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vnovikov1-Visitor
1-Visitor
April 17, 2017
Question

Problem with ODEsolve

  • April 17, 2017
  • 2 replies
  • 6375 views

Hello everyone!

I have a question, how can I solve a system of equation, which is consists of three equation (two of them are Differential and other one is not) using ODEsolve or there is some other way?


So I had a system of two differential equation and everything worked just fine, but then I need to add one more equation and this equation is not differential. So I don't know are the ODESOLVE able to solve this kind of systems or not. It keeps telling me, that I have too few initial conditions. File 9 is a file where only 2 equation are solved.


Thanks for your help

2 replies

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-MFra-21-Topaz II
21-Topaz II
April 18, 2017

Hi Vladimir Novikov,

first of all you should do the following changes:

vnovikov.jpg

W
Werner_E25-Diamond I
25-Diamond I
April 18, 2017

.) The whole sheet is unitlesse - otherwise I guess we would experience severe problems with odesolve in Mathcad 15 (thats better in Prime). So Vladimir decided to write the supposed units next to the values in a textbox or separate math region for documentation and reference purpose only. This has no effext on the calculations.

I guess Valery would define every unit as being :=1 and then attached them in the unit field.

.) the equations with the "j" also are for refernce purpose only and have no effect on the calculations.

At the moment I haven't digged far enough in the calculations to be able to provide a solution but I guess the problem is, as Vladimir already guessed, that one of the equations is not an ODE.

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    -MFra-21-Topaz II
    21-Topaz II
    April 18, 2017

    I would say that not all the units are written in text boxes, on the contrary, many are written as if they were variables. The first (° C) for sure.

    W
    Werner_E25-Diamond I
    25-Diamond I
    April 18, 2017

    Attach is an absolutely unsatisfactory solution by turning phik into a function in an extra solve block.

    I had to limit tau1 because of a strange error if its too large and the phi solve block takes quite long to calculate for higher values of tau.

    Maybe its a tolerance problem because of too hight or too low values - when I set TOL to a lower value liek 10^-9 the odesolveblock fails for lower values of tau1, too.

    Hope somebody has a better idea to tackle with that system.

    V
    vnovikov1-VisitorAuthor
    1-Visitor
    April 18, 2017

    Снимок1.JPG

    your solution doesn't seems to work fine for me(((

    W
    Werner_E25-Diamond I
    25-Diamond I
    April 19, 2017

    Vladimir Novikov wrote:

    your solution doesn't seems to work fine for me(((

    Yes, The solution sure is unsatisfactory. I guess that you had changed the value of the system variable TOL from the default 10^-3 to something smaller. Try TOL:=10^-3 at the top of your sheet.

    You may also set tau1 to a value lower than the 250 which worked for me.

    But as your original sheet demanded to solve up to tau1=2000 (or was it even 20000?) I guess this would not be a satisfactory option.

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