Turbulence intensity plummets

Piedrapablo · February 3, 2017, 15:50

Hi everyone,

I am trying to analyze the external turbulence flow over a car (Re~10e7 and k-epsilon model).

I set the Turbulence Intensity to 16% and the turbulence viscosity ratio to 10 at the inlet, but when plotting the converged solution I can see that the turbulence intensity immediatly plummets to basically 0% right after the inlet (even the final turbulence intensity at the inlet drops to ~0% while the velocity stands at V0).

How do I solve this?What does it mean? It seems as if the turbulence instantaniuosly "dissapear" as epsilon does not increase....

Thanks a lot

LuckyTran · February 5, 2017, 23:28

It is real.

Your problem is the turbulent viscosity ratio of 10 is not realistic for what you are trying to simulate. Garbage in => garbage out.

The "turbulence" is not being generated by anything so that it can only decay. It would be a lot easier to understand if you switched to intensity and length scale method. You have basically specified a very small length scale so that the turbulence decays rapidly. It disappears over a very short length which gives the illusion that the turbulence has disappeared. What you need to do is specify a larger length scale so that it takes longer to decay.

February 3, 2017, 15:50	Turbulence intensity plummets	#1
Piedrapablo New Member Join Date: Jan 2017 Posts: 3 Rep Power: 9	Hi everyone, I am trying to analyze the external turbulence flow over a car (Re~10e7 and k-epsilon model). I set the Turbulence Intensity to 16% and the turbulence viscosity ratio to 10 at the inlet, but when plotting the converged solution I can see that the turbulence intensity immediatly plummets to basically 0% right after the inlet (even the final turbulence intensity at the inlet drops to ~0% while the velocity stands at V0). How do I solve this?What does it mean? It seems as if the turbulence instantaniuosly "dissapear" as epsilon does not increase.... Thanks a lot

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February 5, 2017, 23:28		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,668 Rep Power: 65	It is real. Your problem is the turbulent viscosity ratio of 10 is not realistic for what you are trying to simulate. Garbage in => garbage out. The "turbulence" is not being generated by anything so that it can only decay. It would be a lot easier to understand if you switched to intensity and length scale method. You have basically specified a very small length scale so that the turbulence decays rapidly. It disappears over a very short length which gives the illusion that the turbulence has disappeared. What you need to do is specify a larger length scale so that it takes longer to decay.