# Mechanisms of Sweep on the Performance of Transonic Centrifugal Compressor Impellers
## 5 Aerodynamic Effects of Sweep Feature
### Effect on the Tip Leakage Vortex  
Limiting streamlines with static pressure contour are presented in Figure 19. In the datum case, the thickened boundary layer downstream the shock radially migrates to the tip, increasing the blockage. The shock is therefore pushed further upstream and away from casing, resulting in a stronger shock effect and thus the shock-boundary layer interaction. In the OSA and the MSA case, the radial migrating process has been successfully suppressed. The forward sweep restricts the radial migration of low-momentum flow by two mechanisms. Firstly, the reduced shock strength results in a thinner boundary layer and decreases the amount of boundary layer low-momentum flow. Secondly, the leading edge near the tip starts earlier when sweeping forward. The high-pressure region of the tip section is moved more upstream, which increases the tip pressure and counteracts with the outward spanwise pressure gradient.  
As observed in the datum case in Figure 20, the radially migrated flow of the boundary layer mixes with the tip leakage flow, forming the tip leakage vortex. The vortex is gradually dissipated, and finally joins the downstream wake region in the passage between main blade and splitter I. However, in the OSA case, the boundary layer flow and the tip leakage flow decouple with each other due to the suppressed radial migration of low-momentum flow. The tip leakage vortex is therefore restrained, and the downstream wake loss is decreased, which corresponds to the gradual efficiency increase from Section I to the exit as illustrated in Figure 11. At impeller exit, the improved efficiency near the casing between main blade and splitter I is captured in Figure 12b.  
Figure 19: Contours of static pressure and streamlines on the suction surface of main blade.  
Figure 18: Contours of relative Mach number at 95% span.  
In the datum case, a tip leakage vortex is also observed near splitter II, as presented in Figure 21. The vortex is formed from the interaction between the incoming main flow and the tip leakage flow. The high momentum main flow immediately separates from the suction surface due to the large incidence at splitter II, which is induced by the slip effect. The tip leakage flow across the tip gap of splitter II is driven by the pressure difference (or tip loading). Then the vortex is gradually dissipated, and finally joins the downstream wake region in the passage between main blade and splitter II. In the OSA case, because of the unloading effect of splitter sweep, both the main flow separation and the tip leakage flow have been constrained. Hence, the tip leakage vortex and the downstream wake loss have been controlled. The efficiency then gradually increases from Section II to the exit with a steeper slope than that from Section I to Section II, as illustrated in Figure 11. It is also confirmed in Figure 12b that the reduced wake loss improved the impeller exit efficiency near the casing between main blade and splitter II. To control flow separations at splitter tip, an alternative choice of splitter sweep is the splitter tip re-cambering, where the incidence is controlled by readjusting the inlet blade angle [39].