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-   -   turbulent flow turns to transition and then to laminar (http://www.cfd-online.com/Forums/cfx/74459-turbulent-flow-turns-transition-then-laminar.html)

asalama March 31, 2010 22:28

turbulent flow turns to transition and then to laminar
 
Hi All,
I'm modeling a case in which the flow starts turbulent and then as time passes the flow decays away and eventually turns to transition and then to laminar. I wonder if CFX is able to doing this and if so, how can I set up this option to work.

ghorrocks April 1, 2010 06:25

CFX does not have a relaminarisation model. However, you may be able to get close enough by either using a laminar model and running a LES/DES style model or by using a k-omega based turbulence model which will degenerate to zero turbulence if given a chance. This will depend on exactly what you are trying model however and you have not explained that yet.

asalama April 1, 2010 06:50

Thanks Glenn for your reply.
I'm modeling an experiment in which we have a flow inside a vertical annulus section heated at the center. The flow is first turbulent and steady state. Then the pump is coasting down and the flow will thus decay away. As it decays, the flow, eventually will turn to laminar before it switches to natural convection. Now, if I start with turbulent flow, I don't know how to make the program switch to laminar model when the Re gets smaller.

ghorrocks April 1, 2010 07:01

What are you trying to get out of the analysis?

As I said previously, there is no relaminarisation model in CFX, so you will not be able to directly model the relaminarisation process. You may be able to get close enough to it using either a LES/DES approach or a k-omega turbulence model, but that depends on what you are trying to achieve with the simulation as to whether they will be appropriate.

asalama April 1, 2010 08:15

I was told that cfx provides a transition model. Does it apply to this case.
And if the SST model may work, does it also work for the natural convection scenario

cfdman April 2, 2010 02:50

SST Transitional Model
 
http://img18.imageshack.us/img18/879...narization.jpg
http://img18.imageshack.us/img18/879...narization.jpg

Hi Asalama,

You can use SST Transitional model in CFX for relaminarization modelling. Above there is a test case graph from Langtry's PhD thesis. So CFX can solve the relaminarization with SST Transitional model. Also you can use transitional model with FLUENT.
My suggestion is, collect some information about Transitional model. You can see Menter and Langty's papers on literature.

Regards.

asalama April 2, 2010 03:49

Dear Arda,
Thanks alot for your reply and the attachment. I'll certainly look at the paper you suggested. I hope it will also work in natural convection scenario. Thanks once again.

ghorrocks April 2, 2010 06:19

Thanks Arda, I did not know of that work. It looks like the turbulence transition model is cleverer than I thought.

But be a little careful of it, the turbulence transition model was tuned for aerofoil flows so the flows you describe may require a little work to get accurate.


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