Warning: Imaginary parts of complex X and/or Y arguments ignored.

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Opariuc Andrei
Opariuc Andrei on 6 Dec 2021
Commented: Jon on 6 Dec 2021
Error when attempting to plot ,the first imaginary result comes from Q3 afterwards everything that involves Q3 results as imaginary , i tried doing q3=sqrt(h3)*k; Q3=real(q3); and got rid of the imaginary result (i think) but when trying to plot my plot is empty
%% initial data
p32 = [1.22 1.225 1.225 1.22 0.765 0.085];
p33 = [0.660 0.660 0.665 0.655 0.435 0.085];
p34 = [0.830 0.830 0.830 0.815 0.540 0.105];
p39 = [0.705 0.705 0.730 0.690 0.460 0.085];
p40 = [0.410 0.410 0.405 0.400 0.285 0.090];
d=0.051;
k=0.064;
niu=1.01*10^(-6);
g=9.81;
L1=0.5 ;
L2= 1;
L=1.5;
%%
h1=p32-p33;
h2=p34-p39;
h3=p39-p40;
Q1=sqrt(h1)*k;
Q2=sqrt(h2)*k;
Q3=sqrt(h3)*k
S=(pi*d^2)/4;
V1=Q1/S;
V2=Q2/S;
V3=Q3/S;
Re1=(V1*d)/niu;
Re2=(V2*d)/niu;
Re3=(V3*d)/niu;
j1=h1/L;
j2=h2/L;
j3=h3/L;
he1=h1+j1*(L1+L2);
he2=h2+j2*(L1+L2);
he3=h3+j3*(L1+L2);
re1=Q1*4/pi*d^2;
re2=Q2*4/pi*d^2;
re3=Q3*4/pi*d^2;
zita1=he1*2*g/(re1.^2);
zita2=he2*2*g/(re2.^2);
zita3=he3*2*g/(re3.^2);
he1=zita1*re1.^2/2*g;
he2=zita2*re2.^2/2*g;
he3=zita3*re3.^2/2*g;
%%
figure
plot(Re1,real(zita1),'c');hold on;
plot(Re2,real(zita2),'r');
plot(Re3,real(zita3)','g');
grid on;axis tight;

Accepted Answer

Jon
Jon on 6 Dec 2021
Haven't tried your code, but most likely one or more of h1, h2, or h3 are negative and when you take the square root of them it generates a complex value. In the future please include the error in the body of your question and put a more descrptive subject in
  1 Comment
Jon
Jon on 6 Dec 2021
I looked at your code a little more. The problem is in the last element of h3. Since p39(4) = 0.085 and p40(4) = 0.090 the last element of h3 = p39-p40 is negative. Then on line 21 you compute Q3 as sqrt(h3)*k which gives a purely imaginary result in the fourth element of Q3, and as you say the problems cascade from there.
Rather than trying to mask the problem by taking just real parts I think you need to figure out what your physical interpertation is of the negative, what looks like a pressure difference, is in the fourth term of h3.
Looks like maybe the flow is reversed. If you just care about the magnitude of the flows and not the directions I would suggest computing
h3 = sqrt(abs(h3))*k

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