Want to set dependent variable in system of ODEs being solved by ode45 to zero

[clustro]

Hi Octave forum,

I have a problem with a system of differential equations I am trying to
solve.

It is a system of nonlinear equations, discretized over length, and solved
forwards in time.

The problem is, when another variable crosses a certain value, I want the
dependent variable of a certain ODE to become 0 immediately and forever.

Unfortunately, I can't get ode45 in Octave to do it; it appears that
internally in the ODE45 code, the previous values for the integrated
variables are being remembered, so putting the zero-condition in my ODE
doesn't do anything at all.

Observe code:

function [t,Cp] = pmin

r = [0:0.5:10]';
Cp0 = 10*exp(-0.1*r);
kI = 0.1;
deltap = 0.1;
kr = 0.1;
params.r = r;
params.kI = 1;
params.deltap = 0.1;
params.kr = 0.1;
[t,Cp] = ode45(@(t,Cp) pCpODE(t,Cp,params),[0:5:250]',Cp0);
[tt,rr] = meshgrid(t,r);
mesh(tt,rr,Cp')

function dCpdt = pCpODE(t,Cp,params)
r = params.r;
kI = params.kI;
deltap = params.deltap;
kr = params.kr;
n = length(Cp);
dCpdt = zeros(n,1);
R = r(end) - kr*t; <- R changes with time; defines zero condition below
for i = 1:n
        if r(i) > R
                Cp(i) = 0; <- Right here is where I set the dependent variable 
to zero
        end
        dCpdt(i) = -kI*Cp(i).*exp(-(R - r(i))/deltap);
end

But it doesn't work. I don't know how to post graphs here, but clearly the
result is not being zeroed. I thought that making Cp global or persistent
would help, but it didn't do anything. Does anyone know how to get that to
work? Or perhaps coding my own RK4 routine is the way to do it, e.g. so I
can make sure the dependent variable gets set to zero in the solver itself?

Thanks!!




--
View this message in context: 
http://octave.1599824.n4.nabble.com/Want-to-set-dependent-variable-in-system-of-ODEs-being-solved-by-ode45-to-zero-tp4646064.html
Sent from the Octave - General mailing list archive at Nabble.com.