# Abstract

This document presents the Underlying flow Regime (UFR) of a turbulent boundary layer separating from a curved surface at moderate Reynolds number (${\displaystyle Re_{\theta }\sim \ 1200}$, based on the momentum thickness ${\displaystyle \ \theta }$ at the domain inlet). The primary focus of this case is on the details of the separation process and the turbulent properties of the separated region, including reattachment. This geometry shows particular 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 geometry of the curved step was designed by a combined effort between University of Manchester, where experiments were conducted, and Imperial College London, where the Large-Eddy Simulations were performed. The geometry is an extended version of that studied by Song & Eaton [‌27], except for the step height, which is increased to produce a larger recirculation bubble. The careful design of the geometry allows an accurate comparison between well-resolved large-eddy simulation and experimental results, for exactly the same flow conditions. All the results available in the database are extracted from the finest Large-Eddy Simulation (LES) presented in the CFD section, and they show very good agreement with the experimental data available. Results of RANS calculations with well-known two-equation models are also presented in graphical form to illustrate the challenges of this test case. A recurring defect of such models is that most predict insufficient levels of turbulence activity in the separated shear layer and thus a serious delay in reattachment and excessive recirculation, shortcomings that reflect an inability of the models to account for the dynamics of the separation process, made worse by the tendency of the models to depress the turbulent stresses in the shear layer bordering the recirculation zone because of the effects of (stabilizing) curvature on the turbulent stresses.

Contributed by: Sylvain Lardeau — CD-adapco, London, UK