Near Wall Turbulence in CFX
 

Hey guys, just a few quick questions about CFX and turbulence models in general.

I am trying to compare the efficiency of wall functions vs. integration to the wall. I have studied up a fair amount on this but sadly my understanding of the material is low.

I am running a simulation of a sharp-edged object around mach 6, and so far i have tried running the k-w turbulence model, the BSL (which is also based on the k-omega model - correct? just multiplied by some factors) and the SAS SST model (again, based on the k-omega model, am I right, except it takes into account the turbulent shear stress).

Now these "omega-based" models have an automatic near-wall treatment as opposed to a scalable wall function treatment. This is where I am confused.
How do I go about simply integrating to the wall instead of using a wall function? There doesn't seem to be an option for that. The closest thing seems to be an automatic near-wall treatment - which still doesn't allow me to choose whether I want to use a full integration to the wall approach or a wall function approach.

Can someone clarify this to me? How do I integrate to the wall using these turbulence models?


Thanks very much in advance.

This is all discussed in the documentation.

When you use automatic wall functions it integrates to the wall if y+<11 or so, and uses wall functions if y+ is above that. There is also a blending between the two so you don't get sharp transitions.

I see, thank you. Perhaps I haven't been careful enough in reading the documentation.

So does this mean the that k-epsilon turbulence models there is no option to integrate to the wall? If I understand this correctly, any turbulence model should be able to use both a wall function which is just an estimation used at a certain distance from the wall, and integrate to the wall which is pretty much a full out integration to the wall.

So what exactly is a scalable wall function?

Also are you looking at the documentation in the solver modeling guide or the solver theory guide?

Thanks a lot ghorrocks for your response :)

You cannot integrate the normal k-e model to the wall. You get a divide by zero so it is not defined. omega based models do not have the problem and therefore can be integrated to the wall. You can put limiters and other weird stuff to integrate k-e to the wall but none of these models are widely used. See a turbulence modellign textbook such as "Turbulence Modelling for CFD" by Wilcox for details.

It is the blend from integrating to the wall at low y+ to wall functions at high y+. See the documentation and the references it lists.

Both, but also look at the references in the theory guide and general turbulence textbooks, eg Wilcox.

Hi,

I too am doing a similar study using CFX regarding near wall treatment. I understand that the wall functions can be "disabled" in CFX by changing the mesh so that I have a y+ that is very much greater than 1. However, wouldn't this affect my results as the mesh would be different when I compare my two sets of results in tandem?

Is there a way of disabling wall functions without changing the mesh? Does anyone have any good references regarding the effect of mesh densities near the wall? Unfortunately, I do not have much experience with CFD....

I studying the vortex shedding effects off a 2D cylinder and a triangular blunt body.

Thanks in advance

Wall functions are "enabled" by increasing the y+ to larger than 11. So your coarse mesh is using wall functions and the finer mesh is not (it is integrating to the wall).

Do a mesh with y+=1 about and then automatic wall treatment will integrate to the wall. You can select the wall function option to over-ride this and use wall functions. This will allow you to compare wall functions to integration to the wall.

Thanks Ghorrocks for the response.

I already have a model in CFX with a y+ < 1. However, when I go to the wall functions option for SST turbulence modeling, I only have the option of "automatic" and nothing else. I've tried setting the option through the command editor from "automatic" to "on" but it gave me an error. I have searched through the documentation and found nothing...

On second thoughts you can't do this as it makes no sense. The wall function approach is only valid above y+>11, so why even let you choose it below 11? Likewise, integrating to the wall is not valid when y+>1 or so, so why let you choose it?

You are trying to compare a valid approach to an invalid approach. This does not seem to make much sense.

"On second thoughts you can't do this as it makes no sense."

Indeed it doesn't make much sense but I am doing this for a project as a study on how bad wall functions perform. In order to do that, I need to keep a consistent mesh so as to not confound my results with the change in densities at the wall. If CFX is capable of doing this, then my next thought would be how it is actually applying it...

Well, put it this way:

The k-e turbulence model is degenerate at the walls, meaning you have to do something to define it at the walls. So you define wall functions and off you go.

The k-omega model is defined at the walls and so has no need for any special treatment. You can simply integrate it to the wall. So wall functions do not exist.

So you can compare a k-e model with wall functions to a k-omega model without. So you are not only comparing wall functions but turbulence models.

Thanks for the help. I am doing just that =D

Dear Frends

I would like to share something about integration to the wall, automatic wall function and scalable wall functions.

1. Automatic wall treatment AWT(as per discussion with F. Menter)

It changes to the integration to wall when y plus is less than 6 and switches to standard wall function when y plus is equal to or greater than 30. In between 6 and 30 it uses the blending function.


2. Integration to wall (ITW)

It simply mean that you have mesh with y plus less than 1 and want to solve the viscous sub-layer as well. Although the automatic wall treatment is similar to ITW but has many critical differences.

a) For ITW you need 35-40 points in the boundary layer
b) for AWT you only need 10-15 points in BL
c) AWT will switch to wall function if y plus is equal to or greater than 30.


c) scalable wall function
In standard wall function you can not refine the mesh as y plus may go to zero at separation or less than 30 due to different velocity scales in domain. This will violate the basic assumption in wall function. Therefore scalable wall function was developed to cure this problem. Now even if you make the mesh with y plus less than 1 and solve it. and Check the y plus and solver y plus contours you will see the y plus contours showing the y plus from 1 or less but the solver y plus will show the y plus 11.06 or greater. That is due to limiter in scalable wall function which does not allow to solver y plus to go to zero. It may be noted that the 11.06 is intersection point of linear law (viscous sub layer) and log law (wall function)

The automatic wall treatment is specifically applicable to SA model (Modified eddy viscosity) and K omega based model (omega variable).

Scalable wall function is designed for epsilon based models e.g. K epsilon, SSG Reynolds stress mode and LRR Reynolds stress model

For more details you may look at the knopp paper

Best Regards
Far