How to give a final value to solve an ODE's, instead of the standard initial values?
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I'm solving a first order differential equation with variable constants and a mass matrix. Therefore, I use ODE113. As shown in the code below, I call 2 functions for defining the ODE and the mass matrix. In order to give in the final value as 'initial condition', in the first call of ode113, the independent variable x is transformed to x_end - x. For comparison, in the second call of ode113, the original x is used with the initial value given from the previous solution. The functions called in the second time are identical as in the first time, except that now x is immediately defined by the function input, and has no transformation. Both solutions were plotted but do not coincide. Why is this, is this the correct way of transforming the pre-programmed initial value programs to final value problems?
Thanks in advance! regards, Kenny
% Limit of the differential eq.:
x_end=2.5;
%
% Parameters:
x0=[0.32;0.15;45];
%
% Final condition is known:
options =odeset('Mass',@(xb) Mass_fromback(x0(1),x0(2),xb,x_end),'MStateDependence','none','RelTol',1e-3);
yfb=ode113(@(xb,y)diffEqn_fromback(x0(1),x0(2),x0(3),xb,y,x_end),[0 x_end], 0,options);
%
% Initial condition from previous solution:
y0=deval(yfb,x_end);
% Solve by giving in initial condition:
options =odeset('Mass',@(xb) Mass(x0(1),x0(2),xb),'MStateDependence','none','RelTol',1e-3);
y=ode113(@(xb,y)diffEqn(x0(1),x0(2),x0(3),xb,y),[0 x_end], y0,options);
with functions:
function M=Mass_fromback(E,nu,xb,x_end)
% Define variable from back to the front, as such, the initial condition
% will be the deformation at the back of the chamber.
x=x_end-xb;
%
hx=1.2-x*tan(3*pi/180);
bx=0.9-x*(2*tan(3*pi/180));
A=-E.*(1-nu);
%
M=A.*hx.*bx;
and
function Mdydx=diffEqn_fromback(E,nu,mu,xb,y,x_end)
%
% Define variable from back to the front:
x=x_end-xb;
%
B=E*nu*(mu+(mu+tan(3*pi/180))./(1-mu*3*pi/180));
D= E*(mu +(mu+8*pi/180)/(1-mu*3*pi/180));
%
Mdydx= B.*x.*y +(1-nu).*D.*x.*log(x+0.1);
For comparison, the two solutions are plotted but they do not coincide:
figure;
plot(x_end-yfb.x,yfb.y,y.x,y.y)
xlabel('x'),ylabel('y'), legend('solved with xb = x\_end - x','solved with x')
1 Comment
Accepted Answer
More Answers (2)
Juan Camilo Medina
on 13 Mar 2013
There are different methods to solve a boundary value problem, which is effectively what you have. The easiest one is the shooting method:
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Sana Ben Hmouda
on 9 Jul 2018
Hello, i have the same problem but i can't get the suggested solutions. Would any one provide me plz with a solution to my problem? In fact i'm solving dxdt=(A+BK)x by means of ode45, the solution x(t) converges to zero, now i would like to set the final value of the solution according to a desired value of xd, someone suggected that i replace dxdt=(A+Bk)x+xd, but the result still converges to zero. Thanks
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