# Effects of \"S\"-Type Bowed Guide Vanes on Unsteady Flow in 1.5-Stage Axial Compressors
## 1 Introduction  
The unsteady flow caused by the relative motion between the moving and static vanes is an intrinsic unsteady phenomenon in fluid machinery, and has certain periodic laws. The outlet aerodynamic parameters of the upstream vane are not uniform, but show a periodic distribution [1], so the downstream moving vane will bear the high-frequency periodic aerodynamic exciting force during rotation. For axial flow compressors, the high-frequency aerodynamic exciting force on the moving vane will increase the high-cycle fatigue fracture risk and reduce the compressor service life. It is also an important part of the compressor aerodynamic noise. Therefore, it is very important to study the unsteady flow phenomena in axial flow compressors and explore efficient measures to reduce the flow exciting force of moving vanes for safe and stable operations [2]. Many scholars have carried out detailed research on unsteady flow in axial flow compressors. [PERSON] et al. [3, 4] experimentally measured the pressure fluctuation on the surface of the moving and stator blades in the subsonic compressor as well as the shape of the moving wake. The results show that the higher-order harmonics of potential flow interaction attenuate very quickly. In most cases, the amplitude of the second harmonic is only 20% to 40% of the first harmonic, and the influence of the higher harmonic on the blade vibration is very small. When the axial clearance of the cascade row is small, the potential flow interference and the wake maintain the same order of magnitude. Thus, increasing the axial clearance can reduce the discrete tone level. The research by [PERSON] [5] shows that, in transonic compressors, with a smaller axial clearance between moving and stator blades,the aerodynamic loss caused by stator-rotor interactions increases gradually, resulting in the decline of axial compressor performance. [PERSON] et al. [6] studied the unsteady flow characteristics generated by the interaction between guide vane wakes and moving blades of axial compressors through experiments, and analyzed the probability distribution of the wake characteristics. The results indicate that the relative position between the moving and stator blades have a strong correlation with unsteady aerodynamic response of the moving vanes, but this correlation would be weakened with the decrease of the flow rate under working conditions. [PERSON] et al. [7] conducted the unsteady measurements of the moving and stator blades for the high load 1.5-stage axial compressor and studied the influence of different working conditions on boundary layer transition. [PERSON] et al. [8] explored the influence of the interaction between upstream blades on the downstream blade boundary layer transition position through a 3-stage axial compressor test rig. The research on unsteady aerodynamic exciting forces can be traced back to the 1960s. [PERSON] [9] used a simple analysis model to predict the pressure fluctuation of adjacent blades. However, due to the lack of cognition at that time, the author could only make rough assumptions about the research model, which pose a limitation on its adaptability. [PERSON] et al. [10] studied the correlation between wake and unsteady aerodynamic forces, and the influence of wake on moving cascades is studied through the combination of the finite element method and the finite difference method. [PERSON] et al. [11] studied the influence mechanism of unsteady blade row interference on unsteady aerodynamic forces of a 4-stage compressor. In this research, the time and frequency domain distribution of aerodynamic forces are analyzed. It is pointed out that the amplitude and shape of aerodynamic forces are not only related to the wake strength and flow field potential, but also affected by the interaction between the wake and the potential flow field. [PERSON] et al. [12] set up a three-stage axial compressor test platform and used high-frequency response pressure sensors to conduct tests.