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

ANSYS CFX Error

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

Reply
 
LinkBack Thread Tools Display Modes
Old   December 15, 2010, 14:34
Question ANSYS CFX Error
  #1
New Member
 
Join Date: Nov 2010
Posts: 9
Rep Power: 6
palofpinaki is on a distinguished road
I am performing 2-way coupled FSI for blood flow in axisymmetric abdominal aortic aneurysm. I am using k-omega turbulence model for analysis. But the following error is showing up everytime:

One or more elements have become highly distorted. Excessive distortion of elements is usually a symptom indicating the need for corrective action elsewhere. Try ramping the load up instead of step applying the load (KBC,1). You may need to improve your mesh to obtain elements with better aspect ratios. Also consider the behavior of materials, contact pairs, and/or constraint equations. If this message appears in the first iteration of first substep,be sure to run shape checking of elements.

If anybody has an idea regarding how to deal with the above error, please give suggestions.

Thanks in advance!!
palofpinaki is offline   Reply With Quote

Old   December 15, 2010, 14:37
Default
  #2
New Member
 
Join Date: Nov 2010
Posts: 9
Rep Power: 6
palofpinaki is on a distinguished road
I have changed the fluid mesh....but the same error occurs again.
palofpinaki is offline   Reply With Quote

Old   December 16, 2010, 02:53
Default
  #3
Senior Member
 
Lance
Join Date: Mar 2009
Posts: 522
Rep Power: 11
Lance is on a distinguished road
some hints:

improve the fluid AND solid mesh
ramp up the pressure, do not apply a step
use better initial conditions
Lance is offline   Reply With Quote

Old   December 16, 2010, 04:26
Question modifications to wilcox's k-Omega model for Low Re Number flows in ANSYS CFX
  #4
New Member
 
Join Date: Nov 2010
Posts: 9
Rep Power: 6
palofpinaki is on a distinguished road
How should I change the mesh....should I make it more fine or coarse??(Sorry for the silly ques....but I really don't understand...I am just a beginner!!)

On a different note, I tried to modify the standard k-omega turbulence model in ANSYS CFX using the advanced controls to make it more suitable for low Re flow. In wilcox's book "Turbulence modelling for CFD", a method is given according to which modifications are made to turbulent viscosity expression, beta star coefficient in k-equation and alpha coeffcient in Omega equation. I tried using CEL expression. All the expressions are functions of turbulent Reynolds number(which is a function of k and omega). But it gives an error saying that the alpha and beta star coefficients can only be constants or they can be expressed in the form of expressions which amount to a constant.

So I just can't figure out how do I implement the necessary modifications to the k-omega model to make it suitable for low Re flows in ANSYS....if anybody has tried it before or has any idea.....please feel free to give suggestions.....
Thanx!!
palofpinaki is offline   Reply With Quote

Old   December 16, 2010, 05:18
Default
  #5
Senior Member
 
Lance
Join Date: Mar 2009
Posts: 522
Rep Power: 11
Lance is on a distinguished road
Improve the mesh quality, i.e. skewness, aspect ratio, etc. This could mean more cells, it depends on how you mesh/geometry look like.

Do not change the turbulence model parameters unless you really know what you are doing.
Lance is offline   Reply With Quote

Old   December 18, 2010, 11:31
Red face
  #6
New Member
 
Join Date: Nov 2010
Posts: 9
Rep Power: 6
palofpinaki is on a distinguished road
Hey Lance...

Thanx for ur suggestion......but I am aware of turbulence....in my problem the Reynolds no is quite low of the order of 2500....so I am pretty sure...I have to modify the coefficients to make it capture the physics well. But I am not sure how....I have adressed my problem in the previous mail....if u have any idea....feel free to help!!!

Thanx
palofpinaki is offline   Reply With Quote

Old   December 19, 2010, 17:08
Default
  #7
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,665
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Does the turbulence transition model help? It is designed to operate in Re numbers in this range, but it aimed at external airfoil flows.
ghorrocks is offline   Reply With Quote

Old   December 20, 2010, 11:03
Unhappy
  #8
New Member
 
Join Date: Nov 2010
Posts: 9
Rep Power: 6
palofpinaki is on a distinguished road
Yes...you guessed it right..... I need to capture the transition to turbulence as the flow isn't laminar for one whole cardiac cycle.....please post any suggestions that might be helpful.... I just wanna know how can I modify the standard k-omega turbulence model in ANSYS CFX to capture transition(you can refer to my previous mails on this thread).....I am little short of time...please include details if possible...

Thanx a lot!!
palofpinaki is offline   Reply With Quote

Old   December 20, 2010, 18:02
Default
  #9
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,665
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Quote:
I just wanna know how can I modify the standard k-omega turbulence model in ANSYS CFX to capture transition
Ha ha ha! And I guess tomorrow you want to invent cold fusion.....

You cannot "simply" modify the k-omega model to capture transition! If it was simple then it would have already be done. The turbulence transition model was developed to do this, but it is definitely not simple and it was developed for external aerodynamic flows so you will have to be careful in applying to your flow.
ghorrocks is offline   Reply With Quote

Reply

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
nonNewtonian viscosity model mhassani OpenFOAM Programming & Development 5 January 7, 2013 10:27
Installation OF1.5-dev ttdtud OpenFOAM Installation 46 May 5, 2009 02:32
user subroutine error CFDUSER CFX 2 December 9, 2006 07:31
user defined function cfduser CFX 0 April 29, 2006 10:58
Problems of Duns Codes! Martin J Main CFD Forum 8 August 14, 2003 23:19


All times are GMT -4. The time now is 13:45.