# I wrote a code to replicate the image to the right but for some reason the M3 and the M2 are on opposite sides and the M1 is completely wrong. Please help me fix this

4 views (last 30 days)
Aijalon Marsh on 1 Dec 2023
Edited: Milan Bansal on 22 Dec 2023
clc
clear all
%Given:
M1 = 3;
G = 1.4;
tot = 180;
Theta= zeros(1,tot);
p2_p1= zeros(1,tot);
n=1;
for i = 1 : 0.01 : tot
Theta(n) = atand(2*cotd(i)*((M1*(sind(i)))^2-1)/(2+M1^2*(G+cosd(2*i))));
p2_p1(n) = 1+(2*G)/((G+1)*(M1*sind(i))^2-1);
n = n+1;
end
error = 1;
i = 0;
while error > 0.001
i=i+1;
error = 20 - Theta(i);
end
d=i;
b2 = 38;
Mn1 = M1*sind(b2);
Mn2 = sqrt((Mn1^(2)+(2/(G-1)))/(2*G/(G-1)*Mn1^(2)-1));
M2 = Mn2/(sind(b2-20));
n = 1;
for i = 30 : 0.01 : 149
theta2(n) = 20 + atand(2*cotd(i)*((M2*(sind(i)))^2-1)/(2+M2^2*(G+cosd(2*i))));
p3_p1(n) = p2_p1(d)*(1+(2*G)/(G+1)*((M2*(sind(i)))^2-1));
n = n+1;
end
error = 1;
i = 0;
while error > 0.001
i = i+1;
error = abs(-15-Theta(i));
end
a=i;
b3 = 32;
Mn1 = M1*sind(b3);
Mn3 = sqrt((Mn1^(2)+(2/(G-1)))/(2*G/(G-1)*Mn1^(2)-1));
M3 = Mn3/sind(b3-15);
s = 1;
for i = 26 : 0.01 : 153
theta3(s) = -15 + atand(2*cotd(i)*((M3*(sind(i)))^2-1)/(2+M3^2*(G+cosd(2*i))));
p4_p1(s) = p2_p1(a)*(1+(2*G)/(G+1)*((M3*(sind(i)))^2-1));
s = s+1;
end
figure(1)
plot(Theta,p2_p1)
hold on
plot(theta2,p3_p1)
hold on
plot(theta3,p4_p1)
title('PD-Diagram')
xlabel('Theta (\theta)')
ylabel('Pressure Ratio')
axis([-45 45 1 17])
legend('M1','M2','M3')

Milan Bansal on 22 Dec 2023
Edited: Milan Bansal on 22 Dec 2023
Hi Aijalon Marsh,
It is my understanding that you are trying to plot the graphs M1, M2 and M3 present in the attached image but the M1 curve is not plotted as expected and M2 and M3 are on the opposite side.
For correcting M1 curve, please modify the "for loop" for M1 in the code as showm in the code snippet below.
% iterate from 20 to 160 instead of 1 to 180
for i = 20 : 0.01 : 160
Theta(n) = atand(2*cotd(i)*((M1*(sind(i)))^2-1)/(2+M1^2*(G+cosd(2*i))));
p2_p1(n) = 1+(2*G)/(G+1)*((M1*(sind(i)))^2-1); % modified equation
n = n+1;
end
The output firgure after the modification is shown below.
The M2 and M3 plots are at the correct positions and M1 plot is in the correct form. Please modify the "M1", "M2" and "M3" variables to change the size of plots and the threshold values to set the positions to replicate the plots given in the image.
Hope it helps!