fluent les sgs effective viscosity

eugene99 · March 31, 2020, 17:24

Hallo,
I can't find the definition or equation of sgs effective viscosity in LES. Could anyone help?

Thank you
Best Regards
Eugene

LuckyTran · March 31, 2020, 21:20

The SGS turbulent viscosity or SGS eddy viscosity is described in SGS models. In Fluent, you are most likely using the Smagorinsky-Lilly or dynamic Smagorinsky approach.

Effective viscosity is the sum of molecular viscosity and turbulent/eddy viscosity. $\mu_{eff}=\mu+\mu_{t}$

We don't normally talk about a SGS effective viscosity. Are you sure that's what you're looking for?

eugene99 · March 31, 2020, 22:03

Hi LuckyTran,

Thank you for your reply. I used Smagorinsky-Lilly for the simulation, there are only subgrid turbulent viscosity and subgrid effective viscosity under the turbulence (fluent19). I'm confused about these names.
mu_eff=mu+mu_t (mu_eff is effective/total viscosity, mu is molecular viscosity and mu_t is turbulent viscosity).

But in this case means mu_eff the subgrid effective viscosity?

LuckyTran · April 2, 2020, 00:34

Where did you even hear the term effective SGS viscosity? It's not a part of any normal LES nomenclature.

The subgrid stress is purely turbulent so there is no notion of molecular SGS and effective viscosity as far as the SGS is concerned.

Ppl that examine effective viscosity in LES are back-calculating the effective viscosity as per the Boussinesq hypothesis (which is no longer specific to the SGS) or they are studying numerical schemes, i.e. a numerical viscosity that they call an effective viscosity. In the latter case, effective viscosity could mean anything.

March 31, 2020, 17:24	fluent les sgs effective viscosity	#1
eugene99 New Member joe Join Date: Jun 2019 Posts: 2 Rep Power: 0	Hallo, I can't find the definition or equation of sgs effective viscosity in LES. Could anyone help? Thank you Best Regards Eugene

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March 31, 2020, 21:20		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	The SGS turbulent viscosity or SGS eddy viscosity is described in SGS models. In Fluent, you are most likely using the Smagorinsky-Lilly or dynamic Smagorinsky approach. Effective viscosity is the sum of molecular viscosity and turbulent/eddy viscosity. $\mu_{eff}=\mu+\mu_{t}$ We don't normally talk about a SGS effective viscosity. Are you sure that's what you're looking for?

March 31, 2020, 22:03		#3
eugene99 New Member joe Join Date: Jun 2019 Posts: 2 Rep Power: 0	Hi LuckyTran, Thank you for your reply. I used Smagorinsky-Lilly for the simulation, there are only subgrid turbulent viscosity and subgrid effective viscosity under the turbulence (fluent19). I'm confused about these names. mu_eff=mu+mu_t (mu_eff is effective/total viscosity, mu is molecular viscosity and mu_t is turbulent viscosity). But in this case means mu_eff the subgrid effective viscosity?

April 2, 2020, 00:34		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	Where did you even hear the term effective SGS viscosity? It's not a part of any normal LES nomenclature. The subgrid stress is purely turbulent so there is no notion of molecular SGS and effective viscosity as far as the SGS is concerned. Ppl that examine effective viscosity in LES are back-calculating the effective viscosity as per the Boussinesq hypothesis (which is no longer specific to the SGS) or they are studying numerical schemes, i.e. a numerical viscosity that they call an effective viscosity. In the latter case, effective viscosity could mean anything.