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turbulent kinetic energy is not zero at the wall

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Old   January 29, 2013, 00:25
Default turbulent kinetic energy is not zero at the wall
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yogendra poul
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hi all
i am solving multiphase turbulent flow problem. velocity profile is correct but turbulent kinetic energy and intensity is not zero at the wall, although it should be zero at the wall. turbulent kinetic energy and intensity is showing maximum on the wall. i do not why is it so?

i need help


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Old   January 29, 2013, 03:14
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i am waiting for guidance from experts. it will really help me a lot.
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Old   January 29, 2013, 05:07
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What kind of wall modeling are you using? If you are using a wall function with Y+ around 60 it is normal that the TKE has a maximum value in the first cell away from the wall. If I recall correctly, the maximum for TKE is occurs at around Y+=30 for a flat plate.
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Old   January 29, 2013, 08:04
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Thank for your guidance

In my case fluid is flowing in a circular pipe. i am using k-omega viscous model. As you told wall model and Y+ value, i am not clear how and where to use wall model and Y+ value. i mean where is the input for Y+ value in gui.

probably you are getting my point



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Old   January 29, 2013, 12:26
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Y+ is not an input parameter.
It is the non-dimensional wall distance of the first cell.
You can evaluate it as a post-processing variable at solid walls.

The only way to influence it (besides the flow conditions) is with the mesh size.
If you want to resolve the boundary layer and thus capture the "correct" behavior of the flow variables near the wall, Y+ should be around 1 as a rule of thumb. Additionally, you will have to use the "enhanced wall treatment" in the turbulence modeling section.
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Old   January 30, 2013, 07:00
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thank you very much Alex S
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Old   January 30, 2013, 10:47
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It may help to use the scalable wall treatment if it is available with the turbulence model you are using. This applies the different wall treatments based on the value of Y+ or Y* as far as i am aware.

I had this problem as well a while ago. Good luck.
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Old   April 28, 2014, 11:03
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Quote:
Originally Posted by yogi06_sati View Post
hi all
i am solving multiphase turbulent flow problem. velocity profile is correct but turbulent kinetic energy and intensity is not zero at the wall, although it should be zero at the wall. turbulent kinetic energy and intensity is showing maximum on the wall. i do not why is it so?

i need help


thank you
Hi guys,
I think the question is to ask why turbulent kinetic energy and turbulence intensity are not zero at the wall, but the replies are all related to Y-plus!

The answer is, FLUENT doesn't implement zero boundary condition for kinetic energy k (k omega based model)! It uses zero gradient boundary condition (the change rate across the wall boundary is zero, instead of fixing a zero value for k), so you will see a finite value of k on the wall. I don't know why FLUENT does this. Among literature, I see some use zero, while some use zero gradient. You probably need a reference discussing boundary condition. The k change very near wall surface may be quite not trustful. One guy from my university shows me the difference from FLUENT and in-house DNS code. He then implements a UDF turbulence model using zero value boundary condition and also finds difference from FLUENT default turbulence models.

Sheng
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