Using the force output calculated in one script as an input to mass/spring damper

Question

Richard Gilchrist 2016-7-4

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/293654-using-the-force-output-calculated-in-one-script-as-an-input-to-mass-spring-damper

回答： Ondrej Piroh 2022-4-6

Hi, I'm very much new to Matlab so would appreciate some guidance with a problem I'm having.

Background: I'm trying to build a very simple model of an offshore wind turbine using a spring and damper model. I have a script whose output is a force that varies with time. I also have a function called 'myspring' that solves the position and velocity of a mass on a spring, considering damping and driving force.

I want to use the force I have calculated using my script as the driving force input to 'myspring', but I can't figure out how I can make it work.

Here is what I have. Firstly the 'Morison' script to calculate the force. The content is not important - it could be any force.

% Morison equation force on body
% Define the variables
H = 3.69;           % Wave height (m)
A = H/2;            % Wave amplitude (m)
Tw = 9.87;           % Wave period (s)
omega = (2*pi)/Tw;  % Angular frequency (rad/s)
lambda = 128.02;    % Wavelength (m)
k = (2*pi)/lambda;  % Wavenumber (1/m)
dw = 25;             % Water depth (m)
Cm = 2;             % Mass coefficient (-)
Cd = 0.7;           % Drag coefficient (-)
rho = 1025;         % Density of water (kg/m^3)
D = 3;              % Diameter of structure (m)
x = 0;              % Fix the position at x = 0 for simplicity
n = 100;            % Number of time steps
t = linspace(0,6*pi,n);   % Define the vector t with n time steps
eta = A*cos(k*x - omega*t); % Create the surface elevation vector with size equal to t
F_M = zeros(1,n); % Initiate an inertia force vector with same dimension as t
F_D = zeros(1,n); % Initiate a drag force vector with same dimension as t
F = zeros(1,n); % Initiate a total force vector with same dimension as t
fun_inertia = @(z)cosh(k*(z+dw));  % Define the inertia function to be integrated
fun_drag = @(z)(cosh(k*(z+dw)).*abs(cosh(k*(z+dw))));   % Define the drag function to be integrated
for i = 1:n
    F_D(i) = abs(((H*pi)/Tw) * (1/sinh(k*dw)) * cos(k*x - omega*t(i))) * ((H*pi)/Tw) * (1/sinh(k*dw)) * cos(k*x - omega*t(i)) * integral(fun_drag,-dw,eta(i));
    F_M(i) = (Cm*rho*pi*(D^2)/4) * ((2*H*pi^2)/(Tw^2)) * (1/(sin(k*dw))) * sin(k*x - omega*t(i)) * integral(fun_inertia,-dw,eta(i));
    F(i) = F_D(i) + F_M(i);
end

and now 'myspring.m':

function pdot = myspring(t,p,c,k,m)
w = sqrt(k/m);
g = sin(t);  % This is the forcing function
pdot = zeros(size(p));
pdot(1) = p(2);
pdot(2) = g - c*p(2) - (w^2)*p(1);
end

where k = spring stiffness, c = damping, m = mass. These values are specified before running ode45 with 'myspring'.

So in summary, is there a way I can replace my 'g = sin(t)' driving function which I have written into 'myspring' with the force calculated in my 'Morison' script? Any support is appreciated.

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Answer 1

Thorsten 2016-7-4

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/293654-using-the-force-output-calculated-in-one-script-as-an-input-to-mass-spring-damper#answer_227626

编辑：Thorsten 2016-7-4

在 MATLAB Online 中打开

Turn your script into a function by adding these lines on top of your script

function F = morison(t)
if nargin == 0 % define default argument
  t = linspace(0,6*pi,n);   % Define the vector t with n time steps
end

and remove from the body of the function

t = linspace(0,6*pi,n);

You can then use the function "morison" as follows:

g = morison(t);

Just like a build-in function like sin.

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Richard Gilchrist 2016-7-5

编辑：Richard Gilchrist 2016-7-5

在 MATLAB Online 中打开

Hi Thorsten,

My lack of understanding is more fundamental. The Morison thing will come later, but what I'm trying to do is make the function 'myspring' accept any arbitrary driving force that I specify when running ode45. I've managed to make it accept k, c and m values, but when I try to give it a driving force it gives me this

In an assignment  A(:) = B, the number of elements in A and B must be the same.
Error in myspring (line 9)
pdot(2) = g - c*p(2) - (w^2)*p(1);
Error in @(t,p)myspring(t,p,c,k,m,g)
Error in odearguments (line 87)
f0 = feval(ode,t0,y0,args{:});   % ODE15I sets args{1} to yp0.
Error in ode45 (line 115)
    odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

If I give it a 'g' that doesn't depend on time (for troubleshooting purposes) it works e.g. g=2 allows it to run.

To get this error message, I had created a variable g = sin(t), which I intended to use as the driving force.

>> k = 2; m = 5; c = 1.5; t = linspace(0,10,100);
>> x0 = [1 0];
>> g = sin(t);
>> [t,x] = ode45(@(t,p)myspring(t,p,c,k,m,g),t,x0);

Here's what 'myspring' looks like now:

function pdot = myspring(t,p,c,k,m,g)
%g = sin(t); % Driving force
w = sqrt(k/m);
pdot = zeros(size(p));
pdot(1) = p(2);
pdot(2) = g - c*p(2) - (w^2)*p(1);
end