To investigate the control effect and flow mechanism of the L-shaped endwall groove on corner separation in the real compressor stage, a single stage axial flow compressor is selected as the research object and the L-shaped grooves are introduced on the stator casing side. First, the experimental measurement is conducted on the optimal L-shaped groove obtained through a full factorial experimental design, and the results demonstrate that the optimal groove has a great control over the endwall flow. Moreover, the peak efficiency is improved by 0.9% and the stall margin is increased by 4.46%. Then, the flow field visualization of numerical results and analysis of variance method are employed to analyze the control mechanism and parameter control law of the L-shaped groove. It is found that the L-shaped groove can guide the skewed inlet endwall boundary layer towards the streamwise direction due to its confinement effect, thereby delaying the onset of corner separation and reducing the size of ring vortex. As a result, the aerodynamic performance of the compressor is enhanced. Furthermore, the groove depth and groove width play a significant role in controlling endwall flow among the three L-shaped groove design parameters. The larger groove depth and smaller groove width enhance the capability of the streamwise groove to constrain the endwall boundary layer, leading to a greater reduction in endwall loss.

To overcome the shortcomings of current vortex generator techniques used to control corner separation in compressors, we investigate the mitigation of corner separations using the streamwise vortex generated by the L-shaped endwall groove in a high-speed compressor cascade. First of all, a convenient and universal parametric modeling method for the L-shaped endwall groove has been proposed by introducing the standard modeling space and function superposition strategy, and the L-shaped endwall groove optimization was conducted based on this method. The simulated flow fields of the baseline cascade and a Pareto-optimal case have been compared. It was found that the groove separation vortex generated by the L-shaped groove could effectively block the endwall cross flow, cut off the supply of low momentum endwall fluid to the corner region, and therefore significantly mitigated the reverse flow in the corner region and avoid the formation of the corner separation vortex, remarkably improving the cascade performance within a wide range of incidence. Calculated results of different Pareto-optimal cases have been compared. The results show that the strength of the groove separation vortex determines the control effect of the groove on the endwall cross flow and the severity of additional loss and blockage caused by the groove, and thus is the key factor influencing the control effect of the L-shaped endwall groove. Secondly, the influence of design parameters on the L-shaped endwall groove and its mechanism have been analyzed through the combined use of the Sobol indices-based sensitivity analysis method and conventional control variate method. It was found that the control effect of the endwall groove was significantly influenced by four design parameters including the pitchwise location of the groove, groove depth, the width and length of the upstream groove. The pitchwise location of the groove determined the distance between the groove separation vortex and suction-side corner region, while the other three mainly influenced the strength of the groove separation vortex. Finally, a guideline for selecting design parameters of the L-shaped endwall groove has been summarized based on the above analyses.

Unsteady flow in the hub endwall region has long been a hot topic in the turbomachinery community. However important it is to the performance of the whole engine, the coherent unsteady flow phenomena are still not well understood. In this paper, the complex flow field in the hub endwall of a cantilevered compressor cascade has been investigated through numerical approach. The predicted results were validated by experimental data. To highlight the dominant flow structures among irregular and chaotic motions of various vortices, a Dynamic Mode Decomposition (DMD) method was utilized. The results show that there exist three dominant periodic flow structures: the oscillation of the leakage vortex, a circumferential migration of a Breakdown Induced Vortex (BIV) and the fluctuation of the passage vortex. These three coherent structures all together form a self-sustained closed loop which accounts for the flow unsteadiness of the studied cascade. During this process, the BIV plays a key role in inducing the flow unsteadiness. Only if the BIV is strong enough to affect the passage vortex, the flow unsteadiness occurs. This study expands current knowledge base of flow unsteadiness in a compressor environment, and shows the efficacy of the DMD method for revealing the origin of flow unsteadiness.