(231h) Secondary Flow Behavior and Charged Particle Transport in Bifurcations

The secondary flow behavior in multi-bifurcation models is complex and not well understood. The numerical solution of oscillatory flow in single bifurcation models yields the familiar counter-rotating Dean's vortices in the daughter tubes in the inspiratory phase, which coexist independently in the parent branch upon expiration. The unsteady secondary flow behavior is known to be subjected to steady streaming effects but this is yet to be investigated in detail. PIV measurements are conducted using oscillatory flows in a physiologically representative bifurcation model, for flow visualization and experimental verification of the phenomenon. The 3-D numerical simulation study on the bifurcation geometry suggests that flow separation may occur on the outer walls of the bifurcation at high Reynolds numbers and large exterior bifurcation angles, and this is experimentally verified using PIV. The presence of a separation bubble is of great importance to particulate transport characteristics, especially particle residence time and deposition profiles.

A separate numerical study has shown that the secondary flow behavior in a double bifurcation model is remarkably different from single bifurcation models. The vortices found in the granddaughter branches are observed to be in opposite sense to the familiar Dean's vortices found in single bifurcations. The transition of vortex sense may be related to some critical Reynolds number. This demonstrates a strong interplay between the transport phenomena in consecutive generations such that the predictions based on a single bifurcation may be inaccurate. This finding has some implications for the secondary transport of inertial particles and the corresponding deposition profile.

Inertial particles are conventionally known to be thrown out of vortex cores due to centrifugal acceleration, resulting in a non-uniform cross-sectional distribution of particles. For highly charged particles (uni-polar), however, inter-particle repulsion forces may result in complex redistribution of the particles, depending on the local competing effects between centrifugal and electrostatic forces. To simulate this phenomenon, the particle tracking method is employed using the equations of motions for individual particles, subjected to fluid drag forces as well as Coulombic interactions.