docs: def Nu_cylinder_Zukauskas(Re, Pr, Prw=None):
      r'''Calculates Nusselt number for crossflow across a single tube at a
      specified Re. Method from [1]_, also shown without modification in [2]_.
  
      .. math::
          Nu_{D}=CRe^{m}Pr^{n}\left(\frac{Pr}{Pr_s}\right)^{1/4}
  
      Parameters
      ----------
      Re : float
          Reynolds number with respect to cylinder diameter, [-]
      Pr : float
          Prandtl number at free stream temperature [-]
      Prw : float, optional
          Prandtl number at wall temperature, [-]
  
      Returns
      -------
      Nu : float
          Nusselt number with respect to cylinder diameter, [-]
  
      Notes
      -----
      If Prandtl number at wall are not provided, the Prandtl number correction
      is not used and left to an outside function.
  
      n is 0.37 if Pr <= 10; otherwise n is 0.36.
  
      C and m are from the following table. If Re is outside of the ranges shown,
      the nearest range is used blindly.
  
      +---------+-------+-----+
      | Re      | C     | m   |
      +=========+=======+=====+
      | 1-40    | 0.75  | 0.4 |
      +---------+-------+-----+
      | 40-1E3  | 0.51  | 0.5 |
      +---------+-------+-----+
      | 1E3-2E5 | 0.26  | 0.6 |
      +---------+-------+-----+
      | 2E5-1E6 | 0.076 | 0.7 |
      +---------+-------+-----+
  
      Examples
      --------
      Example 7.3 in [2]_, matches.
  
      >>> Nu_cylinder_Zukauskas(7992, 0.707, 0.69)
      50.523612661934386
  
      References
      ----------
      .. [1] Zukauskas, A. Heat transfer from tubes in crossflow. In T.F. Irvine,
         Jr. and J. P. Hartnett, editors, Advances in Heat Transfer, volume 8,
         pages 93-160. Academic Press, Inc., New York, 1972.
      .. [2] Bergman, Theodore L., Adrienne S. Lavine, Frank P. Incropera, and
         David P. DeWitt. Introduction to Heat Transfer. 6E. Hoboken, NJ:
         Wiley, 2011.
      '''
      if Re <= 40:
          c, m = 0.75, 0.4
      elif Re < 1E3:
          c, m = 0.51, 0.5
      elif Re < 2E5:
          c, m = 0.26, 0.6
      else:
          c, m = 0.076, 0.7
      if Pr <= 10:
          n = 0.37
      else:
          n = 0.36
      Nu = c*Re**m*Pr**n
      if Prw:
          Nu = Nu*(Pr/Prw)**0.25
      return Nu