Study of flow in a planar asymmetric diffuser using large-eddy simulation

Large-eddy simulation (LES) has been used to study the flow in a planar asy mmetric diffuser. The wide range of spatial and temporal scales, the presen ce of an adverse pressure gradient, and the formation of an unsteady separa tion bubble in the rear part of the diffuser make this flow a challenging t est case for assessing the predictive capability of LES. Simulation results for mean flow, pressure recovery and skin friction are in excellent agreem ent with data from two recent experiments. The inflow consists of a fully d eveloped turbulent channel flow at a Reynolds number based on shear velocit y, Re-tau = 500. It is found that accurate representation of the inflow vel ocity field is critical for accurate prediction of the how in the diffuser. Although the simulation in the diffuser is well resolved, the subgrid-scal e model plays a significant role for both mean momentum and turbulent kinet ic energy balances. Subgrid-scale stresses contribute a maximum of 8% to th e local value of the total shear stress with the maximum values found in th e inlet duct and along the flat wall where the flow remains attached. The s ubgrid-scale model adapts to the enhanced turbulence levels in the rear par t of the diffuser by providing more than 80% of the dissipation rate for tu rbulent kinetic energy. The unsteady separation excites large scales of mot ion which extend over the major part of the duct cross-section and penetrat e deeply into the core of the flow. Instantaneous flow reversal is observed along both walls immediately behind the diffuser throat which is far upstr eam of the location of main separation. While the mean how profile changes gradually as the flow enters the expansion, turbulent stresses undergo rapi d changes over a short streamwise distance along the deflected wall. An exp lanation is offered which considers the strain field as well as the influen ce of geometry changes. The effect of grid resolution and spanwise domain s ize on the flow field prediction has been documented and this allows an ass essment of the computational requirements for carrying out such simulations .