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

Mass accumulation in transient Simulation

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 23, 2010, 07:52
Default Mass accumulation in transient Simulation
  #1
New Member
 
Join Date: Aug 2009
Location: Switzerland
Posts: 11
Rep Power: 7
Arevik is on a distinguished road
I work on a transient simulation in CFX. It is a Taylor-Couette flow in a narrow gap between two concentric cylinders, with the inner cylinder rotating. I get the expected flow phenomena (wavy taylor vortices).
However, the total pressure at the monitoring points constantly rises. The domain imbalance for mass ist ok (0.002%) also the other imbalances. Residuals after 4 iterations per timestep are ok (Max.Residual 10e-4..10-5), only H-Energy doesn't converge (Max. Res.= 10e-2). As boundary conditions at the cylinder ends I set first translational periodicity, I also tried with symmetry or free slip walls. There is no inlet or outlet. It is a laminar flow, however, I also tried with SST turbulence once.
Whatever I tried out, total pressure /mass was rising. Can anyone tell me what is wrong?
Arevik is offline   Reply With Quote

Old   August 24, 2010, 05:35
Default
  #2
New Member
 
CCTech
Join Date: Jul 2010
Posts: 20
Rep Power: 7
CCTech_Pune is on a distinguished road
Hi

What is the time-step you are using. Generally you should choose 5/RPM ass the timestep. And also check if you have residuals in one or two cells only. Have you checked grid dependency?


Thanks
CCTech Pune
CCTech_Pune is offline   Reply With Quote

Old   August 24, 2010, 06:49
Default
  #3
New Member
 
Join Date: Aug 2009
Location: Switzerland
Posts: 11
Rep Power: 7
Arevik is on a distinguished road
Hi CCTech Pune

Thanks for you ideas.
The grid should be ok, I did a study about this before starting the transient simulation.
My timesteps were much smaller, I used Courant-Nr ~= 1. An increase of timestep by a factor of 5 didn't change anything to my problem. I will set up a new simulation with ~5 dt/rpm and see what happens. Why do you say 5/rpm? Is there a special theory about that?
I think, this will solve the problem. In the meanwhile I finished my simulation of turbulent Taylor vortex flow. there is still some mass accumulation, but total pressure in monitor points didn't rise. There I used much higher timesteps (~1.5/rpm).
Arevik is offline   Reply With Quote

Old   August 26, 2010, 09:32
Default
  #4
New Member
 
Join Date: Aug 2009
Location: Switzerland
Posts: 11
Rep Power: 7
Arevik is on a distinguished road
Unfortunately, increasing the timestep didn't help. Total Pressure / mass still rose (despite the closed system). When I wanted to increase it even more, the simulation broke up with errors only few timesteps after the adaption.

ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| c_fpx_handler: Floating point exception: Overflow |

+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| Stopped in routine c_fpx_handler: |


Are there any other suggestions, what I could try?
Arevik is offline   Reply With Quote

Old   August 26, 2010, 09:52
Default
  #5
New Member
 
CCTech
Join Date: Jul 2010
Posts: 20
Rep Power: 7
CCTech_Pune is on a distinguished road
Hi

Can you please send me the complete out file to consultancy@cctech.co.in

I could not figure out what could be reason for crash.

Thanks
CCTech Pune
CCTech_Pune 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
Time step in transient simulation shib FLUENT 0 June 17, 2010 13:07
transient simulation of a rotating rectangle icesniffer CFX 1 August 8, 2009 07:25
Transient Angle of Attack Simulation Not Displaying in Post Josh CFX 35 August 5, 2009 09:16
mass accumulation Daria CD-adapco 0 April 26, 2005 05:15
Transient using target mass flow Z. T. FLUENT 0 May 2, 2003 04:54


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