I'm getting the error "Array indices must be positive integers or logical values." and it seems like I structures the for loop to handle the equations. Can anyone help me with this?

ds=zeros(length(y),1);
dAs=zeros(length(y),1);
Ac=zeros(length(y),1);
for j=1:N
 t1=(t(y(j))+t(y(j+1/2)))/2; %average thickness of CV on side closer to y=0
 t2=(t(y(j-1/2))+t(y(j)))/2; %avgerage thickness of CV on side closer to y=L
 ds(j)=sqrt((t(y(j+1))-t(t(j-1))).^2+(2*dy).^2)/2; %using central difference
 dAs(j)= (w+((t1+t2)/L))*ds(j)*dx; %surface area
 Ac(j)=(t(y(j)))*dx; %Cross sectional area 
end

[Matt Gaidica]

What are you expecting from y(j+1/2), is this possibly an order of operations issue? It's hard to debug without providing t, y, and N.

[Matthew Williams]

The codes functionality is solving for heat transfer in a triangular fin at each control volume and each volume has a changing suface area and cross sectional area. At the boundary y=0 the CV of interest is half compared to a centralized control volume, so I have it modeled as j+1/2 as opposed to j+1.

Earlier in the beginning of the code I'm defining the thickness t as a function of y because along the y direction the thickness is changing wrt y, tt is the given thickness at the base of the triangular Fin and L is the given length of the fin from the base to the tip.

t= @(y)(tt/L)*y;

%% Discretization information

M= 41; %number of nodes in the x direction

N= 31; %number of nodes in the y direction

dx=w/(M-1); %spacing between nodes, m

x=[0:dx:w];

dy=L/(N-1); %spacing between nodes, m

y=[0:dy:L];

[Walter Roberson]

You cannot index at half-integers . You can index a value and divide the result by 2, though.

Answer 1

[Gouri Chennuru]

Hi Matthew,

As per my understanding, you are trying to extract the variable "y" with indices (j+1/2) and (j-1/2), from the following lines.

 t1=(t(y(j))+t(y(j+1/2)))/2; 
 t2=(t(y(j-1/2))+t(y(j)))/2;

You cannot point arrays to a particular decimal index.As a workaround make sure that array Index is always a positive integer or a logical value.

Regards,

Gouri Chennuru