Migration of an intruder particle in a boundary driven shear flow
Department of Mechanical Engineering
Doctor of Philosophy
Rosato, Anthony D.
Blackmore, Denis L.
Discrete element method
This study reports on three-dimensional, discrete element simulations of a single large spherical intruder in a Couette shear flow composed of uniform sized particles. The simulation results are useful in providing a numerical reproduction of the experiments for size segregation and mixing. This in turn has a major importance in many industries which are concerned with handling of particles and powders, such as pharmaceutical manufacturing, agriculture, chemical and mineral processing.
Discrete element simulations are carried out using the "soft sphere" model of Walton, et. al., which provides a method for obtaining the information at a macroscopic level from multi-body collisions within each computational step. A granular shearing flow is induced by allowing the upper and lower bumpy walls to move with the same constant speed in opposite directions. The wall particles are in square arrangements and have the same size as the regular flow particles. The typical transport properties of this flow are characterized by the depth profile of granular temperature, mean velocity and granular pressure which are related to the wall roughness, shear gap height and particle inelasticity measured by a constant normal restitution coefficient. By means of autocorrelation and spectral analysis, the vortex-like structure of velocity field has been revealed, which coincides with the results from the wavelet analysis.
In the micro-gravity study case, an "intruder" with different size ratio has been added in the uniform shear flow described above. It has been observed to migrate away from the walls and finally become trapped in the central area with a small fluctuation around the equilibrium location. The amplitude of the fluctuation has a relation with the intruder size ratio. Computations indicated that the intruder's motion is induced by the depth distribution of granular pressure which is higher near the moving boundary and lower in the center of the gap. The differences of the normal pressure on the depth profile could be represented by the fluctuation of net force on both sides of the intruder. Also, the circulation pattern in the velocity field may enhance this trend.
Simulations of the annular shear cell device shows us that the motion of the "intruder" is very sensitive to gravity. Similar to the experimental studies, the "intruder" will eventually migrate to the top of the shear flow with a velocity proportional to the size ratio. A further investigation also revealed that the migration of the "intruder" may have a relation with the pattern of the velocity field in the cross-section of the shear direction.
njit-etd2004-028 (156 pages ~ 7,191 KB pdf)
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Created July 26, 2004