# Contributions to turbulent kinetic energy in les

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 November 12, 2021, 13:15 Contributions to turbulent kinetic energy in les #1 New Member   Join Date: Apr 2020 Posts: 7 Rep Power: 4 Hi, I am learning LES, and I see that it is important to estimate the modelled turbulent kinetic energy (k_sgs) and the resolved kinetic energy (k_res) There is an equation to calculate k_res with the given outputs in fluent using the time average of the velocity fluctuations. One can also calculate with a little more effor using udf k_sgs. Finally there is another output in fluent that is the turbulent kinetic energy (tke) My question is that I am not sure if tke=k_res+k_sgs or there are other sources of tke that I am missing. On the other hand I’ve seen variables in older versions fluent called subgrid eddy viscosity that I haven’t found un fluent 2021r2

 November 12, 2021, 15:15 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,062 Rep Power: 60 Your understanding is correct, you need to find kres + ksgs. There is almost always going to be a subgrid eddy viscosity whenever you do LES because that is exactly what the subgrid model is. The only time you do not have a subgrid eddy viscosity, is if you are doing LES without a subgrid model. Sometimes you need to fetch the eddy viscosity in order to calculate ksgs using the subgrid model but that is its own problem. I am assuming of course that you somehow manage to correctly get ksgs from your udf. There shouldn't be an option for tke unless you are doing a hybrid RANS/LES or DES. But if you are doing one of these, then the tke is entirely modeled and there is no resolved tke. If you in fact do see such a thing as a tke when you are doing LES, you should be very very careful indeed.

November 12, 2021, 16:17
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 Originally Posted by LuckyTran Your understanding is correct, you need to find kres + ksgs. There is almost always going to be a subgrid eddy viscosity whenever you do LES because that is exactly what the subgrid model is. The only time you do not have a subgrid eddy viscosity, is if you are doing LES without a subgrid model. Sometimes you need to fetch the eddy viscosity in order to calculate ksgs using the subgrid model but that is its own problem. I am assuming of course that you somehow manage to correctly get ksgs from your udf. There shouldn't be an option for tke unless you are doing a hybrid RANS/LES or DES. But if you are doing one of these, then the tke is entirely modeled and there is no resolved tke. If you in fact do see such a thing as a tke when you are doing LES, you should be very very careful indeed.

As it usually happens to me in CFD the answers to my questions only bring more questions…

I am indeed running a Hybrid RANS/LES model (SBES) and , that must be why, as you said, I am having tke as an output as well. However I am a bit confused when you said that if I am using RANS/LES everything is modelled. There is no k_sgs in hybrid models?

I guess in a Hybrid RANS/LES tke=K_modelled+k_res where k_modelled=k_rans+k_sgs. That supports the idea of having an UDF to calculate k_sgs separately. I also think in the regions of the domain far from the walls ideally Tke=K_sgs+k_res

Last edited by sonsonst; November 13, 2021 at 05:56.

 Tags hybrid, les, rans