Large-eddy simulation of turbulent boundary layer separation from a rounded step

Abstract

Highly resolved large-eddy simulation (LES) is used to investigate the characteristics of a canonical boundary layer separating from a curved step in a channel of height 8.5 times that of the step. The flow is treated as statistically spanwise homogeneous, in line with the conditions of a related experimental study in a large aspect ratio channel, undertaken within a companion research programme. Primary attention focuses on the details of the separation process and the properties of the separated region, including reattachment. Results are reported and analysed, from a flow physical perspective, for a wide variety of properties, including wall pressure and skin friction, mean velocity, Reynolds stresses and related anisotropy maps, two-point-correlation functions, unsteadiness indicators, budgets of the Reynolds stresses and length scales characterising the turbulence, and mean strain fields. The study highlights a range of distinctive features of separation from gently curved surfaces: the separation process is highly unsteady in time and space; turbulence is highly non-local in character; the mean reverse-flow region is thin and highly elongated; no part of the flow is reversed at all times; the level of production is extremely high following separation, resulting in massive departures from turbulence energy equilibrium, very high anisotropy and a trend towards one-component turbulence in the separated shear layer. The result, apart from offering insight into the physics of separation, constitutes a valuable data set for benchmarking model solutions and investigating statistical turbulence-closure proposals.