def vdowham(eta, vel_entrain, e_eff, r_eff):
    """

    Calculate the velocity parameter of the contact problem according to
    Dowson-Hamrock.

    Parameters
    ----------
    eta: ndarray, scalar
        The dynamic viscosity of the lubricant.
    vel_entrain: ndarray, scalar
        The entrainment velocity of the contact problem.
    e_eff: ndarray, scalar
        The effective modulus of the contact problem.
    r_eff: ndarray, scalar
        The effective radius of the contact problem.

    Returns
    -------
    param_velocity: ndarray, scalar
        The velocity parameter of the contact problem.

    """
    param_velocity = eta * vel_entrain / (e_eff * r_eff)
    return param_velocity