bvp4c guess value

Hi,

I'm trying to solve a boundary value problem using bvp4c. My equation is as folows.

G'= k1*rw*uw;

Here k1 is a constant. rw is a known (1x100) vector. uw and G' are unknowns. The boundary conditions are

G'(0)= Ginf*0.99 and

G'(L)= Ginf*0.01;

I tried to solve this using the following code, but it gives me the following error.

"The boundary condition function BCFUN should return a column vector of length 2."

Following is my code.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% function [y1,uw] = testbvp

rhoW= [0.912669843 0.912669843 0.913062531 0.913082624 0.913083242 0.913083252 0.913083251 0.913083251 0.91308325 0.91308325];

a= 0.0015; Ginf= 2.3881;

options = [];

solinit = bvpinit(linspace(0,1,10),1,1);

sol = bvp4c(@testode,@testbc,solinit,options,a,rhoW,Ginf);

y1= deval(sol,x);

% -----------------------------------------------------------------

function dydx = testode(x,y,a,rhoW,~,uw)

k1= -4/a;

dydx = k1*rhoW.*uw;

% -----------------------------------------------------------------

function res = testbc(ya,yb,~,~,Ginf,~)

res = [ya(1)-Ginf*0.99

       yb(1)-Ginf*0.01];

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

I need to find the unknown values of G' and uw. Thanks in advance.

Luke

Answer 1

[Walter Roberson]

Your testode function needs to return a column vector the same size as y, but you are ignoring x (a scalar) and y (a column vector) completely. Possibly what you are doing is okay, but it looks wrong to me at the moment. Please check the size of your dydx against the size of y in testode.

I do not immediately see a reason why it would have a problem in testbc(), not unless it was finding that Ginf was the empty set. Please check the size of your inputs and outputs for testbc()