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cfdbeginner February 28, 2006 11:42

local time stepping
 
Hello,

I would like to ask something about local time stepping and I hope somebody can help me.

Would you say that this method is applied to a solver to increase stability - so a steady problem can be treated as "unsteady" with a time step which is dependent on the CFL number?

Thanks.

CFD_ACE February 28, 2006 15:03

Re: local time stepping
 
No. Local time stepping is used as a solution acceleration method - it can occasionally lead to robustness problems as the solution is no longer entirely 'physical', except in the limit t -> infinity (i.e. the 'steady' solution).

Aditya March 1, 2006 00:04

Re: local time stepping
 
you actually lose the temporal accuracy if you use the local time stepping. It is mainly used when we have a grid clustered on one side. So for large cells the CFL does not demand the small time step that a small cell would need. Hence local time stepping is used to accelarate the steady state convergence. It actually means that the different cells move in different time regimes if at all it makes any sense !!!. No physics here. A considerable lot of people are not comfortable with the local time stepping due to this aspect. You can use Dual time stepping to regain the temporal accuracy and use local time stepping for unsteady problems.


cfdbeginner March 1, 2006 02:06

Re: local time stepping
 
Sorry, but now I am completely mixed up.

Could you just try to give me another short explanation (or a free reference)?

Mohammad March 3, 2006 04:26

Re: local time stepping
 
Hello..

Whatever the informations which we are applying interms boundary conditions(Physics of the problem)... cant move more than one grid space per unit time. So we have to give optimum timestep sothat the information shouldnt jump more than one grid space per unit time...

This is the funda of CFL number... Next Local and Global timestep..

Local timestep:: In each cell the available information moves with local velocity.. so Local timestep= (Size of the cell/ Local speed)

Global timestep is calculated based on max speed of information otherwise min timestep required :: Global timestep= Min. all( size of cell/ corres.infor speed)

I hope it will help

Regards Mohammed



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