Best practice guidelines for turbomachinery CFD
From CFD-Wiki
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. Which turbulence model people use often also has as much to do with beliefs and feelings as with knowledge and facts. There are many diffrent schools. Howeever, below follows at least some advices that most CFD engineers in the turbomachinery field tend to agree upon.
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 
. 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
For on-design simlations without any large separated regions it is often sufficient to use a wall-function model close to the wall, preferably using some form of non-equilibrium wall-function that is sensitised to streamwise pressure gradients.
For off-design simulation, or simulations involving complex secondary flows and separations, it is often necessary to use a low-Re model. There exists many low-Re models that have been used with success in turbomachinery simulations. A robust and often good choice is to use a one-equation model, like for example the Wolfstein model, in the inner parts of the boundary layer. There are also several Low-Re models that work well. Just make sure they don't suffer from the problem with overproduction of turbulent energy in regions with strong acceleration or decelleration. In the last few years Menter's low-Re 
model has gained increased popularity.
Transition prediction
Transition refers to the process when a lamainar boundary layer becomes unstable and transitions to a turbulent boundary layer. There are two types of transition - natural transition, where inherent instabilities in the boundary layer cause the transition and by-pass transition, where convection and diffusion of turbulence from the free-stream into the boundary layer causes transition.
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
