
Re: How does Flow Simulation judge convergence?
Bill McEachern Mar 8, 2011 12:22 PM (in response to Dan Cook)FS runs the time averaged formulation of the NS equations which means it essentially does a transient analysis (time explicit) solution so there is no real "converging" going on. Rather, for a (quasi) steady state solution (they don't call it fluid dynamics for nothing) the program runs till the (typically) impulsive start up has died out and it gets more or less "steady" (not varying much in time). So I think your question is how does it know when this has happended? Well, if you do nothing it has preset termination criterea for given problem types  like 4 "travels" I think is the default. A travel is the number of iterations it takes for a distrubance to fully propagate across the domain. Further, you can add goals and the program can use those and will stop when the goals you have assign stop changing (the criteria is determined by the program or you can change it to whatever you like, though I would not advise this right out of the gate as a newbie). When you use goals (the recommended approach) you will be able to see the "convergence" trajectories (really they are time trajectories). When they flatten out and remain essentially invariant for some number of iterations (you can be the judge or you can let the program decide) then you can consider the particular result "achieved" or "converged". Given that FS is a "cart" code (cartesian) is suffers much reduced numerical diffusion issues compared to many body fitted codes (such as Fluent) and will typically "converge" for an incompressible flow problem in the order of low hundreds of iterations where as a similar problem in Fluent may require several thousand at very much higher cell counts  as a fluent guy once remarked "How does FS get the same asnwer as with 200,000 cells and and 2
hours as Fluent with 2 million cells and 18 hours?" Then again FS doesn not have all the physics that Fluent supports but for what it does do it does a very fine job at it most of the time, at least in my experience.

Re: How does Flow Simulation judge convergence?
Shahrul 88 May 15, 2012 10:28 PM (in response to Bill McEachern)Hi All,
I'm using solidworks to do convection heat transfer in the microcomputer. The problem is my understanding on the number of iterations.
2 simulations are done through 2 different setting on the iteration:
1) goals convergence (result obtained as in Pic 1 attached  stop at iteration 56)
2) maximum iterations = 42 (result obtained as in Pic 2 attached  stop at iteration 42)
How do I explain about the differences of data ( in this case temperature) at iteration 20 and iteration 30??
For goals convergence; what is mean by goal convergence?? is it equilibrium state/ condition??
This number of iterations are not explained about time, am I right?
Thanks
