UFR 3-31 Best Practice Advice: Difference between revisions

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== Application Uncertainties ==
== Application Uncertainties ==
{{Demo_UFR_BPA4}}
A careful assessment of the effect of the inflow condition on the results is
necessary. The advice is to use the mean flow and turbulent kinetic energy
extracted from the LES database if possible, but the length-scale should be
extracted from a preliminary computation with the same model as the one tested,
in order to match the boundary layer thickness and the wall-shear stress up to
the separation point.
 
== Recommendations for Future Work ==
== Recommendations for Future Work ==
{{Demo_UFR_BPA5}}
{{Demo_UFR_BPA5}}

Revision as of 11:58, 1 June 2012

Flow over curved backward-facing step

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Description

Test Case Studies

Evaluation

Best Practice Advice

References

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

  • As the separation region is very thin, a fine wall-normal resolution is necessary everywhere in the flow domain.
  • 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 ) close to the theoretical separation region.
  • 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.
  • The inflow boundary condition is also important for consistency, and two possible definitions are detailed in the CFD section.

Physical Modelling

  • Both LES approaches, with dynamic SGS and mixed time-scale modelling, yield good results,
  • Low-Reynolds number models are better at predicting the attached boundary layer upstream of separation and should thus be preferred to high-Reynolds number models with wall-functions. The effect of curvature is also important in this respect and specific treatment are needed if not using a Reynolds-stress model.

Application Uncertainties

A careful assessment of the effect of the inflow condition on the results is necessary. The advice is to use the mean flow and turbulent kinetic energy extracted from the LES database if possible, but the length-scale should be extracted from a preliminary computation with the same model as the one tested, in order to match the boundary layer thickness and the wall-shear stress up to the separation point.

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

Front Page

Description

Test Case Studies

Evaluation

Best Practice Advice

References


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