Particle dynamics in inhomogeneous flow at moderate to high Reynolds number.

摘要

In order to obtain an accurate parameterization of the hydrodynamic force acting on a spherical particle in complex inhomogeneous flows at moderate particle Reynolds numbers (Re = 10–600), we develop a DNS technique that resolves the smallest scales in the ambient flow and in the particle wake, and provides detailed microscale information on flow-particle interaction. We address the following sequence of problems. (a) Irrotational flow: We perform DNS of stationary and freely moving particles in straining flows. We show that the spatial nonuniformity in the ambient flow can substantially enhance drag and lift. We explore the mechanism of these forces and relate them to the structure of the wake. A universal description of the added-mass force, and a generalized invariant representation of the viscous force are presented. An improved parameterization, comprising of the generalized viscous and added-mass forces, is shown to predict the DNS results very accurately. (b) Particle rotation: We perform DNS of a freely rotating particle in linear shear flow. We observe that under the torque-free condition, the rotation-rate of the particle decreases rapidly with Re following a power law. The effect of rotation on the drag is negligible, while that on the lift is to generate the Magnus force. DNS of a freely translating and rotating particle shows that free rotation has little effect on the unsteady motion. (c) Shear- vs. vortex-induced lift force: We perform DNS of a particle in a shear flow and in a pure rotational flow. We observe that the lift force in a pure rotational flow is two orders of magnitude higher than that in a shear flow. We explore the mechanism of the difference, and its implication on the particle/bubble migration in a vortex. (d) Particle-turbulence interaction: We perform DNS of a particle subjected to an isotropic turbulent flow. We explore different estimates of the mean and instantaneous drag and compare them with the DNS results. The mean and instantaneous wake structure, wake oscillation and vortex shedding in turbulent flow are studied to understand the mechanism of turbulence modulation in the wake.