On the effect of inlet conditions on particle/droplet dispersion in a shear layer

Abstract

The impact of the inlet conditions on particle/droplet dispersion in a post-transitional shear layer is investigated using two-dimensional numerical simulations. The flow inlet conditions are varied between a monotonically changing inlet velocity profile (errorfunction) and one that exhibits a wake deficit reminiscent of the presence of the upstream splitter plate. The particle field inlet conditions are varied by altering the particle inlet location. Monodisperse dilute particle fields of various diameters are considered. The numerical models for both the time-dependent and un-averaged flow and particle fields are Lagrangian yielding a totally grid-free approach. The well known relationship between the particle Stokes number and dispersion is verified. Results also indicate that, under most conditions, dispersion is increased if particles are introduced from the fast stream side and as close as possible to the layer centerline. This trend is more pronounced for the wake-modified than the error-function inlet profile flow. The impact of the Stokes number on this trend is secondary. This suggests that the origin of this behavior lies in the flow dynamics which is known to favor entrainment from the fast stream. For the error-function case the dispersion trend is reversed if particles are introduced far enough from the shear layer so that they do not get entrained into the rotational flow. This is due to the tendency of the shear layer to tilt towards the slow stream. For the wake-modified case, however, this reversal does not occur. This, and the above noted enhanced asymmetry in the dispersion patterns for this case are attributed to the presence of positive vorticity on the slow speed side of the shear layer. This vorticity, which is counter-rotating with respect to the overall vorticity field has its origin in the inlet condition. While its effect on the large scale flow structure is secondary, it tends to weaken the entrainment patterns locally and thus diminishes the entrainment and subsequent dispersion of particles. Copyright © 1998 by M. C. Soteriou. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. T Assistant Professor, member AIAA. ? Graduate Student.