Hello,

I am interested in evaluating the losses and flow field in hydraulic machinery and I would like to know what if some of you would be kind enough to share your experience with the optional production limiters (Clip Factors or Kato-Launder) available with the k-epsilon turbulence model in CFX 11.0.

For those who investigated the subject, I am interested to know if you usually turn these limiters on, or if you avoid using them. Did you notice any real impact in the losses (especially the ones occuring at stagnation points) ? What is the clip factor value to use ? Is there any reason to use something else than C.F.=10 - the default value of the SST model. Does the Kato-Launder performs better ?

Any good reference paper on the subject will also be appreciated !

Thank you very much,

Felix

Hi,

Unless you have a strong background in turbulence modelling I would just stick to the default options for the various turbulence models. As for whether the SST or Kato-Launder perform better - you would have to compare against experimental results but I would guess the SST would be superior.

Glenn Horrocks

Hi I have tried some case of production limitation,but not some obvious different.i think there will be differents in zone of vortex. is that right?

wayne

Hello Glenn & Wayne

I personally think that optional parameters are there to be used by trained specialists. If we don't even consider trying them then maybe we should just stick to colorfull fluid dynamics. ;-) I might no have a *strong* background in turbulence modelling but I do have a good knowledge of the involved equations and implications of using one or the other turbulence models, and also on how to use them.

To make a long story short, I sometimes prefer the k-epsilon model over the SST but I know that there is a "stagnation point anomaly" with the k-epsilon model. Since I want to be as accurate as possible, I need to know it's implications and try avoiding any source of error.

CFX support advised me to use a Clip Factor whenever there is a stagnation point or a recirculating flow is expected (i.e. virtually all the cases I'm interested to) but I fear I might underestimate the losses and be over-optimistic in the efficiency evaluation. Also, if we should use this parameter as often as they say, why isn't it included by default in the model ? And what value should we use ?

As Wayne pointed out, I did not notice important differences so far but the tetra meshes I used for those quick checks were quite coarse. I will do other verifications soon but I am also interested to have a more general view of the problem : other geometries, mesh types, Reynolds numbers, etc...

Thanks again to share your experience !

Felix

Does the little number of answers received mean that nobody uses a production limiter ?