Abstract of the article :
The fall velocity of a dense large ball in a suspension of neutrally buoyant non-Brownian particles subjected to horizontal oscillatory shear is studied. As the strain amplitude is increased, the velocity increases up to a maximum value before decreasing to the value that it would have in a resting suspension. The higher the frequency is, the stronger the effect is. The falling ball velocity can be largely increased in the presence of the oscillatory cross-shear flow. For instance, for a particle volume fraction of Φ=0.47 it reaches four times the value it has in the unsheared suspension. At small strain amplitudes, it turns out that the velocity of the falling ball is determined by a balance between the steady drag flow, which drives the apparent suspension viscosity toward a high value, and the oscillatory cross-shear, which lessens it. A simple model is proposed to explain the experimental observations at small strain amplitude. The velocity decrease observed at larger amplitude is not completely understood yet.
This chronophotography shows the settling of a 1 mm steel ball in a concentrated suspension of neutrally buoyant 30 micrometers hard spheres. The steel ball moves at the centerline of the gap of a cylindrical Couette cell which is mounted on a rheometrer. An oscillatory horizontal cross-shear can be applied during the fall. Without cross-shear, the velocity of the ball is inversely proportional to the suspension viscosity, as predicted by Stokes’ law. In a Newtonian fluid the sedimentation rate of the falling ball is unaffected by the presence of an oscillatory cross-shear. Surprisingly this is not the case in a concentrated suspension. When the oscillatory cross-shear is applied, the vertical velocity is significantly increased, up to four times. Parameters of the picture : Suspension volume fraction 47% - suspension viscosity 58.9 Pa.s - oscillatory cross-shear frequency 15Hz - Amplitude of the strain 0.04 - Time between two successives frames 20s (i.e 300 cycles of oscillation).
Reference : Frédéric Blanc, Elisabeth Lemaire and François Peters (2014). Tunable fall velocity of a dense ball in oscillatory cross-sheared concentrated suspensions . Journal of Fluid Mechanics, 746, R4 doi:10.1017/jfm.2014.160
Fluides & Matériaux Complexes, (Enseignants-)Chercheurs, Rhéologie des Suspensions
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