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

CFX Pre/Double Precision

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 21, 2009, 15:31
Default CFX Pre/Double Precision
  #1
New Member
 
Join Date: Jun 2009
Posts: 13
Rep Power: 17
Scott Nordsen is on a distinguished road
I am working on a case with small finite volumes (~1e-18 m^3).

To run in fluent I used double precision solver since single precision had issues with the mesh.

Now, I am trying to run the same case in CFX.

I see how to run a double precision simulation with cfx5solve.

Question is how do I setup the case using cfx5pre?
-double is not an option.
When I read msh file into cfx5pre I get mesh errors as I did with fluent single precision.

The only difference is fluent is one tool (pre/solve/post) that I can start as dp from the start.

Thanks

Scott
Scott Nordsen is offline   Reply With Quote

Old   July 21, 2009, 19:57
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,850
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Hi,

The fact that you have small finite volumes should not cause grief (my control volumes in my current work are 1e-20 m^3!), providing all the control volumes are a similar order of magnitude in size. There is no loss in precision of 1e-18 versus 1e0 as the significand has the same number of bits representing its size.

The problem is when you have a large range in sizes. What is the largest and smallest finite volume in your model?

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Old   July 22, 2009, 10:02
Default
  #3
New Member
 
Join Date: Jun 2009
Posts: 13
Rep Power: 17
Scott Nordsen is on a distinguished road
Have a geom with a far field ~20D away down to the geometry that has edges ~0.1".
D~10 ft

So my cells range from 10m^3 to 1e-18 m^3. So as you would say I have a problem.
Scott Nordsen is offline   Reply With Quote

Old   July 22, 2009, 12:12
Default
  #4
Senior Member
 
ckleanth's Avatar
 
George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 18
ckleanth is on a distinguished road
an easy way of refining could be to run it first steady state with a coarse mesh and use a few adaptive mesh steps but I'm not sure how you will manage to get low volume aspect ratio.. I suppose your best bet is if you can create a nice hexa mesh so it wont distort during mesh adaptation
__________________
Top 4 tips
1. Knowledge is everything and Ignorance is dangerous.
2. Understand your limitations and try to eliminate them.
3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window.
4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials
ckleanth is offline   Reply With Quote

Old   July 22, 2009, 19:45
Default
  #5
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,850
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Hi,

Cell volumes ranging over 19 orders of magnitude is not a good idea. That is what your problem is. If your fine cells really need to be that fine then you are going to have to refine your coarse cells. Yes, that probably means you need a supercomputer but hey, this is CFD and why CFD needs supercomputers.

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Reply

Tags
double precision, preprocessor


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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pros and Cons for CFX, CFdesign, COMSOL Val Main CFD Forum 3 June 10, 2011 03:20
double precision cfx pat CFX 5 November 11, 2008 07:45
what's wrong about my code for 2d burgers equation morxio Main CFD Forum 3 April 27, 2007 11:38
CFX 64bit single-double precision Tom CFX 2 December 19, 2006 16:39
Double precision CFX Vanessa CFX 8 August 13, 2006 05:34


All times are GMT -4. The time now is 20:59.