Effect of the subgrid scales on particle motion

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