From help-request at octave dot org Fri Jan  7 06:14:10 2005
Subject: Re: ODE system solving
From: Geraint Paul Bevan <g dot bevan at eng dot gla dot ac dot uk>
To: "homer_jay at katamail dot com" <homer_jay@katamail.com>
CC: help at octave dot org
Date: Fri, 07 Jan 2005 12:04:57 +0000

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homer_jay at katamail dot com wrote:
| hi everybody
|
| i need to solve a differential equation system, 1 first order equation
and 1 second order equation.
|
| they are in the form
|
| w"=f(z,w,w',T,T')
| T'=f(z,w,T)
|
| w,w',w",T,T' are function of z.
| i have 3 initial condition w0,w'0, T0.
| in the rfirst equation i make the substitution
|
| w --> y1
| w'--> y2, so w" --> y2'
| T --> y3, so T' --> y3'
|
| the system became a 3 first order equation system, is it right?

Yes.

| this is the m-file section:
|
| SysO(1)=y(2);
|
|
SysO(2)=y(1)*y(2)/(3*Nup)-(g/(3*Nup))+(2*tau/(3*Nup*rho*Rz))+((y(2)^2)/y(1)*3*Nup)-(1/Nup)*dNup*y(2)*(-2*hc*(y(3)-Ta)/(Rz*rho*Cp*y(1)));
|
| SysO(3)=-2*hc*(y(3)-Ta)/(Rz*rho*Cp*y(1));
|
|
| as you can see, the second equation (SysO(2)) is dependent of T': in
fact the last term of the second equation is the same of the third equation.
| the question is: is this m-file correct?

As long as all the rates are expressed in terms of states only (which
they are), this should not cause any problems for ODE solvers.

The equations look fine to me.

| of course in the m-file i have all the other equations (Rz, Nup....).
they are function of w and T.
|
| in solving the system i receive a "tolerance not met" error message.
can you help me?!?!?

The problem is almost certainly numerical so it isn't really possible to
give a definite solution unless you provide the complete m-file and tell
us how you are trying to solve the equations. Are you using lsode? With
what options (if any)?

You could try relaxing the tolerance using lsode_options. Alternatively,
it often helps if you tighten the tolerance (slightly counter-intuitive,
perhaps).


- --
Geraint Bevan
http://www.mech.gla.ac.uk/~gbevan

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