Study of the turbulence modulation in particle-laden flows using LES

One of the most interesting problems in fluid dynamics is the prediction of particleladen turbulent flows. These flows are as diverse as pollutant dispersion in the atmosphere and contaminant transport in industrial applications. An issue of primary importance for moderately dense suspensions concerns how particles affect the turbulent flow itself, the so-called two-way coupling. It is known that the addition of particles to a turbulent flow may change the intensity significantly, even at very low volume fraction. The principal difficulty in the prediction of particle-laden turbulent flows is that traditional approaches model particle transport using the Reynolds-averaged Navier-Stokes (RANS) equations. The RANS methods do not accurately predict the Eulerian turbulence field, and it is known that accurate prediction of particle transport is strongly dependent upon providing a realistic description of the velocity field encountered along particle trajectories. Although the most accurate approach to representing the structure of turbulence – including particle transport – is direct numerical simulation (DNS), is not practical for use as a predictive tool because it remains restricted to relatively low Reynolds numbers. An approach which is not as severely restricted in the range of Reynolds number as DNS is large eddy simulation (LES). LES predictions are less sensitive to modeling errors than RANS calculations and, since the subgrid scales are more universal than large scales, it is also possible to represent the effect of the subgrid scales using relatively simple models. A significant advantage of LES over RANS methods is that it permits a much more accurate accounting of particle-turbulence interactions. If modulation of the turbulence by particles is negligible and if the particle relaxation time is of the order of the turbulent time macro-scales, LES of gas-particle flows can be expected to be as accurate as in single-phase flow. In contrast, if two-way coupling effects are important, then the subgrid turbulence model might require modification. The principal objective of this work is application of large eddy simulation to computation of a well defined turbulent shear flow, fully developed channel flow, for which experimental results – Kulick et al. (1994) – exist for comparison. Also, a subgrid model which takes into account the presence of particles is proposed and evaluated.