Variable-geometry turbine (VGT) vanes involve partial tip and hub clearance leakage flows, while pivot-induced wake losses interact with leakage flows and complicate endwall secondary-flow structures, necessitating a reevaluation of conventional control strategies. In this study, numerical simulations are performed on an annular turbine stator cascade incorporating a pivot structure, where winglet and squealer-tip configurations with two characteristic scales are examined at vane angles of −10°, 0°, and +10°. The control effectiveness is assessed through comparisons of total pressure loss and its spanwise distribution, while the underlying mechanisms are analyzed using three-dimensional flow structures and clearance leakage mass-flow characteristics. Particular emphasis is placed on the evolution and interaction of the in-cavity vortex (ICV) and tip leakage vortex (TLV)/hub leakage vortex (HLV) under varying loading conditions. The results show that the presence of the pivot structure significantly alters the development of leakage flow, leading to a pronounced sensitivity of control effectiveness to vane-angle-induced loading variations. Under high-loading conditions, large-scale winglet and squealer configurations exhibit clear advantages over their smaller-scale counterparts, providing greater reductions in total pressure loss. Compared with squealer tips, winglet configurations deliver superior overall performance across all operating conditions by increasing the effective flow-path resistance of the leakage flow and suppressing the radial growth of leakage vortices. In addition, the interaction mechanisms between the ICV and leakage vortices in squealer configurations are clarified, revealing the influence of vane-angle-dependent non-uniform partial radial clearance on control effectiveness. The findings provide physical insight into leakage-flow control mechanisms and guidance for endwall aerodynamic design in variable geometry turbines.
Chen et al. (Mon,) studied this question.