docs: def Nu_horizontal_cylinder_Kuehn_Goldstein(Pr, Gr):
      r'''Calculates Nusselt number for natural convection around a horizontal
      cylinder according to the Kuehn-Goldstein [1]_ correlation, also shown in
      [2]_. Cylinder must be isothermal.
  
      .. math::
          \frac{2}{Nu_D} = \ln\left[1 + \frac{2}{\left[\left\{0.518Ra_D^{0.25}
          \left[1 + \left(\frac{0.559}{Pr}\right)^{3/5}\right]^{-5/12}
          \right\}^{15} + (0.1Ra_D^{1/3})^{15}\right]^{1/15}}\right]
  
      Parameters
      ----------
      Pr : float
          Prandtl number [-]
      Gr : float
          Grashof number [-]
  
      Returns
      -------
      Nu : float
          Nusselt number, [-]
  
      Notes
      -----
      [1]_ suggests this expression is valid for all cases except low-Pr fluids.
      [2]_ suggests no restrictions.
  
      Examples
      --------
      >>> Nu_horizontal_cylinder_Kuehn_Goldstein(0.69, 2.63E9)
      122.99323525628186
  
      References
      ----------
      .. [1] Kuehn, T. H., and R. J. Goldstein. "Correlating Equations for
         Natural Convection Heat Transfer between Horizontal Circular Cylinders."
         International Journal of Heat and Mass Transfer 19, no. 10
         (October 1976): 1127-34. doi:10.1016/0017-9310(76)90145-9
      .. [2] Boetcher, Sandra K. S. "Natural Convection Heat Transfer From
         Vertical Cylinders." In Natural Convection from Circular Cylinders,
         23-42. Springer, 2014.
      '''
      Ra = Pr*Gr
      return 2./log(1 + 2./((0.518*Ra**0.25*(1. + (0.559/Pr)**0.6)**(-5/12.))**15
                    + (0.1*Ra**(1/3.))**15)**(1/15.))