# Can anyone help me of using K-Epsilon

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 October 3, 2012, 23:20 Can anyone help me of using K-Epsilon #1 New Member   Hope Join Date: Oct 2012 Posts: 8 Rep Power: 7 Hi, When I use the K-Epsilon in CFX, the result is strange. My first node is 2e-7 m from wall. After exporting the result, I find that V(y=0)=0, V(y=2e-7)=5, it's almost half of the highest one. The velocity is growing too fast. Can anybody tell me that how can I fix that? Thank you Hope

 October 3, 2012, 23:33 #2 New Member   Hope Join Date: Oct 2012 Posts: 8 Rep Power: 7 Basically my problem is the hybrid and conservative are same expect the wall velocity. And the velocity grows too fast. Could anyone help me and tell me how to fix that? Thank you! Hope

 October 4, 2012, 07:40 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,318 Rep Power: 110 The parameter you want to look at is y+, the non-dimensional distance from the wall. The distance of the first node does not mean anything by itself. And why do you say the velocity grows too fast? Do you have analytical or experimental results to show it should be slower?

October 4, 2012, 08:15
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Hope
Join Date: Oct 2012
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Quote:
 Originally Posted by ghorrocks The parameter you want to look at is y+, the non-dimensional distance from the wall. The distance of the first node does not mean anything by itself. And why do you say the velocity grows too fast? Do you have analytical or experimental results to show it should be slower?
Thank you, Horrocks.

What's y+?
my width is about 0.1m, and the first node is at 2e-7 m.
Yes, I have experimental results.
my case is a wind tunnel, velocity inlet, pressure=0 at outlet, and non-slip smooth wall.
Look my attachment. The green one the experimental results, and red one is my result. at the very beginning it grows too fast. but it's better on the second node. And I don't know why.
Attached Images
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 October 4, 2012, 11:50 #5 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 718 Rep Power: 14 Like Glenn said, plot your "y plus" (not solver y plus) for the wall. I'm guessing it is below 11 and k-epsilon is using wall functions to approximate your wall improperly since the y plus is too small. I'd recommend switching to a different turbulence model that will fully integrate the walls, like SST. Try to get your "y plus" values below 1 if you are trying to compare the boundary layers near the wall, you only use wall functions when you don't care much about the boundary layer. This is pretty basic, CFD 101; Your homework: Read Chapter 4 of the CFX solver modeling guide "Turbulence and Near-Wall Modeling" Last edited by evcelica; October 4, 2012 at 11:53. Reason: This is pretty basic, CFD 101; Read Chapter 4 "Turbulence and Near-Wall Modeling"

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