In this case you view a cross section in the xy-plane of the actual 3D geometry. The geometry is mathematically extended to infinity in both directions along the
z-axis, assuming no variation along that axis. All the total flows in and out of boundaries are per unit length along the
z-axis. A simplified way of looking at this is to assume that the geometry is extruded one unit length from the cross section along the
z-axis. The total flow out of each boundary is then from the face created by the extruded boundary (a boundary in 2D is a line).
The spatial coordinates are called r and
z, where
r is the radius. The flow at the boundaries is given per unit length along the third dimension. Since this dimension is a revolution, you have to multiply all flows with
αr, where
α is the revolution angle (for example, 2
π for a full turn).