Random Walk Models for Particle Diffusion in Free-Shear Flows

Abstract

The main objectives were to establish and investigate discontinuous and continuous random walk models appropriate for free-shear e ows with regard to turbulent particle diffusion. The models were designed to capture the crossing trajectories effect, the continuity effect, and the inertial-limit effect, all for the case of heavy particles whose densities are much greater than that of the surrounding e uid. In addition, both techniques included an isotropic drift velocity to account for inhomogeneous turbulence. The computational efe ciency of the continuous random walk models is improved by utilizing local time stepping, which effectively e lters out high-frequency velocity e uctuations that do not have a signie cant ine uence on particle diffusion. The predictive performances of these two random walk models were examined through comparison with experimental data and idealized test conditions. The results indicate that both models agree well with experimental data for a nearly homogeneous turbulent wake and an inhomogeneous turbulent axisymmetric jet (although the continuous random walk model performs somewhat better for the inhomogeneous e ows ). It was also found that the proposed drift velocity models are important to ensure continuity when simulating particle diffusion with inhomogeneous turbulence.