## 3 Results
### Typical Flow Structures in the Stator Hub Corner  
From above, there were five typical flow structures in the stator hub corner as the stator hub clearance varies, as shown in Figures 10 and 11. The first one, named Type A, features as the traditional hub corner separation, which mainly existed at large mass flow rate conditions with zero stator hub clearance. When the stator hub corner was predominated by Type A, the induced flow blockage at the  
Figure 9: The evolution of flow topologies on the suction surface and hub with the variation of hub clearance at the near stall condition (NS) condition. stator outlet was very small. The second one, named Type B, features as the traditional hub corner stall, which mainly exists at small mass flow rate conditions with zero stator hub clearance. When the stator hub corner is predominated by Type B, the induced flow blockage at the stator outlet is relatively large. No matter for Type A or Type B, the hub corner flow blockage region at the stator outlet was more like a triangle, i.e., the transition from the hub corner to the blade wake is relatively smooth. The third one, named Type C, features as the leakage flow, which mainly exists at large mass flow rate conditions with large stator hub clearances. When the stator hub corner was predominated by Type C, the induced flow blockage region at the stator outlet is relatively small and the shape of the blockage region is more like an ellipse, of which the long axis was along the pitchwise direction and the short axis was along the spanwise direction, besides, the transition from the hub corner to the blade wake was abrupt. The fourth one, named Type D, features as the coexistence of the leakage flow and the hub corner separation, which mainly exists at small mass flow rate conditions with large stator hub clearances or large mass flow rate conditions with very small stator hub clearances. When the stator hub corner was predominated by Type D, the induced flow blockage region at the stator outlet was between the one of Type A and Type B, no matter in the scale or the shape. The last one, named Type E, features as the coexistence of the leakage flow and the hub corner stall, which mainly exists at small mass flow rate conditions with small stator hub clearances. When the stator hub corner was predominated by Type E, the induced flow blockage region at the stator outlet was much like the one of Type B.  
Figure 11: Secondary flows that dominate the stator hub corner. Figure 10: Typical flow topologies inside the stator passage. From Figure 11 it can be seen that when the stator hub clearance was big enough, the corner stall did not occur in the whole compressor operating range, and from the rear analysis it can be seen that the stator incidence almost does not change with the variation of the stator hub clearance, which means that the critical incidence phenomenon was not applicable again when stator hub clearance exists (when the stator hub clearance is not very small). Actually, corner separations usually originate from the migrant of the low energy boundary layer flow in the end wall under the transverse pressure gradient from the blade pressure side to the blade suction side, then with the effect of the streamwise adverse pressure gradient, the streamline turned to the upstream direction. With the increment of the blade loading, the streamwise adverse pressure gradient increases, and the streamline curvature becomes severer and severer, until the streamline turns 180\({}^{\circ}\), the corner stall will occur without warning and large flow blockage and loss will be introduced [2].