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Why my calculated epsilon is different from FLUENT calculation? 

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April 30, 2020, 21:48 
Why my calculated epsilon is different from FLUENT calculation?

#1 
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Min Zhang
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Hello All,
For the "Velocity Inlet" boundary condition, I specified both Turbulent Intensity and Hydraulic Diameter. For the "Solution Initialization", I chose to "Compute from inlet". But the "Turbulent Dissipation Rate (epsilon)" calculated from FLUENT is different from my calculation. I am wondering why? This is my calculation and my reference is here: https://www.afs.enea.it/project/nept...ug/node238.htm (1) I calculated Reynolds number at the inlet Re = D_pipe*U/nu = 553852.1841 (2) I calculated Turbulence Intensity, I using equation (7.31) I = 0.03063344 = 3.0633% which is the value I specified for my "Velocity Inlet" (3) I calculated Turbulence length scale (L) using L=0.07*D_pipe (7.32) L = 0.0072m which is the value I specified for my "Velocity Inlet" (4) I calculated Turbulent Kinetic Energy using equation (7.34) k = 0.0411m2/s2 (the same as FLUENT calculation = 0.04104477) (5) I calculated Turbulence Dissipation Rate (epsilon) using eqaution (7.35) epsilon = 0.1898 m2/s3 However, the value calculated by FLUENT is 2.711054 m2/s3. 

May 1, 2020, 14:59 
Version

#2 
Senior Member

It is because you might be using one of the current versions while referring the documentation, which is quite old. Current formulation is
And is included in .
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May 1, 2020, 17:01 
Current version of Ansys documentation

#3 
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Min Zhang
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Hello vinerm,
Could you give me more details about the expression for l, and how C_mu is included? Could you point me to the current version of Ansys documentation? Thanks! 

May 2, 2020, 16:07 
Documentation

#4 
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May 3, 2020, 20:00 
Don't have access to the Documentation

#5 
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Min Zhang
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It seems that, as a free student academic version, I don't have the permission to access the document.


May 4, 2020, 16:21 
Help Manual

#6 
Senior Member

Yes, students do not have access to that. The formulation is still same, except that is considered within . So, the value of itself changes.
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May 4, 2020, 16:36 
Formula of Turbulence length scale

#7 
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Min Zhang
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I am wondering whether you could post the formula for Turbulence length scale.
Thanks! 

May 5, 2020, 10:07 

#9 
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Min Zhang
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Based on the formula you gave, substitute l into epsilon, then it will be the same as equation (7.35) in the old documentation. Is that right?


May 5, 2020, 10:35 
Epsilon

#10 
Senior Member

Yes, the formula is still the same, however, the length scale is not.
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May 5, 2020, 10:37 

#11 
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Min Zhang
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But it won't affect the calculation for epsilon.
Therefore, my calculated epsilon is still different from the FLUENT calculation. 

May 5, 2020, 11:05 
Epsilon

#12 
Senior Member

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May 6, 2020, 17:49 
I still could not get the same epsilon value as FLUENT?

#13 
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Min Zhang
Join Date: Mar 2017
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This is my current calculation based on the latest formula you posted.
(1) I calculated Reynolds number at the inlet Re = D_pipe*U/nu = 553852.1841 (2) I calculated Turbulence Intensity, I using equation (7.31) I = 0.03063344 = 3.0633% which is the value I specified for my "Velocity Inlet" (3) I calculated Turbulence length scale (L) using your equation L=0.07*D_pipe/C_mu^(3/4) L = 0.0439m which is the value I specified for my "Velocity Inlet" Here I have an assumption that Hydraulic Diameter == Turbulence length scale. (4) I calculated Turbulence Dissipation Rate (epsilon) using your equation epsilon = 0.1898 m2/s3 (same as my previous calculation since essentially the formula is the same) However, the value calculated by FLUENT (this time) is 0.4446375 m2/s3. 

May 7, 2020, 05:33 
Hydraulic Diameter

#14 
Senior Member

Length Scale and Hydraulic Diameter are not same, otherwise you would not require different options.
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May 12, 2020, 22:53 
Now I know why!

#15 
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Min Zhang
Join Date: Mar 2017
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Finally, I got to know why.
1. The reason is I assumed "Hydraulic Diameter == Turbulence length scale". When I use "Hydraulic Diameter =D_pipe", then I got the same epsilon value as FLUENT. 2. The formula in the old reference is consistent with the latest ones. Thanks! 

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