The unsteady characteristic in the tip region of an axial compressor has been numerically studied with the help of the dynamic mode decomposition analysis. The characteristics of frequency and dynamic modes are compared and discussed under different operating points and different parameters, such as tip clearance and rotating speeds. For the flowfield structures in the tip region, such as tip leakage flow, separation flow and shock wave, their relationships with the unsteadiness are studied in detail. Except for the unsteadiness caused by the interaction between rotating rotor and the stationary boundaries, it is found that the unsteadiness is attributed to the moving of the low-velocity cell. Based on the generation and the development of the low-velocity cell, the unsteady characteristics in tip region are divided into 4 types: BPF-dominated, shedding-dominated, self-induced and separation-dominated. When the tip leakage flow is weak, the unsteadiness in the tip region is only triggered by the blade sweeping. As the tip leakage flow gets stronger to a certain extent, the low-velocity cell is shed into the flow passage and mixed with the main-flow. When the main-flow is weaker under the low flowrate condition, the interaction between the low-velocity cell and the pressure side occurs and generates a new low-velocity cell near the leading-edge of the neighboring blade. The frequency of the new cell generation is actually the self-induced frequency. In the zero and small clearance model, the low-velocity is shed by the separation in the leading-edge and the casing-suction corner. By understanding these unsteady characteristics, the change tendency of the leading frequency in the rotor tip is easily explained and forecasted. Furthermore, under the transonic operation condition, the low-velocity cell is decelerated and eliminated by the shock wave in the unsteadiness of the self-induced type and the separation-dominated type, respectively. Thus, the leading frequency in the tip flow field is moderated.

借助于动态模式分解分析，对轴向压缩机的尖端区域中的非稳态特性进行了数值研究。在不同的工作点和不同的参数（例如叶尖间隙和转速）下，对频率和动态模式的特性进行了比较和讨论。对于尖端区域内的流场结构，如尖端泄漏流，分离流和冲击波，详细研究了它们与不稳定性的关系。除了由旋转的转子和静止边界之间的相互作用引起的不稳定之外，发现不稳定还归因于低速单元的运动。根据低速电池的产生和发展，尖端区域的不稳定特征可分为4种类型：以BPF为主，脱落为主，自我诱导和分离为主。当尖端泄漏流微弱时，尖端区域的不稳定性仅由叶片扫掠触发。当尖端泄漏流在一定程度上变强时，低速单元会掉入流道并与主流混合。当在低流量条件下主流较弱时，低速单元与压力侧之间会发生相互作用，并在相邻叶片的前缘附近生成一个新的低速单元。新一代细胞的频率实际上是自感应频率。在零间隙和小间隙模型中，低速是由前缘和机壳吸角之间的分隔而产生的。通过了解这些不稳定的特征，转子尖端超前频率的变化趋势很容易解释和预测。此外，在跨音速操作条件下，在自感应型和分离控制型的不稳定状态下，低速电池分别被冲击波减速和消除。因此，尖端流场中的超前频率被缓和。