Effect of the subgrid scales on particle motion

In this study, the effects of small-scale velocity fluctuations on the motion of tracer particles is investigated by releasing particles in a turbulent channel flow at Re-tau=175, and following their motion in time. Two types of numerical experiments were carried out: first, the Eulerian velocity field was evaluated by the direct numerical simulation (DNS) and the particles were advanced in time using the resolved and several filtered velocity fields to study the effect of the subgrid-scale velocity fluctuations on particle motion without the influence of modeling errors. In the second stage, the particle-motion study was performed using independent DNS and large-eddy simulations (LES), thus including the effect of interpolation and subgrid-scale stress modeling errors on the dispersion statistics. At this Reynolds number the small scales were found to have a limited effect on the statistics examined (one-particle dispersion, one-particle velocity autocorrelation, Lagrangian integral time scale, turbulent diffusivity, and two-particles rms dispersion). Only when a significant percentage of the energy was removed from the velocity field some differences were observed between filtered and unfiltered data (especially near the wall). It was found that when the dynamic eddy-viscosity model was employed, modeling errors did not affect the results as much as the filtering itself; the use of the Smagorinsky model, on the other hand, gave inaccurate results. Additional computations for finite-inertia particles have shown that these results represent a conservative estimate, in the sense that actual particles with inertia are less sensitive than the tracer particles examined in the first part of the investigation, and that LES provides improved results when particles with inertia are used. (C) 1999 American Institute of Physics. [S1070-6631(99)02610-0].