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March 30, 2009, 05:07 
LES wall function

#1 
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Hi all, and good Monday morning for everyone (at least for those close to my meridian time) !
About the nuSgsWallFunction, I still have some doubts. Do the yPlus values have to be higher than say 11 for the wall function to be effective ? Or can LES wall function cope with yPlus values from say 1 to 30 ? Also, if I don't use wall function, am I right applying 'zeroGradient' for nuSgs boundary condition at walls ? Thanks in advance, Fabien 

March 31, 2009, 04:36 

#2 
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Cedric DUPRAT
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Hi Fabien,
In OpenFOAM, the nuSgsWallFunction correspond the Spalding's law. It's a fit of the laminar and logarithmic regions for an equilibrium flow. So, if your boundary layer is in equilibrium, you can use the wall model for a large value of y+. But because it is never the case, I would advise to resolve as accurately as possible your boundary layer by putting your first of the wall grid point in the viscous sublayer (Y+<7) the point is that between 7 and 30, it's the buffer layer and .... people don't really know what happend there that's why people usually avoid these values. I hope this will help you Cedric PS: more information can be found in the turbulenceSIG wiki http://openfoamwiki.net/index.php/Si...Implementation 

March 31, 2009, 07:55 

#3 
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Hi Cedric,
Thanks for the information. I looked after the Spalding's law and now I get it. Regards, Fabien 

April 4, 2014, 18:41 

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Quote:
I have a question about the wall functions of LES in Openfoam. Theoretically, when wall function is used, the velocity at the wall is not zero and instead the velocity is friction velocity which is solved from the loglaw or linear law. Then the wall shear stress is obtained tau_w=rho*u_tau**2. Then this wall shear stress can be used for the discretization of the momentum equations. However, in openfoam's specifications, when wall function is applied, the velocity at the wall is still (0 0 0). Why can we still use (0 0 0) for the wall velocity here? Thank you in advance. OFFO 

April 11, 2014, 06:50 

#6  
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Hi cedric_duprat,
About the LES wall function in openfoam, there is a question I need to ask you: in nuSgsWallFunction, the velocity at the first cell near the wall is instantaneous, not mean. Is there any possible to implement the time averaged velocity for that quantity? OFFO Quote:


September 4, 2014, 15:44 
Wallfunction in LES

#7 
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Ashvin Chaudhari
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Hi OFFO,
I doubt it’s not possible to implement the timeaveraged velocity for the wallfunction in LES, because LES resolves the instantaneous flow not the mean (or timeaveraged) flow. Opposite to RANS. But in any case, one use the wallfunction approach based on the instantaneous flow in LES. People often say it’s not good to use the wallfunction in LES, but what about the high Re flows, such as the atmospheric flow having the Reynolds number~10^(89). It is simply too much computationally demanding even using the modern supercomputers. Most importantly, when you use the wallfunction try to put your first node at, say y^+>50 from the wall. Moreover, the nuSgsWallFunction is available for LES in OF. But be carefully this wall function is poor in predicting the flow separation. Hope this helps! BR, Ashvin 

October 1, 2015, 16:51 

#8  
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Dear Ashvin,
Thank you. In "muSgsUSpaldingWallFunction", the mu_w (which is for the boundary wall surface) is updated as follows: Code:
muSgsw[facei] = max ( rhow[facei]*sqr(utau)/magFaceGradU[facei]  muw[facei], 0.0 ); Quote:


October 7, 2015, 10:56 

#9 
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Ashvin Chaudhari
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Hi,
In current framework of OpenFOAM, all the wallfunctions are implemented using subgridscale (or eddy) viscosity concept. We therefore write like (e.g. in your case): Code:
wall { type muSgsUSpaldingWallFunction; Cmu 0.09; kappa 0.41; E 9.8; value uniform 0; } Code:
muSgsw [facei] = max ( rhow[facei]*sqr(utau)/magFaceGradU[facei]  muw[facei], 0.0 ); Last edited by ashvinc9; October 27, 2015 at 18:27. 

April 23, 2016, 08:10 

#10  
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Thank you for your reply and the link for the thesis. The rough wall function is investigated. Do you have any comment about the exisiting wall function in OpenFOAM for smooth walls? I mean if you also tested the spalding's law in LES of near wall turbulence?
cheer, OFFO Quote:


June 15, 2016, 03:30 

#11 
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Ashvin Chaudhari
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Hi, Sorry for my late reply. Yes, I have made a comparison between the two (rough and smooth) wall functions. In the above mentioned thesis, see Figure 2.3 for smooth and 2.4 for rough wall functions results on simple openchannel flow. In more detail, Figures 3.8, 3.10 and 3.11. However, it must be clear that because of the lack of roughness, the smooth wallfunction (i.e. nuSgsUSpaldingWallFunction) should not perform as good as the rough one. For this reason the agreement with windtunnel measurements using the smooth wallfunction is poor.


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