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

CFX-solver crash with function defined material properties from data sheets

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 26, 2016, 06:15
Default CFX-solver crash with function defined material properties from data sheets
  #1
New Member
 
Join Date: Sep 2015
Posts: 6
Rep Power: 3
oscol227 is on a distinguished road
Hi!

I get the Floating point exception: Zero divide error in the CFX-solver.

I have implemented functions from data-sheets that describes material data (density, specific heat capacity, dynamic viscosity and thermal conductivity) based on the pressure an temperature which is then used in the materials-tab for a new defined material. However when i start the solver it stucks at the first iteration loop for about 5 minutes and then crashes.

I have tried different meshes, inital conditions, different number of maximum points in the materials-tab and extrapolation of temp and pressure. Nothing helps!

When i change the material to ideal gas air it works just fine so it must be the new material i have described that is causing the problem. If I evaluate the functions and plot for different ranges i cannot see anything that seems wrong.

I hope someone here can help me
Regards
Oscar
oscol227 is offline   Reply With Quote

Old   February 26, 2016, 09:42
Default
  #2
Senior Member
 
Join Date: Jun 2009
Posts: 573
Rep Power: 13
Opaque is on a distinguished road
For a start, try using one of the "data sheets" at a time to isolate which is giving you trouble.

Not sure what equation of state you are trying to model, but you should be very aware that inputs for density and specific heat capacity are not independent and must satisfy specific thermodynamic relationships (see ANSYS CFX documentation for details); otherwise, the material database will not be correct if it can be computed at all.

Hope the above helps,
Opaque is offline   Reply With Quote

Old   February 26, 2016, 11:43
Default
  #3
New Member
 
Join Date: Sep 2015
Posts: 6
Rep Power: 3
oscol227 is on a distinguished road
Thanks for the answer!

Have located the density to be the problem. If I do not use the data for the density the solver does not crash.

If I plot the data I have for density for different pressures and temperatures I do however not see anything wrong with the plot. It looks smooth and fine, it does not contain so much data points for certain pressures and temperatures which maybe the problem. Does anyone know who CFX interpolates the data? Can the few number of data points at a certain range be a problem? Note that the density only have minor changes at those locations.
oscol227 is offline   Reply With Quote

Old   February 27, 2016, 04:38
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
CFX uses a table to interpolate the physical properties. You can control the settings of this interpolation table in the materials tab.

Be aware that in early stages of the simulation you can have weird values of flow parameters which lead to wild physical properties. If the physical properties model goes unstable during this time the simulation can crash, even though the final converged result is fine. If this is causing problems then you need to use a better initial condition or "bend" your material model to eliminate the instability during convergence.
ghorrocks is offline   Reply With Quote

Old   February 27, 2016, 13:54
Default
  #5
New Member
 
Join Date: Sep 2015
Posts: 6
Rep Power: 3
oscol227 is on a distinguished road
Okey, the strange thing is that I start from initial values that are from an earlier solution which are close to what i expect the results to look like. The earlier solution is however obtain when using a function for the density instead of the data sheet, but it is not as accurate as the data sheet. I have also tried to modified the interpolation settings in the materials-tab but changing ranges and/or number of points does not help.
oscol227 is offline   Reply With Quote

Old   February 28, 2016, 06:15
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
I recommend you do a simulation where you write out a res file on the time step before it crashes. You probably have the density going bezerk somewhere, so you need to find out where and how.
ghorrocks is offline   Reply With Quote

Old   February 29, 2016, 03:06
Default
  #7
New Member
 
Join Date: Sep 2015
Posts: 6
Rep Power: 3
oscol227 is on a distinguished road
Forgot to mentioned it is a steady state simulation I am doing.

The problem is that it crashes for the first iteration so there is no results to investigate and the inital start is not containing any wierd values, as mentioned earlier it is started from an earlier solution case that is very close to the expected results.
oscol227 is offline   Reply With Quote

Old   February 29, 2016, 05:27
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,832
Rep Power: 100
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
In that case the hints listed in this FAQ are applicable: http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F

And be extra-especially specially careful that the density function you have defined is well behaved in the range of densities you expect to see.
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
OpenFOAM static build on Cray XT5 asaijo OpenFOAM Installation 9 April 6, 2011 12:21
Version 15 on Mac OS X gschaider OpenFOAM Installation 120 December 2, 2009 11:23
error using combination of step function xujjun CFX 1 January 15, 2008 17:46
Two-Phase Buoyant Flow Issue Miguel Baritto CFX 4 August 31, 2006 12:02
CFX 5.5 Roued CFX 1 October 2, 2001 16:49


All times are GMT -4. The time now is 08:16.