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Old   March 30, 2009, 05:07
Default LES wall function
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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
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Old   March 31, 2009, 04:36
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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 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
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Old   March 31, 2009, 07:55
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Hi Cedric,

Thanks for the information.

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

Regards,
Fabien
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Old   January 25, 2011, 16:05
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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
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Old   April 4, 2014, 18:41
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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
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Old   April 11, 2014, 06:50
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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:
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
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Old   September 4, 2014, 15:44
Default Wall-function in LES
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Hi OFFO,

I doubt itís 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 itís 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
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Old   October 1, 2015, 16:51
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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
                );
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
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Old   October 7, 2015, 10:56
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Ashvin Chaudhari
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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
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Last edited by ashvinc9; October 27, 2015 at 18:27.
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Old   April 23, 2016, 08:10
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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:
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
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Old   June 15, 2016, 03:30
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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 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.
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