CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

LES wall function

Register Blogs Community New Posts Updated Threads Search

Like Tree3Likes
  • 1 Post By cedric_duprat
  • 2 Post By ashvinc9

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 30, 2009, 06:07
Default LES wall function
  #1
New Member
 
Join Date: Mar 2009
Posts: 29
Rep Power: 17
ayoros is on a distinguished road
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
ayoros is offline   Reply With Quote

Old   March 31, 2009, 05:36
Default
  #2
Senior Member
 
Cedric DUPRAT
Join Date: Mar 2009
Location: Nantes, France
Posts: 195
Rep Power: 17
cedric_duprat is on a distinguished road
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 sub-layer (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
amir.a.aliabadi likes this.
cedric_duprat is offline   Reply With Quote

Old   March 31, 2009, 08:55
Default
  #3
New Member
 
Join Date: Mar 2009
Posts: 29
Rep Power: 17
ayoros is on a distinguished road
Hi Cedric,

Thanks for the information.

I looked after the Spalding's law and now I get it.

Regards,
Fabien
ayoros is offline   Reply With Quote

Old   January 25, 2011, 16:05
Question
  #4
Senior Member
 
maysmech's Avatar
 
Join Date: Jan 2010
Posts: 347
Blog Entries: 2
Rep Power: 17
maysmech is on a distinguished road
Quote:
the point is that between 7 and 30, it's the buffer layer
Hi,
Which B.C should be used for nu and K for yPlus 20?
Thanks in advance
maysmech is offline   Reply With Quote

Old   April 4, 2014, 19:41
Default
  #5
Senior Member
 
Join Date: Jan 2013
Posts: 372
Rep Power: 14
openfoammaofnepo is on a distinguished road
Quote:
Originally Posted by cedric_duprat View Post
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 sub-layer (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
Hi All,

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 log-law 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
openfoammaofnepo is offline   Reply With Quote

Old   April 11, 2014, 07:50
Default
  #6
Senior Member
 
Join Date: Jan 2013
Posts: 372
Rep Power: 14
openfoammaofnepo is on a distinguished road
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:
Originally Posted by cedric_duprat View Post
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 sub-layer (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
openfoammaofnepo is offline   Reply With Quote

Old   September 4, 2014, 16:44
Default Wall-function in LES
  #7
New Member
 
Ashvin Chaudhari
Join Date: Aug 2011
Location: Finland
Posts: 23
Rep Power: 15
ashvinc9 is on a distinguished road
Hi OFFO,

I doubt it’s not possible to implement the time-averaged velocity for the wall-function in LES, because LES resolves the instantaneous flow not the mean (or time-averaged) flow. Opposite to RANS. But in any case, one use the wall-function approach based on the instantaneous flow in LES.

People often say it’s not good to use the wall-function in LES, but what about the high Re flows, such as the atmospheric flow having the Reynolds number~10^(8-9). It is simply too much computationally demanding even using the modern super-computers.

Most importantly, when you use the wall-function 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
ashvinc9 is offline   Reply With Quote

Old   October 1, 2015, 17:51
Smile
  #8
Senior Member
 
Join Date: Jan 2013
Posts: 372
Rep Power: 14
openfoammaofnepo is on a distinguished road
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
                );
My understanding is: at the walls mu_w should be zero since there will be no any turbulence there. The mu that should be calculated from the wall function should be for the near wall cells. Could you please give me some comments for what I say above. Thanks!

Quote:
Originally Posted by ashvinc9 View Post
Hi OFFO,

I doubt it’s not possible to implement the time-averaged velocity for the wall-function in LES, because LES resolves the instantaneous flow not the mean (or time-averaged) flow. Opposite to RANS. But in any case, one use the wall-function approach based on the instantaneous flow in LES.

People often say it’s not good to use the wall-function in LES, but what about the high Re flows, such as the atmospheric flow having the Reynolds number~10^(8-9). It is simply too much computationally demanding even using the modern super-computers.

Most importantly, when you use the wall-function 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
openfoammaofnepo is offline   Reply With Quote

Old   October 7, 2015, 11:56
Default
  #9
New Member
 
Ashvin Chaudhari
Join Date: Aug 2011
Location: Finland
Posts: 23
Rep Power: 15
ashvinc9 is on a distinguished road
Hi,

In current framework of OpenFOAM, all the wall-functions are implemented using subgrid-scale (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;
    }
Therefore, muSgs=0 on the wall boundary means "NO wall-function at all". In fact, the mu that is calculated on the wall using

Code:
 muSgsw [facei] =            
     max ( rhow[facei]*sqr(utau)/magFaceGradU[facei] - muw[facei],                     0.0 );
is for the near-wall cell centers. I agree that there is no turbulence on the wall-surface and this can be defined by applying the velocity U=0 on the wall. To understand the OF wall-function theory (for LES and rough wall-fucntion) in bit detail see e.g. page no. 31 of this thesis: http://www.doria.fi/handle/10024/103100
fumiya and openfoammaofnepo like this.

Last edited by ashvinc9; October 27, 2015 at 18:27.
ashvinc9 is offline   Reply With Quote

Old   April 23, 2016, 09:10
Default
  #10
Senior Member
 
Join Date: Jan 2013
Posts: 372
Rep Power: 14
openfoammaofnepo is on a distinguished road
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:
Originally Posted by ashvinc9 View Post
Hi,

In current framework of OpenFOAM, all the wall-functions are implemented using subgrid-scale (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;
    }
Therefore, muSgs=0 on the wall boundary means "NO wall-function at all". In fact, the mu that is calculated on the wall using

Code:
 muSgsw [facei] =            
     max ( rhow[facei]*sqr(utau)/magFaceGradU[facei] - muw[facei],                     0.0 );
is for the near-wall cell centers. I agree that there is no turbulence on the wall-surface and this can be defined by applying the velocity U=0 on the wall. To understand the OF wall-function theory (for LES and rough wall-fucntion) in bit detail see e.g. page no. 31 of this thesis: http://www.doria.fi/handle/10024/103100
openfoammaofnepo is offline   Reply With Quote

Old   June 15, 2016, 04:30
Default
  #11
New Member
 
Ashvin Chaudhari
Join Date: Aug 2011
Location: Finland
Posts: 23
Rep Power: 15
ashvinc9 is on a distinguished road
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 open-channel 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 wall-function (i.e. nuSgsUSpaldingWallFunction) should not perform as good as the rough one. For this reason the agreement with wind-tunnel measurements using the smooth wall-function is poor.
ashvinc9 is offline   Reply With Quote

Old   February 9, 2017, 07:34
Default
  #12
New Member
 
bangun
Join Date: Feb 2015
Posts: 16
Rep Power: 11
eb19 is on a distinguished road
Hi All,

Do we still need to use nuSgSpaldingWall if we use dynamic Smagorinsky in OpenFOAM? I use homogenousDynamic smagorinsky model. Let's say the boundary layer in my model is not resolved.

Please help. Thanks

Eb19
eb19 is offline   Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[blockMesh] BlockMesh FOAM warning gaottino OpenFOAM Meshing & Mesh Conversion 7 July 19, 2010 15:11
Problem with compile the setParabolicInlet ivanyao OpenFOAM Running, Solving & CFD 6 September 5, 2008 21:50
LES wall treatment Valerio FLUENT 0 April 26, 2008 08:12
Droplet Evaporation Christian Main CFD Forum 2 February 27, 2007 07:27
Wall function tod Phoenics 1 May 19, 2003 06:05


All times are GMT -4. The time now is 23:34.