A Numerical Study on Vortex Dynamics Over Ripple Geometry with Defects

Abstract:

Wave-generated ripples in natural environment rarely display a regular geometry. More often, they display a transient 3D geometry characterized by the presence of defects (the bedform irregularities, e.g., bifurcation and termination). Defect interactions help shape the ripple geometry and accelerate their evolution. Despite their role, it is still not well understood how defects affect the hydrodynamics over local and neighbouring ripples. We carried out high-resolution Large Eddy Simulations to study the three-dimensional hydrodynamics over regular and irregular (with defects) wave-generated ripples. We considered two types of irregularities in ripple crestlines, ripples with a termination defect and with a bifurcation defect. A case with regular ripple geometry is set up for comparison. Results show that the flow and vortex structures vary in the spanwise direction, displaying evident three-dimensional vortex structures. The presence of defects affects the maximum velocity and vortex structures over both local and neighbouring crestlines. The presence of “rib” patterns (long tube-like flow circulation in the spanwise direction) is also affected by the presence of defects. Finally, the shear stress is asymmetrically distributed due to the existence of defects, which can potentially drive bedform evolution.