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

Centrifugal pump convergence issue

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 25, 2011, 00:21
Default Centrifugal pump convergence issue
  #1
Member
 
D
Join Date: Nov 2010
Posts: 50
Rep Power: 16
bhatiadinesh is on a distinguished road
Dear All,
I am doing a centrifugal pump simulation in CFX general mode ( not using the turbo module ).
My B.C's are Total Pressure at inlet ( Atmospheric ) and mass flow outlet and the flow in incompressible.
I am facing this in my simulation. Initially the residuals drop rapidly without any issues and my monitor of total pressure rise across the pump gradually converges to a value with almost a flat curve. But after some 300 iterations or so the residual and pressure rise monitor start fluctuating with the amplitude increasing as the simulation progresses. Shown below are the pics showing various residuals and the total pressure rise monitor. My pressure rise fluctions are of the order of 4-5% of the mean value. Also what I observe is that the frequency of fluctuations is exactly the same in all the monitors and residuals and this value is close to half the frquency of my rotor revolution, are these related ?
Can someone tell me what might the possible reasons be ? and HOW to GET Rid of these fluctuations

Thanks in Advance,
Dinesh
Attached Images
File Type: jpg Circ_int_MAX_res_plot.jpg (87.1 KB, 55 views)
File Type: jpg Circ_int_RMS_res_plot.jpg (83.9 KB, 46 views)
File Type: jpg Circ_int_TKE_plot.JPG (87.9 KB, 40 views)
File Type: jpg Circ_int_TOT_PRESS_RISE_plot.JPG (77.3 KB, 39 views)
bhatiadinesh is offline   Reply With Quote

Old   February 25, 2011, 06:51
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,867
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
http://www.cfd-online.com/Wiki/Ansys...gence_criteria
ghorrocks is offline   Reply With Quote

Old   February 26, 2011, 00:41
Default
  #3
Member
 
D
Join Date: Nov 2010
Posts: 50
Rep Power: 16
bhatiadinesh is on a distinguished road
Hi,
I tried playing around with the time scale to get the convergence. In the simulation shown above I had given a time scale of 1/2w = 0.0015, then after the post I changed it to 0.003(1/w) and got better convergence, then I reduced it further to 0.005 and got huge drop in the residuals.
The CFX documentation says 1/w is a good estimate for the physical time scale. My doubt is what are the criteria based on which we should decide the physical time scale and most of all WHAT ARE THE LIMITS OF PHYSICAL TIME SCALE THAT WE SHOULD NOT CROSS ???

Thanks in Advance,
Dinesh
Attached Images
File Type: jpg Pressure-Mass_residuals_time_scale_var.jpg (88.1 KB, 62 views)
bhatiadinesh is offline   Reply With Quote

Old   February 26, 2011, 16:38
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,867
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
There are no general limits on timescale. For a steady state flow the time scale you use should be irrelevant - the flow is the same in 1 nanosecond and 1 year.

But practically, larger timesteps mean that the turbulence modelling works better. If the timesteps are in the range of the turbulence timescale then the turbulence modelling does not work well and you get convergence problems. Bigger time steps fix this problem - as you found in your case.
ghorrocks is offline   Reply With Quote

Old   February 28, 2011, 00:22
Default
  #5
Member
 
D
Join Date: Nov 2010
Posts: 50
Rep Power: 16
bhatiadinesh is on a distinguished road
Thanks for the clarification.
bhatiadinesh is offline   Reply With Quote

Old   April 3, 2020, 07:19
Default
  #6
New Member
 
Dileep S
Join Date: Nov 2017
Posts: 7
Rep Power: 9
dileeps is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
There are no general limits on timescale. For a steady state flow the time scale you use should be irrelevant - the flow is the same in 1 nanosecond and 1 year.

But practically, larger timesteps mean that the turbulence modelling works better. If the timesteps are in the range of the turbulence timescale then the turbulence modelling does not work well and you get convergence problems. Bigger time steps fix this problem - as you found in your case.
But sir, what is the relevance of physical timescale in steady analysis?
dileeps is offline   Reply With Quote

Old   April 3, 2020, 07:57
Default
  #7
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,867
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
The physical time scale is only used as a way of advancing towards convergence. If you take big time steps you progress faster but risk divergence, if you take small time steps you progress slower but are more numerically stable. Once you have a converged solution the time step makes no difference as all the time derivatives are zero.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
release of the ERCOFTAC centrifugal pump - Fourth OpenFOAM Workshop olivier OpenFOAM 8 October 29, 2018 08:49
Problem with centrifugal pump calculation Michiel Main CFD Forum 2 June 17, 2012 22:32
Centrifugal Pump and Turbulence Model Michiel CFX 12 January 25, 2010 04:20
centrifugal pump and centrifugal fan Mangesh Main CFD Forum 3 January 3, 2006 12:21
stator-rotor interaction in the centrifugal pump G.H.Lee Main CFD Forum 4 May 25, 1999 08:33


All times are GMT -4. The time now is 00:51.