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-   -   Openfoam nut and k_sgs equation (https://www.cfd-online.com/Forums/openfoam-programming-development/228702-openfoam-nut-k_sgs-equation.html)

EnigmaFluids July 10, 2020 16:11

Openfoam nut and k_sgs equation
 
Greetings, I have noticed while completing LES that openfoam uses the equation

\nu_t=C_k\Delta\sqrt{k_{sgs}}

or if you check WALE.C

this->nut_ = Ck_*this->delta()*sqrt(this->k(fvc::grad(this->U_)));

but where does this equation come from? I have been reading the original smagorinsky model and also WALE model papers, but I don't see it contained there. I am looking for a literature source because I want to know if this k_sgs is actually a physical quantity or just an intermediate calculation on the way to get nut.

For example, in the original WALE paper by Nicoud and Ducros (1999), we notice that the model gives nut alone and not k_sgs. However, I assume that openfoam calculates k_sgs first and then nut after, for coding convenience. I would like to know whether the openfoam k_sgs is a real quantity so I can estimate whether 80% of the turbulence kinetic energy is being resolved in my field via

\frac{k_{resolved}}{k_{resolved}+k_{sgs}}

So once again, where does this equation come from? Thanks!

Tibo99 July 11, 2020 21:25

That is a good question.

As I expect to use the WALE model in the near future, I would be interested to know the answer also.

superkelle July 14, 2020 16:46

Maybe this helps:

https://caefn.com/openfoam/wale-sgs-model

EnigmaFluids July 17, 2020 17:07

The actual WALE model as originally written does not use a relationship between nut and k. However, I believe that Openfoam uses the equation

\nu_t=c_k\Delta\sqrt{k_{sgs}}

for coding convenience.

I believe that this is from the so-called One-equation TKE model. Specifically read section 3.4 of https://link.springer.com/content/pd...016-9751-4.pdf

This paper references Bardina et. al. (1980) where a similar equation is found in Table 3 and Equation 15

This is all to say that once again, the WALE model does not use said equation, but rather I believe Openfoam has implemented it as a convenient way to get both k_sgs and nut during runtime. From this conclusion, I assume then that we inherit all the assumptions of the one-equation turbulence model if we are using the openfoam nut field to recover k_sgs. This is important since I am comparing k_sgs with a calculated k_resolved from the instantaneous fluctuations to determine how if 80% of the TKE is being resolved.

Tibo99 July 21, 2020 18:13

That being said, it's a calculation? And not use for resolving the turbulence model?


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