Transition: which function to identify it?

sarav · October 20, 2009, 10:44

Hi
I'm testing the transition model gammaRetheta in star-ccm+.
I would know how I can identify the point at which transition starts. I could use:
-a plot of skin friction coefficient
-a plot of tubolence intensity
-a plot of eddy viscosity
-other (tell me

)

What do you think it's the best choice?
The main problem is that using skin friction coefficient the transition starting point is not very evident; instaed if I use a plot with intensity I get a transition point which is different from the one I get using a plot with eddy viscosity!

Thanks!

SKK · October 21, 2009, 08:19

Gamma is intermittency factor varying from 0 to 1, ie fully laminar to turbulent condition. So, if the option is available, you cans see at what point your gamma starts to increase in the 'boundary layer', denoting start of transition. I would use this rather than turbulent viscosity. Note that in gamma Re_theta model (of Menter) gamma can be greater than 1 for separated flow transition.

sarav · October 22, 2009, 03:35

I want to make it clear first that I'm studying now turbulence models and transition models so I'm not so expert...so I might commit mistakes...

I can't visualize gamma but I can visualise directly Intermittency...is the same thing? It's values is 0.02 on the laminar part and after transition It growes up to 0.035...It might be correct?
Thanks for all your advices!
Sara

SKK · October 22, 2009, 03:49

Hi Sara, I've worked with transition modeling for my research, and I still wouldn't call myself an expert, so don't worry

Yes, gamma is intermittency. Ideally in laminar flow it should be zero, but it is possible for some models to have non-zero values either due to stability requirement or because they include a model for laminar fluctuation. In your fully turbulent region intermittency should be 1 or thereabout according to traditional definition. I suggest you read the paper by Menter, "A correlation-based transition model using local variables-Part I: Model Formulation". Also for general transition understanding the paper by Mayle is excellent, "The role of laminar-turbulent transition in gas turbine engines".

Hope this helps.

sarav · October 22, 2009, 03:57

Thanks for all!
This paper"A correlation-based transition model using local variables-Part I: Model Formulation" is here on my desptop!

And now I'm going to search the other one!

October 20, 2009, 10:44	Transition: which function to identify it?	#1
sarav New Member Join Date: Jul 2009 Posts: 25 Rep Power: 16	Hi I'm testing the transition model gammaRetheta in star-ccm+. I would know how I can identify the point at which transition starts. I could use: -a plot of skin friction coefficient -a plot of tubolence intensity -a plot of eddy viscosity -other (tell me ) What do you think it's the best choice? The main problem is that using skin friction coefficient the transition starting point is not very evident; instaed if I use a plot with intensity I get a transition point which is different from the one I get using a plot with eddy viscosity! Thanks!

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October 21, 2009, 08:19		#2
SKK Member Join Date: Mar 2009 Posts: 55 Rep Power: 17	Gamma is intermittency factor varying from 0 to 1, ie fully laminar to turbulent condition. So, if the option is available, you cans see at what point your gamma starts to increase in the 'boundary layer', denoting start of transition. I would use this rather than turbulent viscosity. Note that in gamma Re_theta model (of Menter) gamma can be greater than 1 for separated flow transition.

October 22, 2009, 03:35		#3
sarav New Member Join Date: Jul 2009 Posts: 25 Rep Power: 16	I want to make it clear first that I'm studying now turbulence models and transition models so I'm not so expert...so I might commit mistakes... I can't visualize gamma but I can visualise directly Intermittency...is the same thing? It's values is 0.02 on the laminar part and after transition It growes up to 0.035...It might be correct? Thanks for all your advices! Sara

October 22, 2009, 03:49		#4
SKK Member Join Date: Mar 2009 Posts: 55 Rep Power: 17	Hi Sara, I've worked with transition modeling for my research, and I still wouldn't call myself an expert, so don't worry Yes, gamma is intermittency. Ideally in laminar flow it should be zero, but it is possible for some models to have non-zero values either due to stability requirement or because they include a model for laminar fluctuation. In your fully turbulent region intermittency should be 1 or thereabout according to traditional definition. I suggest you read the paper by Menter, "A correlation-based transition model using local variables-Part I: Model Formulation". Also for general transition understanding the paper by Mayle is excellent, "The role of laminar-turbulent transition in gas turbine engines". Hope this helps.

October 22, 2009, 03:57		#5
sarav New Member Join Date: Jul 2009 Posts: 25 Rep Power: 16	Thanks for all! This paper"A correlation-based transition model using local variables-Part I: Model Formulation" is here on my desptop! And now I'm going to search the other one!