
[Sponsors] 
April 26, 2004, 04:25 
wall function and grid dependency

#1 
Guest
Posts: n/a

Hi I am new at modelling
I have question about turbulence modeling and mesh independency. I am using kepsilon and wall functions to resolve the flow. Now I have to optimize the mesh for my case. How can I attin the gris independency Can someone help me out. Pratap 

April 26, 2004, 04:49 
Re: wall function and grid dependency

#2 
Guest
Posts: n/a

Grid independency and wallfunctions is a bit extra tricky. You can not just refine your grid more and more without any care. Wall functions only work when the first node has a certain distance from the wall. This nondimensional distance, y+, should be around 50, or at least not below 15 or above 300).
To do a griddependence study on a wallfunction grid you have to make sure that you do not refine the grid close to the wall so much that your y+ distance becomes too small for wallfunctions. When you reach y+ around 50 you should stop refining the normal walldistance of the first node and instead refine your grid in other regions and in the direction parallell to the wall. 

April 26, 2004, 06:12 
Re: wall function and grid dependency

#3 
Guest
Posts: n/a

thank you jonas, Now I have another simple question whether this applies to all the walls. when you have flow over a sphere while the sphere is in channel so now you have channel walls and sphere walls.
Thanks bob 

April 26, 2004, 06:40 
Re: wall function and grid dependency

#4 
Guest
Posts: n/a

Yes, in theory it applies to all walls where you use wallfunctions. However, depending on the case some walls will of course be more critical. Typically the walls where viscous effect are important for your overall flow field will be most critical.


April 27, 2004, 03:30 
Re: wall function and grid dependency

#5 
Guest
Posts: n/a

Another question,
Let us suppose you perform simulations and take some mesh and check y+ plus values for all the walls after some iterations(let us say 5 iterations).le us say they are in the range now you continue the simulations till it reaches the steady state now you calculate the y+ plus values again. and suppose that they are not in the range (above 300). Does it mean that simulations is wrong so do I have to check y+ plus values at every iterations. Bob 

April 27, 2004, 11:57 
Re: wall function and grid dependency

#6 
Guest
Posts: n/a

I don't quite understand why wall functions won't work when your value of y+ is too small? I thought, in most codes, the wall functions come into action only when the y+ values are large (say >15) and no extrapolation is done when y+ values are small. Is there some text where this is well described?


April 27, 2004, 13:41 
Re: wall function and grid dependency

#7 
Guest
Posts: n/a

Hi Does anybody know ofsome paper where refinement of near wall grid is discussed for recirculating flows.
Bob 

April 27, 2004, 13:48 
Re: wall function and grid dependency

#8 
Guest
Posts: n/a

You will only get correct y+ values when your solution is converged. You need a fully developed boundary layer in your solution before you can compute y+. Hence, you have to let the simulation converge, then check your y+, and if necessary remesh and rerun the case. Checking y+ after a few iterations will not produce anything useful.
Btw, there is a small tool on the net which you can use to guess the y distance of the first node based on the desired y+. You can find a link to it in the Resources/Calcs/Misc section here at CFD Online. 

April 27, 2004, 13:59 
Re: wall function and grid dependency

#9 
Guest
Posts: n/a

The problem is that most turbulence models do not work well when y+ goes below 15 or so and you come close to the viscous sublayer. kepsilon in particular does not produce correct results if used in this region and in order to get it to work you need to add dampingfunctions etc. to make it a lowRe model.
komega works down through the viscous sublayer. However, it is not at all trivial to automatically switch between a wallfunction approach and a fully resolved lowRe approach based just on your local grid resolution  you need to match two different types of wall treatment and that creates a whole range of new problems. There are such models that can switch between lowRe and wallfunctions though. 

April 27, 2004, 15:55 
Re: wall function and grid dependency

#10 
Guest
Posts: n/a

I was hoping I'd be able to avoid this discussion In my PhD work I had to do mesh refinement with wall functions and I've hit some limitations mentioned before. if you carry on refining (especially in 2D where you've got the chance of producing enough mesh resolution), you end up with an unfortunate situation where the numerical error is localised in the cells next to the wall and the wall functions rules won't allow you to touch those cells. The reference (if you fancy having a look) is:
@PhdThesis{Jasak:PhD, author = {Jasak, H.}, title = {Error analysis and estimation in the Finite Volume method with applications to fluid flows}, school = {Imperial College, University of London}, year = 1996 } As for the second issue (why wall functions go wrong), it is quite straightforward. Consider a fully developed boundary layer on a flat plate. Near the wall k changes with wall distance squared and epsilon with wall distance cubed. This, obviously, requires a LOT of mesh to resolve. Secondly, the standard kepsilon model without the wall function treatment does NOT reproduce this behaviour: lowRe models (which do) use a bunch of damping functions and sufficient mesh resolution near the wall (otherwise they blow up!). What wall functions do is simply switch off the correction on turbulent viscosity for the wall face for y+ < 11.6 (that's the famous crossover). Thus, you end up with laminar viscosity on the wall and the bulk kepsilon model that does not reproduce the nearwall physics at all. In otherwords, the result is wrong. Hope this is clear, Hrv 

April 28, 2004, 02:15 
Re: wall function and grid dependency

#11 
Guest
Posts: n/a

So Based upon the suggestions I am conlcuding the following I have to first mesh the geometry and then run the case let the flow developed its boundary layer(when steady state is reached). then check the y+ plus values if they are under 30300 then its is okay other wise I have to correct the mesh and re run the case till I get the yplus values of 30300.
When you simulate wall bounded flow and recirculating flows and use standard kepsilon turbulence model this restriction is big problem I guess. Do somebody have this paper where mesh refinement in recirculating flows is discussed ? From this disscussion mesh refinement in reciruclting flows is notorious task. Correct me if I am wrong somewhere. bob 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
wall function and grid independency?????  scott  FLUENT  3  October 26, 2006 12:31 
wall function and grid  Y  FLUENT  3  March 17, 2005 07:48 
Onera Wing Wall function?  Dianhong  Main CFD Forum  0  November 21, 2003 21:19 
Grid Independent Solution  Chuck Leakeas  Main CFD Forum  2  May 26, 2000 11:18 
Wall function in adverse pressure gradients  stephane baralon  Main CFD Forum  11  September 2, 1999 04:05 