docs: def Nu_horizontal_cylinder_Morgan(Pr, Gr):
      r'''Calculates Nusselt number for natural convection around a horizontal
      cylinder according to the Morgan [1]_ correlations, a product of a very
      large review of the literature. Sufficiently common as to be shown in [2]_.
      Cylinder must be isothermal.
  
      .. math::
          Nu_D = C Ra_D^n
  
      +----------+----------+-------+-------+
      |  Gr min  |  Gr max  |  C    |  n    |
      +==========+==========+=======+=======+
      | 10E-10   |  10E-2   | 0.675 | 0.058 |
      +----------+----------+-------+-------+
      | 10E-2    |  10E2    | 1.02  | 0.148 |
      +----------+----------+-------+-------+
      | 10E2     |  10E4    | 0.850 | 0.188 |
      +----------+----------+-------+-------+
      | 10E4     |  10E7    | 0.480 | 0.250 |
      +----------+----------+-------+-------+
      | 10E7     |  10E12   | 0.125 | 0.333 |
      +----------+----------+-------+-------+
  
      Parameters
      ----------
      Pr : float
          Prandtl number [-]
      Gr : float
          Grashof number [-]
  
      Returns
      -------
      Nu : float
          Nusselt number, [-]
  
      Notes
      -----
      Most comprehensive review with a new proposed equation to date.
      Discontinuous among the jumps in range. Blindly runs outside if upper and
      lower limits without warning.
  
      Examples
      --------
      >>> Nu_horizontal_cylinder_Morgan(0.69, 2.63E9)
      151.3881997228419
  
      References
      ----------
      .. [1] 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.
      .. [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
      if Ra < 1E-2:
          C, n = 0.675, 0.058
      elif Ra < 1E2:
          C, n = 1.02, 0.148
      elif Ra < 1E4:
          C, n = 0.850, 0.188
      elif Ra < 1E7:
          C, n = 0.480, 0.250
      else:
          # up to 1E12
          C, n = 0.125, 0.333
      return C*Ra**n