code: def vel_grad_avg(self):
          
          return ((u.standard_gravity * self.HL) /
                 (pc.viscosity_kinematic(self.temp) * self.Gt)).to(u.s ** -1)
docs: Calculate the average velocity gradient (G-bar) of water flowing
          through the flocculator.
          :returns: Average velocity gradient (G-bar)
          :rtype: float * 1 / second