Graduate Seminar in Fluid Mechanics: EN.530.807

September 18, 2020 @ 4:00 pm – 5:00 pm
Join online via Zoom

Department of Mechanical Engineering
Join on-line via Zoom:
Friday, September 18, 2020
4:00 p.m. – 5:00 p.m. (EDT)

“The Dynamics of Settling Particles in Channel Flows: Gravity, Lift and Particle Clusters”

(Adviser: Prof. Tamer Zaki)

Settling of finite-size particles in a vertical channel flow of Newtonian and viscoelastic fluids was studied using particle resolved simulations. Despite a small solid-to-fluid density ratio, the results highlight remarkable differences from previous studies of neutrally buoyant conditions. Dense particles experience a sustained slip velocity that lead to significant lift forces and to clustering. Conditional trajectory-averaged statistics highlight that the net lift forces play a dominant role in particle migration and, as a result, momentum transfer in the wall-normal direction. A competition between the rotation- and shear- induced lift forces leads to accumulation of the particles near the wall. The Voronoi diagram is used for identifying particle clusters in that region. We will show that the near-wall clusters are formed due to the preferential transport of aggregated particles towards the wall — an effect which is also controlled by the lift forces. Finally, the correlation between the normalized Voronoi areas and particle velocities highlights the difference between the motions of isolated and clustered particles.

“Time Evolution and Effect of Dispersant on the Morphology and Viscosity of Water-in-Crude Oil Emulsions”

(Adviser: Prof. Joseph Katz)

This study examines the time evolution and effects of adding dispersant (Corexit 9500) at varying concentrations on the microscopic morphology and bulk viscosity of salt water-in-crude oil (Louisiana) emulsions. Rheology is used for measuring the viscosity at varying strain rates, and microscopy, followed by machine-learning-based analysis, is used for characterizing the size and spatial distribution of the water droplets in the mechanically mixed emulsion. Initially, the water droplets appear as a multi-scale lattice with mean diameter of 2.7μm and polydispersity of 0.44, with small droplets aggregating around large ones. The corresponding bulk viscosity is one to two orders of magnitude higher than that of the crude oil, decreasing with increasing shear rate. After seven days, the number of submicron droplets increases and the nearest neighbor distance decreases, indicating preferential aggregation. At high shear rates (5-100 s-1), the viscosity increases by 60-130% compared to the initial values. Test performed after 14 and 21 days show that as the droplets coalesce and many of the clusters are lost, the bulk viscosity decreases. These trends suggest that aggregation contributes to the increase in viscosity. Subsequent analysis uses previously-developed models for the increase in viscosity to characterize the effect of aggregation on the properties of the emulsion. Adding dispersant without mixing generates Marangoni-driven flows as the water droplets coalesce. In time, part of the water separates, a fraction forms clouds of submicron droplets, and the rest remains unchanged. Mixing dispersant at low concentration with the emulsion accelerates the coalescence and phase separation. The removed water fraction increases with dispersant concentration, reaching 77% for dispersant concentration of 10-3. The remaining emulsion consists of fine droplets with Newtonian viscosity that is still four times higher than that of the crude oil.

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