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Best practice guidelines for turbomachinery CFD

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== Turbulence modeling ==
== Turbulence modeling ==
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Spalart-Allmares
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Selecting a suitable turbulence model for turbomachinery simulations can be challenging task. Obviously there is no single model which is suitable for all types of simulations. For attached flows close to the design point a simple algebraic model like the [[Baldwin-Lomax model]] can be used. Another common choice for design-iteration type of simulations is the one-equation model by [[Spalart-Allmaras model | Spalart-Allmaras]]. The big advantage with both the [[Baldwin-Lomax model]] and the [[Spalart-Allmaras model]] over more advanced models are that they are very robust to use and rarely produce complete unphysical results.
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In order to accurately predict more difficult cases, like flows that are close to or even fully separated, rotating flows, flows strongly affected by secondary flows etc. it is often necessary to use a more refined turbulence model. Common choices are a two-equation models like the <math>k-\epsilon</model>. <math>k-\epsilon</math> models can give good results but this type of models need to include some form of correction to avoíd over-production of turbulent energy in regions with strong acceleration or decelleration. Typical such corrections are some form or [[realizability constraint]] or the [[Kato-Launder modification]]. Antoher common choice in turbulence model is Menter's [[SST k-omega model]] or the slightly more elaborate [[v2f model]] by Durbin.
=== Near Wall Treatment ===
=== Near Wall Treatment ===

Revision as of 18:00, 7 September 2005

Contents

Deciding what type of simulation to do

2D, Quasi-3D or 3D

Inviscid or viscid

Transient or Stationary

Meshing

Boundary conditions

Turbulence modeling

Selecting a suitable turbulence model for turbomachinery simulations can be challenging task. Obviously there is no single model which is suitable for all types of simulations. For attached flows close to the design point a simple algebraic model like the Baldwin-Lomax model can be used. Another common choice for design-iteration type of simulations is the one-equation model by Spalart-Allmaras. The big advantage with both the Baldwin-Lomax model and the Spalart-Allmaras model over more advanced models are that they are very robust to use and rarely produce complete unphysical results.

In order to accurately predict more difficult cases, like flows that are close to or even fully separated, rotating flows, flows strongly affected by secondary flows etc. it is often necessary to use a more refined turbulence model. Common choices are a two-equation models like the k-\epsilon</model>. <math>k-\epsilon models can give good results but this type of models need to include some form of correction to avoíd over-production of turbulent energy in regions with strong acceleration or decelleration. Typical such corrections are some form or realizability constraint or the Kato-Launder modification. Antoher common choice in turbulence model is Menter's SST k-omega model or the slightly more elaborate v2f model by Durbin.

Near Wall Treatment

Numerical considerations

Multi-stage analysis

Heat transfer predictions

What to trust and what not to trust

CFD is generally quite good at predicting surface static pressure distributions. With care CFD can also be used to predict performance, total-pressure losses and blade turning.

Predicting separation, stall and off-design performance can be a challenge and results with non-attached flows should be interpreted with care.

Heat transfer is often very difficult to predict accurately and it is common to obtain heat-transfer coefficients that are 100% wrong or more. Validation data is critical in order to be able to trust heat transfer simulations.

Transition is almost impossible to predict accurately in genereal. However, there exists models that have been tuned to predict transition and these tend to give acceptable results for cases close to the ones they were tuned for.

External links

QNET-CFD Best Practise Advice for Turbomachinery Internal Flows

My wiki