Effect of hydrodynamic forces on sandy sediments in oscillatory boundary layer flow

Abstract

In the present work, the forces exerted on spherical sand particles by oscillatory boundary layer flows at Re=25,000 and Re=55,000 are examined by means of Large Eddy Simulations. The particles move within the carrier fluid under the influence of the Stokes Drag, Gravity, Added Mass, Pressure Drag and Basset forces. The model has been validated for the cases of a particle accelerating within a stagnant fluid under the influence of gravity and for a particle within an oscillatory flow. The Basset force is computed in an efficient way, by calculating the Basset integral only for a specific time in the particle’s immediate history, called “window time”. In order to define the window time, existing literature approaches have been implemented. The rest of the particle’s history is calculated by an efficient second order methodology suitable for a large number of particles moving within the carrier fluid. The importance of each of the hydrodynamic forces has been examined when the particles are released far from the horizontal bed and when they are released in the boundary layer region, in the absence of gravity. The results indicate that Stokes Drag, Basset force and Pressure Drag are equally important. The contribution from the Added Mass force was found to be orders of magnitude smaller and the acceleration components in the other directions are also negligible. Furthermore, in the boundary layer region, a near-wall zone of negative mean Stokes Drag and positive mean Basset force has been identified. Moving away from the wall but still in the boundary layer region, another zone of positive mean Stokes Drag and negative mean Basset have been identified.