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

Timestep selection

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 20, 2005, 07:21
Default Timestep selection
  #1
Jindra Kosprdova
Guest
 
Posts: n/a
Hi everybody,

We are solving a flow through an impeller of centrifugal compressor.

We use STEADY STATE as a SIMULATION TYPE.

The problem was solved for 3 different PHYSICAL TIMESTEP (t1=2E-4,t2=3E-4,t3=2.23E-2).

The history of MAX RESIDUUM was bouncy for the first PHYSICAL TIMESTEP. For the last one the history was monotonic.

When checking solutions we found that all three solutions are the same.

The question is what is the influence of PHYSICAL TIMESTEP on the results of solutions?

What is determinative for the first estimation of the PHYSICAL TIMESTEP?

Thank you a lot,

Jindra and Petr.

  Reply With Quote

Old   April 20, 2005, 10:32
Default Re: Timestep selection
  #2
Robin
Guest
 
Posts: n/a
Hi Jindra and Petr,

This is a good observation. The Physical Timestep has no influence on the final solution, only the convergence. Even though you are solving for a steady state, CFX keeps the transient term in the equations to stabilize the equations. This is similar to using an underrelaxation factor, but has a more physical behavior.

Since the solver is fully implicit, there are no timestep restrictions. A larger timestep will generally give you faster convergence, but may be a little unstable. A good tech tip on how to use the timestep is available from the CFX Community Site at http://www-waterloo.ansys.com/cfxcom...onvergence.htm.

Best regards, Robin
  Reply With Quote

Old   April 20, 2005, 10:36
Default Re: Timestep selection
  #3
Ian
Guest
 
Posts: n/a
Assuming it's subsonic the timestep should be anywhere between 0.1/omega to 10/omega, where omega is the angular velocity in rad/s. On the lower end 0.1 should only be used at startup for unstable cases but be increased to 1 for proper convergence.

If the timestep is too small the solution might still have low frequency error and convergence will be slow. Increasing the timstep speeds up convergence and removes low frequency errors to a point where the solver will become unstable. I've found an optimal timestep for subsonic centrifugal compressors to be 2/omega.
  Reply With Quote

Old   April 20, 2005, 10:38
Default Re: Timestep selection
  #4
Ian
Guest
 
Posts: n/a
Hey Robin I thought it was against company policy for you to post to CFD Online from work?
  Reply With Quote

Old   April 20, 2005, 17:44
Default Re: Timestep selection
  #5
John
Guest
 
Posts: n/a
I guess answering question for cfx users is Robin's job.

We need Robin here. His/Her answers are always simple but clear and helpful.

John
  Reply With Quote

Old   April 21, 2005, 03:36
Default Re: Timestep selection
  #6
Jindra a Petr
Guest
 
Posts: n/a
Hi,

thanks a lot for your answers. They help.

Jindra and Petr.
  Reply With Quote

Old   April 25, 2005, 15:11
Default Re: Timestep selection
  #7
Robin
Guest
 
Posts: n/a
Actually, it's the other way around. The short wavelenght (relative to the mesh) error is always reduced well. The timestep will effect how quickly large wavelength errors are reduced. Large wavelength errors are essentially the transients.

For a single component, values between .1 and 10/omega work well. We have found that for multi-component systems, a larger timestep is needed, on the order of 10 to 100/omega. A small timestep may actually cause the solver to diverge.

Regards, Robin
  Reply With Quote

Old   April 26, 2005, 23:26
Default Re: Timestep selection
  #8
Bak_Flow
Guest
 
Posts: n/a
Robin,

I think that is what he said ..... large time step reduces the large wavelength errors best.

In a time advance under-relaxation to SS method ALL wavelengths are transients. IF the objective is a steady state simulation there is no interest in resolving the high frequency (short wavelength transients) that is why large time steps work fast....would resolving fine features cause divergence...not sure...why would it??

However all non-linear problems have some information that is lagged. And not all inter-equation coupling is active. IF a lot of information (eddy viscosity always, density for compressible flows, mass fluxes at GGI's, etc) then you are limited by the time step you can run.

Mesh quality is also an issue and the discretization scheme.....Hi Res adds another level of non-linearity as well.

Therefore sometimes an upper limit exists....I think that is what Ian was getting at!!!

What do you mean by multi-component....multi-stage?

Be careful to work out the details. ;-)

Bak_Flow
  Reply With Quote

Old   April 27, 2005, 09:57
Default Re: Timestep selection
  #9
Robin
Guest
 
Posts: n/a
Hi Bak_Flow,

Sorry, you're right, he did say low frequency errors. Regarding your question "would resolving fine features cause divergence...not sure...why would it??". If your initial guess is far off, a large timestep can smooth the errors out quickly, effectively removing the large peaks and troughs. If you use too small a timestep, these very large errors may cause the solver to diverge locally.

A practical upper limit may exist for a given problem, above which the solver may diverge or non-linear terms, such as the beta term in the high res scheme, may cause sharp oscillations. But formally, there is no limit since the formulation is fully implicit. There are many cases which have no upper limit and you can increase you timestep infinitely, essentially removing the transient terms from the equations.

By multi-component I mean the same as multi-stage, i.e. several rotating parts, like an entire compressor.

-Robin
  Reply With Quote

Old   April 28, 2005, 07:41
Default Re: Timestep selection
  #10
Jindra
Guest
 
Posts: n/a
Hi Robin, we have no idea what wavelength errors mean. Could you explain it to us. Thanks Jindra a Petr
  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
Run time Selection Mechanism - Some help required to understand jaswi OpenFOAM Programming & Development 3 October 29, 2015 14:42
Restart 2-way FSI with different timestep? Lance CFX 10 April 17, 2013 01:37
How to interrupt unsteady calculation and carry on without timestep increase? aleisia FLUENT 1 March 19, 2011 00:02
Adequate timestep selection for multidomain problem gerardosrez CFX 6 November 28, 2010 18:50
timestep selection Florian CFX 15 December 9, 2003 19:26


All times are GMT -4. The time now is 07:43.