Large eddy simulation of particle-laden turbulent flow over a backward-facing step

Abstract

The present study is a two-dimensional numerical prediction of an incompressible particle-laden turbulent flow over a backward facing step. The motion of the gas phase is treated by large eddy simulation (LES) and the particle phase is solved by a Lagrangian method. The governing equations for the gas phase are solved with Chorin's fractional-step projection method, and the standard Smagorinsky sub-grid scale model is used. The direct solver for the pressure correction Poisson's equations is the Buneman variance of the cyclic odd-even reduction algorithm. Particle motion is traced by a particle track model. Numerical simulations of the two-phase flow are performed with one-way coupling which means the influence of particles on fluid field is neglected. The validation of LES methodology is assessed through comparison of recirculation lengths over backward facing steps with published experimental results for the range of Reynolds numbers between 70 and 7,500. Good agreement is established between computational and experimental results. The predicted mean velocity profile for low Reynolds number of 1,290 and a high value of 18,400 also agrees well with the experimental results. Temporal and spatial growths of vortical structures in the flow of high Reynolds number have been successfully simulated as well using LES. Extensive features of the vortical structures in the turbulent flow are identified in the turbulent flow field, including rolling up, growing, pairing, merging, breaking up and separation of vortices. Motion of particles carried by the fluid flow field is simulated by introducing 70 um copper spheres and 150 um glass spheres in a backward facing step channel flow of air at high Reynolds number 18,400. The computed statistical mean properties of the two phases are in good agreement with the experimental results. Further investigation has been carried out on the dispersion of particles with different Stokes numbers in the vortical turbulent structure of the fluid field. Motion of glass spheres with diameters 2 um, 20 um, 50 um, 100 um and 200 um respectively introduced in the gas flow field is simulated. The predicted patterns of instantaneous dispersion of particles reveal that the phenomenon of preferential concentration of particles will occur at a certain range of Stokes number. Subsequently, the effects of initial velocity slip between the two phases and the action of gravity on particle dispersion are also investigated. This study has successfully demonstrated the value of LES in predicting the evolution of a flow field within a relatively complex channel. It also indicates that one-way coupling of fluid-particle interaction is sufficient to predict the particle flow field and dispersion in the channel flow.