MATLAB: Getting wrong answer,

if statementor else

So I have these formula's and information given to me. From question 1 I find the Zi values which I have done using Sym Z and solve, I found what B was no problem (I dont have what Go and Rs are in the pictures but it's in my script.) I am also given the first Pi values which is 101325, from that I put it into the equation and get what P(2) is. However my script is outputting me very different numbers to the answers of Pi in equation 5 and I cant figure out why.

Any help is appreciated!

Best Answer

Hi,

In your code, there are a few errors with loop indexes. Here is the fixed part of the script that gives all correct answers:

...
    
Pa=Pi; HH=diff(hi);  TT=Ti; AA=Ai_1;  % New variables are introduced not to get confused with the variable names containing i :). This is optional though!
for i= 1:length(ai)
    if AA(i) ==0  
       Pa(i+1)= real(Pa(i).*exp(-B*HH(i)/TT(i)));  % Pressure values are saved
    else Pa(i+1)=real(Pa(i)*(TT(i)/(TT(i)+ai(i)*HH(i))).^(B/AA(i)));  % Pressure values are saved 
    end
end

Related Solutions

MATLAB: Loop answers slightly out from correct values

Are the differences significant? We are unable to check all the numbers that went into the calculations given in the text. There is at least one difference between the table at the top of the text and the output table (T5 is given as 270.67 in the former and 270.65 in the latter, though this makes little practical difference). The largest relative error in Pr is only 0.03%

You don't need symbolic maths here, you can solve for Z directly in a single statement - see the listing below. I've tidied up some of the coding as well.

hi = [0 , 11 , 20, 32 , 47, 51 , 71 , 84.852 , 90 ];
Ti = [288.15, 216.65 , 216.65, 228.65 , 270.65, 270.65 , 214.65 , 186.946, 186.946];
ai = diff(Ti)./diff(hi);
go = 9.80666;
Rs = .287054;
B =(go/Rs);
Re = 6356.766;
Pi= 101325.0000;
Pr=Pi;
Hh=diff(hi);
Z = Re*hi./(Re - hi);  % solve for Z directly
for i= 1:length(ai)
    if ai(i) == 0  
       Pr(i+1) = Pr(i).*exp(-B*Hh(i)/Ti(i));  
    else
        Pr(i+1) = Pr(i)*(Ti(i)/(Ti(i)+ai(i)*Hh(i))).^(B/ai(i));
    end
end
fprintf('Z\n')
fprintf('%8.5f\n', Z)
fprintf('\n') 
fprintf('Pr\n')
fprintf('%12.5f\n', Pr)
fprintf('\n') 
 
fprintf('P_at_h\n')
P_at_h = Pr./(Rs.*Ti);
fprintf('%10.5f\n',P_at_h)
fprintf('\n')
% Text values
Prt = [101325.0000, 22632.0587, 5474.8862, 868.0180, 110.9062, 66.9388, 3.9564, 0.3734, 0.1457];
RelativeError = (Pr./Prt - 1)*100;
fprintf('Relative error (percent)\n')
fprintf('%5.4f\n',RelativeError)

MATLAB: Inverse Laplace contains a complex number

Hi Karl

You show one of the terms, and I assume you have another one that is the complex conjugate that one so that the entire result is real. Here is a simple example of what is probably going on.

syms s a
F = 2*s/(s^2+a^2);
f = ilaplace(F)
  f = 2*cos(a*t)
G = 1/(s+i*a) + 1/(s-i*a)    % same as F, in partial fractions
g = ilaplace(G)
  g = exp(-a*t*1i) + exp(a*t*1i)

( s^2 + a^2 ) has two complex roots, and ilaplace does not recombine the resulting complex expressions in G. However,

g1 = simplify(g)
  g1 = 2*cos(a*t)

You can also try simplifying before doing ilaplace

G2 = simplify(G)
  G2 = (2*s)/(a^2 + s^2)         % same as F
g2 = ilaplace(G2)
  g2 = 2*cos(a*t)

Best Answer

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MATLAB: Loop answers slightly out from correct values

MATLAB: Inverse Laplace contains a complex number

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