On the role of infiltration and exfiltration in swash zone boundary layer dynamics

Abstract

Boundary layer dynamics are investigated using a 2‐D numerical model that solves the Volume‐Averaged Reynolds‐Averaged Navier‐Stokes equations, with a VOF‐tracking scheme and a k ‐ ϵ turbulence closure. The model is validated with highly resolved data of dam break driven swash flows over gravel impermeable and permeable beds. The spatial gradients of the velocity, bed shear stress, and turbulence intensity terms are investigated with reference to bottom boundary layer (BL) dynamics. Numerical results show that the mean vorticity responds to flow divergence/convergence at the surface that result from accelerating/decelerating portions of the flow, bed shear stress, and sinking/injection of turbulence due to infiltration/exfiltration. Hence, the zero up‐crossing of the vorticity is employed as a proxy of the BL thickness inside the shallow swash zone flows. During the uprush phase, the BL develops almost instantaneously with bore arrival and fluctuates below the surface due to flow instabilities and related horizontal straining. In contrast, during the backwash phase, the BL grows quasi‐linearly with less influence of surface‐induced forces. However, the infiltration produces a reduction of the maximum excursion and duration of the swash event. These effects have important implications for the BL development. The numerical results suggest that the BL growth rate deviates rapidly from a quasi‐linear trend if the infiltration is dominant during the initial backwash phase and the flat plate boundary layer theory may no longer be applicable under these conditions.

Large eddy simulation of dam‐break‐driven swash on a rough‐planar beach

“…Unlike turbulence production, turbulence dissipation occurs near the Kolmogorov scale and only DNS can resolve the complete turbulence dissipation process. In LES, turbulence dissipation rate, e, is estimated from the resolved (e) and subgrid (e ts ) dissipation rate [Pham and Sarkar, 2014] as…”

“…Pintado‐Patiño et al . [] further validated a 2DV Volume‐Averaged Reynolds Averaged Navier‐Stokes (VARANS) model [ Hsu et al ., ; Losada et al ., ] for the infiltrated/exfiltration of flows inside permeable beaches [ Kikkert et al ., ]. The mean vorticity was employed to estimate the boundary layer thickness since it responds to momentum gradients.…”

“…Moreover, it was found that the local acceleration is not well correlated with the horizontal pressure gradient due to nonnegligible turbulence and advective transport in the momentum balance. Pintado-Patiño et al [2015] further validated a 2DV Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) model [Hsu et al, 2002;Losada et al, 2008] for the infiltrated/ exfiltration of flows inside permeable beaches [Kikkert et al, 2012[Kikkert et al, , 2013. The mean vorticity was employed to estimate the boundary layer thickness since it responds to momentum gradients.…”

A 2D hydrodynamic model for shallow water flows with significant infiltration losses

“…Kikkert et al () conducted an experimental study of bore‐driven swash hydrodynamics on permeable rough beaches in the Fluid Mechanics Laboratory at the University of Aberdeen. It is worth emphasizing that this experiment has not been investigated by any depth‐averaged models along with the GA equation hitherto, and although Pintado‐Patiño et al () reproduced this experiment using a depth‐resolving model, the depth‐averaged model still remains a practical tool for engineering use at present (Briganti et al, ).…”

“…Briganti et al () reviewed the state‐of‐the‐art in swash zone dynamic modelling and stated that depth‐resolving models improve the flow description with significantly higher computational costs, whereas depth‐averaged models are practical tools for engineering use. Kikkert et al () carried out a bore‐driven swash experiment on permeable slopes, which has been investigated only by depth‐resolving models so far (Pintado‐Patiño, Torres‐Freyermuth, Puleo, & Pokrajac, ).…”