Large Eddy Simulation of the ventilated wave boundary layer

A Large Eddy Simulation (LES) of (1) a fully developed turbulent wave boundary layer and (2) case 1 subject to ventilation (i.e., suction and injection varying alternately in phase) has been performed, using the Smagorinsky subgrid-scale model to express the subgrid viscosity. The model was found to reproduce experimental results well. However, in case 1, the near-bed ensemble averaged velocity is underestimated during the acceleration stage, probably due to the Smagorinsky subgrid-scale model not being able to capture the physics well in that region. Also, there is a general overestimation of the streamwise turbulence intensity, while an underestimation of the intensities in the two other directions. This may be an effect from the stretched computational mesh in the streamwise direction, since the Smagorinsky subgrid viscosity assumes proportionality to one scalar expressing the overall (local) grid size. The results indicate that the large eddies develop in the resolved scale, corresponding to fluid with an effective viscosity decided by the sum of the kinematic and subgrid viscosity. Regarding case 2, the results are qualitatively in accordance with experimental findings. Injection generally slows down the flow in the full vertical extent of the boundary layer, destabilizes the flow and decreases the mean bed shear stress significantly; whereas suction generally speeds up the flow in the full vertical extent of the boundary layer, stabilizes the flow and increases the mean bed shear stress significantly. Ventilation therefore results in a net current, even in symmetric waves.