Bias Flow Liner Research Articles

Dense U-Net for Limited Angle Tomography of Sound Pressure Fields

Tomographic reconstruction allows for the recovery of 3D information from 2D projection data. This commonly requires a full angular scan of the specimen. Angular restrictions that exist, especially in technical processes, result in reconstruction artifacts and unknown systematic measurement errors. We investigate the use of neural networks for extrapolating the missing projection data from holographic sound pressure measurements. A bias flow liner was studied for active sound dampening in aviation. We employed a dense U-Net trained on synthetic data and compared reconstructions of simulated and measured data with and without extrapolation. In both cases, the neural network based approach decreases the mean and maximum measurement deviations by a factor of two. These findings can enable quantitative measurements in other applications suffering from limited angular access as well.

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 52 μm 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 BFL's 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.

Aeroacoustic analysis using natural Helmholtz–Hodge decomposition

Abstract. The analysis of aeroacoustic phenomena is crucial for a deeper understanding of the damping mechanisms of a sound-absorbing bias flow liner (BFL). For this purpose, simultaneous measurements of the sound field and the flow field in a BFL are required. The fluid velocity can serve as the measurand, where both the acoustic particle velocity and the aerodynamic flow velocity contribute and, thus, can be acquired simultaneously. However, there is a need to separate these two quantities to distinguish between them. This is challenging because they generally coincide with each other in the time domain. Due to the interaction of sound and flow in a BFL, both velocities also overlap in the temporal frequency domain, having a coherent oscillation at the acoustic frequency. For this reason, the recently developed natural Helmholtz–Hodge decomposition (NHHD) is applied to separate both quantities from the measured oscillation velocity field in the spatial domain. The evaluation of synthetic vector field data shows that the quality of the decomposition is enhanced when a smaller grid size is chosen. The velocity field in a generic BFL, necessarily recorded within a three-dimensional region of interest at more than 4000 measurement locations, is evaluated using NHHD. As a result, the measured oscillation velocity in the BFL is dominated by the flow that is related to vortices and also by irrotational aerodynamic flow. Moreover, indications for an aeroacoustic source near the facing sheet of the liner are revealed.

Spectral Analysis of Velocity Fluctuations in the Vicinity of a Bias Flow Liner With Respect to the Damping Efficiency

Bias flow liners are perforated walls with an additional bias flow through the perforation. A common application is their integration in walls of hot gas areas, e.g. in combustion chambers of aero engines, for cooling. It is wellknown for a long time that the bias flow additionally provides a substantial amount of acoustic damping. However, until now the detailed damping mechanisms are not fully understood. Therefore, this study takes a closer look at the energy transfer from a propagating acoustic wave to the flow velocity field, which can be measured in the vicinity of the perforated wall. Two laser-optical measurement techniques, Doppler global velocimetry with sinusoidal laser frequency modulation (FM-DGV) and acoustic particle image velocimetry (A-PIV), were applied to a bias flow liner. The time-continuous velocity signal measured by FM-DGV was used to perform a spectral analysis of the fluctuating velocity components above the liner surface. This was done for different acousticexcitation frequencies, where the liner exhibits different dissipation rates. After removal of the acoustic Content from the power spectral density, the hydrodynamic velocity components feature a spectral augmentation in the vicinity of the excitation frequency. The effect appears downstream of the holes. The amount of spectral Augmentation correlates to the dissipation rate of the liner at the different excitation frequencies. Consequently, a certain transfer of acoustic energy into turbulent velocity fluctuations occurs in different frequency regimes. This energy transfer is, as a matter of fact, a function of the dissipation rate of the liner. Moreover, it has a strong dependence on the spatial location with respect to the orifice of the bias flow liner.

Aeroacoustic near-field measurements with microscale resolution

In order to analyse aeroacoustic phenomena at near-fields, e.g. the sound–flow interaction at aircraft engine liners, measurements of the flow velocity and the acoustic particle velocity (APV) with microscale resolution are required. To this end, the APV measurement with a high spatial resolution of 10 µm was conducted by means of a laser Doppler velocity profile sensor. For validation of the APV measurements using the profile sensor in a superposed flow, a good agreement with indirect microphone measurements as a reference was achieved, up to a maximum Mach number of 0.25. Aeroacoustic measurements at a minimum distance of 350 µm to the perforation of a bias flow liner were performed using the profile sensor. As a result, acoustically induced velocity oscillations near the rim of the orifice were detected with microscale resolution. The phase-resolved oscillation field indicates vortex shedding from the perforation, which is initiated by the sound–flow interaction. Thus, it is demonstrated that the profile sensor is a valuable tool for analysing aeroacoustic phenomena at near-fields, down to the Kolmogorov scale.

Spectral analysis of the flow sound interaction at a bias flow liner

The interaction between the flow field and the sound field is responsible for the sound absorption at perforated acoustic liners with bias flow and has to be investigated contactlessly. Based on the optically measured flow velocity spectrum, an energy analysis was performed. As a result, the generation of broadband flow velocity fluctuations in the shear layer surrounding the bias flow caused by the flow sound interaction has been observed. In addition, the magnitude of this acoustically induced flow velocity oscillation exhibits a correlation with the acoustic dissipation coefficient of the bias flow liner. This supports the assumption that an energy transfer between the flow field and the sound field is responsible for the acoustic damping.

Multi-scale measurement of acoustic particle velocity and flow velocity for liner investigations

The optimization of liners for noise reduction lacks a thorough comprehension of the inherent aeroa- coustic damping phenomena. Therefore, the interaction of the acoustic particle velocity (APV) and the flow velocity field at liners has to be investigated. Hence, the simulta- neous measurement of both quantities, differing by several orders of magnitude, is aspired. However, the required high dynamic range is challenging. In order to analyse the energy transfer from the acoustic wave to the flow turbu- lence, turbulence spectra must be resolved which necessi- tates a high measurement rate as well. Doppler global velocimetry with sinusoidal laser frequency modulation (FM-DGV) fulfils these demanding requirements. First, the capability of FM-DGV for APV measurements was suc- cessfully validated with microphone measurements. Finally, a multi-point measurement at a bias flow liner was performed with a high dynamic range of 7,000 and a high measurement rate of 50 kHz. Hence, a deeper under- standing of the aeroacoustics at liners can be gained in future investigations.