Particle dispersion in organized vortex structures within turbulent free shear flows

A discrete vortex model is used to approximate adequately large eddy featur es in turbulent free shear flows and the effects of such eddy structures on Lagrangian particle trajectories and particle dispersion are investigated. Two independent scaling parameters (i) he ratio of the particle's aerodyna mic response time to the characteristic flow time, the Stokes number S-t = rho(p)d(p)(2)Delta U(1 + C-m/gamma)/18 mu delta; and (ii) the ratio of iner tia to gravitational forces, the Froude number Fr = Delta U/root g delta, t ogether with the mass ratio parameter gamma, have been introduced to determ ine the particle dynamics. The Stokes number has been modified to account f or the effect of the density ratio of particles to the carrier fluid on par ticle dispersion. It is demonstrated that St, Fr and gamma, which constitut e a dominant single scaling group, can be used to characterise particle tra nsport dynamics in turbulent free shear flows. In seeking quantification of the particle dispersion, the Eulerian approach based on the particle numbe r fluxes at different downstream cross-sections of the mixing layer in term s of ensemble trajectory statistics and Lagrangian approach based on the pa rticle mean square displacement are adopted. The simulations show the exist ence of different parameter regimes, in which the particle motion is domina ted by both the large-scale vortices and gravity. At intermediate Stokes nu mbers, the particles acquire larger dispersion than the particles do at oth er ranges of the Stokes number. The particle dispersion patterns obtained a re consistent with earlier experimental observations and numerical simulati ons (Wen, Kamalu, Chung, Crowe & Troutt, 1992; Crowe, Chung & Troutt, 1993) that heavy particles accumulate at the periphery of the large-scale Vortic es and particularly near the braid stagnation points between the consecutiv e vortices. (C) 1999 Elsevier Science Ltd. All rights reserved.