WALE Subgrid scale model: changing the model constants

em17 · January 22, 2019, 07:53

Hi all,

I am using the WALE SGS model and have a query about the model constants. I would like to change the constant Cw, however, when I looked at the source code I found that the model is also dependent on constants Ck and Ce. It seems that Ce is only used in the calculation for epsilon, so I am not too bothered about this.

I am presuming if I want to change the value of Cw, then I must also change Ck? Does anyone know the relationship between these two constants?

Thank you!
Em

navidmt · November 11, 2019, 16:24

Hi Emily

Have you found any answer for your question? I came up with the same question and I don't know if I need to run my simulation using the default values of OpenFoam or it is essential to change the values of Cw and consequently the Ck.

I would appreciate if you share your possible solution here with me. Thank you

clapointe · November 13, 2019, 15:00

If you look at the publication described in the .H file, the authors suggest a "universal" Cw of somewhere between 0.55 and 0.6 if I remember correctly. I am not sure where the default coefficient came from, but you can change it by adding to turbulenceProperties :

Code:

WALECoeffs
{
    Cw 0.55;
}

As for Ck, they have an expression for nut in the paper that is some function of Cw and the velocity gradient tensor. If you look at the foam implementation and do some math, you'll likely find that nut is computed such that Ck cancels out.

Caelan

em17 · November 14, 2019, 08:42

Hi,

Caelan is correct! I ended up working through the equations in the source code (WALE.C) to figure it out.

Ck (which is actually the Smagorinsky constant) is used to calculate the subgrid-scale turbulent kinetic energy,

. This

is then used to calculate the turbulent eddy viscosity, $\nu_t$ , which is written in your time directories. During the calculation of $\nu_t$ , the Ck terms will cancel out. So if you are only interested in $\nu_t$ then you only need to update Cw. However if you are interested in the subgrid-scale turbulent kinetic energy

, then you'll need to estimate Ck as well.

You'll also see that Ce is simply used to calculate epsilon, so if you are not interested in epsilon then Ce is irrelevant.

Also, based on my personal experience I have found the OpenFOAM 'standard' constants to work well (Cw=0.325). Although if you have the time/resources available then it's always good to test different values to see how sensitive your simulations are.

I hope this helps,
Emily

navidmt · November 15, 2019, 14:08

Thank you so much I followed as you suggested.

January 22, 2019, 07:53	WALE Subgrid scale model: changing the model constants	#1
em17 New Member Emily Join Date: Nov 2017 Posts: 24 Rep Power: 9	Hi all, I am using the WALE SGS model and have a query about the model constants. I would like to change the constant Cw, however, when I looked at the source code I found that the model is also dependent on constants Ck and Ce. It seems that Ce is only used in the calculation for epsilon, so I am not too bothered about this. I am presuming if I want to change the value of Cw, then I must also change Ck? Does anyone know the relationship between these two constants? Thank you! Em

November 13, 2019, 15:00		#3
clapointe Senior Member Join Date: Aug 2015 Posts: 494 Rep Power: 15	If you look at the publication described in the .H file, the authors suggest a "universal" Cw of somewhere between 0.55 and 0.6 if I remember correctly. I am not sure where the default coefficient came from, but you can change it by adding to turbulenceProperties : Code: WALECoeffs { Cw 0.55; } As for Ck, they have an expression for nut in the paper that is some function of Cw and the velocity gradient tensor. If you look at the foam implementation and do some math, you'll likely find that nut is computed such that Ck cancels out. Caelan navidmt likes this.

November 14, 2019, 08:42		#4
em17 New Member Emily Join Date: Nov 2017 Posts: 24 Rep Power: 9	Hi, Caelan is correct! I ended up working through the equations in the source code (WALE.C) to figure it out. Ck (which is actually the Smagorinsky constant) is used to calculate the subgrid-scale turbulent kinetic energy, . This is then used to calculate the turbulent eddy viscosity, $\nu_t$ , which is written in your time directories. During the calculation of $\nu_t$ , the Ck terms will cancel out. So if you are only interested in $\nu_t$ then you only need to update Cw. However if you are interested in the subgrid-scale turbulent kinetic energy , then you'll need to estimate Ck as well. You'll also see that Ce is simply used to calculate epsilon, so if you are not interested in epsilon then Ce is irrelevant. Also, based on my personal experience I have found the OpenFOAM 'standard' constants to work well (Cw=0.325). Although if you have the time/resources available then it's always good to test different values to see how sensitive your simulations are. I hope this helps, Emily navidmt and dweaver123 like this.

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November 11, 2019, 16:24		#2
navidmt New Member navid toussi Join Date: Nov 2015 Posts: 20 Rep Power: 11	Hi Emily Have you found any answer for your question? I came up with the same question and I don't know if I need to run my simulation using the default values of OpenFoam or it is essential to change the values of Cw and consequently the Ck. I would appreciate if you share your possible solution here with me. Thank you

November 15, 2019, 14:08		#5
navidmt New Member navid toussi Join Date: Nov 2015 Posts: 20 Rep Power: 11	Thank you so much I followed as you suggested.