# convergence error in transient simulations

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

 November 29, 2000, 10:29 convergence error in transient simulations #1 Ossi Kaario Guest   Posts: n/a Hi, How to make sure in transient simulations that e.g. momentum, turbulence, energy or scalar equations are sufficiently convergenced !? And what to do if it seems that they are not ? Thanks

 November 29, 2000, 12:28 Re: convergence error in transient simulations #2 John C. Chien Guest   Posts: n/a (1). Very good question. (2). The answer is : I don't know. (3). I think, the real solution of transient problem changes from time to time, and problem to problem. (4). If you have analytical solution of an oscillating plate, then, it is possible to know whether the simulation is converged for each time step. But ,in general, it is not easy to find out whether the solution is converged for each time step. (5). At the same time, you could change the number of inner loop iterations, or even the mesh arrangement and density, to see how the solution react to these changes. (6). In the explicit methods, reducing the time step is the only way to improve the accuracy.(converge to real transient solution) (7). In implicit methods, I guess, you could also reduce the time step to improve the convergence and accuracy of the solution. (8).On the other hands, if the mesh is fixed, and the flow field is changing rapidly, then I guess, you will always have some kind of convergence problem.(in addition to the accuracy problem.)

 November 30, 2000, 06:38 Re: convergence error in transient simulations #3 Ossi Kaario Guest   Posts: n/a Hi, if one has made grid independency as well as time step independency checks, how should one then charge for the convergence for each equation during each time step ?

 November 30, 2000, 09:43 Re: convergence error in transient simulations #4 Pat Neuman Guest   Posts: n/a Convergence is a very difficult parameter to quantify. The two parameters to watch closely in the star.info file are the PISO corrections and the CORRAUNT number. The PISO corrections are the number of inner loops that John mentioned. They should be between 3 and 5 for a stable solution. If they are big, the time step should be reduced. The CORRAUNT number is the number of cells a molecule passes through in a time step. Values from 10 to 100 are reasonable. If these two numbers are reasonable, you should be fairly confident that each iteration is converging.

 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 Tom Main CFD Forum 1 May 22, 2008 11:06 Luk Main CFD Forum 0 October 19, 2007 10:09 Target CFX 12 September 25, 2006 07:20 bob Main CFD Forum 0 October 1, 2003 03:54 Steve Aboagye FLUENT 2 February 21, 2000 11:55

All times are GMT -4. The time now is 23:43.