Treatment of RANS-LES boundary in Embedded LES

clarice · September 6, 2018, 09:11

I'm modeling an axial fan in Fluent with the goal of determining the acoustics using Ffowcs-Williams Hawkings. I have an upstream duct, then the fan, followed by a downstream duct. The solution is steadily converged using k-epsilon realizable, and now I want to enable embedded LES in the fan cell zone with the purpose of activating the acoustics model when the flow has reached a statistically steady state.

I read that using solve/initialize/init-instantaneous-vel generates an instantaneous velocity field out of the steady state results, which I did, but now I am struggling to figure out how to specify a spectral sythesizer / random vortex approach at the RANS to LES interface boundary. Does anyone have any advice on how I would go about doing that? Or does the solve/initialize/init-instantaneous-vel command take care of the creation of turbulence at the RANS/LES interface?

LuckyTran · September 7, 2018, 12:26

The init-instantaneous-vel command adds perturbations onto your RANS field to generate a suitable initial condition. If you don't add perturbations, you tend to get a flow that remains laminar rather than producing any turbulence.

You of course need to also specify the perturbations at domain boundaries, otherwise your flow will likely become laminar if the incoming flow does not have any perturbations. Since it's rare that anyone has time-accurate velocity, temperature, and pressure profiles at boundaries some tools are needed. The spectral synthesizer is one such tool. However, here we are talking about inlets at the domain, not the RANS|LES interface.

Unfortunately, I don't recall what is the treatment at the RANS-LES interface. I haven't used e-LES in a long long time.

clarice · September 12, 2018, 10:25

Thanks! I found some documentation and guidelines on the use of the Vortex Method that Fluent recommends for the treatment of the RANS-LES interface, but I can't find it anywhere in Fluent in order to activate/use it.

LuckyTran · September 12, 2018, 11:54

Did you miss it in the Fluent manual? There's step by step instructions.

Fluent Manual

Which part are you stuck on? Do you have embedded LES already activated?

clarice · September 27, 2018, 04:38

I have seen the instructions, and I have activated embedded LES in a certain zone, but the interfaces between the LES and RANS regions do not give me the option to change the boundary condition from interface to rans-les interface. All the usual options are there, except the rans-les interface option.

LuckyTran · September 27, 2018, 16:06

Since you said interface... do you have more than 1 cell zone condition? The RANS and LES region need to be in the same cell zone. And that surface will should not be an interface.

September 6, 2018, 09:11	Treatment of RANS-LES boundary in Embedded LES	#1
clarice New Member Clarice du Plessis Join Date: Aug 2017 Location: Stellenbosch University Posts: 9 Rep Power: 9	I'm modeling an axial fan in Fluent with the goal of determining the acoustics using Ffowcs-Williams Hawkings. I have an upstream duct, then the fan, followed by a downstream duct. The solution is steadily converged using k-epsilon realizable, and now I want to enable embedded LES in the fan cell zone with the purpose of activating the acoustics model when the flow has reached a statistically steady state. I read that using solve/initialize/init-instantaneous-vel generates an instantaneous velocity field out of the steady state results, which I did, but now I am struggling to figure out how to specify a spectral sythesizer / random vortex approach at the RANS to LES interface boundary. Does anyone have any advice on how I would go about doing that? Or does the solve/initialize/init-instantaneous-vel command take care of the creation of turbulence at the RANS/LES interface?

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September 7, 2018, 12:26		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	The init-instantaneous-vel command adds perturbations onto your RANS field to generate a suitable initial condition. If you don't add perturbations, you tend to get a flow that remains laminar rather than producing any turbulence. You of course need to also specify the perturbations at domain boundaries, otherwise your flow will likely become laminar if the incoming flow does not have any perturbations. Since it's rare that anyone has time-accurate velocity, temperature, and pressure profiles at boundaries some tools are needed. The spectral synthesizer is one such tool. However, here we are talking about inlets at the domain, not the RANS\|LES interface. Unfortunately, I don't recall what is the treatment at the RANS-LES interface. I haven't used e-LES in a long long time.

September 12, 2018, 10:25		#3
clarice New Member Clarice du Plessis Join Date: Aug 2017 Location: Stellenbosch University Posts: 9 Rep Power: 9	Thanks! I found some documentation and guidelines on the use of the Vortex Method that Fluent recommends for the treatment of the RANS-LES interface, but I can't find it anywhere in Fluent in order to activate/use it.

September 12, 2018, 11:54		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Did you miss it in the Fluent manual? There's step by step instructions. Fluent Manual Which part are you stuck on? Do you have embedded LES already activated?

September 27, 2018, 04:38		#5
clarice New Member Clarice du Plessis Join Date: Aug 2017 Location: Stellenbosch University Posts: 9 Rep Power: 9	I have seen the instructions, and I have activated embedded LES in a certain zone, but the interfaces between the LES and RANS regions do not give me the option to change the boundary condition from interface to rans-les interface. All the usual options are there, except the rans-les interface option.

September 27, 2018, 16:06		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Since you said interface... do you have more than 1 cell zone condition? The RANS and LES region need to be in the same cell zone. And that surface will should not be an interface.