[Anna Tian]

Hi,

In CFX, we can't use different turbulent model to different fluid domain. We have to choose only one to all of them. But for my case, some of my fluid domain have laminar flow and the others are turbulent. So I have to use turbulent model to not only the turbulent part but also the laminar part. May I ask will that bring any inaccuracy?

Thank you very much.

[ghorrocks]

Yes, it will cause an inaccuracy. It can be minimised by using a turbulence model which degenerates nicely to zero turbulence. k-e get a divide by zero error at zero turbulence (unless it has a low Re correction), but SST and k-w work fine with zero turbulence.

So I recommend you use the SST or k-w turbulence models as they handle turbulence levels of zero (or very low levels of turbulence) gracefully, and will only add a small amount of dissipation to the model.

[evcelica]

I believe you can apply different turbulence models to different domains.
EDIT > OPTIONS > GENERAL > ENABLE BETA FEATURES > then uncheck CONSTANT DOMAIN PHYSICS

But maybe you don't need to if you take Glenn's advice.

[ghorrocks]

I remember years ago I compared a flow which was quite laminar with a simulation using laminar and SST turbulence models. The SST model ran fine and the results were close enough to be useful. But I recommend you check this for your configuration.

I do not think you can use non-constant domain physics if the domains are connected. The interface will not be well specified and I am sure the simulation will crash. But it might work if the domains are not connected.

[Anna Tian]

Originally Posted by ghorrocks

Yes, it will cause an inaccuracy. It can be minimised by using a turbulence model which degenerates nicely to zero turbulence. k-e get a divide by zero error at zero turbulence (unless it has a low Re correction), but SST and k-w work fine with zero turbulence.

So I recommend you use the SST or k-w turbulence models as they handle turbulence levels of zero (or very low levels of turbulence) gracefully, and will only add a small amount of dissipation to the model.

Could Reynolds stress model also be okay for laminar and transitional flow?

[Anna Tian]

Originally Posted by ghorrocks

I remember years ago I compared a flow which was quite laminar with a simulation using laminar and SST turbulence models. The SST model ran fine and the results were close enough to be useful. But I recommend you check this for your configuration.

I do not think you can use non-constant domain physics if the domains are connected. The interface will not be well specified and I am sure the simulation will crash. But it might work if the domains are not connected.

What parameter did you use to compare these two models' simulation results? Could SST gives a good estimation of the wall friction loss? Could it resolve laminar boundary layer well?

[ghorrocks]

Could Reynolds stress model also be okay for laminar and transitional flow?

No. The issue is what happens when the turbulence level goes to zero. The important thing is the behaviour of epsilon/omega. The turbulence energy is simple, it just goes to zero. But epsilon is badly behaved as turbulence goes to zero but omega is.

What parameter did you use to compare these two models' simulation results? Could SST gives a good estimation of the wall friction loss? Could it resolve laminar boundary layer well?

I cannot remember the model, but it simply compared a laminar and SST simulation for a flow which was actually laminar. SST should be able to resolve most laminar flow features, but it will have more dissipation than a true laminar model so you will need to validate it carefully. Or even better use a laminar flow model.

[ghorrocks]

Could Reynolds stress model also be okay for laminar and transitional flow?

No. The issue is what happens when the turbulence level goes to zero. The important thing is the behaviour of epsilon/omega. The turbulence energy is simple, it just goes to zero. But epsilon is badly behaved as turbulence goes to zero but omega is.

What parameter did you use to compare these two models' simulation results? Could SST gives a good estimation of the wall friction loss? Could it resolve laminar boundary layer well?

I cannot remember the model, but it simply compared a laminar and SST simulation for a flow which was actually laminar. SST should be able to resolve most laminar flow features, but it will have more dissipation than a true laminar model so you will need to validate it carefully. Or even better use a laminar flow model.