Dust resuspension by the flow around an impacting sphere

Abstract

A rigid body colliding with a layer of dust is capable of resuspending dust through two distinct mechanisms: a ballistic mechanism, where kinetic energy is transferred from the impacting body to dust particles through direct contact, and a hydrodynamic mechanism, where dust particles are resuspended by the flow disturbance generated by the body. In this paper, we study the hydrodynamic resuspension mechanism by considering the flow around a sphere moving either towards or away from a wall. Experiments were performed using a sphere translating at a constant velocity for Reynolds number, Re, in the range 300 to 3500, and at varying angles of approach and departure from a wall. A wider range of Re was investigated by releasing dense rigid spheres above the wall. The high Reynolds number flow past a steadily translating sphere is characterized by a recirculating wake region behind the sphere. When the sphere approaches the wall and stops on making contact with it, the wake vortex which is initially behind the sphere threads over the sphere's surface, generating a secondary vortex ring. The coherent structure, composed of the wake and secondary vortices, strikes the wall and pushes fluid or dust, initially adjacent to the wall, to one side. The resuspension of dust particles of diameter b which are initially at rest on the wall is governed by a particle Shields' parameter, θ_p, based on the sphere's impact velocity, U: θ_p = ρ_fU²/ (ρ_p−ρ_f)bg, where ρ_p and ρ_f are respectively the density of the dust particles and fluid. The resuspension criterion is a function of particle Reynolds number, Re_p, based on the diameter and fall velocity of the dust particles and occurs when θ_p [ges ] θ_p,c where θ_p,c ≈ 3.0 for Re_p [gsim ] 1, and θ_p,c ≈ 5.0/Re^1/2_p for Re_p [lsim ] 1. The geometry of the region of dust resuspended by the sphere was studied as a function of the impact velocity, angle of impact and the properties of the dust particles. When the sphere impacts a thick layer of dust, the volume concentration of resuspended dust is sufficiently high to generate a particle-driven gravity current which transports the dust far from the point of impact. The dynamics of the gravity current were determined as a function of dust particle properties and size of the impacting sphere.

A sphere moving impulsively from rest away from a wall is found not to play a significant role in the resuspension of dust; however trailing vorticity generated on the surface of the sphere advects a large volume of fluid away from the wall, which may contain dust already in suspension.