# The Characteristics and Mechanisms of High-Intensity Sound in a High-Speed Multistage Compressor
## 6 Conclusions  
1. An experiment in a multistage high-speed compressor was conducted to synchronously investigate the phenomenon of high-level vibration on R1 with high noise. Different testing positions of the casing were set to monitor the casing pressure and vibration status of R1. The noise spectrum of R1 was characterized as discrete multi-tone at the time that the occurrence of abnormal vibration appeared. Characteristic frequency gradually increased with the increasing rotational speed, and there was a phenomenon that characteristic frequency kept constant with discrete rotor speed. The evolution of the displaced blade vibration and the sound pressure of the characteristic frequency against the rotor speed showed synchronicity; several peak frequencies around character frequencies with an equal space of about 166 Hz matched with the rotating speeds of the rotors, which shows that there was a certain relation between frequency and the vibration of the rotor blades. The assumption that rotational noise sound is the reason for the vibration of rotor blades was made, and its accuracy was verified through theoretical calculation;
2. The mechanisms and characteristics of sound induced by shedding vortices in the pipeline based on acoustic analogy are proposed. A rectangular cavity pipeline model was established to analysis the distribution characteristics of the vortex system and the acoustic mode distribution in the pipeline under flow conditions. The flow was separated at the leading edge of the rectangular cavity, a large-scale vortex was generated and developed in the cavity until it reached the rear edge of the cavity, and then this flowed to the circular pipe. The vortices were mainly concentrated in the rectangular cavity region. The calculated results of the acoustic modal frequency values of each order were in line with the experimental results, and the value of the maximum calculation error was 2.7%, which verified the applicability of the numerical method;
3. A simplified compressor model based on a duct model with a locked plate was established to study the characteristics and mechanisms of high-intensity sound with the acoustic analogy method. The motion laws of the wake vortices of the plate were obtained, presenting a typical Karman vortex street structure. The shedding vortex frequency gradually increased with the flow velocity, and there was a frequency-locked area at a special range of flow velocity which was similar to the features of the experimental results in the compressor. The acoustic modal frequency, i.e., the pure tone component f\({}_{\rm{s}}=511\) Hz, was 3.6% different from the corresponding frequency of 530 Hz given by Welsh. The maximum value of sound pressure level was up to 145 dB at the inlet velocity of 32 m/s, and the high-intensity sound appeared in the pipe at the same time and remained in a limit velocity range of 30-33 m/s. The sound pressure distribution, when high-intensity sound appears in this model, presents the \(\beta\) mode form of the Parker model. **Author Contributions:** Conceptualization, F.Z. and B.C. ; methodology, F.Z. ; software, B.C. ; validation, F.Z. and H.L. ; formal analysis, F.W. and Y.S. ; investigation, H.L. and Y.S. ; resources, F.W., H.L. and Y.S. ; data curation, Y.S. and F.W. ; writing--original draft preparation, F.Z. and B.C. ; writing--review and editing, F.Z. and B.C. All authors have read and agreed to the published version of the manuscript. This research was supported by the China Aviation Development Shenyang Engine Research Institute for the experimental test of compressor noise. Additionally, it was supported by the Basic Research Project of Science and Technology Department of Liaoning Province (No. JYT2020009), the Scientific Research Foundation for Introduced Scholars of Shenyang Aerospace University (No. 21 YB01), the Liaoning Natural Science Foundation Guiding Plan of China (No. 2019-ZD-0237), and the National Science and Technology Major Project (No. J 2019-II-0006-0026). **Data Availability Statement:** Not applicable.