# Mesh size with y+ > 30 criterion too rough

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 October 22, 2015, 10:39 Mesh size with y+ > 30 criterion too rough #1 New Member   Gerrit Horstmann Join Date: Oct 2014 Posts: 3 Rep Power: 3 Hello everybody, I'm creating a workflow for shape optimization of a cylindrical nozzle using pisoFoam. I've finished as a first step a laminar simulation, know I'm trying to create an adequate mesh for a turbulent simulation. I will use the k - epsilon model, therefore I have to ensure that the height of my fist cell near to the wall is higher than y+ = 30 to meet the log-law-region, because I want to use the openFoam standard wall functions. My inflow-velocity of interest causes a Re-number of 10000 at the inlet with a diameter of 1m. So I can assume a fully developed turbulent flow. My Problem is now, that when I estimate the height of the first cell height using the y+ calculater e.g. http://www.pointwise.com/yplus/ I get a size of 0.05 m for y+ = 30. That is much too rough, so I can't adequate dissolve the flow. So what I can do? Is my Re-number too low for wall functions or can I use the wall functions with finer sizes without causing trouble? Of course I can use y+ < 1 and solve the viscous sublayer without using wall functions, but later I will simulate flows with higher Re numbers and I want prevent the additional numerical cost, since the boundary layer are not important for my optimization. Thank you very much for your help! Best regards Gerrit Last edited by Stokes23; October 26, 2015 at 04:26.

 October 22, 2015, 16:52 #2 Member   Davi Barreira Join Date: Apr 2014 Location: Fortaleza Posts: 58 Rep Power: 3 [You can use the low Reynolds formulation, it is appropriate for y+ around 1 (it should be in the viscous sublayer, which would be y+<5). So use kLowReWallFunction, epsilonLowReWallFunction and nutLowReWallFunction.] Sorry, I think what I said up here is incorrect as pointed by Fabian in the post below. The kLowReWallFunction seems to be only applicable to the v2f model. Stokes23 and vitocorleone like this. Last edited by davibarreira; November 18, 2015 at 15:23.

 October 26, 2015, 04:27 #3 New Member   Gerrit Horstmann Join Date: Oct 2014 Posts: 3 Rep Power: 3 Thanks, that's working fine!

 October 26, 2015, 05:41 #4 Senior Member     Fabian Roesler Join Date: Mar 2009 Location: Bad Friedrichshall, Germany Posts: 185 Rep Power: 8 Moreover, you should consider using a lowRe turbulence model. As far as I know, kEpsilon is only highRe. But this strongly depending on your case. Try LaunderSharmaKE. Besides, kLowReWallFunction wallFunction is only for v2f model. Have a look into this thread Question about k-epsilon turbulence model for a backward-facing step? Cheers Fabian davibarreira likes this.

 October 26, 2015, 11:57 #5 New Member   Gerrit Horstmann Join Date: Oct 2014 Posts: 3 Rep Power: 3 Thanks for this hint! To be on the safe side I will simply use k = 0 instead of the kLowReWallFunction in combination with LaunderSharma. Unfortunately, I have a y+ value around 4, I hope this bc is still accurate. With y+ = 1 my mesh was much too big, while using y+ around 30 and using highRe models my mesh was too rough with respect to my application. It's hard to find the balance. Best regards Gerrit

 November 18, 2015, 13:46 #6 Member   Davi Barreira Join Date: Apr 2014 Location: Fortaleza Posts: 58 Rep Power: 3 Precisely, Fabian. Although, I haven't been able to actually figure if the kLowReWallFunction and epsilonLowReWallFunction are actually only for the v2f, im taking a look at the code.

November 19, 2015, 08:23
#7
Senior Member

Fabian Roesler
Join Date: Mar 2009
Location: Bad Friedrichshall, Germany
Posts: 185
Rep Power: 8
Quote:
 Originally Posted by davibarreira Precisely, Fabian. Although, I haven't been able to actually figure if the kLowReWallFunction and epsilonLowReWallFunction are actually only for the v2f, im taking a look at the code.
Hi,

I'd didn't say that epsilonLowReWallFunction is only suitable for v2f model. Only kLowReWallFunction is for v2f. epsilonLowReWallFunction can be used with other kEpsilon lowRe models such as LaunderSharma etc.

Cheers

Fabian

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