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

2-D solutions in cfx???

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 3, 2008, 12:04
Default 2-D solutions in cfx???
  #1
Ravi
Guest
 
Posts: n/a
Dear friends..

In FAQ of this site, some one has explained how to solve 2-D problems using CFX.But, for an axisymmtric problem, it has been told not to go beyond 5 degree angle. What is the reason for this??

Thank you.
  Reply With Quote

Old   April 3, 2008, 14:24
Default Re: 2-D solutions in cfx???
  #2
andy2O
Guest
 
Posts: n/a
In CFX axisymmetric calculation is performed by simulating a wedge containing a single layer of cell. The thickness of the wedge increases as you go away from the axis. This makes the area of the cell faces and the volume of the cells increase as you move away from the axis. This increase in area is crucial! It is the effect of this area change that makes the solution behave as axi-symmetric, instead of just 2D. (If you've done a university maths course you might remember all the r and 1/r terms in the Laplacian in cylindrical polar coordinates. The increase in area of the wedge with radius plays the same role in CFX's numerical method as the r and 1/r terms play in the Laplacian)

So, why a small angle?

Well, imagine a 90 degree wedge with one layer of mesh on it (Note only one layer as you want a single layer of cells in a 2-D mesh). It would look like a big triangle from 'above' (there is only one layer of mesh, so the curved surface would become a single straight edge). The areas of the mesh would still increase with radius, and so would the volumes, but not in quite the right way - the ratio of volume/area for each cell face would be wrong. The ratio would be different by a factor of sqrt(2) from the ratio in a real curved wedge. This would cause errors in the CFD results.

Instead, now imagine a 5 degree wedge with one layer of mesh. Now the difference between the perfect curved wedge and the 1-layer mesh is much smaller. It's not 100% perfect, but it is close enough to get good CFD results.

If the angle is <5% the error is even smaller.

So, you want to be working with the 5 degree wedge instead of the 90 degree wedge to get good results! Infact many people recommend using less than 5 degrees to keep accuracy. I don't know what the best angle is, but I use 1 degree often myself.

Regards, andy
  Reply With Quote

Old   April 23, 2008, 09:45
Default Re: 2-D solutions in cfx???
  #3
sbaa laazab
Guest
 
Posts: n/a
etude de cavitation sur un profil NACA 0015 2d
  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
CFX Treatment of Laminar and Turbulent Flows Jade M CFX 6 January 26, 2013 11:11
High Resolution (CFX) vs 2nd Order Upwind (Fluent) gravis ANSYS 3 March 24, 2011 03:43
CFX pressure in Simulations problem nasdak CFX 1 April 14, 2010 13:22
Different flow pattern between OpenFOAM and CFX AirS OpenFOAM 0 January 12, 2010 08:08
CFX 4.4 installation problem Pandu Sattvika CFX 1 December 1, 2001 05:07


All times are GMT -4. The time now is 14:32.