# Dual Time stepping

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 November 11, 2005, 01:25 Dual Time stepping #1 Aditya Guest   Posts: n/a Does Dual time stepping with larger physical time step accelarates convergence for steady state solutions than Local time stepping? Please comment Regards Aditya

 November 11, 2005, 07:17 Re: Dual Time stepping #3 Aditya Guest   Posts: n/a Hi Ganesh, I knew it did not make a reasoning but was rather a wishful thinking that DTS would help. Actually the problem is that in our code, the residues make a 2 to 3 decade fall and almost remain stagnant after that. Can DTS give me a further fall in residues. So thats not exactly a convergenc accelaration i guess. Aditya

 November 11, 2005, 15:38 Re: Dual Time stepping #4 Mani Guest   Posts: n/a Without dual time, you are choosing the global minimum as the time step everywhere in the field. Local time stepping (using dual time) simply speeds up convergence by increasing the pseudo time step based on local stability. It's very unlikely that this will give you a lower residual. In fact, the convergence with global time stepping will likely be more benign, and local time stepping would likely worsen the situation. There is, however, a small chance that local time stepping might help. If your convergence troubles are caused by a physical instability (unsteadiness), local time stepping may possibly counter-act that problem, because you are not solving in real-time anymore. It's possible, but I wouldn't count on it. Anyway, considering the advantages of local time stepping (and the relative ease of implementation), it's imperative that you implement it in any case. No need to contemplate on possible effects on residual stagnation. Just do it and find out.

 November 12, 2005, 01:38 Re: Dual Time stepping #5 ganesh Guest   Posts: n/a Dear Aditya, The residue fall for the outer loop (or physical time) is of no relevance because the problem is unsteady. The relevant residue fall is in DTS, which should theoretically be zero and practically be of the order of 1e-6. However, researchers have different opinion on this tolerance and even 1e-2 have been adopted. Generally, this happens when you are on a finer grid, and more so for a viscous problem especially, so it does not come as a big surprise. Therefore, to make sure that your code is really bug-free for an inviscid problem and otherwise , you could check out DTS on a coarser grid and see if a good convergence is obtained, after incorporating local time stepping, in case you have not. Hope this helps Regards, Ganesh

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