docs: def Geldart_Ling(mp, rhog, D, mug):
      r'''Calculates saltation velocity of the gas for pneumatic conveying,
      according to [1]_ as described in [2]_ and [3]_.
  
      if Gs/D < 47000, use equation 1, otherwise use equation 2.
  
      .. math::
          V_{salt} = 1.5G_s^{0.465}D^{-0.01} \mu^{0.055}\rho_f^{-0.42}
  
          V_{salt} = 8.7G_s^{0.302}D^{0.153} \mu^{0.055}\rho_f^{-0.42}
  
          Fr_s = 15\mu^{0.25}\left(\frac{d_p}{D}\right)^{0.1}
  
          Fr_s = \frac{V_{salt}}{\sqrt{gD}}
  
          \mu = \frac{m_p}{\frac{\pi}{4}D^2V \rho_f}
  
          G_s = \frac{m_p}{A}
  
      Parameters
      ----------
      mp : float
          Solid mass flow rate, [kg/s]
      rhog : float
          Gas density, [kg/m^3]
      D : float
          Diameter of pipe, [m]
      mug : float
          Gas viscosity, [Pa*S]
  
      Returns
      -------
          V : float
              Saltation velocity of gas, [m/s]
  
      Notes
      -----
      Model is rearanged to be explicit in terms of saltation velocity
      internally.
  
      Examples
      --------
      >>> Geldart_Ling(1., 1.2, 0.1, 2E-5)
      7.467495862402707
  
      References
      ----------
      .. [1] Weber, M. 1981. Principles of hydraulic and pneumatic conveying in
         pipes. Bulk Solids Handling 1: 57-63.
      .. [2] Rabinovich, Evgeny, and Haim Kalman. "Threshold Velocities of
         Particle-Fluid Flows in Horizontal Pipes and Ducts: Literature Review."
         Reviews in Chemical Engineering 27, no. 5-6 (January 1, 2011).
         doi:10.1515/REVCE.2011.011.
      .. [3] Gomes, L. M., and A. L. Amarante Mesquita. "On the Prediction of
         Pickup and Saltation Velocities in Pneumatic Conveying." Brazilian
         Journal of Chemical Engineering 31, no. 1 (March 2014): 35-46.
         doi:10.1590/S0104-66322014000100005
      '''
      Gs = mp/(pi/4*D**2)
      if Gs/D <= 47000:
          V = 1.5*Gs**0.465*D**-0.01*mug**0.055*rhog**-0.42
      else:
          V = 8.7*Gs**0.302*D**0.153*mug**0.055*rhog**-0.42
      return V