Fan Noise Reduction by Cascade Clocking for Next Generation Supersonic Transport

2002 Fiscal Year Final Research Report Summary

• Project/Area Number: 12555272
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 展開研究
• Research Field: Aerospace engineering
• Research Institution: The University of Tokyo
• Principal Investigator: KAJI Shojiro Graduate School of Engineering, Professor, 大学院・工学系研究科, 教授 (80013704)
• Co-Investigator(Kenkyū-buntansha): WATANABE Toshinori Graduate School of Engineering, Associate Professor, 大学院・工学系研究科, 助教授 (10201211)
• Project Period (FY): 2000 – 2002
• Keywords: Cascade Clocking / Rotor-Stator Interaction / Fan Tone Noise / Axial Compressor / Noise Reduction / Supersonic Transport / Blade Passing Frequency Tone

Research Abstract

Cascade clocking is a concept to reduce the total pressure loss incurred in a multi-stage axial compressor by adjusting properly the circumferential position of the downstream stator vane relative to the upstream stator vane. The head investigator discovered the fact that the same concept can be applied to reduce fan tone noise in the case of a multi-stage fan such as used for the propulsion system of the next generation supersonic transport where the fan bypass ratio is low accordingly, the fan pressure ratio is high and a two or three stage fan is required.
In order to apply the cascade clocking concept to tone noise reduction, the number of rotor blades should be the same for multiple stages and also the number of stator vanes should be the same. Then, the tone frequency is the same for multiple stages and the circumferential mode of the tone becomes identical. There is a possibility to cancel the pressure wave of the tone generated by the first stage by the pressure wave of the tone generated by the second stage based on the principle of superposition.
The purpose of this project is to confirm experimentally the possibility to reduce tone noise by the concept of cascade clocking and also to find out numerically the condition necessary for the cascade clocking to be optimized.
A two-stage axial compressor having 32 rotor blades for the first and the second stages and 38 stator vanes for both stages were used for the experiment. It has the ability of stator vane clocking in continuous fashion and that of rotor blade clocking at every 1/8 pitch.
Results show that stator vane clocking can reduce the tone noise by as much as 18 dB. The level of noise reduction depends on rotational speed of the compressor for a fixed rotor blade setting for both stages. However, the combination of rotor blade clocking and stator vane clocking can give the maximum reduction mentioned above. Numerical calculation well explains the tone noise level variation against cascade clocking as obtained by experiments.