Experiments and large eddy simulations of oscillatory flow over vortex ripples at high Reynolds number

Long-crested sand ripples are ubiquitous seabed features in shallow coastal environments, characterized by the alternating generation of spanwise coherent vortices (SCVs) on either side of ripple crests. While previous studies have elucidated SCV dynamics at moderate Reynolds numbers ($Re\le 10^4$), a range that is common at many beaches and can persist for long wave periods. Nonetheless, their applicability to higher Reynolds number conditions ($Re\sim 10^5$) remains uncertain. This investigation combines wall-modeled large eddy simulations (WMLES) and oscillating water tunnel experiments to examine SCV formation at high Reynolds numbers ($Re\sim O\left(10^5\right)$). The WMLES approach employs a logarithmic wall model for rough surfaces, achieving accurate SCV representation with computational efficiency. Experimental validation demonstrates good agreement in both phase-averaged flow fields and turbulence statistics, confirming the model’s fidelity. Key findings reveal a Reynolds number dependence analogous to the drag crisis phenomenon: SCV intensity diminishes significantly for smooth ripples at $Re=O\left(10^5\right)$, while surface roughness preserves vortex coherence. Our analysis of numerical results uncovers a positive feedback mechanism governing SCV development, where the residual SCV from the preceding half-cycle promotes early flow separation at ripple crests, facilitating vorticity accumulation and subsequent SCV formation. Analysis of the initial-cycle simulation (starting from a quiescent initial condition) shows that lee-side boundary layer must separate intrinsically during the deceleration phases of the first half-cycle to initiate this positive feedback loop. Both low Reynolds numbers and surface roughness can contribute to this first-half-cycle separation by increasing momentum deficit in the lee-side boundary layer.