error in computing log values

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F SHEFIN 2021-12-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1601035-error-in-computing-log-values

评论： F SHEFIN 2021-12-6

Hi sir..My simulink model runs Good for certain values of input...but for some values it gives a complex computation error.I have used a algebraic constraint solver.

An error occurred while running the simulation and the simulation was terminated

Caused by:

Domain error. To compute complex results from real x, use 'log(complex(x))'.
Error in log.m (line 14) coder.internal.error('Coder:toolbox:ElFunDomainError',mfilename); Error in 'ExitMoles/OutletEnthalpy' (line 20)

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F SHEFIN 2021-12-2

编辑：Walter Roberson 2021-12-2

在 MATLAB Online 中打开

function [h1,h2,h3,h4,h5] = OutletEnthalpy(Tad)
CH4 = 0;
N2 = 0;
O2 = 0;
CO2 = 0;
H2O = 0;
if Tad < 1000
    % CH4
    a1ch4 = -1.7669e+05;
    a2ch4 = 2.7862e+03;
    a3ch4 = -12.0258;
    a4ch4 = 0.0392;
    a5ch4 = -3.6191e-05;
    a6ch4 = 2.0269e-08;
    a7ch4 = -4.9767e-12;
    b1ch4 = -2.3313e+04;
    b2ch4 = 89.0432; %#ok<NASGU>
    % H of CH4
    CH4 = (((-a1ch4)/Tad^2)+((a2ch4*log(Tad))/Tad)+a3ch4+((a4ch4*Tad)/2)+((a5ch4*Tad^2)/3)+((a6ch4*Tad^3)/4)+((a7ch4*Tad^4)/5)+(b1ch4/Tad))*(8.31*Tad);
else 
    
    a1ch4 = 3730042.76000000;
    a2ch4 = 1034972.09600000;
    a3ch4 = -2412.69856200000;
    a4ch4 = 4.64611078000000;
    a5ch4 = 0.00229199830700000;
    a6ch4 = -6.83683048000000e-07;
    a7ch4 = 9.42646893000000e-11;
    b1ch4 = -4.82238053000000e-15;
    b2ch4 = -121.912488900000; %#ok<NASGU>
    % H of CH4
    CH4 = (((-a1ch4)/Tad^2)+((a2ch4*log(Tad))/Tad)+a3ch4+((a4ch4*Tad)/2)+((a5ch4*Tad^2)/3)+((a6ch4*Tad^3)/4)+((a7ch4*Tad^4)/5)+(b1ch4/Tad))*(8.31*Tad);  %HERE
end
h1 = CH4;

Here i get error in the bolded line...my simulation stops at this line...for certain values of input..

Walter Roberson 2021-12-2

Simulink models can be attached to posts here using the paperclip icon of the editor.

F SHEFIN 2021-12-2

ExitMoles.slx

Sir...i have attached the model..

there are 4 inputs..actualyy...T,Ratio,Fuel,Per the output is next to the solver...

pls keep T as 100 at every trail.

For all values of Ratio @ 1 per i am getting my answer..

When ratio changes to 1 simulink doesnt compute @ 0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9 ...but computes @ 1 per

when ratio changes ti 2 , simulink doesnt compute @ 0.3 to 0.9....but computes @ 0.1, 0.2,1 per.

similarly for ratio 3 and 4 some values are alone getting computed,

But when ratio is 5 i am able to compute @ all the per values.

It is not simulating for some values...i hope i was clear withe the problem. Thank you

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回答（1 个）

Mathieu NOE 2021-12-2

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1601035-error-in-computing-log-values#answer_845920

ExitMoles.slx

hello

the "log" error was just a consequence of an algebraic loop of OutletEnthalpy bloc : (its input is dependant of the P output)

so probably log of zero or NaN or negative Tad would trigger the error.

simply added a memory block on the P input signal of the OutletEnthalpy bloc and seems the simulation runs fine now (as far I can tell)

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Mathieu NOE 2021-12-2

在 MATLAB Online 中打开

ok I removed the memory block

the simulation seems to work for a few Per values (either low 0 to 0.2 ) or high (0.9 and 1)

In between those two areas we get an error

now I wanted to test the continuity of the laws that are coded in the OutletEnthalpy block

if I look at the first equations for CH4 and plot it for the two case (Tad<1000 or >=1000) , there is a significant discontinuity at tad = 1000 - and that worries me. Shouldn't be the two laws coincident at Tad = 1000 ?

% test CH4
 Tad = 1:10:1000;
    % CH4
    a1ch4 = -1.7669e+05;
    a2ch4 = 2.7862e+03;
    a3ch4 = -12.0258;
    a4ch4 = 0.0392;
    a5ch4 = -3.6191e-05;
    a6ch4 = 2.0269e-08;
    a7ch4 = -4.9767e-12;
    b1ch4 = -2.3313e+04;
    b2ch4 = 89.0432; %#ok<NASGU>
     % H of CH4
    CH4 = (((-a1ch4)./Tad.^2)+((a2ch4*log(Tad))./Tad)+a3ch4+((a4ch4*Tad)/2)+((a5ch4*Tad.^2)/3)+((a6ch4*Tad.^3)/4)+((a7ch4*Tad.^4)/5)+(b1ch4./Tad)).*(8.31.*Tad);
 
    plot(Tad,CH4);hold on 
    
Tad = 1001:10:2000;    
    a1ch4 = 3730042.76000000;
    a2ch4 = 1034972.09600000;
    a3ch4 = -2412.69856200000;
    a4ch4 = 4.64611078000000;
    a5ch4 = 0.00229199830700000;
    a6ch4 = -6.83683048000000e-07;
    a7ch4 = 9.42646893000000e-11;
    b1ch4 = -4.82238053000000e-15;
    b2ch4 = -121.912488900000; %#ok<NASGU>
     % H of CH4
    CH4 = (((-a1ch4)./Tad.^2)+((a2ch4*log(Tad))./Tad)+a3ch4+((a4ch4.*Tad)/2)+((a5ch4*Tad.^2)/3)+((a6ch4*Tad.^3)/4)+((a7ch4*Tad.^4)/5)+(b1ch4./Tad)).*(8.31.*Tad);
    plot(Tad,CH4);hold off 