plotting using fsolve in a for loop (2 variables)

I want to plot two equations in terms of one parameter z that goes from 0.1 to 1. For that, I used fsolve in a for loop. When I run my code, it says that the equation is solved but there is no line line on my graph.

Here is my code:

P_1;
k=25;
a=-0.0075;
b=0.1750;
c=4;

z=linspace(0.1,1);
for z=1:length(z);
f@(x) compressorcharacteristic(x, z, k, P_, a, b, c);
xguess=[1.1,10];
sol=fsolve(f,xguess);
mdot_sol=sol(1);
P_sol=sol(2);
end

plot(z,mdot_sol);
hold on
plot(z,P_sol);
hold off

function y=compressorcharacteristic(x,P_,k,a,b,c,z)
mdot=x(1);
P=x(2);
y(1)=z.*k.*sqrt(P-P_)-mdot;
y(2)=a.*(mdot.^2)+b.*mdot+4-P;
end

Can someone pinpoint the problem ?

Answer 1

[Walter Roberson]

I had to guess the value of P_

Your equations appear to have complex-valued solutions

P_ = 1;
k=25;
a=-0.0075;
b=0.1750;
c=4;

opts = optimoptions('fsolve', 'display', 'none');
zvals = linspace(0.1,1);
Nz = length(zvals);
mdot_sol = zeros(Nz, 1);
P_sol = zeros(Nz, 1);
for zidx = 1 : Nz
    z = zvals(zidx);
    f = @(x) compressorcharacteristic(x, z, k, P_, a, b, c);
    xguess=[1.1,10];
    sol = fsolve(f, xguess, opts);
    mdot_sol(zidx) = sol(1);
    P_sol(zidx) = sol(2);
end
subplot(2,2,1)
plot(zvals, real(mdot_sol)); title('mdot - real');
subplot(2,2,2)
plot(zvals, real(P_sol)); title('P - real');
subplot(2,2,3)
plot(zvals, imag(mdot_sol)); title('mdot - imaginary');
subplot(2,2,4)
plot(zvals, imag(P_sol)); title('P - imaginary');

function y=compressorcharacteristic(x,P_,k,a,b,c,z)
mdot=x(1);
P=x(2);
y(1)=z.*k.*sqrt(P-P_)-mdot;
y(2)=a.*(mdot.^2)+b.*mdot+4-P;
end

[Nora]

You got it right for the P_ value, sorry I forgot to put an "=". Anyway, thank you so much !