Inlet boundary conditions for turbulence changing

robboflea · September 24, 2013, 05:11

hi all,

I noticed a thing in fluent which I don't understand and I'll be happy if somebody could explain it to me.

I was working on a CFD simulation for a turbine vane with transitional SST turb model and I was setting the inlet turbulence boundary conditions in the form of turbulence intensity and length scale.

My idea was that these values were used in fluent and converted to the corresponding values of k and w which were actually imposed as boundary conditions at the inlet.
However, after my CFD has converged I found out, by plotting the contours of k and w at the inlet that their value is not even close to the value they should have from the turbulence intensity and length scale.

If I specify directly k and w as boundary conditions then they are kept constant at the inlet.

So my question is: how are the turbulence intensity and length scale information at the inlet handled in fluent? Why do they give way to different values of k and w?

I hope I've been clear enough.
Thanks for your answer.

Cheers,

Rob

Kokemoor · September 24, 2013, 12:33

Do the measured values match what it calculates if you use "Calculate from..." on the initialization tab?

How are you calculating k and omega from intensity and length scale? I believe $k=\frac{3}{2}(UI)^2$ and $\omega=\frac{\sqrt{k}}{\lambda_T}$ where

is the intensity and $\lambda_T$ is the length scale. Are you using a moving reference frame? If so, try it with both the absolute and relative velocity for

; it's probably absolute, but I'm not certain.

robboflea · September 24, 2013, 12:38

Quote:

Originally Posted by Kokemoor

Do the measured values match what it calculates if you use "Calculate from..." on the initialization tab?

Nope, they do not match them.

Quote:

Originally Posted by How are you calculating k and omega from intensity and length scale? I believe [math

k=\frac{1}{2}(UI)^2[/math] and $\omega=\frac{\sqrt{k}}{\lambda_T}$ where

is the intensity and $\lambda_T$ is the length scale. Are you using a moving reference frame? If so, try it with both the absolute and relative velocity for

; it's probably absolute, but I'm not certain.

I didn't use any formula myself, I just referred to the calculated values from Fluent in the "calculate from inlet" function in the initialize tab.
I am not using a moving reference frame, everything is stationary.

Kokemoor · September 24, 2013, 14:28

If you use those formulas, do you get results matching either the data or the initialization computation?

robboflea · September 25, 2013, 11:02

nope...they don't match any of them

robboflea · October 8, 2013, 06:14

also I did notice that, after the calculations are completed the inlet values of turbulent intensity are completely different from the values I specified at the inlet.

Anybody has any idea on why is this happening?

Reza Zarghanishiraz · April 7, 2022, 15:37

Hey all,
as an answer to this problem, you should be mindful of two things:
1- your turbulent properties can change when you standard initialize the domain. make sure that you enter the right K and Omega (I am using the SST K-Omega model) values in the panel.
2- make sure that you have the right values in the "reference values" panel (accessible through the physics tab on top). Intensity is calculated based on K and turbulence length scale and hydraulic diameter so you should have correct length values set in that panel.

September 24, 2013, 05:11	Inlet boundary conditions for turbulence changing	#1
robboflea Senior Member Join Date: Nov 2011 Posts: 109 Rep Power: 15	hi all, I noticed a thing in fluent which I don't understand and I'll be happy if somebody could explain it to me. I was working on a CFD simulation for a turbine vane with transitional SST turb model and I was setting the inlet turbulence boundary conditions in the form of turbulence intensity and length scale. My idea was that these values were used in fluent and converted to the corresponding values of k and w which were actually imposed as boundary conditions at the inlet. However, after my CFD has converged I found out, by plotting the contours of k and w at the inlet that their value is not even close to the value they should have from the turbulence intensity and length scale. If I specify directly k and w as boundary conditions then they are kept constant at the inlet. So my question is: how are the turbulence intensity and length scale information at the inlet handled in fluent? Why do they give way to different values of k and w? I hope I've been clear enough. Thanks for your answer. Cheers, Rob

September 24, 2013, 12:33		#2
Kokemoor Senior Member Andrew Kokemoor Join Date: Aug 2013 Posts: 122 Rep Power: 14	Do the measured values match what it calculates if you use "Calculate from..." on the initialization tab? How are you calculating k and omega from intensity and length scale? I believe $k=\frac{3}{2}(UI)^2$ and $\omega=\frac{\sqrt{k}}{\lambda_T}$ where is the intensity and $\lambda_T$ is the length scale. Are you using a moving reference frame? If so, try it with both the absolute and relative velocity for ; it's probably absolute, but I'm not certain. Last edited by Kokemoor; September 24, 2013 at 12:41. Reason: Correction: k=3/2(UI)^2, not 1/2(UI)^2

April 7, 2022, 15:37		#7
Reza Zarghanishiraz New Member Reza Zarghanishiraz Join Date: Sep 2021 Posts: 4 Rep Power: 5	Hey all, as an answer to this problem, you should be mindful of two things: 1- your turbulent properties can change when you standard initialize the domain. make sure that you enter the right K and Omega (I am using the SST K-Omega model) values in the panel. 2- make sure that you have the right values in the "reference values" panel (accessible through the physics tab on top). Intensity is calculated based on K and turbulence length scale and hydraulic diameter so you should have correct length values set in that panel. arashjkh likes this.

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September 24, 2013, 14:28		#4
Kokemoor Senior Member Andrew Kokemoor Join Date: Aug 2013 Posts: 122 Rep Power: 14	If you use those formulas, do you get results matching either the data or the initialization computation?

September 25, 2013, 11:02		#5
robboflea Senior Member Join Date: Nov 2011 Posts: 109 Rep Power: 15	nope...they don't match any of them

October 8, 2013, 06:14		#6
robboflea Senior Member Join Date: Nov 2011 Posts: 109 Rep Power: 15	also I did notice that, after the calculations are completed the inlet values of turbulent intensity are completely different from the values I specified at the inlet. Anybody has any idea on why is this happening?