def Rayleigh(Pr, Gr):
    """Calculates Rayleigh number or `Ra` using Prandtl number `Pr` and
    Grashof number `Gr` for a fluid with the given
    properties, temperature difference, and characteristic length used
    to calculate `Gr` and `Pr`.

    .. math::
        Ra = PrGr

    Parameters
    ----------
    Pr : float
        Prandtl number []
    Gr : float
        Grashof number []

    Returns
    -------
    Ra : float
        Rayleigh number []

    Notes
    -----
    Used in free convection problems only.

    Examples
    --------
    >>> Rayleigh(1.2, 4.6E9)
    5520000000.0

    References
    ----------
    .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
       Eighth Edition. McGraw-Hill Professional, 2007.
    .. [2] Cengel, Yunus, and John Cimbala. Fluid Mechanics: Fundamentals and
       Applications. Boston: McGraw Hill Higher Education, 2006.
    """
    return Pr * Gr
