Only getting zero as my output for ode45

Question

Harry Parker 2020-11-23

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/659038-only-getting-zero-as-my-output-for-ode45

评论： Harry Parker 2020-11-26

Hi,

So I'm producing a model based on a paper I've recently reviewed. When entering it into ode45 it only ouputs 0 as the value of X. I believe this is because X0 = 0. Though this is the correct initial condition is there anyway to work around this.

function dxdt = microalgal_model(t, X)
    uxmax = 0.275; %h-1
    %fxS variables
    kxs = 0.0624; %gSL-1  
    kixs = 12.40; %gSL-1
    %fxN variables
    kxn = 0.0812; %gNL-1
    kixn = 0.6250; %gNL-1
    %fxI variables
    kxi = 45.50; %uEm-2s-2
    kixi = 297.75; %uEm-2s-2
    o = 0.7419; %gX-1Lm-1
    l = 0.25; %m
    %fxT variables
    aox = 0.6175; %h-1
    eax = 25.9243; %Kcalmol-1
    r = 0.0099858775; %Kcalmol-1
    to = 293; %K
    box = 0.1101; %h-1
    ebx = 48.0151; %Kcalmol-1
    S = 1.05;
    N = 0.098;
    Io =125;
    T = 303;
    fxS = S / (S + kxs + ((S^2)/kixs));
    fxN = N / (N + kxn + ((N^2)/kixn));
    Il = Io*exp(-o*X*l);
    fxI = Il / (Il + kxi + ((Il^2)/kixi));
    fxT = aox * exp((-eax/r)*((1/T)-(1/to))) - box * exp ( (-ebx/r)*((1/T)-(1/to)));
    ux = uxmax * fxS * fxN * fxI * fxT;
    dxdt = ux * X;
end

tspan = [0  192];
X0 = 0;
[t, X] = ode45(@(t,X) microalgal_model(t,X), tspan, X0);
plot(t,X)

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Answer 1

Jon 2020-11-23

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/659038-only-getting-zero-as-my-output-for-ode45#answer_553548

The "problem" is that x0 = 0 is an equilibrium point of your system dx/dt = 0 when x = 0.

So if you start the system at x0 = 0 then it doesn't go anywhere, it just stays at zero.

Please try for a non-zero initial condition and you will see that it infact evolves to another value.

If you don't think x0 should be an equilibrium point then you need to look further into your equations in microalgal_model as currently they evaluate to zero when x = 0