def uch_yang(xy, d_pipe, gs, rhos, ut):
    """
    Provide functions to solve for voidage and choking velocity based on
    equations from Yang 1975. Requires SciPy fsolve function.

    Parameters
    ----------
    xy
        Solve for ep and uch using SciPy fsolve where
        ep = voidage [-] and uch = choking velocity [m/s]
    d_pipe : float
        Internal pipe diameter [m]
    gs : float
        Solids flux [kg/(s m^2)]
    rhos : float
        Density of particle [kg/m^3]
    ut : float
        Terminal velocity [m/s]

    Returns
    -------
    f1, f2 : fuctions
        Functions to solve for ep and uch

    References
    ----------
    Yang, 1975. A Mathematical Definition of Choking Phenomenon and a
    Mathematical Model for Predicting Choking Velocity and Choking Voidage.
    AIChE Journal, 21, 5, 1013-1015.
    """
    ep, uch = xy
    g = 9.81
    f1 = 2 * g * d_pipe * (ep ** -4.7 - 1) - 0.01 * (uch / ep - ut) ** 2
    f2 = gs - (uch / ep - ut) * rhos * (1 - ep)
    return f1, f2
