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Topic: ODE stiffness problems
Replies: 3   Last Post: Apr 22, 2013 9:49 PM

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Marc

Posts: 108
Registered: 7/2/10
Re: ODE stiffness problems
Posted: Apr 22, 2013 9:49 PM
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"Torsten" wrote in message <khunje$be2$1@newscl01ah.mathworks.com>...
> "Matthew" wrote in message <khtlea$ba3$1@newscl01ah.mathworks.com>...
> > I removed the abs and signs by putting a constant low flow rate just to see if it would work. However it doesn't make any change it still stops at the same point.
>
>
> I solved your problem with another software and it worked without problems.
>
> The problem with ODE15s seems to be the abrupt change of the variables
> in the time span from t=0.95 to t=1. This is caused by the parameter Av.
>
> A remedy (I also used within my ODE integrator) is to call ODE15s seperately for
> the time span from t=0 to t=0.05, then from t=0.05 to t=0.95 and then from t=0.95 to t=1.
> Do it in a loop and set the inital conditions of the variables for a new time span to the final values of the variables from the preceeding time span.
>
> Best wishes
> Torsten.


Torsten,

Is this "other" integrator you speak of Limex? If so, I found setting ODE15s MaxOrder to 2 sometimes help get similar/almost identical results. For my system, ODE23t also got me similar results.

I also used a similar "trick" of breaking up the time span manually.

Another set of integrators I have had nice luck with is the sundials ode integrators, code and ida (for DAEs) which are free and have a nice mex setup in Matlab.

Not 100% sure why setting the MaxOrder to 2 worked but it did.

My 2 cents....



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