged so as to induce a strong upward flow toward the water surface.