Active Control of Blade Tip Flow in Turbomachinery

离散叶尖泄漏涡的起始点位于距前缘一定距离的吸力面上。高涡度涡核是由泄漏射流与主流之间的剪切层产生的。泄漏流中的流线在涡核周围盘绕。叶尖泄漏涡核内的涡线都与表面边界层内的涡线相连。吸力面边界层中的其他涡线与压力面边界层和铸壁边界层中的涡线相连。降低叶尖泄漏涡强度的机制有两种：一种是由于吸力面与铸壁边界层之间的涡线联动导致的排气涡量的降低，另一种是叶尖泄漏涡量的扩散。在低于峰值压升工况下，转子内部发生泄漏涡击穿。击穿的特点是存在一个滞止点，其后是一个气泡状的再循环区。击穿的发生使叶尖泄漏涡的性质发生了显著的变化：旋涡大幅膨胀，集中在旋涡中的流向涡量消失。膨胀对前缘上游有极大的堵塞作用。集中涡量的消失导致转子下游涡没有卷起，铸件上的压力槽消失。研究发现，泄漏涡击穿对近失速工况下转子的性能特性起着重要作用。在峰值压升工况下，随着流量的减小，沿流向、展向和俯仰方向的击穿区迅速增大。击穿的增长导致堵塞和损失的急剧增加。

The onset of the discrete tip leakage vortex is located on the suction surface at some distance from the leading edge. The vortex core with high vorticity is generated from a shear layer between the leakage jet flow and the main flow. The streamlines in the leakage flow are coiling around the vortex core. All the vortex-lines in the tip leakage vortex core link to ones in the surface boundary layer. The other vortex-lines in the suction surface boundary layer link to the vortex-lines in the pressure surface boundary layer and in the casting wall boundary layer. There are two mechanisms to reduce intensity of the tip leakage vortex : one is reduction of discharge vorticity caused by the linkage of vortex-lines between the suction surface and casting wall boundary layers, and another is diffusion of vorticity from the tip leakage vortex.The breakdown of the leakage vortex occurs inside the rotor at a lower flow rate than the peak pressure rise operating condition. The breakdown is characterized by the existence of the stagnation point followed by a bubble-like recirculation region. The onset of breakdown causes significant changes in the nature of the tip leakage vortex : large expansion of the vortex and disappearance of the streamwise vorticity concentrated in the vortex. The expansion has an extremely large blockage effect extending to the upstream of the leading edge. The disappearance of the concentrated vorticity results in no rolling-up of the vortex downstream of the rotor and the disappearance of the pressure trough on the casting. It is found that the leakage vortex breakdown plays a major role in characteristic of rotor performance at near-stall conditions. As the flow rate is decreased from the peak pressure rise operating condition, the breakdown region grows rapidly in the streamwise, spanwise and pitchwise directions. The growth of the breakdown causes the drastic rises in the blockage and the loss.