CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Can anyone help me of using K-Epsilon

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   October 3, 2012, 23:20
Default Can anyone help me of using K-Epsilon
  #1
New Member
 
Hope
Join Date: Oct 2012
Posts: 7
Rep Power: 4
hopehope is on a distinguished road
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
hopehope is offline   Reply With Quote

Old   October 3, 2012, 23:33
Default
  #2
New Member
 
Hope
Join Date: Oct 2012
Posts: 7
Rep Power: 4
hopehope is on a distinguished road
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
hopehope is offline   Reply With Quote

Old   October 4, 2012, 07:40
Default
  #3
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,960
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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?
ghorrocks is offline   Reply With Quote

Old   October 4, 2012, 08:15
Default
  #4
New Member
 
Hope
Join Date: Oct 2012
Posts: 7
Rep Power: 4
hopehope is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
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
File Type: jpg Capture.JPG (11.7 KB, 14 views)
hopehope is offline   Reply With Quote

Old   October 4, 2012, 11:50
Smile
  #5
Senior Member
 
Erik
Join Date: Feb 2011
Location: Earth (Land portion)
Posts: 487
Rep Power: 10
evcelica is on a distinguished road
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"
evcelica is offline   Reply With Quote

Reply

Tags
k-epsilon cfx

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
SimpleFoam k and epsilon bounded nedved OpenFOAM Running, Solving & CFD 13 November 4, 2013 15:13
Large Epsilon 3D lepsilon Main CFD Forum 0 July 10, 2012 10:43
Orifice Plate with a fully developed flow - Problems with convergence jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 05:24
How to get reference to k and epsilon in the epsEqn and kEqn cfd_explorer OpenFOAM 0 March 10, 2011 10:58
SimpleFoam k and epsilon bounded nedved OpenFOAM Running, Solving & CFD 1 November 25, 2008 21:21


All times are GMT -4. The time now is 12:51.