Three-dimensional oscillatory flow over steep ripples

Abstract

The process which leads to the appearance of three-dimensional vortex structures in the oscillatory flow over two-dimensional ripples is investigated by means of direct numerical simulations of Navier–Stokes and continuity equations. The results by Hara & Mei (1990a), who considered ripples of small amplitude or weak fluid oscillations, are extended by considering ripples of larger amplitude and stronger flows respectively. Nonlinear effects, which were ignored in the analysis carried out by Hara & Mei (1990a), are found either to have a destabilizing effect or to delay the appearance of three-dimensional flow patterns, depending on the values of the parameters. An attempt to simulate the flow over actual ripples is made for moderate values of the Reynolds number. In this case the instability of the basic two-dimensional flow with respect to transverse perturbations makes the free shear layer generated by boundary layer separation become wavy as it leaves the ripple crest. Then the amplitude of the waviness increases and eventually complex three-dimensional vortex structures appear which are ejected in the irrotational region. Sometimes the formation of mushroom vortices is observed.

Particle based modelling and simulation of natural sand dynamics in the wave bottom boundary layer

“…However, significant three-dimensional small-scale structures are also evident throughout the cycle in the form of entangled vortex filaments around the larger two-dimensional vortex cores and streaks along the bed surface, similar to Natural sand dynamics in the wave bottom boundary layer observations from other numerical studies (e.g. Vittori & Verzicco 1998;Scandura, Vittori & Blondeaux 2000;Zedler & Street 2006;Schmeeckle 2014). These coherent vortical features have a clear influence on particle motion throughout the cycle: at Natural sand dynamics in the wave bottom boundary layer 367 off-onshore flow reversal (figure 10a), a small vortex is detected at the bottom of the stoss slope in an area of high suspended sediment concentration (see also figure 8a), also observed by Van der Werf et al (2007).…”

Direct numerical simulations of flow over two-dimensional and three-dimensional ripples and implication to sediment transport: Steady flow

“…With the current advances in computational resources, and with novel numerical methods such as immersed-boundary-methods, it is now becoming feasible to use DNS as a research tool for complex geometries (Bhaganagar et al, 2004). The DNS results will provide fundamental understanding of turbulent flow over bedforms, comprehensive database for various flow conditions (De Angelis et al, 1997;Cherukat et al, 1998;Scandura et al, 2000) and improved closures for LES and RANS modeling approaches.…”

Large eddy simulation of sediment transport over rippled beds

“…Models based on Reynolds averaged NS equations have also been commonly used (RANS; e.g., Eidsvik, 2006); Chang and Scotti (2004), for instance, compared RANS techniques with LES for modeling flows over ripples. Direct simulations of NS equations (DNS) of flows over ripples have also been performed (e.g., Scandura et al, 2000;Blondeaux et al, 2004), but there are stringent limits on the flow Reynolds number that can realistically be computationally achieved. Vortex ripples are found in a range of dimensions, but are characterized by flow separation in the lee of each ripple crest (e.g., Fig.…”