Forces on particles in oscillatory boundary layers

Abstract

The lift and drag forces on an isolated particle resulting from an oscillating wall- bounded flow, are approximated using direct numerical simulation and extrapolation techniques. We also confirm the existence of anomalies in the lift force, which arise from the interaction of the vortical field with the particle. Anomalies can also occur for computational reasons and these are discussed as well.This study was motivated by a long-standing question about the importance of lift forces in the dynamics of sediments in oceanic settings. To answer this question we use the numerically generated data as well as extrapolations to compute the ratio of the lift to buoyancy forces on a particle. This analysis suggests that for particles and oceanic conditions typical of the nearshore, the lift force can play a role in the dynamics of sedimentary beds.

Direct Numerical Simulation of Oscillatory Flow Over a Wavy, Rough, and Permeable Bottom

“…More details on the numerical approach can be found in Mazzuoli and Vittori (). The reliability of both the numerical approach and of the code, which implements it, is verified (Mazzuoli et al, ; Mazzuoli & Vittori, ) through comparisons with experimental measurements (Keiller & Sleath, ) and other numerical results (Fischer et al, ; Fornarelli & Vittori, ). In particular, both the velocity field and the hydrodynamic forces acting on solid spherical particles were considered in the comparisons and fair agreement with previous results was found.…”

Wave boundary layer over a stone-covered bed

“…Sarpkaya (1975) noted that no correlation was found between Re and the force coefficients, evidently revealing the preceding argument. The second set of data is from the work of Fischer et al (2002). The latter authors studied forces on a bottommounted single sphere exposed to an oscillating flow, using direct numerical simulation.…”

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

“…The bottom is assumed to be made up of spherical sediment grains of size d * resting on a plane wall located at x * 3 = 0. Because of numerical reasons, as in Fischer et al (2002), the small spheres, which mimic the sediment grains, do not touch but they are * apart from the bottom. Then, a much larger spherical object, characterized by a diameter D * , is laid down on the sediment bed (see figure 1).…”

“…Much less is known on the dynamics of the three-dimensional vortex structures shed by an object lying on a flat wall and subject to an oscillatory flow. Fischer et al (2002) made direct numerical simulations of the oscillatory flow around a sphere laying on a plane wall. The investigation of Fischer et al (2002) was inspired by the experiments described by Rosenthal & Sleath (1986) and was aimed at providing more information on the lift and drag forces acting on a sediment grain at the bottom of sea waves.…”

“…The investigation of Fischer et al (2002) was inspired by the experiments described by Rosenthal & Sleath (1986) and was aimed at providing more information on the lift and drag forces acting on a sediment grain at the bottom of sea waves. Unlike the results of , , Asmolov (1999) and Asmolov & McLaughlin (1999), the numerical result of Fischer et al (2002) are not restricted to relatively small values of the Reynolds number or to disparate diffusive, convective and oscillatory length scales. However, attention was focused on the forces acting an the sphere and the velocity and vorticity field around the sphere as well as the shear stress acting on the bottom were not analysed.…”