Find Critical Mach number graphically

Hey,

I have to plot two curves to find the critical mach number in matlab. Very rusty at Matlab (been a few years). For the first curve I have to plot the Critical Pressure coefficient value vs. the Critical Mach number and the other curve is the Pressure coefficient (calculated with the minimum pressure coefficient at low speed) vs object mach number

my code thus far is:

Cpo = -0.65; Mcr = linspace(1,8,20); Minfin = linspace(0,1,20);

Ccrit = (2/(1.4*Mcr.^2))*(((((2+(.4*Mcr.^2))/2.4))^3.5)-1); Cp = Cpo/(sqrt(1-Minfin.^2));

plot(Mcr,Ccrit) plot(Minfin,Cp) grid

and I keep getting an error in the Ccrit and Cp lines. Any help is appreciated thanks

[Brian B]

What does the error tell you?

[Kyle]

error using /

[Brian B]

Did you try breaking down your equations until you found which division produces the error?

[Kyle]

I took out the *(((((2+(.4*Mcr.^2))/2.4))^3.5)-1); section and still gave me an error on Ccrit. I apologize if it is an elementary error, just rusty at Matlab

[Brian B]

No worries. But there's nothing like a little debugging to refresh the memory. I'm not trying to be annoying; this is how I learned MATLAB, too. (And I'm doing my own homework right now, as well.)

In the error

 Error using /

you'll notice that the slash is bold and underlined. Click on it. Compare with the function rdivide.

[Kyle]

Sorry but still don't completely understand where the error is completely

[Brian B]

You've probably figured it out by now, but you need to change use

 Ccrit = (2./(1.4*Mcr.^2)) ...*(((((2+(.4*Mcr.^2))/2.4)).^3.5)-1);
 %         ^                                            ^
 %         |                                            |
 % Note the periods before the "/" and before the "^".

You will need to make a similar change to the equation for Cp.

Answer 1

[Teja Muppirala]

You probably just need to change "/" to "./"

Remember, the dot indicates elementwise operations.

This works:

1./[1 2 3]

This does not:

1/[1 2 3]

Answer 2

[Youssef Khmou]

hi Kyle : try this :

 Cpo = -0.65;
Mcr = linspace(1,8,20);
Minfin = linspace(0,1,20);

 Ccrit = (2./(1.4*Mcr.^2)).*(((((2+(.4*Mcr.^2))./2.4)).^3.5)-1);
%keyboard,
Cp = Cpo./(sqrt(1-Minfin.^2));

 figure,
plot(Mcr,Ccrit),xlabel('Critical Mach number'), ylabel('Critical Pressure coefficient value')
figure,grid
plot(Minfin,Cp,'r'), xlabel('object mach number'), ylabel(' Pressure coefficient' );grid

[Youssef Khmou]

The erro exist only in power operator "^" you have to use element-wise ".^" .

Answer 3

[Kyle]

Thank you everyone, got it working. @ Youssef KHMOU, you are a saint thank you