code: def cylinder(target, throat_diameter='throat.diameter'):
      r
      diams = target[throat_diameter]
      value = _pi/4*(diams)**2
      return value
docs: r"""
      Calculate throat cross-sectional area for a cylindrical throat
  
      Parameters
      ----------
      target : OpenPNM Object
          The object which this model is associated with. This controls the
          length of the calculated array, and also provides access to other
          necessary properties.
  
      throat_diameter : string
          Dictionary key of the throat diameter values