# Very technical question about solving wall boundary layer ...

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 April 15, 2011, 09:05 Very technical question about solving wall boundary layer ... #1 New Member   Join Date: Apr 2011 Posts: 3 Rep Power: 7 I am reading the fluent help manual since one day and I don't manage to find a precise answer on the resolution of the boundary layer for FLUENT. I am using Enhanced wall treatment : - for KE , I understood, if the mesh is suffisently small (Y+=1) then the 2 layers model will be used. If the mesh is coarse a wall function using : Kader [168] + White and Cristoph [418] and Huang et al. [147] will be used. Is it ok ? - But for kw-SST it become really really unclear : (chapter 4.4.4. Wall Boundary Conditions for Kw-SST) "This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds-number boundary conditions will be applied" (this is all right ;-)) and after :Analytical solutions can be given for both the laminar sublayer (eq : 4–118) and the logarithmic region (4–119). Therefore, a wall treatment can be defined for the w equation, which switches automatically from the viscous sublayer formulation to the wall function, depending on the grid. This blending ..." Are these equation used for a modeling the boundary layer or are they wall functions ? I am quite deasapointed because in Chapter :4.13.5.( Enhanced Wall Treatment ω-Equation (EWT-ω)) it is stated : "the Kw-equation can be integrated through the viscous sublayer without the need for a two-layer approach. This feature can be utilized for a y* -insensitive wall treatment by blending the viscous sublayer formulation and the logarithmic layer formulation based on y*,This formulation is the default for all Kw equation based models." And finaly in 4.13.4. (Enhanced Wall Treatment ε-Equation (EWT-ε)) "Please note that an alternative blending compared to the Kader-blending is used for the improved near wall treatment for turbulence models based on the w equation, which is the new default near wall treatment." I do not understand what is the wall function and what are the equations used for modeling (or blending ...) the complete BL ? If somebody get the point ... Thanks in Advance JB

 April 18, 2011, 03:50 #2 New Member   Join Date: Apr 2011 Posts: 3 Rep Power: 7 No idea ???

April 19, 2011, 04:09
#3
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Raashid Baig
Join Date: Mar 2010
Location: Bangalore, India
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Quote:
 Originally Posted by jlb001 I am reading the fluent help manual since one day and I don't manage to find a precise answer on the resolution of the boundary layer for FLUENT. I am using Enhanced wall treatment : - for KE , I understood, if the mesh is suffisently small (Y+=1) then the 2 layers model will be used. If the mesh is coarse a wall function using : Kader [168] + White and Cristoph [418] and Huang et al. [147] will be used. Is it ok ? - But for kw-SST it become really really unclear : (chapter 4.4.4. Wall Boundary Conditions for Kw-SST) "This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds-number boundary conditions will be applied" (this is all right ;-)) and after :Analytical solutions can be given for both the laminar sublayer (eq : 4–118) and the logarithmic region (4–119). Therefore, a wall treatment can be defined for the w equation, which switches automatically from the viscous sublayer formulation to the wall function, depending on the grid. This blending ..." Are these equation used for a modeling the boundary layer or are they wall functions ? I am quite deasapointed because in Chapter :4.13.5.( Enhanced Wall Treatment ω-Equation (EWT-ω)) it is stated : "the Kw-equation can be integrated through the viscous sublayer without the need for a two-layer approach. This feature can be utilized for a y* -insensitive wall treatment by blending the viscous sublayer formulation and the logarithmic layer formulation based on y*,This formulation is the default for all Kw equation based models." And finaly in 4.13.4. (Enhanced Wall Treatment ε-Equation (EWT-ε)) "Please note that an alternative blending compared to the Kader-blending is used for the improved near wall treatment for turbulence models based on the w equation, which is the new default near wall treatment." I do not understand what is the wall function and what are the equations used for modeling (or blending ...) the complete BL ? If somebody get the point ... Thanks in Advance JB
Can you please simplify as to what is your question. Don't quote FLUENT manual help. Just post your question.

 April 19, 2011, 04:27 #4 New Member   Join Date: Apr 2011 Posts: 3 Rep Power: 7 Dear cfd_newbie, To be simple, my question is just about how Fluent is dealing with wall treatment of Kw-SST models. When Fluent is using a wall function (if the mesh is not fine enought) what are the equations ? When FLUENT is using a wall treatment (by modelling), does it solve the NS equations until the wall or is it using a blending function ? Thanks in advance JB

December 26, 2014, 09:09
wall treatment k-w SST in Fluent
#5
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Ali.E
Join Date: Sep 2010
Location: Lisboa
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Hello,
Do you understand that how Fluent is dealing with wall treatment of k-w SST model?

Quote:
 Originally Posted by jlb001 Dear cfd_newbie, To be simple, my question is just about how Fluent is dealing with wall treatment of Kw-SST models. When Fluent is using a wall function (if the mesh is not fine enought) what are the equations ? When FLUENT is using a wall treatment (by modelling), does it solve the NS equations until the wall or is it using a blending function ? Thanks in advance JB

December 27, 2014, 06:18
#6
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Anh
Join Date: Sep 2014
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Quote:
 Originally Posted by behest Hello, Do you understand that how Fluent is dealing with wall treatment of k-w SST model?
As I understand, SST k-o model is kind of low Reynolds number, so It can be calculate the viscous sub-layer. So no need two layer approach, hence, this model use blending function if y+ in the range of buffer layer

December 27, 2014, 06:56
#7
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Ali.E
Join Date: Sep 2010
Location: Lisboa
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Thank you very much for your commend; but I need more details. I need to understand very well for writting the code. please explain better.

In the Fluent Theory guide is written as bellow:
"The wall boundary conditions for the k equation in the k-w models are treated in the same way as the k equation is treated when enhanced wall treatments are used with the k-e models. This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds number boundary conditions will be applied."

It said that Fluent uses enhanced wall function for k. But it is not very clear for me. What do you think about it?
Quote:
 Originally Posted by dinhanh As I understand, SST k-o model is kind of low Reynolds number, so It can be calculate the viscous sub-layer. So no need two layer approach, hence, this model use blending function if y+ in the range of buffer layer

 Tags boundary layer, enhanced wall traitment

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