Complex results when using my annonymous function

Hey Guys,

I am using runge-kutta-method to integrate function 1, after that i integrate function 2. But I always get complex results for function 2 (except the first iteration). Can anybody tell my why it is how it is? All other variables make sense, so the error should be anywhere else.

Thank you!!

while betrag >  tol  
for i=1:integrationsende             
    T_H2O_vektor(i+1) = T_H2O_vektor(i)+h;
    
    X_sat(i) = 0.622 * p_sat_Antoine(T_H2O_vektor(i))/(p - p_sat_Antoine(T_H2O_vektor(i)));
    
    h_Luft_sat(i) = (cp_Luft + cp_Dampf * X_sat(i)) * (T_H2O_vektor(i))+X_sat(i)*h_verdampfung;
    
    Massenstromverhaeltnis = M_H2O_ein/M_Luft*(1 - M_Luft/M_H2O_ein*(X_2 - X_vektor(i)));
    
    %Funktion 1
    f1 = @(T_H2O,h_Luft) cp_H2O * Massenstromverhaeltnis *(1+(X_sat(i)-X_vektor(i))*cp_H2O...
                          *T_H2O/(h_Luft_sat(i)- h_Luft +((0.865^(2/3)*(((0.622+X_sat(i))/...
                         (0.622+X_vektor(i))-1)/(log((0.622+X_sat(i))/(0.622 + ...
                         X_vektor(i))))))-1) *(h_Luft_sat(i)- h_Luft-(X_sat(i)-... 
                         X_vektor(i))*(h_verdampfung +cp_Dampf*T_H2O)) - (X_sat(i) ...
                         - X_vektor(i))*cp_H2O*T_H2O));
    
    
    l1 = f1((T_H2O_vektor(i)        ),(h_Luft_vektor(i)           ));
    l2 = f1((T_H2O_vektor(i) + 0.5*h),(h_Luft_vektor(i) + 0.5*h*l1));
    l3 = f1((T_H2O_vektor(i) + 0.5*h),(h_Luft_vektor(i) + 0.5*h*l2));
    l4 = f1((T_H2O_vektor(i) +     h),(h_Luft_vektor(i) +     h*l3));
    h_Luft_vektor(i+1) = h_Luft_vektor(i) + h/6*(l1+2*l2+2*l3+l4);
    
    %Funktion 2
    f2 = @(T_H2O,X) cp_H2O * Massenstromverhaeltnis * ((X_sat(i) - X)/(h_Luft_sat(i)...
                    - h_Luft_vektor(i) + ((0.865^(2/3)*(((0.622+X_sat(i))/(0.622+X)-1)/...
                    (log((0.622+X_sat(i))/(0.622+X)))))-1)*(h_Luft_sat(i) - ...
                    h_Luft_vektor(i) -(X_sat(i) - X)*(h_verdampfung + cp_Dampf*T_H2O))) ...
                    - (X_sat(i) - X)*cp_H2O * T_H2O);
    
    k1 = f2((T_H2O_vektor(i)        ),(X_vektor(i)           ));
    k2 = f2((T_H2O_vektor(i) + 0.5*h),(X_vektor(i) + 0.5*h*k1));
    k3 = f2((T_H2O_vektor(i) + 0.5*h),(X_vektor(i) + 0.5*h*k2));
    k4 = f2((T_H2O_vektor(i) +     h),(X_vektor(i) +     h*k3));
    X_vektor(i+1) = X_vektor(i) + h/6*(k1+2*k2+2*k3+k4);  
    
    X_2R = X_vektor(i) + h/6*(k1+2*k2+2*k3+k4);
    betrag = abs(X_2R - X_2);
end

[Star Strider]

Is the argument to the log function here:

log((0.622+X_sat(i))/(0.622+X))

ever negative?

That is the only part of ‘f2’ I can see that could produce a complex result.

Answer 1

[Michael Käufl]

Your answer didn't solve the problem, but I found my problem because I had a look at the part you were talking of. It was a stupid parenthesis error. Thanks!

[Star Strider]

My pleasure!