Solving multiple cubic equations in loop
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I have 'l' cubic equations, and I want to extract the imaginary part of the root of these equations.
Below is my code. The part in the for loop is where i'm having trouble. I don't understand how to collect the imaginary part of the roots.
close all
clear all
clc
global a0 a1 a2 a3 We U
rhog = 1.2;
rhol = 1.0e3;
r = rhog/rhol;
sigma = 0.072; % surface tension
D = 0.0025; % diameter of jet
U = 1500; %cm/s
q = 3.886*10e-4 ;%cm2
x=1/2; %cm
h = q/x; %cm
% non-dimensional
We = 1/(rhol*(U/100)^2*h/(100*sigma)) % 1/Weber number
%------End of User defined constants------------------4
omega_max = 20000;
for l= 1:omega_max
w(l) = l+1; %frequency
a0(l) = (-r*w(l)^2/((U^2)*h*(1-We))); %Equation: a3*x^3 + a2*x^2 + a1*x + a0 = 0
a1(l) = (-w(l)^2/((U^2)*(1-We)));
a2(l) = (2*w(l)/(U*(1-We)));
a3(l) = 1;
k(l) = roots([a3(l) a2(l) a1(l) a0(l)]);
k_im(l) = imag(k(l))*ij;
end
plot( w/(2*22/7),k_im,'r*')
xlabel('frequency(Hz)')
ylabel('k imaginary (cm^-^1)')
1 个评论
Vladimir Sovkov
2020-1-21
A cubic equation has 3 complex roots provided by the function "roots" but you are trying to deliver it to one scalar element of a vector k(i), that is erroneous.
采纳的回答
Walter Roberson
2020-1-21
k(l) = roots([a3(l) a2(l) a1(l) a0(l)]);
That will fail because there are going to be 3 roots of a cubic and you are trying to write all 3 into a single slot k(l)
k(l, :) = roots([a3(l) a2(l) a1(l) a0(l)]);
k_im(l, :) = imag(k(l,:))*ij;
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