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, θp,
based on the sphere's impact velocity, U: θp =
ρfU2/
(ρp−ρf)bg,
where ρp and ρf are respectively the density of
the dust particles and fluid. The resuspension criterion is a function of particle Reynolds
number, Rep, based on the diameter
and fall velocity of the dust particles and occurs
when θp [ges ] θp,c where
θp,c ≈ 3.0 for Rep [gsim ] 1, and
θp,c ≈ 5.0/Re1/2p
for Rep [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.
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