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

Steady-State solution by time-stepping?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 15, 2015, 23:03
Default Steady-State solution by time-stepping?
  #1
New Member
 
John
Join Date: Jan 2015
Posts: 18
Rep Power: 2
mavguy is on a distinguished road
This might be a silly question, but I wrote a CFD code (compressible, unstructured mixed element in 2D and 3D) that I am having trouble converging in 3D for relatively low Mach numbers (~0.1).

I am aware that the N-S and Euler equations become stiff at low Mach numbers. How are these cases typically dealt with? Are solutions to complex 3D problems typically time stepped to a steady-state solution? This will certainly make the linear system easier to solve, but seems rather inefficient. Is seems that straight-up Newton-Krylov would be better.

I'm using GMRES to solve the linear system and have put in a lot of work to get a good preconditioner, to the point that I don't think I can do any better on the preconditioning side of things.

Any advice would be great!

Last edited by mavguy; February 16, 2015 at 01:25.
mavguy is offline   Reply With Quote

Old   February 16, 2015, 08:14
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,662
Rep Power: 23
FMDenaro will become famous soon enough
Quote:
Originally Posted by mavguy View Post
This might be a silly question, but I wrote a CFD code (compressible, unstructured mixed element in 2D and 3D) that I am having trouble converging in 3D for relatively low Mach numbers (~0.1).

I am aware that the N-S and Euler equations become stiff at low Mach numbers. How are these cases typically dealt with? Are solutions to complex 3D problems typically time stepped to a steady-state solution? This will certainly make the linear system easier to solve, but seems rather inefficient. Is seems that straight-up Newton-Krylov would be better.

I'm using GMRES to solve the linear system and have put in a lot of work to get a good preconditioner, to the point that I don't think I can do any better on the preconditioning side of things.

Any advice would be great!
at low Mach you can use pre-conditioning techniques or, if accettaple, using the incompressible model. If the steady state really exists, you can reach it by using a time-integration method untile the time derivatives become smaller (in some norm) than some tolerance
FMDenaro is offline   Reply With Quote

Old   February 16, 2015, 10:44
Default
  #3
New Member
 
John
Join Date: Jan 2015
Posts: 18
Rep Power: 2
mavguy is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
If the steady state really exists, you can reach it by using a time-integration method untile the time derivatives become smaller (in some norm) than some tolerance
Shouldn't steady-state be reachable without time-stepping? In 2D, I use Newton's method and reach steady-state just fine. Is the initial guess for Newton typically just too far away from the solution in 3D for Newton to converge?
mavguy is offline   Reply With Quote

Old   February 16, 2015, 12:09
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,662
Rep Power: 23
FMDenaro will become famous soon enough
Again, you can directly solve the steady-state solution. Sometimes, there is not a numerical solution to the discrete system and that can be a signal that the flow is unsteady.
I suppose that for converging, the Newton method require you start from an initial guess not very far from the solution
FMDenaro is offline   Reply With Quote

Reply

Tags
steady-state convergence, time-stepping

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
Steady state solution as an initial condition for a transient problem adnanakhtar FLUENT 5 January 19, 2015 14:26
How to write k and epsilon before the abnormal end xiuying OpenFOAM Running, Solving & CFD 8 August 27, 2013 15:33
mixerVesselAMI2D's mass is not balancing sharonyue OpenFOAM Running, Solving & CFD 6 June 10, 2013 09:34
same geometry,structured and unstructured mesh,different behaviour. sharonyue OpenFOAM Running, Solving & CFD 13 January 2, 2013 23:40
AMI speed performance danny123 OpenFOAM 19 October 24, 2012 07:44


All times are GMT -4. The time now is 14:47.