DYNAMICS OF SMALL, SPHERICAL-PARTICLES IN VORTICAL AND STAGNATION POINT FLOW-FIELDS

The transport in vortical and stagnation point flow fields is analyzed for particles across the entire range of density ratios, based on the Maxey-Riley equation [Phys. Fluids 26, 883 (1983)] without history ef fects. For these elementary flow fields, the governing equations simpl ify substantially, so that analytical progress can be made towards qua ntifying ejection/entrapment trends and accumulation behavior. For a s olid body vortex, the analysis shows that optimal ejection or entrapme nt occurs for all density ratios, as the difference between inward and outward forces reaches a maximum for intermediate values of the Stoke s number. The optimal Stokes number value is provided as a function of the density ratio. Gravity is shown to shift accumulation regions, wi thout affecting the entrapment or ejection rates. For a point vortex f low, the existence of up to three different regimes is demonstrated, w hich are characterized by different force balances and ejection rates. For this flow, optimal accumulation is demonstrated for intermediate Stokes numbers. The stagnation point how gives rise to optimal accumul ation for heavy particles, whereas light particles do not exhibit opti mal behavior. The analysis furthermore indicates that nonvanishing den sity ratios give rise to a finite Stokes number regime in which the pa rticle motion is oscillatory. Above and below this regime, the motion is overdamped. (C) 1997 American Institute of Physics.