Structures formation by migration of particles in suspension flows
Seminar Room 1, Newton Institute
AbstractFlowing dense particle mixtures give rise to fascinating phenomena that are ubiquitous in nature and in engineering applications: migration, segregation, anisotropic viscosity and dilatancy. Relevant examples are quick sands, lubricants, colloids, erosion flows, magma, debris flow, biological fluids and detergents. One of the most intriguing features of dense suspension flows is particle migration. Experimental and simulation studies to date with smooth and neutrally buoyant particles which are sufficiently coarse (mm range) so that Brownian motion can be neglected show that in slow shearing channel flows, the particles migrate to the centre of the channel where the shear rates are low; see for example Morris et al (1999), Miller & Morris (2006). The migration of much smaller colloidal (micron to nano size range) are also affected by the bulk of fluid shear strain gradients but to a lesser extent due to the Brownian motion created by the thermal fluctuations in the carrier fluid; see Frank et al (2003). The ration of these two effects is given by the Peclet number. At low Peclet numbers, the Brownian motion dominates and affects the structure's formation process in such a way that destroys and diffuses away the structures whilst at much higher Peclet numbers, the migration is believed to become more pronounced. For coarse particles in suspension, Jenkins and Koenders (2005) have proposed a lubrication interaction with additional collisional mechanics with particles separated by gap sizes of the order of the roughness dimension. The lubrication interactions between micro and nano-size particles are much less understood due to the anisotropes introduced by the non-uniformity of the Brownian motion on the fluctuating temperatures and the stress and strain rates thus resulting in heterogeneity of particle structures. A hybrid numerical simulation strategy has been developed to probe the particle-particle and particle-fluid interactions in dense suspensions of fine particles with a view to aid the development of appropriate lubrication interaction models for such systems. The simulations start off with simple MD calculations using Lennard Jones potentials to simulate the flows of fluid particles incorporating thermal diffusivity effects to be followed by frictional DEM simulations of collisional dynamics flows of solid particles. The hybrid scenario is aimed to simulate the two-phase flow by the interaction of the lubrication forces between the particles and the solvent molecules. The project is funded by the Leverhulme Trust and the progress to date with numerical simulations will be presented in the early stages of the current investigations. Challenges associated with simulation strategies and the relaxing of simplifying assumptions will be discussed.
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