docs: def Nu_cylinder_Whitaker(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_D = (0.4 Re_D^{0.5} + 0.06Re_D^{2/3})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
      -----
      Developed considering data from 1 to 1E5 Re, 0.67 to 300 Pr, and range of
      viscosity ratios from 0.25 to 5.2. Found experimental data to generally
      agree with it within 25%.
  
      Examples
      --------
      >>> Nu_cylinder_Whitaker(6071, 0.7)
      45.94527461589126
  
      References
      ----------
      .. [1] Whitaker, Stephen. "Forced Convection Heat Transfer Correlations for
         Flow in Pipes, Past Flat Plates, Single Cylinders, Single Spheres, and
         for Flow in Packed Beds and Tube Bundles." AIChE Journal 18, no. 2
         (March 1, 1972): 361-371. doi:10.1002/aic.690180219.
      .. [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 = (0.4*Re**0.5 + 0.06*Re**(2/3.))*Pr**0.3
      if mu and muw:
          Nu *= (mu/muw)**0.25
      return Nu