how to solve "Matrix dimensions must agree."

When i run this program it give me error in line 32. MathLab code attach here please guide me.

i need to sum all the Rm & Rk in the for loops.

for m=1:3
    r_m=dr*m+dr*(m-(m-1))/2; %% r_m for R_m+ %%
    Rm(m)=dr/((r_m+dr/2)*dteta*dz*k_pipe); %% r direction %%
    for z=1:5
        Rk(z)=dz/(r_m*dteta*dr*k_water); %% z direction %%
    end
    Rh=1/(h_out*A);
    if (m==1)
        v=pi()*(dr*m)^2*dz; %%%
    elseif (m>1&&m<3)
        v=pi()*((dr*m)^2-(dr*(m-1)^2))*dz;  %% r element volume %%
        Cm=c*rau_p*v; %% rau_p/w?
        line 32      s=sum(Rm+Rk+Rh); %??
        dt=Cm/(1/s));     %%   ??   
    end
end
for p=1:10
    
    if (z==1)
        T(P+1,m,k)=(dt/Cm)*(((T(p,m+1)-T(p,m))/Rm)+((T(p,m)-T(p,m+1))/Rm)+((T(p,z+1)-T(p,z))/(Rk)))+T(p,m,k);
    elseif (z>1&&z<5)
        T(P+1,m,k)=(dt/Cm)*(((T(p,m+1)-T(p,m))/Rm)+((T(p,m+1)-T(p,m))/Rm)+((T(p,z+1)-T(p,z))/(Rk)+((T(p,z)-T(p,z-1))/(Rk))))+T(p,m,k);
    elseif (z==5)
        T(P+1,m,k)=(dt/Cm)*(((T(p,m+1)-T(p,m))/Rm)+((T(p,m)-T(p,m+1))/Rm)+((T(p,z)-T(p,z-1))/(Rk)))+T(p,m,k);
    end
end


disp('T out [K]: ');disp('T(Z_n_elem)');  
plot(dz*(1:Z_n_elem),T);
xlabel('x (m) '),ylabel('T (K) ');

[Torsten]

Rm and Rk are vectors of different size - thus they cannot be added.

[ofek kadoshy]

T=size(1,1,1); Rm=size(1); Rk=size(1); Rh=size(1);s=size(1);

in here they in the same size? what i'm missing?

[Torsten]

for m=1:3
    r_m=dr*m+dr*(m-(m-1))/2; %% r_m for R_m+ %%
    Rm(m)=dr/((r_m+dr/2)*dteta*dz*k_pipe); %% r direction %%     %size 3

and

Rh=1/(h_out*A);                                                  % size 1

and

for z=1:5
    Rk(z)=dz/(r_m*dteta*dr*k_water); %% z direction %%           % size 5
end

[Jan]

@ofek kadoshy: What does this mean: "T=size(1,1,1); Rm=size(1); Rk=size(1); Rh=size(1);s=size(1);" ?

Rm is a vector with 3 or up to 3 elements:

Rm(m)=dr/((r_m+dr/2)*dteta*dz*k_pipe);

Rk is a vector with 5 elements:

for z=1:5
    Rk(z)=dz/(r_m*dteta*dr*k_water); %% z direction %%
end

I cannot guess, what Rh is:

Rh=1/(h_out*A)

The error message tells you clearly, that the sizes do not match. So simply check the sizes.

Please post running code. If the code cannot be executed at all, it is impossible to understand, why it does not do, what you expect:

dt=Cm/(1/s))

This line prevents, that the code is executed, because the parentheses do not match.

The simpler the code, the easier is the debugging. Compare:

if (z==1)
    T(P+1,m,k)=(dt/Cm)*(((T(p,m+1)-T(p,m))/Rm) + ...
        ((T(p,m)-T(p,m+1))/Rm)+((T(p,z+1)-T(p,z))/(Rk)))+T(p,m,k);
elseif (z>1&&z<5)
    T(P+1,m,k)=(dt/Cm)*(((T(p,m+1)-T(p,m))/Rm) + ...
        ((T(p,m+1)-T(p,m))/Rm)+((T(p,z+1)-T(p,z))/(Rk)+((T(p,z)-T(p,z-1))/(Rk))))+T(p,m,k);
elseif (z==5)
    T(P+1,m,k)=(dt/Cm)*(((T(p,m+1)-T(p,m))/Rm) + ...
        ((T(p,m)-T(p,m+1))/Rm)+((T(p,z)-T(p,z-1))/(Rk)))+T(p,m,k);
end

with

if z == 1
   tmp = (T(p,m+1) - T(p,m)) / Rm + ...
         (T(p,m) - T(p,m+1)) / Rm + (T(p,z+1) - T(p,z)) / Rk;
elseif z < 5
   tmp = (T(p,m+1) - T(p,m)) / Rm + ...
         (T(p,m+1) - T(p,m)) / Rm + (T(p,z+1) - T(p,z)) / Rk + ...
         (T(p,z) - T(p,z-1)) / Rk;
else  % z == 5
   tmp = (T(p,m+1) - T(p,m)) / Rm + ...
         (T(p,m) - T(p,m+1)) / Rm + (T(p,z) - T(p,z-1)) / Rk;
end
T(P+1,m,k) = (dt/Cm) * tmp + T(p,m,k);

r_m=dr*m+dr*(m-(m-1))/2;  % ??? Why not:
r_m = dr * m + dr / 2;    % (m-(m-1)) == 1 !

[ofek kadoshy]

this is my fixed code.

in line 50 it says: Index in position 2 is invalid. Array indices must be positive integers or logical values.

%%% Inputs %%%

L=1; %% tube legnth [m] %%

k_n_elem=5; %% number of elements in z %%

m=3; %% number of elements in r %%

T_out=298; %% air temperature [K]-25c %%

D_pipe=0.3; %% [m] pipe diameter %%

dr=0.05; %% [m] pipe delta radius %%

t_pipe=0.002; %% [m] pipe thikness %%

dz=L/k_n_elem; %% element length z %%

T=size(1,1,1); Rm=size(1); Rk=size(1); Rh=size(1);s=size(1); Cm=size(1);

A=pi()*D_pipe*dz; %% pipe to air convection %%

k_pipe=10; %% [W/mK] %%

h_out=10; %% [W/m^2K] %%

k_water=0.6668; %% [W/mK] %%

dteta=pi(); %% const %%

rau_p=1020; %% [kg/m^3] %% pvc density%%

rau_w=1000; %% [kg/m^3] %% water density%%

c=1000; %% [j/kgK] %%

for m=1:m

if (m==1)

r_m=dr/2;

elseif (m>2&&m<m)

r_m=dr*(m-1)-((dr*m-dr*(m-1))/2); %% r_m for R_m+ %%

end

Rm(m)=dr/((r_m+dr/2)*dteta*dz*k_pipe); %% r direction %%

for k=1:5

Rk(k)=dz/(r_m*dteta*dr*k_water); %% z direction %%

end

Rh=1/(h_out*A);

if (m==1)

v=pi()*(dr*m)^2*dz; %%%

elseif (m>1&&m<3)

v=pi()*((dr*m)^2-(dr*(m-1)^2))*dz; %% r element volume %%

Cm(m)=c*rau_p*v; %% rau_p/w?

s=sum(Rm)+sum(Rk)+Rh; %??

end

tau=Cm/s;

dt=min(tau);

for p=1:10

if (k==1)

T(1,1,1)=348; %% in water temperature [K]-75c %%

T(P+1,m,k)=(dt/Cm)*(((T(p,m)-T(p,m-1))/Rm)+((T(p,m+1)-T(p,m))/Rm)+((T(p,k+1)-T(p,k))/Rk))+T(p,m,k);

elseif (k>1&&k<5)

T(P+1,m,k)=(dt/Cm)*(((T(p,m+1)-T(p,m))/Rm)+((T(p,m+1)-T(p,m))/Rm)+((T(p,k+1)-T(p,k))/Rk+((T(p,k)-T(p,k-1))/Rk)))+T(p,m,k);

elseif (k==5)

line 50%% T(P+1,m,k)=(dt/Cm(m))*(((T(p,m)-T(p,m-1))/Rm(m))+((T(p,m+1)-T(p,m))/Rm(m))+((T(p,k)-T(p,k-1))/Rk(k)))+T(p,m,k);

end

end

end

disp('T out [K]: ');disp('T(Z_n_elem)');

plot(dz*(1:Z_n_elem),T);

xlabel('x (m) '),ylabel('T (K) ');

[ofek kadoshy]

Rm(m)=dr/((r_m+dr/2)*dteta*dz*k_pipe); %% r direction %%

Rk(k)=dz/(r_m*dteta*dr*k_water); %% z direction %%

Rh=1/(h_out*A);

in here r_m is function that i built and the others known(dteta/dz/k_pipe/drh_out/a/k_water).

when i use the size function it return the number of the size .am i wrong?

[Walter Roberson]

You are invoking your r_m function with no parameters??

[ofek kadoshy]

if (m==1)

r_m=dr/2;

elseif (m>2&&m<m)

r_m=dr*(m-1)-((dr*m-dr*(m-1))/2); %% r_m for R_m+ %%

end

dr is a parameter .

[Walter Roberson]

"in here r_m is function that i built"

You repeatedly invoke your function with no parameters.

Rk(k)=dz/(r_m*dteta*dr*k_water); %% z direction

Notice the r_m has no () after it. In MATLAB when you define a true function and mention its name without the @ operator, that invokes the function passing in no parameters.

Answer 1

[Walter Roberson]

T(P+1,m,k)=(dt/Cm(m))*(((T(p,m)-T(p,m-1))/Rm(m))+((T(p,m+1)-T(p,m))/Rm(m))+((T(p,k)-T(p,k-1))/Rk(k)))+T(p,m,k);

nothing prevents that from executing when m==1

Before that point you test m>1 but you have "end" on that test so you are no longer protected.