# how to solve a steady problem explicitly

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 September 20, 2008, 17:30 how to solve a steady problem explicitly #1 meda Guest   Posts: n/a Dear all, I am a little bit confused how I can solve a steady-state problem EXPLICITLY. do I have to write code for an unsteady situation and then give a long time? thanks. Meda

 September 21, 2008, 05:39 Re: how to solve a steady problem explicitly #2 Paolo Lampitella Guest   Posts: n/a Yes, until the time derivatives will be 0

 September 21, 2008, 10:55 Re: how to solve a steady problem explicitly #3 ztdep Guest   Posts: n/a yes, you need to specify a time step, and it must satisfy the stability condition, then let it run until the difference between two consecutive time step is very small.

 September 22, 2008, 06:30 Re: how to solve a steady problem explicitly #4 Bart Guest   Posts: n/a In a steady state solution you can use local time stepping. Each cell progresses at it's own CFD criterium. As the solution is steady state the final converged outcome does not depend on the (local) timestep. This can significantly decrease your CPU time. In normal unsteady problems (explicit) your timestep is determined by the smallest cell with the largest velocity (via CFL criterium giving you a small overall timestep). Bart

 September 22, 2008, 14:36 Re: how to solve a steady problem explicitly #5 meda Guest   Posts: n/a hi Bart.how can I define the local time stepping for each cell? doesnt it take more time to compute the criteria for each cell than to define a genaral criteria?

 September 23, 2008, 03:27 Re: how to solve a steady problem explicitly #6 Bart Guest   Posts: n/a I general criteria also requires you calculate a CFL timestep for every cell in between timesteps. I mean, how do you determine your timestep now and would it really make a big difference? However if your velocity scales and cell sizes in your domain vary only slightly then it may not make sense to use local timestepping.

 September 25, 2008, 06:53 Re: how to solve a steady problem explicitly #7 bkjohn Guest   Posts: n/a You can also employ local-preconditioning, which alters PDE such that all waves travel at an equal speed, thereby maximizing the minimum local time step (local time step is limited by the slowest wave speed of the PDE system). See this van Leer's paper --> Download at http://citeseerx.ist.psu.edu/viewdoc...10.1.1.43.8861 Good luck, bkjohn

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