docs: def Weber(mp, rhop, dp, rhog, D, Vterminal=4):
      r'''Calculates saltation velocity of the gas for pneumatic conveying,
      according to [1]_ as described in [2]_, [3]_, [4]_, and [5]_.
  
      If Vterminal is under 3 m/s, use equation 1; otherwise, equation 2.
  
      .. math::
          Fr_s = \left(7 + \frac{8}{3}V_{terminal}\right)\mu^{0.25}
          \left(\frac{d_p}{D}\right)^{0.1}
  
          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}
  
      Parameters
      ----------
      mp : float
          Solid mass flow rate, [kg/s]
      rhop : float
          Particle density, [kg/m^3]
      dp : float
          Particle diameter, [m]
      rhog : float
          Gas density, [kg/m^3]
      D : float
          Diameter of pipe, [m]
      Vterminal : float
          Terminal velocity of particle settling in gas, [m/s]
  
      Returns
      -------
          V : float
              Saltation velocity of gas, [m/s]
  
      Notes
      -----
      Model is rearanged to be explicit in terms of saltation velocity
      internally.
  
      Examples
      --------
      Examples are only a self-test.
  
      >>> Weber(mp=1, rhop=1000., dp=1E-3, rhog=1.2, D=0.1, Vterminal=4)
      15.227445436331474
  
      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] Setia, G., S. S. Mallick, R. Pan, and P. W. Wypych. "Modeling
         Minimum Transport Boundary for Fluidized Dense-Phase Pneumatic Conveying
         Systems." Powder Technology 277 (June 2015): 244-51.
         doi:10.1016/j.powtec.2015.02.050.
      .. [4] Bansal, A., S. S. Mallick, and P. W. Wypych. "Investigating
         Straight-Pipe Pneumatic Conveying Characteristics for Fluidized
         Dense-Phase Pneumatic Conveying." Particulate Science and Technology
         31, no. 4 (July 4, 2013): 348-56. doi:10.1080/02726351.2012.732677.
      .. [5] 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
      '''
      if Vterminal <= 3:
          term1 = (7 + 8/3.*Vterminal)*(dp/D)**0.1
      else:
          term1 = 15.*(dp/D)**0.1
      term2 = 1./(g*D)**0.5
      term3 = mp/rhog/(pi/4*D**2)
      V = (term1/term2*term3**0.25)**(1/1.25)
      return V