Difference between revisions of "UFR 4-08 Best Practice Advice"

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{{UFR|front=UFR 4-08|description=UFR 4-08 Description|references=UFR 4-08 References|testcase=UFR 4-08 Test Case|evaluation=UFR 4-08 Evaluation|qualityreview=UFR 4-08 Quality Review|bestpractice=UFR 4-08 Best Practice Advice|relatedACs=UFR 4-08 Related ACs}}
{{UFR|front=UFR 4-08|description=UFR 4-08 Description|references=UFR 4-08 References|testcase=UFR 4-08 Test Case|evaluation=UFR 4-08 Evaluation|qualityreview=UFR 4-08 Quality Review|bestpractice=UFR 4-08 Best Practice Advice|relatedACs=UFR 4-08 Related ACs}}


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The best practice advice is based on the only known and available numerical computation performed by Wang (1991).
The best practice advice is based on the only known and available numerical computation performed by Wang (1991).


''Key Physics:''
== Key Physics ==
 
The flow is characterised by an axi-symmetric separation at a sharp-edged circular orifice, leading to a highly turbulent shear layer which is subject to streamline curvature. The shear layer re-attaches some distance downstream from the orifice, producing a region of flow recirculation. The length of the recirculation region is a good overall indicator of predictive performance. This recirculation region has its greatest radial extent a short distance downstream from the orifice, at the 'vena contracta'. Overall there is a large loss in static pressure, due to substantial turbulence generation and subsequent energy dissipation.


* turbulent flow with streamline curvature, axi-symmetric separation and flow re-attachment
== Numerical Modelling Issues ==


''Numerical Modelling Issues:''
To capture the main flow features, a two-dimensional axi-symmetric computation can be undertaken.


* an two-dimensional axi-symmetric calculation captures the main features of the flow
To obtain a mesh-independent prediction of the re-attachment length, a grid of at least 128 x 28 cells, refined near the orifice, is required - at least when combined with a flux-blended scheme for convection discretisation (50% upwind, 50% central difference).


* a grid with 128x28 cells with a refinement near the orifice allowed an accurate prediction of the re-attachment length
== Physical Modelling ==


* a second order scheme for the diffusion terms and a hybrid scheme for the convective terms is advisable
The standard k-ε turbulence model with wall functions can be used to provide a very good prediction of the reattachment length, mean velocity field and wall friction coefficient. Whilst the turbulent kinetic energy field will be less well-predicted, it will still be found to match measurements quite well at most locations.


* the computational domain needs to insure a long enough inflow and outflow region
== Application Uncertainties ==


''Physical Modelling:''
The mean velocity and turbulence field is likely to be quite sensitive to the specification of the dissipation rate at the inlet cross-section. This can be specified according to fully-developed pipe flow.


* the standard k-ε turbulence model with wall functions can provide a reasonable good prediction of the turbulent kinetic energy field
== Recommendations for Future Work ==


* the result for mean velocities and turbulence intensities seems to be quite sensitive to the specification of the dissipation rate at the inlet cross-section
The application of a full Reynolds stress turbulence model should, in principle, provide more meaningful predictions of the turbulence field - in particular turbulence anisotropy.


''Recommendation for Future Work:''


* the application of a full Reynolds stress turbulence model should be able also to predict the anisotropy of the turbulence and will also allow to predict the shear stresses


<font size="-2" color="#888888">© copyright ERCOFTAC 2004</font><br />
<font size="-2" color="#888888">© copyright ERCOFTAC 2004</font><br />
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{{UFR|front=UFR 4-08|description=UFR 4-08 Description|references=UFR 4-08 References|testcase=UFR 4-08 Test Case|evaluation=UFR 4-08 Evaluation|qualityreview=UFR 4-08 Quality Review|bestpractice=UFR 4-08 Best Practice Advice|relatedACs=UFR 4-08 Related ACs}}
{{UFR|front=UFR 4-08|description=UFR 4-08 Description|references=UFR 4-08 References|testcase=UFR 4-08 Test Case|evaluation=UFR 4-08 Evaluation|qualityreview=UFR 4-08 Quality Review|bestpractice=UFR 4-08 Best Practice Advice|relatedACs=UFR 4-08 Related ACs}}
[[Category:Underlying Flow Regime]]

Latest revision as of 14:18, 12 February 2017

Front Page

Description

Test Case Studies

Evaluation

Best Practice Advice

References




Orifice/deflector flow

Underlying Flow Regime 4-08               © copyright ERCOFTAC 2004


Best Practice Advice

Best Practice Advice for the UFR

The best practice advice is based on the only known and available numerical computation performed by Wang (1991).

Key Physics

The flow is characterised by an axi-symmetric separation at a sharp-edged circular orifice, leading to a highly turbulent shear layer which is subject to streamline curvature. The shear layer re-attaches some distance downstream from the orifice, producing a region of flow recirculation. The length of the recirculation region is a good overall indicator of predictive performance. This recirculation region has its greatest radial extent a short distance downstream from the orifice, at the 'vena contracta'. Overall there is a large loss in static pressure, due to substantial turbulence generation and subsequent energy dissipation.

Numerical Modelling Issues

To capture the main flow features, a two-dimensional axi-symmetric computation can be undertaken.

To obtain a mesh-independent prediction of the re-attachment length, a grid of at least 128 x 28 cells, refined near the orifice, is required - at least when combined with a flux-blended scheme for convection discretisation (50% upwind, 50% central difference).

Physical Modelling

The standard k-ε turbulence model with wall functions can be used to provide a very good prediction of the reattachment length, mean velocity field and wall friction coefficient. Whilst the turbulent kinetic energy field will be less well-predicted, it will still be found to match measurements quite well at most locations.

Application Uncertainties

The mean velocity and turbulence field is likely to be quite sensitive to the specification of the dissipation rate at the inlet cross-section. This can be specified according to fully-developed pipe flow.

Recommendations for Future Work

The application of a full Reynolds stress turbulence model should, in principle, provide more meaningful predictions of the turbulence field - in particular turbulence anisotropy.


© copyright ERCOFTAC 2004



Contributors: Martin Sommerfeld - Martin-Luther-Universitat Halle-Wittenberg


Front Page

Description

Test Case Studies

Evaluation

Best Practice Advice

References