New test facility to investigate the noise generated by shrouded fans for small aircraft applications

Abstract

The development of fan noise reduction measures that specifically address the sources requires detailed understanding of the aerodynamic noise generation mechanisms for the specific fan application. Due to the complex dependencies on the details of the flow field, this task is ideally approached by combining experiment and modelling. For the implementation of thorough experimental analyses, the novel fan test environment CRAFT (Counter Rotating Acoustic Fan Test Rig) is established. It enables comprehensive studies at low costs. Rotor and stator can quickly be exchanged. By varying the rotor-stator distance and measuring the isolated rotor, individual noise sources can be highlighted and the aeroacoustic generation mechanisms can be specifically investigated. The key parameters of the CRAFT reference fan are diameter at fan case 453 mm, blade tip Mach number up to 0.3, mass flow rates up to 12 kg/s, pressure ratio up to 1.1 and maximum shaft speed of 4500 rpm. It is representative for the shrouded fan stage of small airplanes to be operated in urban air mobility missions by scale 1:2. Since disturbances of the fan inflow can occur in UAM applications and cause considerable noise contributions, the CRAFT test stand was extended by a device for generating representative inlet disturbances. The identification of the relevant aerodynamic parameters of fan noise excitation is enabled by comprehensive aerodynamic and acoustic measurements, as e.g. with total pressure rakes, multi-hole probes, hot-wire anemometry and acoustic mode decomposition techniques. The high-resolution data can be used as input for numerical modeling methods, e.g. for extended interpretation or validation purposes. The presentation gives an overview of the measurement and analysis possibilities and shows first results of a test series with a fan under disturbed inflow conditions.