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September 1, 2009, 12:35 
Blending funtction F1 for SST turbublence model

#1 
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I am confused about the blending function for SST model:
in Menter's paper, he said, F1 switch to one inside of boundary layer? But in my case i found in the free surface, I got F1=one also. Is that ok? or something wrong? Thanks in advance! 

September 2, 2009, 04:48 

#2 
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F1 is formulated so that it is 1 in boundary layer and in regions of adverse pressure gradient. This switches the kw part of the model in these situations. In the freestream it should be one. Check your formulation and the variables of F1. I would suggest have a look at the value of vorticity in the problematic area to see if it is behaving the way it should.


September 2, 2009, 12:50 

#3 
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Thanks for your reply!
I checked the formula, it was right and also the wall distance calculated rightly. For my cases I got a good convergence with regular kw, but if i use sst kw it did not convergence very well and F1 is one in some other region besides boundary layer. Last day i tired to set F1=one and F2=zero everywhere and switch the SST boundary to Kw boundary conditions, It was supposed to get the same results as regular komega, but I did not. And I found somethings confused at wall boundary for omega equation: 1, for regular Kw, w=7.2*Nu/Y**2 at y=y1 2, for SST, w=10*6*Nu/beta_1/Y**2=800*Nu/Y**2 at y=0 (beta_1=0.075) My question are: 1, what does y=y1 and y=0 mean? what i understand is: y=y1 mean we specify this value to the first cell above the wall boundary, and y=0 mean we need to specify this value at wall face. Is that right? 2, cause w in SST model is 1/0.09 times w in the regular Kw model, so those two values are not identical.If they are same, the SST model w should equal to 6*Nu/beta_1/Y**2. SO why they are different? Any suggestions will be appreciated Thanks! 

September 4, 2009, 04:06 

#4 
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I don't remember much detail about the Wilcox model, but according to the paper by Menter http://ntrs.nasa.gov/archive/nasa/ca...1993013620.pdf , in Wilcox's model, y is the distance from cell to the wall. It is the same in SST model, but in SST at the near wall cell, the value for Wilcox's w is multiplied by 10 as it gives much more accurate value for w. In general, because w is used to dampen the turbulence near wall, w values are expected to work better when they are high. Menter modified the w values to increase accuracy and to make it easier to implement in traditional CFD environment.
Last edited by SKK; September 4, 2009 at 04:40. 

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