How do I stop ode solver when output is complex number?

Question

Vikash Pandey 2018-11-15

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/430089-how-do-i-stop-ode-solver-when-output-is-complex-number

评论： Umar Abdurrehman 2021-2-16

在 MATLAB Online 中打开

I am currently working on a coupled ode problem, and using ode45 solver to solve it.

My code is something like this:

bubble.m

function rdot = f(t, r)
%Some mathematical expressions
....
...
...
rdot(1) = r(2); % first derivative of r1
rdot(2) = (X11 - X21 - (X31*(X41 - X51)))/X81; % second derivative of r1
rdot(3) = r(4); % first derivative of r2
rdot(4) = (X12 - X22 - (X32*(X42 - X52)))/X82; % second derivative of r2
rdot = rdot';

And,

bubble_plotter.m

clc;
clear all;
%close all;
time_range = [0 1d-4];
r1_eq = 10d-6;
r2_eq = 1d-6;
initial_conditions = [r1_eq 0.d0 r2_eq 0.d0];
[t, r] = ode45('bubble', time_range, initial_conditions);
r1_us=1000000*r(:,1);
r1dot=r(:,2);
r2_us=1000000*r(:,3);
r2dot=r(:,4);
%% Recovery of double derivatives
for i=1:numel(t)
  t_actual = t(i);
  r_actual = r(i,:);
  rdot(i,:) = bubble_mettin(t_actual,r_actual);
end
r1ddot = rdot(:,2);
r2ddot = rdot(:,4);
figure(101);
plot(normalized_time, r1_us, 'b-', normalized_time, r2_us, 'k-.');

For certain input parameter values, ode solver successfully runs only for first few t-values and outputs corresponding real positive values for r, which in the code is r(1) and r(3). But after that the solver starts outputing negative or complex values (that is, imaginary numbers) for r(1) and r(3). I wish the program stops right before it starts yielding complex or negative values. Also the so far calculated values of r(1), r(2), r(3), r(4), rdot(2) and rdot(4) must be receoverable.

Another condition to stop the solver could be that r(1) + r(3) should not excced a given value.

4 个评论
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Juan 2020-11-24

Would you please share how did you solve this issue?

Umar Abdurrehman 2021-2-16

please share how you solved it

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Answer 1

madhan ravi 2018-11-15

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/430089-how-do-i-stop-ode-solver-when-output-is-complex-number#answer_347143

编辑：madhan ravi 2018-11-15

在 MATLAB Online 中打开

see

https://www.mathworks.com/matlabcentral/answers/282449-how-to-stop-ode45-when-one-of-states-reach-certain-value#answer_220745

http://www.mathworks.com/help/matlab/ref/odeset.html#input_argument_d0e709348

use conditions as

isreal(r(1))==0 | r(1)<0 %an example isreal tells whether the number is real returns 0 if it's not

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madhan ravi 2018-11-15

在 MATLAB Online 中打开

Opt    = odeset('Events', @myEvent); %add this before ode45
[t, r] = ode45('bubble', time_range, initial_conditions);
save the below function separately:
function [value, isterminal, direction] = myEvent(t, r)
value      = isreal(r(1))==0 | r(1)<0 | isreal(r(3))==0 | r(3)<0;
isterminal = 1;   % Stop the integration
direction  = 0;
end