def Nu_vertical_cylinder_Griffiths_Davis_Morgan(Pr, Gr, turbulent=None):
    r'''Calculates Nusselt number for natural convection around a vertical
    isothermal cylinder according to the results of [1]_ correlated by [2]_, as
    presented in [3]_ and [4]_.

    .. math::
        Nu_H = 0.67 Ra_H^{0.25},\; 10^{7} < Ra < 10^{9}

        Nu_H = 0.0782 Ra_H^{0.357}, \; 10^{9} < Ra < 10^{11}

    Parameters
    ----------
    Pr : float
        Prandtl number [-]
    Gr : float
        Grashof number [-]
    turbulent : bool or None, optional
        Whether or not to force the correlation to return the turbulent
		 result; will return the laminar regime if False; leave as None for
        automatic selection

    Returns
    -------
    Nu : float
        Nusselt number, [-]

    Notes
    -----
    Cylinder of diameter 17.43 cm, length from 4.65 to 263.5 cm. Air as fluid.
    Transition between ranges is not smooth.
    If outside of range, no warning is given.

    Examples
    --------
    >>> Nu_vertical_cylinder_Griffiths_Davis_Morgan(.7, 2E10)
    327.6230596100138

    References
    ----------
    .. [1] Griffiths, Ezer, A. H. Davis, and Great Britain. The Transmission of
       Heat by Radiation and Convection. London: H. M. Stationery off., 1922.
    .. [2] Morgan, V.T., The Overall Convective Heat Transfer from Smooth
       Circular Cylinders, in Advances in Heat Transfer, eds. T.F. Irvin and
       J.P. Hartnett, V 11, 199-264, 1975.
    .. [3] Popiel, Czeslaw O. "Free Convection Heat Transfer from Vertical
       Slender Cylinders: A Review." Heat Transfer Engineering 29, no. 6
       (June 1, 2008): 521-36. doi:10.1080/01457630801891557.
    .. [4] Boetcher, Sandra K. S. "Natural Convection Heat Transfer From
       Vertical Cylinders." In Natural Convection from Circular Cylinders,
       23-42. Springer, 2014.
    '''
    Ra = Pr*Gr
    if turbulent or (Ra > 1E9 and turbulent is None):
        Nu = 0.0782*Ra**0.357
    else:
        Nu = 0.67*Ra**0.25
    return Nu
r'''Calculates Nusselt number for natural convection around a vertical
    isothermal cylinder according to the results of [1]_ correlated by [2]_, as
    presented in [3]_ and [4]_.

    .. math::
        Nu_H = 0.67 Ra_H^{0.25},\; 10^{7} < Ra < 10^{9}

        Nu_H = 0.0782 Ra_H^{0.357}, \; 10^{9} < Ra < 10^{11}

    Parameters
    ----------
    Pr : float
        Prandtl number [-]
    Gr : float
        Grashof number [-]
    turbulent : bool or None, optional
        Whether or not to force the correlation to return the turbulent
		 result; will return the laminar regime if False; leave as None for
        automatic selection

    Returns
    -------
    Nu : float
        Nusselt number, [-]

    Notes
    -----
    Cylinder of diameter 17.43 cm, length from 4.65 to 263.5 cm. Air as fluid.
    Transition between ranges is not smooth.
    If outside of range, no warning is given.

    Examples
    --------
    >>> Nu_vertical_cylinder_Griffiths_Davis_Morgan(.7, 2E10)
    327.6230596100138

    References
    ----------
    .. [1] Griffiths, Ezer, A. H. Davis, and Great Britain. The Transmission of
       Heat by Radiation and Convection. London: H. M. Stationery off., 1922.
    .. [2] Morgan, V.T., The Overall Convective Heat Transfer from Smooth
       Circular Cylinders, in Advances in Heat Transfer, eds. T.F. Irvin and
       J.P. Hartnett, V 11, 199-264, 1975.
    .. [3] Popiel, Czeslaw O. "Free Convection Heat Transfer from Vertical
       Slender Cylinders: A Review." Heat Transfer Engineering 29, no. 6
       (June 1, 2008): 521-36. doi:10.1080/01457630801891557.
    .. [4] Boetcher, Sandra K. S. "Natural Convection Heat Transfer From
       Vertical Cylinders." In Natural Convection from Circular Cylinders,
       23-42. Springer, 2014.