docs: def Nu_cylinder_Perkins_Leppert_1962(Re, Pr, mu=None, muw=None):
      r'''Calculates Nusselt number for crossflow across a single tube as shown
      in [1]_ at a specified `Re` and `Pr`, both evaluated at the free stream
      temperature. Recommends a viscosity exponent correction of 0.25, which is
      applied only if provided. Also shown in [2]_.
  
      .. math::
          Nu = \left[0.30Re^{0.5} + 0.10Re^{0.67}\right]Pr^{0.4}
          \left(\frac{\mu}{\mu_w}\right)^{0.25}
  
      Parameters
      ----------
      Re : float
          Reynolds number with respect to cylinder diameter, [-]
      Pr : float
          Prandtl number at free stream temperature, [-]
      mu : float, optional
          Viscosity of fluid at the free stream temperature [Pa*s]
      muw : float, optional
          Viscosity of fluid at the wall temperature [Pa*s]
  
      Returns
      -------
      Nu : float
          Nusselt number with respect to cylinder diameter, [-]
  
      Notes
      -----
      Considered results with Re from 40 to 1E5, Pr from 1 to 300; and viscosity
      ratios of 0.25 to 4.
  
      Examples
      --------
      >>> Nu_cylinder_Perkins_Leppert_1962(6071, 0.7)
      49.97164291175499
  
      References
      ----------
      .. [1] Perkins, Jr., H. C., and G. Leppert. "Forced Convection Heat
         Transfer From a Uniformly Heated Cylinder." Journal of Heat Transfer 84,
         no. 3 (August 1, 1962): 257-261. doi:10.1115/1.3684359.
      .. [2] Sanitjai, S., and R. J. Goldstein. "Forced Convection Heat Transfer
         from a Circular Cylinder in Crossflow to Air and Liquids." International
         Journal of Heat and Mass Transfer 47, no. 22 (October 2004): 4795-4805.
         doi:10.1016/j.ijheatmasstransfer.2004.05.012.
      '''
      Nu = (0.30*Re**0.5 + 0.10*Re**0.67)*Pr**0.4
      if mu and muw:
          Nu *= (mu/muw)**0.25
      return Nu