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

steady unsteady

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 27, 2004, 16:02
Default steady unsteady
  #1
subhra
Guest
 
Posts: n/a
Hi, I am a graduate student..Can somebody help me find an answer to following two doubts I have:

1) Do Problems that give rise to physical instabilities will always give rise to numerical instability?

2) What can be the expected consequence of trying to solve numerically the following laminar problem with a steady-state solver?

Flow in a 2-D channel with a cylinder installed at its center(Re=200, say)

An actual experiment with shows a periodic shedding of vortices, the solution never gets time-independent..
  Reply With Quote

Old   March 27, 2004, 18:25
Default Re: steady unsteady
  #2
Jim Park
Guest
 
Posts: n/a
1. No. Damping (numerical viscosity) can overpower physical instability.

2. Depends on the amount of numerical viscosity. The examples I've seen (hardly all of course) show a steady-state wake (separation, recirculation) behind the obstruction.
  Reply With Quote

Old   March 27, 2004, 19:22
Default Re: steady unsteady
  #3
subhra
Guest
 
Posts: n/a
Thank you very much. Should I conclude :

(1) With high numerical viscosity what I get from the crazy idea of steady-statesolving is soln. corresponding to much lower reynold's no. than 200 (say 20).

(2) With a low numerical viscosity (say by maintaining second or higher order everywhere ), no steady numerical solution should be possible. (divergence!)
  Reply With Quote

Old   March 28, 2004, 16:56
Default Re: steady unsteady
  #4
Jim Park
Guest
 
Posts: n/a
1. Yes, that's what I've seen. A high level of damping will force a 'steady state' solution that would have at least some of the features of a steady flow at a much lower Reynolds number.

2. I don't know what would happen in this case. And I sure would be skeptical of any solution obtained.
  Reply With Quote

Old   March 29, 2004, 04:32
Default Re: steady unsteady
  #5
P. Birken
Guest
 
Posts: n/a
1) What do you mean with physical instability? A blowup? The classical Runge Kutta method can solve the equation x'=x perfectly well. What makes numerical computations very difficult is, if changes in the initial data have a huge impact on the solution. Then you have to be very careful, but it is still possible to compute good solutions.

2) I don't know
  Reply With Quote

Old   March 29, 2004, 04:47
Default Re: steady unsteady
  #6
Tom
Guest
 
Posts: n/a
1. Such a large drop in the Reynolds number would imply that your solution is not converged. In a viscous flow simulation the numerical viscosity should be smaller than the real viscosity.

2. There are two possible outcomes depending on your steady state solver: (i) you reach a steady state - which is a solution to the problem but may not be physically realizable or (ii) the iteration scheme oscillates and so does not converge.
  Reply With Quote

Old   March 29, 2004, 05:58
Default Re: steady unsteady
  #7
Anton Lyaskin
Guest
 
Posts: n/a
What you get from steady-state solving of such problem - depends on the solver. I've made such an experiment myself once. The result was pretty weird - I've got a "frozen" Karman vortex street which was "switching" half a period per iteration
  Reply With Quote

Old   March 29, 2004, 13:12
Default Re: steady unsteady
  #8
subhra
Guest
 
Posts: n/a
The 'switching' really interesting and makes some sense too...
  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
Unsteady DPM with steady solver elobb FLUENT 3 May 31, 2015 04:11
steady to unsteady wlt_1985 FLUENT 6 December 4, 2010 17:17
Steady needs unsteady. nico Main CFD Forum 0 September 21, 2007 04:50
Turbulent: Steady or Unsteady: confusion prem FLUENT 0 March 30, 2006 10:40
steady or unsteady? (in dpm) winnie FLUENT 1 April 28, 2003 11:30


All times are GMT -4. The time now is 09:43.