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Hydrodynamic dispersion in concentrated sedimenting suspensions


Lester, Julia C. (1988) Hydrodynamic dispersion in concentrated sedimenting suspensions. Dissertation (Ph.D.), California Institute of Technology.


The hydrodynamic dispersion in concentrated sedimenting suspensions is investigated by numerical simulation. The particle Reynolds number is zero, and the Peclet number is infinite (the particles are non-Brownian). Particle trajectories are calculated by Stokesian dynamics. Stokesian dynamics is a molecular-dynamics-like simulation that provides an accurate representation of the suspension hydrodynamics. Detailed in this thesis is a technique that accelerates the convergence of the mobility interactions among particles in an infinite suspension. The simulations are of a monolayer of identical spheres sedimenting in the plane of the monolayer. Relative motion among the spheres arises from hydrodynamic interactions. The displacement related to this relative motion may constitute a random walk, giving rise to diffusive behavior of the spheres. This hydrodynamically induced self-diffusivity has been seen in sheared suspensions of non-Brownian, neutrally buoyant spheres.

Results of the numerical simulations show that the motion of spheres in sedimenting suspensions is also diffusive. The diffusion coefficient is relatively insensitive to the nature of the microstructure, as expressed by the pair-distribution function and the short-time, self-diffusion coefficient. The coefficient of diffusion decreases as the concentration increases for concentrated suspensions (it increases in the shear case). The ratio of the diffusion coefficient to the velocity variance of the spheres should be proportional to the time scale of the diffusive interactions. The diffusion time scale and the diffusion velocity scale (the square root of the velocity variance) both decrease as the concentration increases. In the shear case, the velocity scale (sphere radius multiplied by the shear rate) is independent of concentration, and the time scale (the product of the square of the concentration and the inverse of the shear rate) increases with increasing concentration. At the lowest concentrations, the spheres whose centers are separated by less than 2.05 radii prefer to align in the direction of sedimentation. At the highest concentrations, the preferred alignment is in the perpendicular direction.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Unknown, Unknown
Thesis Committee:
  • Unknown, Unknown
Defense Date:23 March 1988
Record Number:CaltechETD:etd-08012006-113003
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2995
Deposited By: Imported from ETD-db
Deposited On:03 Aug 2006
Last Modified:26 Dec 2012 02:56

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