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Communication Dans Un Congrès Année : 2015

Fall velocity of a dense ball in cross-sheared concentrated suspensions: oscillatory and step shear flows

Résumé

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 mean 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 = 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 through the variation of an apparent viscosity. The velocity decrease observed at larger amplitude is not completely understood yet. However, the time variation of the velocity during one cycle suggests that it is not sufficient to consider only an apparent viscosity. Indeed, the vertical motion of the ball is found oscillatory with a frequency twice the frequency of the external shear rate. The amplitude of variation can be larger than the mean velocity, meaning that the dense ball moves upwards during part of one period. Another experiment consists in tracking the vertical position of the dense ball after a shear step is applied to the suspension. The ball is observed to begin to move upwards over a distance of the order of its size, then to fall and to reach the stationary velocity it has in a suspension at rest.
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Dates et versions

hal-01179634 , version 1 (23-07-2015)

Identifiants

  • HAL Id : hal-01179634 , version 1

Citer

Elisabeth Lemaire, François Peters, Frédéric Blanc. Fall velocity of a dense ball in cross-sheared concentrated suspensions: oscillatory and step shear flows. 10th Annual European Rheology Conference, Apr 2015, Nantes, France. ⟨hal-01179634⟩
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