# CFX turbulence intensity, turbulence length scale, Turbulence Eddy Dissipation

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 December 14, 2020, 23:53 CFX turbulence intensity, turbulence length scale, Turbulence Eddy Dissipation #1 New Member   duwenhai Join Date: Dec 2020 Posts: 3 Rep Power: 4 Using CFX to calculate the airfoil by changing the I (inlet turbulence intensity) and the inlet Re number, but the K (Turbulent Kinetic Energy), Epsilon (Turbulence Eddy Dissipation) calculated by the CFX are different from the results calculated by myself using the equations. K=3/2*(I*U)^2 Tu Re U K(Cfx) K(My) epsilon(CFX) epsilon(My) 5% 1.5e5 216 163 175 9.5e+5 9.23e+5 10% 1.5e5 216 676 697 1.72e+5 1.58e+5 5% 4.5e5 215 138 172 2.1e+6 2.08e+6 10% 4.5e5 205 580 630 4e+6 3.72e+6 The CFX output the inlet of K and epsilon, are different from the values calculated by the equations from the CFX guide documents, what are the reasons, guys? Thanks, Harry

 December 15, 2020, 04:13 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,326 Rep Power: 138 How are you defining U and averaging it across the inlet? And how are you averaging k across the inlet? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

December 15, 2020, 08:50
#3
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duwenhai
Join Date: Dec 2020
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Rep Power: 4
Quote:
 Originally Posted by ghorrocks How are you defining U and averaging it across the inlet? And how are you averaging k across the inlet?
I define the U at inlet by using the mass averaged value, It is the same for the K. The similar results I can get by using the area averaged values.

The other thing is the turbulence length scale:
According the equation: L= 0.038*Dh
It means length scale should be the same for the different Re and Tu, but it is found that L (turbulence length scale) change from from 7e-4 to 6.5e-3 in my CFX calculated results.

It is really weird

 December 15, 2020, 14:48 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,326 Rep Power: 138 Make sure you are aware of conservative values and what this could be doing to your averages and integrated quantities due to wall effects. What happens if you choose a single point inside the boundary and compare the values there (rather than sing averaged/integrated values over the whole boundary)? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

December 16, 2020, 03:11
#5
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duwenhai
Join Date: Dec 2020
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Quote:
 Originally Posted by ghorrocks Make sure you are aware of conservative values and what this could be doing to your averages and integrated quantities due to wall effects. What happens if you choose a single point inside the boundary and compare the values there (rather than sing averaged/integrated values over the whole boundary)?
Hi, Ghorrocks
I output these values at the inlet boundary by using mass averaged value, including the boundary layer, CFX has this option.
When I change the U value, I can get the similar value of K, epsilon, it seems that the U calculated at inlet is not the same one applied in the CFX calculations.
I decide to implement the Tu and eddy viscosity ratio as the input value for the turbulent boundary condition

 December 16, 2020, 05:33 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,326 Rep Power: 138 Repeating my post #4: Conservative and hybrid variables can cause unexpected results when you average across the flow field due to boundary layer effects distorting the mean. Make sure you understand what CFX is doing when you calculate averages, so read the documentation on hybrid and conservative values. Second point: To avoid the distorting effect of hybrid and conservative values at the boundary, I recommend you put a point (maybe a monitor point, or a post-processing point) in the middle of the flow. You are then just taking the variable values at that point with no averaging. Can you tell us what you get when you do this? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 Tags turbulence intensity, turbulent kinetic energy