Abstract

Oscillatory flow around a large sphere resting on a plane and rough bottom is simulated by means of direct numerical simulations of continuity and Navier-Stokes equations. The bottom roughness is provided by a layer of spherical particles, the size of which is much smaller than the radius of the large sphere, arranged in a hexagonal pattern. To investigate the conditions at the bottom of a propagating sea wave, the flow is forced by a spatially uniform and temporally oscillating pressure gradient. The numerical approach will determine the magnitude and spatial distributions of vorticity and bottom stress, which are difficult to measure in an experimental apparatus. Attention is focused on the dynamics of the vortex structures originated by the free shear layer shed by the surface of the large sphere and on the interaction of the vortex structures with the bottom roughness. In particular, the bottom shear stress in the region close to the large sphere was evaluated suggesting the tendency of the small spheres to be moved and transported by the flow. The results show that peaks of the bed stress originate in the lateral regions as an effect of the flow acceleration around the spherical object.

Introduction

Erosional and accretional morphological patterns are often observed near objects resting on the seafloor. Indeed, vorticity is shed by the object surface and the resulting vortex structures cause a local increase of the bed shear stress and initiate sediment transport. Once the sediments move, the divergence of the sediment transport rate gives rise to erosion and deposition processes and the erosional and accretional patterns may spread both down-current and up-current and give rise to small scale bedforms (ripples).

The scour around objects and their self-burial have been widely studied for unidirectional steady flows. In particular, attention has been focused on the scour around piles of bridges and piers that protrude from the sediment bed and on scour and obstacle marks, which are generated around individual objects. In the latter case, the scour can give rise to the burial of the object (self-burial). Less is known on the scour due to oscillatory flows generated by the propagation of sea waves. Vortices shed during the first half a cycle of a sea wave interact with the vortices shed during the following half cycle and the nonlinear interaction of these vortices gives rise to a complex flow field. The phenomenon is quite important when mines or unexploded ordnance (UXO) are considered. For example, Baeye et al. (2012) pointed out that the bed of the North Sea is covered with UXO dating back to World Wars I and II and resting on and buried in the sandy bottom. Nowadays, because of increasing human activities (e.g. navigation, fisheries, sand extraction), the buried mines and UXO are a threat to public safety and remediation is becoming a priority in Dutch, Belgian and French waters.

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