Fig. 34 shows the streamline patterns in and around the impeller region [30]. The streamlines in the impeller region are seen in the rotating frame, while the streamlines in the remaining regions are seen in the absolute frame. In the through-flow region, the figure shows that the impeller acts as a well-behaved cascade. In particular, the blades provide good flow guidance, with low flow incidence and deviation. Note that progressive incidence variation (positive and negative) and deviation occur at the boundaries of region A. Within the vortex cavity regions, large vortical/re-circulating flows exist and the cascade approximation is clearly not applicable there. It should be pointed out that the vortex region in this particular fan is quite large and the fan housing does not appear to be optimal in terms of minimizing losses in regions B and C. As will be discussed in Section 4.2, simplified modeling methods must be able to represent all flow regions in a coupled fashion as they all affect fan performance.