What's wrong with my ode45 function?

Question

Yuxing Zhang 2018-11-27

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/432163-what-s-wrong-with-my-ode45-function

评论： Yuxing Zhang 2018-11-27

I am using the ode45 to solve streamfunction and I compelete my functions and main script but always have wrong in ode 45 function, can anyone tell me where is it wrong, A,B,C are just matrix provided ( they are all correct). Thank you very much!

clear all,clc
% initial conditions
n=64;
tspan= 0:0.5:4;
v= 0.001;
L=20;
x2=linspace(-L/2,L/2,n+1);
xspan=x2(1:n);
dx = xspan(2)-xspan(1);
x=x2(1:n);
y=x;
w0=exp(-x.^2-((y.^2)/20));
wvec=reshape(w0,n,1);
% matrix A,B,C ( you can ignore this)
N=n*n; % total size of matrix
e0=zeros(N,1); % vector of zeros
e1=ones(N,1); % vector of ones
e2=e1; % copy the one vector
e4=e0; % copy the zero vector
for j=1:n
e2(n*j)=0; % overwrite every n^th value with zero
e4(n*j)=1; % overwirte every n^th value with one
end
e3(2:N,1)=e2(1:N-1,1); e3(1,1)=e2(N,1); % shift to correct
e5(2:N,1)=e4(1:N-1,1); e5(1,1)=e4(N,1); % positions
a=spdiags([e1 e1 e5 e2 2*e1 e3 e4 e1 e1], ...
[-(N-n) -n -n+1 -1 0 1 n-1 n (N-n)],N,N);
A=full(a)./((dx)^2);
b=spdiags([e1 -e1 e0 e1 -e1],[-n*(n-1) -n 0 n n*(n-1)],N,N);
B=full(b)./(2*dx);
c=spdiags([e5 -e2 e0 e3 -e4 ],[ -n+1 -1 0 1 n-1],N,N);
C=full(c)./(2*dx);
[t,wtsol]=ode45(@(t,wvec)rhseq(t,wvec,psivec,A,B,C,v),tspan,w0);
function wtsol=rhseq(t,wvec,psivec,A,B,C,v)
psivec=A\wvec;
wtsol=(C*psivec).*(B*wvec)-(B*psivec).*(C*wvec)+(v*A*wvec);
end

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Answer 1

Stephan 2018-11-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/432163-what-s-wrong-with-my-ode45-function#answer_349085

you want to pass psivec to the function, but it is calculated inside the function.
A is size 4096x4096 and wvec is size 64x1. Due to this A\wvec wont work.