Take the surfaces 
, 
, 
, and 
 to be
  one unit length in the 
-direction.  (To figure out the correct
  direction of the normal vector 
 at a given surface point,
  note that the control volume in this case is the right half of the
  region in between two cylinders of radii 
 and 
 and of unit
  length in the 
-direction.  The vector 
 is a unit normal
  vector sticking out of this control volume.)