Transient Heat Transfer vs. RANS Heat Transfer

hob · October 5, 2015, 11:24

Hi,

I have an issue when simulating (LES) heat transfer of flow over a flat plate. Initally I have tried a simple RANS heat transfer case (similar to: https://confluence.cornell.edu/displ...t+Plate+step+6), which also references Reynolds, W.C., Kays, W.M., Kline, S.J. "Heat Transfer in the Turbulent Incompressible Boundary Layer." NASA Memo 12-1-58W. December 1958.

I initially just used a constant wall temperature (around 400K) with inlet temperature of 300K, the Nusselt vs. Reynolds number matched with experimental.

Switching to LES and time-averaged statistics in Fluent results in the Nu. number (calculated using the heat-flux) being approximately 1/3 that of the RANS case (for constant wall temperature).

Is there any reason for the difference?

Regards,

Hob

LuckyTran · October 5, 2015, 19:27

This is an oversimplification but if you don't give perturbations to an LES simulation it relaminarizes and becomes an unsteady laminar flow. Do you have any perturbations? Check to see if you ended up with the laminar Nu and check to verify that you are actually doing LES and not an unsteady laminar simulation.

hob · October 5, 2015, 19:44

Quote:

Originally Posted by LuckyTran

This is an oversimplification but if you don't give perturbations to an LES simulation it relaminarizes and becomes an unsteady laminar flow. Do you have any perturbations? Check to see if you ended up with the laminar Nu and check to verify that you are actually doing LES and not an unsteady laminar simulation.

I did have inlet pertubations specified via the Fluctuating Velocity Algorithm, I specified Intensity and Hydraulic Diameter. The turbulence intensity is quite low (1%) and it is indeed a laminar Nu profile, I hadn't thought to check that! Thanks for the info

October 5, 2015, 11:24	Transient Heat Transfer vs. RANS Heat Transfer	#1
hob New Member Jimmi Join Date: Dec 2012 Posts: 11 Rep Power: 13	Hi, I have an issue when simulating (LES) heat transfer of flow over a flat plate. Initally I have tried a simple RANS heat transfer case (similar to: https://confluence.cornell.edu/displ...t+Plate+step+6), which also references *Reynolds, W.C., Kays, W.M., Kline, S.J. "Heat Transfer in the Turbulent Incompressible Boundary Layer." NASA Memo 12-1-58W. December 1958*. I initially just used a constant wall temperature (around 400K) with inlet temperature of 300K, the Nusselt vs. Reynolds number matched with experimental. Switching to LES and time-averaged statistics in Fluent results in the Nu. number (calculated using the heat-flux) being approximately 1/3 that of the RANS case (for constant wall temperature). Is there any reason for the difference? Regards, Hob

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October 5, 2015, 19:27		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,673 Rep Power: 65	This is an oversimplification but if you don't give perturbations to an LES simulation it relaminarizes and becomes an unsteady laminar flow. Do you have any perturbations? Check to see if you ended up with the laminar Nu and check to verify that you are actually doing LES and not an unsteady laminar simulation.