Numerical Simulations of Acceleration-Skewed Oscillatory Flows

This paper deals with the oscillatory flow that characterizes the boundary layer at the bottom of forward leaning waves that can be observed in the near shore region. At the outer edge of the boundary layer such flow exhibits an acceleration skewness, which have a strong impact on the sediment transport. This flow is here studied by means of direct numerical simulations of the Navier-Stokes Equations in order to gain insights into several hydrodynamics aspects. The results show that turbulent fluctuations emerge explosively at the end of the accelerating phase giving rise to a sudden increase of the bottom shear stress, which becomes much larger than that in the laminar regime. The positive peak of the wall shear stress is larger than the negative one and their difference increases with the acceleration skewness. Contrarily to analogous experiments carried out in water tunnels, in the present numerical simulations the time average of the bottom shear stress vanishes. The difference between experiments and numerical simulations is attributed to the mass conservation in conjunction with the finite dimensions of the experimental facilities.