def Prandtl(Cp=None, k=None, mu=None, nu=None, rho=None, alpha=None):
    """Calculates Prandtl number or `Pr` for a fluid with the given
    parameters.

    .. math::
        Pr = \\frac{C_p \\mu}{k} = \\frac{\\nu}{\\alpha} = \\frac{C_p \\rho \\nu}{k}

    Inputs can be any of the following sets:

    * Heat capacity, dynamic viscosity, and thermal conductivity
    * Thermal diffusivity and kinematic viscosity
    * Heat capacity, kinematic viscosity, thermal conductivity, and density

    Parameters
    ----------
    Cp : float
        Heat capacity, [J/kg/K]
    k : float
        Thermal conductivity, [W/m/K]
    mu : float, optional
        Dynamic viscosity, [Pa*s]
    nu : float, optional
        Kinematic viscosity, [m^2/s]
    rho : float
        Density, [kg/m^3]
    alpha : float
        Thermal diffusivity, [m^2/s]

    Returns
    -------
    Pr : float
        Prandtl number []

    Notes
    -----
    .. math::
        Pr=\\frac{\\text{kinematic viscosity}}{\\text{thermal diffusivity}} = \\frac{\\text{momentum diffusivity}}{\\text{thermal diffusivity}}

    An error is raised if none of the required input sets are provided.

    Examples
    --------
    >>> Prandtl(Cp=1637., k=0.010, mu=4.61E-6)
    0.754657
    >>> Prandtl(Cp=1637., k=0.010, nu=6.4E-7, rho=7.1)
    0.7438528
    >>> Prandtl(nu=6.3E-7, alpha=9E-7)
    0.7000000000000001

    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.
    .. [3] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
       Berlin; New York:: Springer, 2010.
    """
    if k and Cp and mu:
        return Cp * mu / k
    elif nu and rho and Cp and k:
        return nu * rho * Cp / k
    elif nu and alpha:
        return nu / alpha
    else:
        raise Exception('Insufficient information provided for Pr calculation')
