Large‐Eddy Simulation of Turbulent Oscillatory Flow Over Three‐Dimensional Transient Vortex Ripple Geometries in Quasi‐Equilibrium

Abstract

Vortex ripples induced by oscillatory flow display straight crests at equilibrium, but they develop transient three‐dimensional (3‐D) bedform geometries when transitioning from one equilibrium state to another due to changes in the oscillatory flow. Large‐eddy simulations were performed of oscillatory flow over 3‐D bedform defects associated with an increase of the orbital motion amplitude, similar to what might happen during the waxing phase of a storm. The objective is to determine the intercorrelation between flow processes and evolution of 3‐D bedform defects toward an equilibrium two‐dimensional (2‐D) state of vortex ripples. Results are presented for oscillatory flows over fixed ripples with sinusoidal crests in the spanwise direction, which introduce slope asymmetries on opposite sides of the crests, during different transient states. It was found that steep‐sloped regions of the bedforms are associated with thicker recirculation areas, whereas in more gently sloped regions thinner recirculation vortices exist. Furthermore, streamwise vortical structures and significant secondary flow develop in the vicinity of sinusoidal crests, relative to straight crests. These flow structures might be responsible for the gradual reformation of the rippled bed, as they induce higher bed shear stresses, that is, increasing bed sediment transport, and they diffuse turbulent kinetic energy to higher levels above crests, that is, increasing suspended sediment transport, relative to equilibrium 2‐D ripples. Finally, the period‐averaged currents around the sinusoidal crests exhibit higher velocities on the steep‐sloped side than on the mild‐sloped one, also likely contributing to sediment transport and bed reformation toward a 2‐D configuration.

High‐Resolution Large Eddy Simulations of Vortex Dynamics Over Ripple Defects Under Oscillatory Flow

“…Further, secondary peaks of the drag coefficient are observed (Figure 16), which were not reported by Chalmoukis et al. (2020). These secondary peaks occur during the flow deceleration phases (π/2–π and 3π/2–2π).…”

“…Andersen et al (2002) stressed the importance of neighboring ripple structures and proposed a model incorporating the relation between local and neighboring ripple spacing. Recently, Chalmoukis et al (2020) used LES to investigate the hydrodynamics over fixed ripples by introducing slope asymmetry along the ripple crest. The spacing and steepness between two individual ripple crests varied along the spanwise direction.…”

ADV-Based Investigation on Bed Level Changes Over a Meso-Macro Tidal Beach

“…At this moment, the lag deposition effect of the suspended sediments and the intensity of vertical sediment exchange was weakened compared with a low-energy steeper ripple environment (Lee and Hanes, 1996;Osborne and Vincent, 1996), which broke the foregoing interrelation between waves, measured SSCs, and bed level (Figure 11) and caused different variation patterns among them (Figures 9B,D,F). During these processes, the bed shear stress induced by mean current (τ c ) exceeded the critical bed shear stress most of the time (Figure 8A), and the current was capable of altering the bedform and dominating the way of the vertical sediment exchange (Chalmoukis et al, 2020). Hence, the influences of mean currents and bedforms on bed level changes were enhanced and highlighted under energetic wave conditions.…”

“…In fact, under moderate wave conditions (T4-T9), the ψ, which originated from the modulation of h on waves, was greater than that under low-energy wave conditions most of the time and even exceeded 240 at T7 and T9. Thus, the dominant way of vertical sediment exchange between the seabed and higher elevation turned to the diffusion process from the advection process, which was due to the bedform transformation from a steep ripple (vortex ripple) to a less steep ripple (post-vortex ripple) or a plane seabed under strong wave-induced currents, characterized by high reference concentrations in the vicinity of the seabed and limited suspended sediments at higher elevations (Dingler and Inman, 1976;Chalmoukis et al, 2020). At this moment, the lag deposition effect of the suspended sediments and the intensity of vertical sediment exchange was weakened compared with a low-energy steeper ripple environment (Lee and Hanes, 1996;Osborne and Vincent, 1996), which broke the foregoing interrelation between waves, measured SSCs, and bed level (Figure 11) and caused different variation patterns among them (Figures 9B,D,F).…”

RANSE Modeling of the Oscillatory Flow Over Two‐Dimensional Rigid Ripples

“…If flow conditions change after or before the ripples have reached equilibrium, the profile of the ripple eventually adapts to the new flow conditions by following routes which have been the subject of recent studies (Chalmoukis et al., 2020; Doucette & O’Donoghue, 2006; Hansen et al., 2001; Jin et al., 2020; Myrow et al., 2018; Nienhuis et al., 2014; Perron et al., 2018).…”