Production TKE in LES

CeesH · March 7, 2016, 10:44

Dear all,

I am currently running a large eddy simulation (so far I have only experience with RANS), and I am noticing a significant underestimation of the turbulence dissipation rate.

It was adviced I'd check the energy budget, i.e. are dissipation and production in balance. But I am a bit lost on how to do this check. How can I calculate the turbulent kinetic energy production from FLUENT?

Thanks in advance!
Cees

LuckyTran · March 8, 2016, 02:10

In the RANS sense (4th term on RHS):
https://en.wikipedia.org/wiki/Turbulence_kinetic_energy

To do it in the instantaneous sense is a bit tricky because of the spatial filtering in LES. You have to keep track of the filtered velocities and the influence of the SGS.

CeesH · March 8, 2016, 03:38

I was afraid it wouldn't be that straightforward.

Anyway, my problem is that energy dissipation is significantly underestimated compared to experiments (and RANS). So I wanted to check whether production and dissipation balance. Do you think the RANS-sense implementation can answer that? (or perhaps, do you see a more straightforward way to do the budget check? I haven't gotten around to learning all the ins and outs on LES yet. For example, is the notion that <k_t> seems to be constant in time sufficient to declare the budget closes?).

March 7, 2016, 10:44	Production TKE in LES	#1
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	Dear all, I am currently running a large eddy simulation (so far I have only experience with RANS), and I am noticing a significant underestimation of the turbulence dissipation rate. It was adviced I'd check the energy budget, i.e. are dissipation and production in balance. But I am a bit lost on how to do this check. How can I calculate the turbulent kinetic energy production from FLUENT? Thanks in advance! Cees

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March 8, 2016, 02:10		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	In the RANS sense (4th term on RHS): https://en.wikipedia.org/wiki/Turbulence_kinetic_energy To do it in the instantaneous sense is a bit tricky because of the spatial filtering in LES. You have to keep track of the filtered velocities and the influence of the SGS.

March 8, 2016, 03:38		#3
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	I was afraid it wouldn't be that straightforward. Anyway, my problem is that energy dissipation is significantly underestimated compared to experiments (and RANS). So I wanted to check whether production and dissipation balance. Do you think the RANS-sense implementation can answer that? (or perhaps, do you see a more straightforward way to do the budget check? I haven't gotten around to learning all the ins and outs on LES yet. For example, is the notion that <k_t> seems to be constant in time sufficient to declare the budget closes?).