# Finite Difference Scheme Error

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DIP on 7 Apr 2017
Commented: DIP on 20 Apr 2017
Hi, I am trying to plot this equation , i am unable to understand how to debug the matrix error on the equation.
% Solution 1: Chemical Species Concentration as a Function of Axial Distance
clc
clear
% Constants
L = 0.2;
N = 31;
T(1:N) = 700;
delx = L/(N-1);
xspan = 0:delx:0.2;
u = 1.5;
A = 7e4;
Ea = 100000;
Ru = 8.3145;
T = 700;
k = 0.035;
rho = 0.95;
NM=3;
C(1) = 0.21;
C(2) = 0.68;
C(3) = 0.06;
R(1) = -A * (exp(-Ea / (Ru*T))) * C(1)/rho * sqrt(C(2)/rho);
R(2) = 0.5 * R(1);
R(3) = -R(1);
D(1) = 0.166;
D(2) = 0.201;
D(3) = 0.177;
for i=2:N-1
for j=2:NM
C(i,j)=((R(j)*delx.^2)*(rho*D(j)).^(-1))+ C(i+1,j)+C(i-1,j) ;
end
end
% Plots
subplot(2,1,1);
plot(x,C(:,1),'b--o',x,C(:,2),'g--+',x,C(:,3),'r--s')
legend('C_{CO}','C_{O2}','C_{CO2}')
xlabel('Axial (x) Direction [m]')
ylabel('Concentrations [mol/m^3]')
title('Molar Concentration vs Distance')
subplot(2,1,2);
plot(x,T,'b--*')
legend('Gas Temperature')
xlabel('Axial (x) Direction [m]')
ylabel('Gas Temperature [K]')
title('Temperature vs Distance')
Any help is appreciated.
Thank You

Torsten on 10 Apr 2017
In the calculation of C(i,j), you can't refer to C(i+1,j) because it's not yet defined.
Best wishes
Torsten.
DIP on 20 Apr 2017
Thanks Torsten , got it.