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-   -   Inlet boundary conditions for turbulence changing (http://www.cfd-online.com/Forums/fluent/123909-inlet-boundary-conditions-turbulence-changing.html)

robboflea September 24, 2013 04:11

Inlet boundary conditions for turbulence changing
 
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 11:33

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

robboflea September 24, 2013 11:38

Quote:

Originally Posted by Kokemoor (Post 453334)
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 I 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 U; 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 13:28

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

robboflea September 25, 2013 10:02

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

robboflea October 8, 2013 05:14

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?


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