Flow over curved backward-facing step

Semi-confined flows

Underlying Flow Regime 3-31

Best Practice Advice

Key Physics

The key physical features of this UFR that need to be correctly modelled are the position and the extent of the separation region. Both are very much a function of the performance of the model in three distinct regions: the turbulent boundary layer upstream of the separation (correct rise of turbulence level even upstream of nominal separation point), sufficient level of turbulent activity in the separated shear layer, and correct representation of the stabilizing effect of curvature to turbulence in separated shear layer.

Numerical Issues and Boundary Conditions

\item As the separation region is very thin, a fine wall-normal resolution is necessary everywhere in the flow domain, \item The flow is also very sensitive to the streamwise location of the separation point: a too late separation will lead to a too short bubble, and a too early one will lead to a too late reattachment. A precise prediction of the separation point is crucial, and this requires a fine mesh (of order $\Delta x^+\sim 8-10$) close to the theoretical separation region. \item The flow behaviour in the separated region is less sensitive to the correct prediction of the upper-wall boundary layer, and either a high-Re (ie. wall-function) formulation or a free-slip conditions can be used there, if computational cost is an issue. \item The inflow boundary condition is also important for consistency, and two possible definitions are detailed in the CFD section.

Physical Modelling

Turbulence modelling
Transition modelling
Near-wall modelling
Other modelling

Application Uncertainties

Summarise any aspects of the UFR model set-up which are subject to uncertainty and to which the assessment parameters are particularly sensitive (e.g location and nature of transition to turbulence; specification of turbulence quantities at inlet; flow leakage through gaps etc.)

Recommendations for Future Work

Propose further studies which will improve the quality or scope of the BPA and perhaps bring it up to date. For example, perhaps further calculations of the test-case should be performed employing more recent, highly promising models of turbulence (e.g Spalart and Allmaras, Durbin's v2f, etc.). Or perhaps new experiments should be undertaken for which the values of key parameters (e.g. pressure gradient or streamline curvature) are much closer to those encountered in real application challenges.

Contributed by: Sylvain Lardeau — CD-adapco