If Statement In ode45() With Symbolic Variables

I have the following system of two Second-Order ODEs:

syms q(t) y(t)
structure_oscillator = diff(y,2) == -2 * zeta * diff(y) - y...
+ U(j)^2 / (4 * pi^2 * mass_ratio) * f * (q - 2 * pi * diff(y) / U(j)) +...
U(j)^2 * CFy / (4 * pi^2 * mass_ratio);

where zeta, mass_ratio and f are constants; U is a velocity vector and CFy varies in each time step.

wake_oscillator = diff(q,2) == 2 * beta * ni * diff(q) * (1 - 4 * f^2 / CLo^2 * q^2)... 
- ni^2 * q + lambda * diff(y,2) + ni^2 * 2 * pi * diff(y) / U(j);

where beta, ni, CLo and lambda are constants.

I solved these equations with:

[V,S] = odeToVectorField(structure_oscillator, wake_oscillator);
M = matlabFunction(V,'vars',{'t','Y'});
sol = ode45(M,[0 250],[0.01 0 0 0]);

The problem is: CFy is determined by polynomials as function of an angle α. These polynomials varies depending on α (I used spline to fit the curve, and it gaves me 13 breaks). So, for each value of α I need an If to determine which equation to use to calculate CFy.

How can I introduce a If statament to calculate CFy as α varies?

Thank you,

Rafael F.

[Walter Roberson]

is alpha effectively constant for any one run, a parameter to the system? or is it something that varies dynamically on external stimulus outside of the current variables? or does it vary dynamically with some of the variables?

[Rafael Fehér]

Hi, Mr. Roberson. Thank you for your reply.

Alpha is a parameter of the system. It is calculated as follow, for each time step:

As varies at each time step of the simulation, so does α. U is the velocity of the incoming flow.