How can i use a while loop in matlab?

Question

I have a problem using a while loop. My main objective is to simulate a Fanno flow problem for a case where the length of the tube is longer than required. This means we have to change the Mach number in the middle. My code is the following.

clc
clear all
close all

P1=1;
T1=273;
Cf=0.005;
Dh=0.15;
G=1.4;
M1=3.0;

Lxstar=0;
M2=1;
Lx=0;
My=0;
Lystar=0;
tol=.001;
L = 6.0;
error=0;


fp = ((1-M1^2)/(G*M1^2))+((G+1)/(2*G))*log(((G+1)*M1^2)/(2*(1+(M1^2*(G-1)/(2)))))

Lstar=(fp*Dh)./(4*Cf)

while Lstar<L
Mx=(M1+M2)./2


    fp1=((1-Mx^2)/(G*Mx^2))+((G+1)/(2*G))*log(((G+1)*Mx^2)/(2*(1+(Mx^2*(G-1)/(2)))))

    Lxstar= (fp1*Dh)./(4*Cf)
    Lx= Lstar-Lxstar
    My=((sqrt((2+(G-1)*Mx.^2)/(2*G*Mx.^2-(G-1)))));
    fp2=((1-My^2)/(G*My^2))+((G+1)/(2*G))*log(((G+1)*My^2)/(2*(1+(My^2*(G-1)/(2)))))

    Lystar=(fp2*Dh)./(4*Cf)
   error= Lx+Lystar-L
   if error<=tol
       break

   else
       Diff=Lx+Lystar
       if Diff<L
           Mx=Mx+.01
       else
           Mx=Mx-.01
       end
   end
end

When I run it goes smoothly - it does everything I want but once it changes the Mx it runs again with the Mx=M1+M2/2 instead of the corrected Mx = Mx+.01 or Mx= Mx-.01

Well, Mx = (M1+M2)./2 is inside your while-loop. So it gets redefined in the beginning of each loop. — Wasabi
– Wasabi, Commented Oct 23, 2015 at 14:24

Carlton · Accepted Answer · 2015-10-23 14:22:21Z

2

This line: Mx=(M1+M2)./2 is executed on every loop iteration, even after you assign the new value to Mx. You need to move it out of the loop, like this:

Mx=(M1+M2)./2
while Lstar<L
  %other code here...
end

Then when you assign the new value to Mx, it won't be immediately overwritten with the old value.

answered Oct 23, 2015 at 14:22

Carlton

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