Large-eddy simulation applied to study the influence of upstream conditions on the time-dependant and averaged characteristics of a backward-facing step flow

Abstract

The unsteady backward-facing step flow is investigated using large-eddy simulation. Two different inlet conditions are tested to study the sensitivity of the separated flow to a modification of the upstream boundary layer. The first inlet condition consists of a mean turbulent profile perturbed by a white noise. The second relies upon a more realistic condition, in which fully turbulent inflow data are derived from an auxiliary simulation of a quasi-temporal boundary layer. The temporal pressure spectra in different locations downstream of the step exhibit four different characteristic frequencies in both cases. Pressure and velocity statistics, supplemented by visualizations, demonstrate how the flow in the shear layer is strongly influenced by the upstream conditions. The turbulent boundary layer triggers a rapid destabilization of the mixing layer, resulting in a shortening of the mean recirculation length. The temporal spectra reveal that the precursor simulation leads to an increase of the characteristic frequencies associated with the Kelvin–Helmholtz vortices and to the flapping of the shear layer. Streaks and quasi-longitudinal vortices created in the turbulent boundary layer induce high- and low-longitudinal velocity modulations upstream of the step edge. The spanwise modulation of the velocity seems to be responsible for the wavy destabilization of the Kelvin–Helmholtz vortices. The dependency of the mean flow and of the characteristic frequencies of pressure fluctuations to the incoming boundary layer clearly demonstrates the importance of defining as realistic boundary conditions as possible for the simulation of 3D separated flows.