Poisson Equation with Mixed Boundary Condition - MatlaB

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Hi guys,
I'm solving a Poisson Equation with Mixed Boundary condition. But I have trouBle with that mixed BC in MATLAB. Anyone can help to fix? Thanks a lot!

dT^2/dx^2+dT/dy^2=-Q(x,y)/k

Rectangular domain (HxL), BC: Top: T(x,H)=Th, Left: dT/dx=0, Bottom: dT/dy=q, Right: dT/dx+B(T-Tinf)=0

Q(x,y) = 200*sin((2*pi/L)*x) + 175*sin((2*pi/H)*y)

nx ny L H q B k Th Tinf
20 30 2 3 -25 1 1 100 50

Coding: I tried 3 ways to define the mixed BC But it doesn't work.


Nx=20; %input('value of Nx:')
Ny=30; %input('value of Ny:')
L=2; %input('value of L:') %input initial value of Nx,Ny,L,H,Q,q,B
H=3; %input('value of H:')
q=-25; %input('value of q:')
B=1; %input('value of B:')
k=1; %input('value of k:')
Th=100; %input('value of Th:')
Tinf=50; %input('value of Tinf:')

err=1; tol=0.0001; iter=0; itmax=5000; %set initial error, tolerance, iteration limitation.
T=zeros(Nx+1,Ny+1);
T(:,Ny+1)=Th; % BOUNDARY CONDITION FOR TOP EDGE
dx=L/Nx; dy=H/Ny;
beta=(dx^2)/(dy^2);
x=0:dx:L;
y=0:dy:H;
Q=zeros(Nx+1,Ny+1);
for k=1:Nx+1
for l=1:Ny+1
Q(k,l)=200*sin((2*pi/L)*x(k)) + 175*sin((2*pi/H)*y(l));
end
end

while (err > tol)
iter=iter+1;
Told=T;
for i=1:Nx,j=1:Ny;
if i==Nx+1 ;
%---Tried 3 ways below but it doesn't work-----
%T(i,j) = (dx^2*Q(i,j)/k+(T(i-1,j)-2*dx*B*(T(i,j)-Tinf))+T(im1,j)+beta*T(i,jp1)+beta*T(i,jm1))/(2+2*beta);
%T(i,j) = (dx^2*Q(i,j)/k+2*T(i-1,j)+2*dx*B*Tinf+beta*T(i,jp1)+beta*T(i,jm1))/(2+2*beta+dx*B);
%T(i+1,j)=T(i-1,j)-2*dx*B*(T(i,j)-Tinf);
else ip1=i+1;
end
if i==1;
im1=2; % dT/dx=0 at left edge
else im1=i-1;
end
for j=1:Ny;
jp1=j+1;
if j==1;
jm1=2;
T(i,j) = (dx^2*Q(i,j)/k+ T(ip1,j)+T(im1,j)+beta*T(i,jp1)+beta*(T(i,jm1)-2*q*dy))/(2+2*beta);
else jm1=j-1;
end
T(i,j) = (dx^2*Q(i,j)/k+ T(ip1,j)+T(im1,j)+beta*T(i,jp1)+beta*T(i,jm1))/(2+2*beta);
%value of T we get from the equation
end
end
err=max(max(T-Told))/max(max(T));
if iter > itmax, break, end
end
Unu_Iterative=T';
[xp,yp]=meshgrid(x,y);
subplot(2,1,1);
surf(xp,yp,T')
subplot(2,1,2);
contour(x,y,T',20)