Spectral Element-based Simulations of a Model Internal Swash Zone: Extending to turbulence-resolving mode*

We report results from high-accuracy/resolution direct numerical simulations of a model internal swash zone in a two-layer stratification at laboratory-scale in three dimensions. The simulations are enabled by an efficiently parallelized hybrid nodal Spectral Element / Fourier-Galerkin flow solver and the use of an Euler-Lagrange method to generate finite-amplitude long internal waves in a nonlinear stratification (Lloret et al. ; Nonlin. Proc. Geophys. 2024 – in press). If a free-slip bottom boundary condition is used plunging breakers develop on the slope leading to an upslope propagating turbulent bore. If instead the more realistic no-slip bottom boundary condition is employed boundary layer separation under the rear of the wave gives rise to a bolus-like feature which detaches itself and moves upslope ahead of the wave ; the wave itself then is subject to a backward collapsing break. The turbulence in the bolus interior and its outer interface, the latter resulting from shear instabilities significantly enhanced by the gravitationally-driven return flow originating further up the slope, are examined both qualitatively and quantitatively. The turbulence within the bolus impresses a non-negligible shear stress structure at the bed and supports strong vertical velocities above it indicating non-trivial potential for resuspension. The turbulence at the outer bolus interface appears to be the one mechanism that can efficiently exchange well-mixed fluid with the interior water column.