Discussion Forum

March 16, 2010 7:14am UTC in response to isabelle kafrouni

Re: pb of variable step for solve stiff ODE

Hi Isabelle

Your translation is OK ;)

First my references when I look into these subject is to go back to a couple of books I like, as they are close to COMSOL math notation:

"Introduction to computation and modeling for Differential Equations", by L. Edsberg, Wiley, 2008

And as COMSOL cookbook well written with math references is the:

"Multiphysics Modelling with Finite Element Methods" by B.J. Zimmerman, World-Scientific, 2006

Further, if I understand your well, your issue is in the transient solver settings, it has, at least in my 3.5a, many options, mostly I let COMSOL choose, and even if I say

t=range(min,step,max)

I understand that the solver decides itself upon the steepness of the gradients in the model to select single or multiple intermediate points, but these are not normally stored in my solution vector, unless you define "Time steps from solver", these options are selected in the time stepping sub-tab. I have noticed that solver step mode "free" might skip short duration features, then I impose rather "strict" and ensure my time steps cover well these features, or I use intermediate, then it set at least one more point inbetween my defined time steps. This is rather well described in the user guide (guide.pdf) in the solver section "Time-Dependent" stepping, in the V3.5a

As for me in V3.5a the adaptatif is there by default, but it might be different with femlab if you have an older version

Good luck
Ivar

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