# Direct numerical simulation of the oscillatory flow around a sphere resting on a rough bottom

## TL;DR

This study uses direct numerical simulations to analyze oscillatory flow around a sphere on a rough bottom, revealing vortex shedding, turbulence generation, and sediment interaction relevant to marine environments.

## Contribution

It introduces a detailed simulation approach for flow around a sphere on a rough seabed, including sediment particles, to study vortex dynamics and sediment transport.

## Key findings

- Vortex structures can break up and generate turbulence at high Reynolds numbers.
- Flow velocity fields enable analysis of vortex dynamics and turbulence.
- Forces and torques on sediment particles and the sphere are quantifiable.

## Abstract

The oscillatory flow around a spherical object lying on a rough bottom is investigated by means of direct numerical simulations of continuity and Navier-Stokes equations. The rough bottom is simulated by a layer/multiple layers of spherical particles, the size of which is much smaller that the size of the object. The period and amplitude of the velocity oscillations of the free stream are chosen to mimic the flow at the bottom of sea waves and the size of the small spherical particles falls in the range of coarse sand/very fine gravel. Even though the computational costs allow only the simulation of moderate values of the Reynolds number characterizing the bottom boundary layer, the results show that the coherent vortex structures, shed by the spherical object, can break-up and generate turbulence, if the Reynolds number of the object is sufficiently large. The knowledge of the velocity field allows the dynamics of the large scale coherent vortices shed by the object to be determined and turbulence characteristics to be evaluated. Moreover, the forces and torques acting on both the large spherical object and the small particles, simulating sediment grains, can be determined and analysed, thus laying the groundwork for the investigation of sediment dynamics and scour developments.