# Steady-State solution by time-stepping?

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 February 15, 2015, 23:03 Steady-State solution by time-stepping? #1 New Member   John Join Date: Jan 2015 Posts: 20 Rep Power: 4 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.

February 16, 2015, 08:14
#2
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,681
Rep Power: 41
Quote:
 Originally Posted by mavguy 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

February 16, 2015, 10:44
#3
New Member

John
Join Date: Jan 2015
Posts: 20
Rep Power: 4
Quote:
 Originally Posted by FMDenaro 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?

 February 16, 2015, 12:09 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,681 Rep Power: 41 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

 Tags steady-state convergence, time-stepping

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post adnanakhtar FLUENT 7 November 25, 2016 06:16 xiuying OpenFOAM Running, Solving & CFD 8 August 27, 2013 15:33 sharonyue OpenFOAM Running, Solving & CFD 6 June 10, 2013 09:34 sharonyue OpenFOAM Running, Solving & CFD 13 January 2, 2013 23:40 danny123 OpenFOAM 19 October 24, 2012 07:44

All times are GMT -4. The time now is 05:34.

 Contact Us - CFD Online - Privacy Statement - Top