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Old   December 15, 2011, 03:45
Default CFX 13 SST reattachment
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Hi,

my supervising tutor at University just asked me, if I know something about the reattachment option for SST-Model in CFX 13. Some of his colleagues mentioned thiming about this. But I couldn't finde a reattachment option.
Or was this an option in the previous versions and is now implemented anyway?

Thanks in advance!
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Old   December 15, 2011, 07:22
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Reattachment Model was implemented for better prediction of the re attachment point in low Reynolds number flows. The superiority of this model is not well established.
http://scitation.aip.org/getabs/serv...ifs=yes&ref=no
It was there from versions after CFX 11. You need to activate them through command line if it is not available on the GUI.
Have a look at this thread
CFX v12 adding reattachment modification to sst model
click like if you are happy with the answer
cheers
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Old   December 15, 2011, 08:25
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thanks k_k!
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Old   December 15, 2011, 10:03
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well this model is not physically correct, instead use the Transition model for reattachment prediction (for the right reasons). You can find the equation in paper by Jason - Canada

Assessment of Turbulence Model Predictions for an Aero-Engine Centrifugal Compressor

J. Turbomach. -- January 2011 -- Volume 133, Issue 1, 011025 (15 pages)
doi:10.1115/1.4001136
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Old   December 15, 2011, 10:10
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Hey,

I thought Transition model is for free surface problems like flow over an airfoil.
My problem is a rib-roughened internal cooling channel. Is the Transition model working here correctly?
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Old   December 15, 2011, 10:38
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Well When I have contacted with Some one in ANSYS I get this answer

"SST RM model ------>1. we do not recommend it 2.for very fine grids it has zero effect". I am also told that due to these reasons it is still included as beta feature (from CFX 11 to CFX 13).

Well we have recently worked on the transition prediction and separation control on low pressure turbine using this model and we got very good results. We have used dimples, hump and step in geometry. One of my friend is working on transition model development for CD adapco at Thailand. His area of interest is transition in ribbed channels.

So I dont see any problem in applying the transition model in your application instead of SST RM model. It must be noted that SST Transition model has three functions:

1. Prediction of laminar separation
2. Correct prediction of transition region
3. Correct prediction of transition /turbulent reattachment (similar to SST RM model, which is designed to predict correctly the turbulent reattachment)

Also see these papers for details about transition modeling in general

1.Predicting Separation and Transitional Flow in Turbine Blades at Low Reynolds Numbers—Part I: Development of Prediction Methodology

J. Turbomach. -- July 2011 -- Volume 133, Issue 3, 031011 (10 pages)
doi:10.1115/1.4001230

2. Predicting Separation and Transitional Flow in Turbine Blades at Low Reynolds Numbers—Part II: The Application to a Highly Separated Turbine Blade Cascade Geometry

J. Turbomach. -- July 2011 -- Volume 133, Issue 3, 031012 (7 pages)
doi:10.1115/1.4001231
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Old   December 15, 2011, 10:40
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Heat Transfer and Pressure Losses of W-Shaped Small Ribs at High Reynolds Numbers for Combustor Liner

J. Eng. Gas Turbines Power -- September 2011 -- Volume 133, Issue 9, 091901 (8 pages)
doi:10.1115/1.4002878

"The slope of heat transfer coefficient becomes constant with increasing Reynolds number because of the laminar-turbulent transition around the ribs, which is considered to occur at Reynolds number based on rib height of about 1000. "

This may explain the validity of transition model.

PS: K-Kl-W transition model is available in Fluent at the moment. While Gamma theta transition model is available in both codes.
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Old   December 15, 2011, 10:44
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Hi Far,

thanks for your answer!
I just finished my study thesis, but probably my supervising tutor will do some more simulations and therefore he could use the transition model.
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