Abstract
Particle dispersion in a periodic channel is studied using the elliptic relaxation hybrid RANS/LES (ER-HRL) model. This approach employs a four-equation linear eddy viscosity (LEV) model while in Reynolds Averaged Navier-Stokes (RANS) mode near the wall, and switches to the LES Smagorinsky dynamic model in the outer flow region. We perform a systematic analysis of the dispersion of six sets of particles having Stokes numbers St = 0.2, 1, 5, 15, 25, 125 at shear Reynolds numbers of Reτ=150, 590. To account for the effect of the unresolved scales on particle dispersion, a novel subgrid-scale model (SGS) is proposed based on the wall-normal RMS of the velocity transport equation. The ER-HRL model is validated against DNS and LES databases, with a globally good agreement. For higher Reynolds number i.e. Reτ = 590, the model, with a much coarser grid, outperforms the LES subgrid stochastic acceleration (LES-SSAM) approach.
Highlights
Out of the three RMS velocity values, we focus on the vertical RMS velocity since it is the one responsible for particle impaction against the wall
It was noticed that the model works in hybrid Reynolds Averaged NavierStokes (RANS)/Large Eddy Simulation (LES) mode only when using the coarse mesh which captures the peak of both the turbulent kinetic energy (TKE) and the wall-normal RMS velocity fairly well (Fig. 1)
Turbulent statistics obtained by the elliptic relaxation hybrid RANS/LES (ER-HRL) were shown to converge to the reference Direct Numerical Simulations (DNS) data at a much lower computational cost than conventional LES
Summary
Discrete particle transport in turbulent flows has a plethora of medical and engineering applications, some of which are: growth of rain drops and cloud formation [10,69], air pollution, sand and dust storms [29,53,56], transport and deposition of particulate flows in respiratory airways [6,36,71,28,66,65], deposition of blood cells in the arteries of human bodies [12], mixing and evaporation of fuel droplets in combustion mechanisms [15,57,52], spout-fluid bed techniques [70], and deposition of fission particles in various components of nuclear reactors following a severe accident [9]. The rationale behind WMLES models is to activate the RANS mode in the near-wall region where both spatial and temporal scales are too small to be resolved, and switch to LES when the mesh size is sufficient to resolve the local large scales up to the free stream With such a methodology, WMLES can tolerate much coarser grids in both streamwise and spanwise directions without significantly compromising accuracy [37]. Unlike DRW models, CRW studies proved to be a more accurate approach as it better represents particle physics It was shown in the works of [2] that particle statistics are better predicted when accounting for Stokes number effect. This is due to the weak shielding over implicit RANS mode in the viscous super-layer, which produces an inaccurate prediction of flow behavior, especially at higher Reynolds numbers This might be attributed to the inability of the eddy viscosity blending criteria to properly correct values for balancing the momentum equation. We summarize the results of the work done
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