The three flow regions are designated A, B and C as shown in Fig. 10. Region A represents the main through-flow in the fan and is where most of the useful work is done. Two-stage action occurs as the flow passes first through the suction arc blading (first stage) and then through the discharge arc (second stage). The flow contracts as it moves across the impeller producing high velocities at the second stage. The flow leaves the impeller and contracts again as it turns and squeezes around the vortex. It then expands rapidly, diffusing to the fan exit. The diffusion process may be augmented by the vortex. The combination of all these effects results in high pressure coefficient capability. The action on the fluid by the blades in region B resembles that of a paddle wheel, so the energy transfer is low. Region B is a necessary consequence of the cross-flow phenomenon; however, it has little effect on the overall performance except with regard to its influence in determining the shape of the through-flow region. Region C represents the eccentric vortex. This region consists entirely of re-circulating flow, so no useful work is done there and its primary affects are energy dissipation and shaping of region A. Note that in the frame of reference of the rotating blades, the flow reverses twice per revolution with blade leading edges becoming trailing edges each time, so regions B and C are fundamentally inefficient, but they are also unavoidable. As noted earlier, the natural flow is characterized by an approximately 90° turn in the mean flow, a key consideration in relation to aircraft application in terms of efficient management of the fan intake and exit flows.