Inquiry about turbulent intensity and turbulent length scale.

KarimSayed · May 1, 2020, 22:48

Hello everyone,
I am using the transition model k-kl-omega in an airfoil 2D simulation. When I specify the turbulent intensity at the inlet at a value let's say 5%, after the solution converges, I find that the turbulent intensity at the inlet has changed to a different value although it is a boundary condition as far as I understand. In addition, I would like to know how the turbulent length scale at the inlet affects the solution. I know that it has an effect on the turbulence decay and the turbulent intensity value that reaches the airfoil, but other than that, are there any other effects?

LuckyTran · May 1, 2020, 23:30

There's two turbulence intensities. There's a turbulence intensity boundary condition and a turbulence intensity field.

Turbulence intensity at the BC isn't a BC per se. The actual boundary condition is being applied to k and omega. The intensity and length scale are alternative inputs, using very specific formulas, that get converted into k and omega as seen here (see 7.3-4 & 7.3-8).

The turbulence intensity field is calculated using a similar but slightly different formula, involving instead a reference velocity which is a a user specified value in the reference values pane. By specifying arbitrary values into the reference values, you can make the turbulence intensity be pretty much anything. If the reference velocity is equal to the velocity at the boundary (keep in mind it would also have to be uniform), then you could get the same I.

Quote:

Originally Posted by KarimSayed

know that it has an effect on the turbulence decay and the turbulent intensity value that reaches the airfoil, but other than that, are there any other effects?

That's not incorrect but it's an oversimplification. You're saying it affects the turbulence at the airfoil: which can mean anything, everything, or nothing. Keep in mind the Butterfly effect.

KarimSayed · May 2, 2020, 00:43

Thanks for your reply. I am using the inlet velcoity as a reference value so I should Get the same turbulence intensity as you have said.

LuckyTran · May 2, 2020, 03:20

So you have a velocity inlet with a constant value?

The other thing you can do is check the k and omega values instead of turbulence intensity.

And how different is different? You also haven't told how you check the turbulence intensity on the outlet, whether it be via a report or plot or what.

KarimSayed · May 2, 2020, 10:32

Yes, I have a uniform inlet velocity, and the k,kl & omega are the same before starting calculations and after reaching the converged solution at the inlet. The difference is in turbulent intensity. I use xy profiles to determine the values of these parameters. To be more specific, an inlet boundary condition of turbulent intensity 10% and turbulent length scale 0.01 m results in 5.74% turbulent intensity after reaching the converged solution at the inlet.

May 1, 2020, 22:48	Inquiry about turbulent intensity and turbulent length scale.	#1
KarimSayed New Member Karim Sayed Join Date: May 2020 Posts: 7 Rep Power: 6	Hello everyone, I am using the transition model k-kl-omega in an airfoil 2D simulation. When I specify the turbulent intensity at the inlet at a value let's say 5%, after the solution converges, I find that the turbulent intensity at the inlet has changed to a different value although it is a boundary condition as far as I understand. In addition, I would like to know how the turbulent length scale at the inlet affects the solution. I know that it has an effect on the turbulence decay and the turbulent intensity value that reaches the airfoil, but other than that, are there any other effects?

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May 2, 2020, 00:43		#3
KarimSayed New Member Karim Sayed Join Date: May 2020 Posts: 7 Rep Power: 6	Thanks for your reply. I am using the inlet velcoity as a reference value so I should Get the same turbulence intensity as you have said.

May 2, 2020, 03:20		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,728 Rep Power: 66	So you have a velocity inlet with a constant value? The other thing you can do is check the k and omega values instead of turbulence intensity. And how different is different? You also haven't told how you check the turbulence intensity on the outlet, whether it be via a report or plot or what.

May 2, 2020, 10:32		#5
KarimSayed New Member Karim Sayed Join Date: May 2020 Posts: 7 Rep Power: 6	Yes, I have a uniform inlet velocity, and the k,kl & omega are the same before starting calculations and after reaching the converged solution at the inlet. The difference is in turbulent intensity. I use xy profiles to determine the values of these parameters. To be more specific, an inlet boundary condition of turbulent intensity 10% and turbulent length scale 0.01 m results in 5.74% turbulent intensity after reaching the converged solution at the inlet.