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## solve stiff ODE with femlab 3.5

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I have made the set of stiff ordinary differential equations system of densities of species in form :dn/dt = source - n*perte, for this my question, for do this,i use the interface "Global equations" , but with step adaptatif wich change at each novel solution.

how can specify this adaptatif step in box of time-dependant of software Femlab 3.5 ???

hoping for a response full with your scientific enthusiasm.

1 Reply Last Post Mar 14, 2010, 5:22 p.m. EDT

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Posted: 1 decade ago
Hello Isabelle

Sorry but I have some problems to follow your explanations:

1) are you in the PDE application mode coefficients "c" or global "g" mode or in an other module ?
2) you add a "Gobal Equation" of the type: dn/dt = source - n*loss, where n is probably the species density, but is this one or several equations (dn1/dt = source1 - n*loss1 & dn2/dt = source2 - n*loss2 ... ) ?
3) as dn/dt is a time derivative, I understand that you are in the transient solver mode, so you must have defined a time of the type t = range(start, step, end), or perhaps a t = log10(range(start_power, step_power, end_power)) if you need a logaritmic stepping
4) I do not understand your adaptive step issue, do you mean that "source" and "loss" are dependent on the time too, and you have a non-linear problem ?, or is it the time stepping you would want to have adaptive ?

I understand that it is not easy to write all this in English, but it is a good exercice, for all of us ;),
in the worst case write a couple of sentences in French too, there will always be someone to translate. But the idea of keeping it in English is to give an opportunity for the majority to follow

Good luck
Ivar

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