How to use a fzero function and subfunctions?

Hi,

I am working on a code that involves using an fzero function but I can't figure out how to make it work. I need to find the density of air. This is what I have thus far.

%Parameters
T=250; Tb=340; Bstr=20.5*10^6; b_4=-0.00168785; b_3=-0.0223299; 
b_2=-0.1704; b_1=-1.94783; b0=2.16059; b1=-0.0222243; c_3=2.50287; 
c_2=-5.52109; c_1=4.29631; c0=1.19665; R=8.314; P=0.01;

tau=T/Tb;
B=Bstr*(b_4*tau^-4+b_3*tau^-3+b_2*tau^-2+b_1*tau^-1+b0*tau^0+b1*tau^1);
C=Bstr^2*(c_3*tau^-3+c_2*tau^-2+c_1*tau^-1+c0*tau^0);
x0=(initalguess);
x=fzero(@func1,x0);
p=0.029*pbar;

[sixwwwwww]

what is your function here? where did you define func1

[Stasha]

I think I forgot to add it in. Here is the updated version.

function [den] = blackgandy_stasha_01(~,~)

    %Parameters
    T=250; Tb=340; Bstr=20.5*10^6; b_4=-0.00168785; b_3=-0.0223299; 
    b_2=-0.1704; b_1=-1.94783; b0=2.16059; b1=-0.0222243; c_3=2.50287; 
    c_2=-5.52109; c_1=4.29631; c0=1.19665; 

    %Equation 2

    tau=T/Tb;
    B=Bstr*(b_4*tau^-4+b_3*tau^-3+b_2*tau^-2+b_1*tau^-1+b0*tau^0+b1*tau^1);
    C=Bstr^2*(c_3*tau^-3+c_2*tau^-2+c_1*tau^-1+c0*tau^0);
    x0=1;
    x=fzero(@(pbar) 1+B*pbar+C*pbar^2,x0)
    p=0.029*pbar;

    %subfunction
    function [] = pbar
    % Parameters
    T=250; R=8.314; P=0.01;
    Z=P/pbar*R*T;
    Z=1+B*pbar+C*pbar^2
    end

[sixwwwwww]

can you please edit your code so that it is clearly viewable? you can edit the code using Code{ } button

[Stasha]

I can't find that button but all I am really needing to understand is how to input the equation in the function under %subfunction to get it to be called up to work for the fzero?

[Matt J]

This button.

It is on the toolbar in every Question/Answer/Comment box.

Answer 1

[Stasha]

%Inputs 
%T = Temperature, P = Pressure
T = input('Required temperature in Kelvin')
P = input('Required pressure in MPa')

%Parameters
Tb=340; Bstr=20.5*10^6; b_4=-0.00168785; b_3=-0.0223299; 
b_2=-0.1704; b_1=-1.94783; b0=2.16059; b1=-0.0222243; c_3=2.50287; 
c_2=-5.52109; c_1=4.29631; c0=1.19665; 

%Equation 2

tau=T/Tb;
B=Bstr*(b_4*tau^-4+b_3*tau^-3+b_2*tau^-2+b_1*tau^-1+b0*tau^0+b1*tau^1);
C=Bstr^2*(c_3*tau^-3+c_2*tau^-2+c_1*tau^-1+c0*tau^0);
x0=1;
x=fzero(@(pbar) 1+B*pbar+C*pbar^2,x0)
p=0.029*pbar;

%Parameters
Ts=132.5; pstr=314.3; H=6.1609*10^-6; A1=1.128516; A_5=2.60661; A0=-1.00; 
A_1=-0.709661; A_2=0.662534; A_3=-0.197846; A_4=0.00770147; B1=0.465601; 
B2=1.26469; B3=-0.511425; B4=0.2746;

%Equation 1 
pr=p/pstr;
Tr=T/Ts;
yt=A1*Tr+A_5*Tr^0.5+A0*Tr^0+A_1*Tr^-1+A_2*Tr^-2+A_3*Tr^-3+A_4*Tr^-4;
yp=B1*pr+B2*pr^2+B3*pr^3+B4*pr^4;
y=H*(yt+yp);
end

%subfunction
function [] = pbar
% Parameters
T=250; R=8.314;
Z=P/pbar*R*T;
Z=1+B*pbar+C*pbar^2
end

Answer 2

[sixwwwwww]

Your code is working but its not giving correct ans because of your initial condition:

%Parameters 
T=250; 
Tb=340; 
Bstr=20.5*10^6; 
b_4=-0.00168785; 
b_3=-0.0223299; 
b_2=-0.1704; 
b_1=-1.94783; 
b0=2.16059; 
b1=-0.0222243; 
c_3=2.50287; 
c_2=-5.52109; 
c_1=4.29631; 
c0=1.19665;
%Equation 2
tau=T/Tb; 
B=Bstr*(b_4*tau^-4+b_3*tau^-3+b_2*tau^-2+b_1*tau^-1+b0*tau^0+b1*tau^1); 
C=Bstr^2*(c_3*tau^-3+c_2*tau^-2+c_1*tau^-1+c0*tau^0); 
x0 = 1; 
x = fzero(@(pbar)1+B*pbar+C*pbar^2 ,x0) ;

[Stasha]

Sixwwwwww, The code you said runs fine is not my complete code. The one above it is and that is the one I'm getting an error on.