Three-dimensional direct numerical simulation of flow induced by an oscillating sphere close to a plane boundary

The dynamic interaction of submerged spherical structures with ocean waves alters the local flow field and is responsible for the movement of sediments over the seabed, which would have detrimental effects on the ecosystems. The present research aims to gain insight into the effect of a plane boundary on the vorticity dynamics and shear modifications on the plane boundary induced by the flow due to an oscillating sphere. The flow field around an oscillating sphere close to a plane boundary is investigated numerically by three-dimensional direct numerical simulation of the Navier-Stokes equations. The sphere oscillates perpendicular to the plane boundary modeling the seabed. The flow structure, resulting mean and fluctuating shear stress distribution on the plane boundary, and the Lagrangian particle transport are investigated as a function of the sphere oscillation amplitude, Reynolds number, and the distance of the sphere from the plane boundary. The systematic correlations for the variation of maximum shear stress on the plane boundary are estimated from scaling parameters. The time-mean results are then linked to the Lagrangian particle transport where it is observed that the Lagrangian path lines track the mean streamlines closely. Further, the particles at the stagnation zone cease drifting and oscillate about a constant mean position. The practical significance of this result is that a ring of zero mean shear on the seabed is found toward where particles congregate. This provides a guide for the optimum location for the mooring anchors, minimizing the risk of anchor scour or undermining.