Measurement of the Local Sound Pressure on a Bias-Flow Liner Using High-Speed Holography and Tomographic Reconstruction

Bias-flow liners (BFL) are used as part of the cooling mechanism in airplane turbines and they also enable the reduction of noise emission. The attenuating principle is based on the interaction between sound waves and air flow. This turbulent aeroacoustic interaction occurs on a microscopic level and under harsh conditions. Therefore, in order to investigate local aeroacoustic phenomena, the distribution of the local sound field impedance, defined as sound pressure over acoustic particle velocity, is measured non-invasively. This work focuses on the non-invasive measurement of the local sound pressure. A fast camera-based holographic measurement system for the two dimensional tomographic measurement of the local sound pressure with high spatial resolution is presented. It features a spatial resolution of with a simultaneous acquisition of up to 1600 measuring points. This enables the observation of sub-mm aeroacoustic effects, which are expected in the near field over the surface, while reducing the measurement duration by three orders of magnitude compared to single point measurement systems. An experimental validation of the camera-based tomographic system is carried out by reference measurements. The system was applied at an acoustical model and an aeroacoustic model. The measured local sound pressure variations are discussed for both models. This work seeks to enable the non-invasive investigation of sound pressure, in order to further understand aeroacoustic effects, their sound attenuating properties and increase the attenuation capability of a BFL without degrading the overall performance of turbines.