CFD Online Discussion Forums - Enhanced Wall Treatment

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Enhanced Wall Treatment

Enhanced Wall Treatment is using by default for k-w SST model.
What is it - Enhanced Wall Treatment? It's a kind of wall function, or some blended condition, that models boundary layer directly on meshes with y+=1 and uses some wall function, when y+>30. If it's so what kind of wall function uses Enhanced Wall Treatment?

Quote:

Originally Posted by paduchev (Post 394541)

The enhanced wall treatment is a blended wall model or wall function. It blends the separate models in the two-layer approach by use of a damping function so that the transition between the two is smoother.

Take for example the wall models for velocity/momentum:

In the two-layer approach;

if y+ < 10 then the linear law of the wall is used:

if y > 10 for the wall adjacent cell then the log law of the wall is used: $u^+ = \frac{1}{\kappa}ln (y^+) + C^+$

In the Enhanced Wall Treatment approach, there is no "check" to see if y+ is greater/less than a certain value. The value of u+ is calculated from a single wall model. The enhanced wall function is simply: $u^+ = e^{\Gamma}u^+_{lam} + e^{\frac{1}{\Gamma}} u^+_{turb}$
where $\Gamma$ is the blending function that allows the two different models to be smoothly blended.

In fluent the blending function is $\Gamma=-\frac{0.01\left(y^+\right)^4}{1+5y^+}$

Keep in mind there is an additional two-layer approach for the k equation (and $\epsilon$ ). The $\omega$ equation does not use a two-layer approach. Additional blending is performed for the turbulence quantities (k, $\epsilon$ ), and that is where the most significant differences are.

So if it's no separation and u+ calculated by single equation in all cases is there specific requirements for grid resolution in a near wall zone for proper using Enhanced Wall Treatment?

Quote:

Originally Posted by paduchev (Post 394560)

So if it's no separation and u+ calculated by single equation in all cases is there specific requirements for grid resolution in a near wall zone for proper using Enhanced Wall Treatment?

No, as long as you are sensible with the grid and have enough cells and your wall adjacent cell is not too far from the wall. The Enhanced Wall Treatment was developed to be flexible and work for all grids. It is one size fits all. But you will get more accurate results with finer grids.

I have same results for meshes with y+=1 and meshes with y+=50 For flow around conic nozzle with separate region after shock wave.
Has this equation $u^+ = e^{\Gamma}u^+_{lam} + e^{\frac{1}{\Gamma}} * u^+_{tur}b$ shows the main equation for EWT or it's specific eq for vrlocity?
I need to show some main EWR equation for using in me PhD thesis

That is only for velocity (u+). You will have to dig up the ones for the turbulence quantities.

The Fluent manual goes into detail the EWT for the k-epsilon models but leaves out a lot of details for the kw-SST model. But you can find them on the CFD-online wiki section.

Hi paduchev,

you can find some additional information here:

http://hpce.iitm.ac.in/website//Manu...ug/node514.htm

Quote:

Originally Posted by LuckyTran (Post 394581)

. But you can find them on the CFD-online wiki section.

I have'nt found anything on wiki((

Quote:

Originally Posted by Sören Sander (Post 394585)

Hi paduchev,

you can find some additional information here:

http://hpce.iitm.ac.in/website//Manu...ug/node514.htm

It's a tutorial for fluent 6.3 have'nt it changes for fluent 13.0?

Hmmm, they should be the same, but you have a point. You should use the newest manual of fluent 13, of course.

Quote:

Originally Posted by paduchev (Post 394589)

It's a tutorial for fluent 6.3 have'nt it changes for fluent 13.0?

I compared the link to manual from 14. They are the same (at least those sections).

Quote:

Originally Posted by paduchev (Post 394588)

I have'nt found anything on wiki((

http://www.cfd-online.com/Wiki/Near-wall_treatment_for_k-omega_models

Quote:

Originally Posted by LuckyTran (Post 394556)

if y > 10 for the wall adjacent cell then the log law of the wall is used: $u^+ = \frac{1}{\kappa}ln (y^+) + C^+$

Quote:

Enhanced wall treatment is a near-wall modeling method that combines a two-layer model with enhanced wall functions. If the near-wall mesh is fine enough to be able to resolve the laminar sublayer (typically Y+~1 ), then the enhanced wall treatment will be identical to the traditional two-layer zonal model (see below for details).

In other words the with Y+ ~ 1, you are solving the low reynolds number K-epsilon model of M. Wolfstein.
"The Velocity and Temperature Distribution of One-Dimensional Flow with Turbulence Augmentation and Pressure Gradient.
Int. J. Heat Mass Transfer, 12:301-318, 1969."

Put in simple words:

1. With Y+~1 , you are solving the low Reynolds number K-epsilon model
2. In original form Wolfstein model is not applicable for the Y+ > 0.2
3. So to over come this we have to use the hybrid wall functions.
4. Enhanced wall treatment is method to implement the hybrid (or enhanced) wall functions for the varying Y+ in the CFD model.
5. Enhanced wall treatment is not needed to implement hybrid (enhanced) wall functions in k-omega model because they are already applicable up-to viscous sub-layer.

Now the question is how does the enhanced (hybrid) wall function work.

They work like

Uplus = (1-blending function) * Uplus (of viscous sub-layer) + blending function * Uplus (of log law)

Blending function = 0 for y+ < 6
Blending function =~ 1 for Y+ > 30-40

So for Y+< 6 you have viscous sub layer and you are using the low Reynolds number model
for Y+> 30 You are using the log law implement ion (aka wall functions)
Between Y+ ~ 6 and 30 one is using the linear some of both profiles according to the relative weightage.

For example in reference http://num.math.uni-goettingen.de/ba...ings/knopp.pdf

Blending functions has the following values (Equation 7 of reference http://num.math.uni-goettingen.de/ba...ings/knopp.pdf)

Y+ = 1 , BF = 0

Y+ = 10, BF = 0.018

Y+ = 12, BF = 0.038

Y+ = 15, BF = 0.094

Y+ = 20, BF = 0.2922

Y+ = 25, BF = 0.626

Y+ = 27, BF = 0.761

Y+ = 30 , BF = 0.909

Y+ = 35, BF = 0.9929

Y+ = 38, BF = 0.9992

Y+ = 40, BF = 0.9998

Blending function is different for different terms. For example in above example, BF was calculated for U+ and Y+. But which ever function is used the basic theory is same.

PS : I have already mentioned in one thread that the enhanced wall treatment is good for the Y+ < 10, because for higher values you have increasing weitage of log law and which is not good at predicting the separation.

Quote:

Originally Posted by paduchev (Post 394541)

Enhanced wall treatment is not used by k-w SST model (see above), rather enhanced wall function (hybrid wall function is used).

Enhanced wall treatment is method to implement the enhanced wall function for the two layer model like K-epsilon model.

It uses the normal viscous sub layer (Y+= 1-5) and log law formulae (wall function with Y+ >30). Only difference is that it uses the blending function to make it useful in the region Y+ from 5-30 (buffer zone).

Quote:

Originally Posted by paduchev (Post 394567)

Do you know why results are same?

Hi,
i am modelling a turbulent flow in a finned tube. I am very interested in the near wall behaviour and my aim is to get values for pressure drop and the surface heat transfer coefficient.
I am using the k-w SST model. As far as i understand this model it combines the advantages of the k-E model for fully turbulent flow far away from the wall and the k-w modell near the wall. As proposed in the fluent 13.0 tutorial 6 for turbulence modelling i set my y+ value to y+=0,25 and i meshed a boundary layer with 15 cells for the inner region flow which is governed by viscous effects.
so my questions are:
1. can i enable the "low Re corrction"? the k-w model does already solve the flow close to the wall?
2. what does the "viscous heating" mean?

thanks

Quote:

Originally Posted by Far (Post 395130)

This is completely wrong

If this is true then they would have used the simple else-if command which was used in the previous version of SA and K-Omega model. Still you can observe in the theory guide of Fluent (I am talking about SA model) they are using the integration to wall approach for the Y+< 2 and wall function approach for the Y+>30 and strongly recommend to make the meshes either with Y+< 2 or Y+> 30 so that they can use the either IWT or wall function approach. Although the actual switch is implemented at the intersection of two profiles i.e. 11.225 (previously it was implemented at 11.06 in version 6.3).

Now lets discuss the theory behind the enhanced wall treatment for K-epsilon and K-omega models.

First question comes into mind why two approaches used for the same effect i.e. implementing the smooth transition between the log-law and viscous sub-layer implementation. This is because:

1. K-epsilon models were not designed for the near wall flow, therefore they require the damping functions to simulate the near wall effects.

2. K-omega based models were designed originally for the near wall region and therefore does not require the damping functions, hence the hybrid wall functions (blending of near wall and log law function) were implemented directly and same is true for SA model. You can find the details of latest work here for the k-omega and SA model with hybrid wall functions here

http://num.math.uni-goettingen.de/ba...ings/knopp.pdf

http://num.math.uni-goettingen.de/ba...ngs/alrutz.pdf

But whether it is two layer approach (K-epsilon) or single model implementation approach (K-omega or SA model) the purpose is same i.e. to remove the short comings of the both models. i.e. the low reynolds number is valid for the Y+ < 0.2 (low reynolds number K-epsilon model) or Y+<2 (K-omega model, I am not writing low reynolds number k-omega becuase K-omega is originally a low Reynolds number model, so no need to define the Rose) and similarly the Y+ > 30 for high Reynolds number K-omega and K-Epsilon model.

To be continued....

Now consider this http://en.wikipedia.org/wiki/Law_of_the_wall

It is clearly written that U+ = Y+ for the Y+ < 5 (you can consider the sublayer up 11.225 but at the Y+ = 12 the error is around 25%) http://en.wikipedia.org/wiki/Law_of_the_wall
Log law is for Y+> 30.
Buffer zone is Y+ = 5 to Y+ = 30 This is problem area where both models (low Reynolds number and high Reynolds number ) don't work.

This is the reason why the hybrid or enhanced wall treatment model was came into existence.

Here is the some material from Fluent user guide:

In other words the with Y+ ~ 1, you are solving the low reynolds number K-epsilon model of M. Wolfstein.
"The Velocity and Temperature Distribution of One-Dimensional Flow with Turbulence Augmentation and Pressure Gradient.
Int. J. Heat Mass Transfer, 12:301-318, 1969."

Put in simple words:

1. With Y+~1 , you are solving the low Reynolds number K-epsilon model
2. In original form Wolfstein model is not applicable for the Y+ > 0.2
3. So to over come this we have to use the hybrid wall functions.
4. Enhanced wall treatment is method to implement the hybrid (or enhanced) wall functions for the varying Y+ in the CFD model.
5. Enhanced wall treatment is not needed to implement hybrid (enhanced) wall functions in k-omega model because they are already applicable up-to viscous sub-layer.

Now the question is how does the enhanced (hybrid) wall function work.

They work like

Uplus = (1-blending function) * Uplus (of viscous sub-layer) + blending function * Uplus (of log law)

Blending function = 0 for y+ < 6
Blending function =~ 1 for Y+ > 30-40

So for Y+< 6 you have viscous sub layer and you are using the low Reynolds number model
for Y+> 30 You are using the log law implement ion (aka wall functions)
Between Y+ ~ 6 and 30 one is using the linear some of both profiles according to the relative weightage.

For example in reference http://num.math.uni-goettingen.de/ba...ings/knopp.pdf

Blending functions has the following values (Equation 7 of reference http://num.math.uni-goettingen.de/ba...ings/knopp.pdf)

Y+ = 1 , BF = 0

Y+ = 10, BF = 0.018

Y+ = 12, BF = 0.038

Y+ = 15, BF = 0.094

Y+ = 20, BF = 0.2922

Y+ = 25, BF = 0.626

Y+ = 27, BF = 0.761

Y+ = 30 , BF = 0.909

Y+ = 35, BF = 0.9929

Y+ = 38, BF = 0.9992

Y+ = 40, BF = 0.9998

Blending function is different for different terms. For example in above example, BF was calculated for U+ and Y+. But which ever function is used the basic theory is same.

PS : I have already mentioned in one thread that the enhanced wall treatment is good for the Y+ < 10, because for higher values you have increasing weitage of log law and which is not good at predicting the separation.

Is the Enhanced wall treatment of Fluent use the same idea as the CFX's Automatic wall treatment? Do they have any difference besides the coefficient values?

Quote:

Is the Enhanced wall treatment of Fluent use the same idea as the CFX's Automatic wall treatment? Do they have any difference besides the coefficient values?

Both are different approaches. EWT treatment is used for the k-epsilon family of turbulence model which are modified to solve the vious sub layer and buffer zone with one equation turbulence model.

In the case of the k-omega and sa model family we dont need special treatment for the boundary layer modelling as these models are valid upto viscous sublayer. So AWT is the method to ease the restrictions of yplus on the IWT approach with the almost same results and reliability.

Quote:

Originally Posted by Far (Post 395130)

PS : I have already mentioned in one thread that the enhanced wall treatment is good for the Y+ < 10, because for higher values you have increasing weitage of log law and which is not good at predicting the separation.

Dear Far,

Thank you very much for your clarifying posts. I wish the ansys fluent manual was so clear.

Can you please justify your assertion (see quote) by providing a reference or something that can be cited within an article?

enhanced wall function

hey
iam modeling an ejector with RNG k-epsilon turbulence viscosity..i used enhanced wall function and then selected the pressure gradient effect but somehow when iam solving the problem the iterating stops with an error says float invalid number...then when i unselect the pressure gradient it works just fine...but i need that pressure gradient for my job.can anyone help??....thanks bro;)