Using trapz doesn't work

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Michael Käufl 2020-1-31

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/502964-using-trapz-doesn-t-work

评论： Michael Käufl 2020-2-4

Hey Guys,

I already read some answers about 'errors using trapz' but i didn't get what's wrong because the lengths of my vektors are similiar. All vectors have the size 1x24000 double, thus dX_dTeta is also 1x24000 double. I need to integrate the function F. Are there any mistakes in it?

Thanks!!!

Here is my code:

%% sektion
for i = 1:(ceil((T_H2O_ein-T_H2O_aus)/h))
dX_dTeta(i) = cp_H2O * Massenstromverhaeltnis(i) * ((X_sat(i) - X_vektor(i))/(h_Luft_sat(i)...
                    - h_Luft_vektor(i) + ((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_vektor(i) -...
                    (X_sat(i) - X_vektor(i))*(h_verdampfung + cp_Dampf*T_H2O_vektor(i))) - ...
                    (X_sat(i) - X_vektor(i))*cp_H2O * T_H2O_vektor(i)));
                
end
temp = linspace(19,43,24000);
F = dX_dTeta(i)/(X_sat(i)-X_vektor(i));
for i = 1:integrationsende
    
    NTU = trapz(temp,F,1);
    
end

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Jakob B. Nielsen 2020-1-31

First, what does your 2nd for loop do? You dont have i anywhere in the loop.

Second - without knowing your various variable values it is hard for us to run the code and see the error you get - so can you either post the error, or include your values in the code strip you posted?

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回答（1 个）

J. Alex Lee 2020-1-31

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/502964-using-trapz-doesn-t-work#answer_413036

F is of size 1x1, not 1x24000

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J. Alex Lee 2020-2-1

Hmm, you might want to re-think your overall strategy. What do you mean you "solved" the upper equation? Eq. 20 and 21 look like coupled ODE...what do the double primes mean

and

? Second derivatives? What are your boundary conditions? Is it possible to combine Eqs. 20, 21, and 22 into a single ODE or integral, or express Eq. 22 differently based on Eqs. 20 and 21?

But for the problem at hand of trapz not appearing to work when it seems like it should, can you provide the variables temp and F in a .mat file?

In general, trapz should be replaceable by manually doing a trapezoid rule, but your suggestion doesn't make a whole lot of sense to me, based on your original inputs being temp and FF...

Michael Käufl 2020-2-4