Jeoffrey R. Fischer

Time-domain delay-and-sum beamforming for time-reversal detection of intermittent acoustic sources in flows

This study focuses on the identification of intermittent aeroacoustic sources in flows by using the time-domain beamforming technique. It is first shown that this technique can be seen as a time-reversal (TR) technique, working with approximate Green functions in the case of a shear flow. Some numerical experiments investigate the case of an array measurement of a generic acoustic pulse emitted in a wind-tunnel flow, with a realistic multi-arm spiral array. The results of the time-domain beamforming successfully match those given by a numerical TR technique over a wide range of flow speeds (reaching the transonic regime). It is shown how the results should be analyzed in a focusing plane parallel to the microphone array in order to estimate the location and emission time of the pulse source. An experimental application dealing with the aeroacoustic radiation of a bluff body in a wind-tunnel flow is also considered, and shows that some intermittent events can be clearly identified in the noise radiation. Time-domain beamforming is then an efficient tool for analyzing intermittent acoustic sources in flows, and is a computationally cheaper alternative to the numerical TR technique, which should be used for complex configurations where the Green function is not available.

Detection of Non-Stationary Aeroacoustic Sources by Time-Domain Imaging Methods

Imaging methods such as the beamforming technique are widely used to localize and identify aeroacoustic sources. However, so far, existing applications in aeroacoustics have mostly been performed in the frequency domain. To tackle the characterization of nonstationary sources (for example, intermittent sources), time-domain imaging methods are more appropriate. Indeed, the spatio-temporal reconstruction of the acoustic fields allows studying the source structure in “real-time”. In this paper, two aeroacoustic problems are investigated with the help of time-domain inverse methods. First, numerical acoustic data obtained from the simulation of the radiation of a 2D mixing layer are studied through a numerical time-reversal method based on the Linearized Euler Equations. The spatio-temporal maxima of the acoustic energy are then detected by observing successive snapshots of the reconstructed acoustic field. These are assumed to correspond to wave focusing and, hence, to be related to the presence of a source. Finally, vorticity field snapshots are observed at the times at which spatio-temporal maxima are found. A conditional average of the flow fields, assuming large acoustic emission, is thus possible in principle. The global structure of the source is found to be quadripolar and each kind of detected maxima corresponds to a fixed vortical structure. Second, experimental data of the noise produced by a forward-facing step in a wind-tunnel flow are analysed by using the timedomain beamforming technique. The detection of spatio-temporal maxima highlights that the broadband noise source produced by the step can be seen as a succession of short duration events scattered around the step edge.

Experimental investigation of trailing edge noise from stationary and rotating airfoils

Trailing edge noise from stationary and rotating NACA 0012 airfoils is characterised and compared with a noise prediction based on the semi-empirical Brooks, Pope, and Marcolini (BPM) model. The NACA 0012 is symmetrical airfoil with no camber and 12% thickness to chord length ratio. Acoustic measurements were conducted in an anechoic wind tunnel using a stationary NACA 0012 airfoil at 0° pitch angle. Airfoil self-noise emissions from rotating NACA 0012 airfoils mounted at 0° and 10° pitch angles on a rotor-rig are studied in an anechoic room. The measurements were carried out using microphone arrays for noise localisation and magnitude estimation using beamforming post-processing. Results show good agreement between peak radiating trailing edge noise emissions of stationary and rotating NACA 0012 airfoils in terms of the Strouhal number. Furthermore, it is shown that noise predictions based on the BPM model considering only two dimensional flow effects, are in good agreement with measurements for rotating airfoils, at these particular conditions.

A Comparison of Microphone Phased Array MethodsApplied to the Study of Airframe Noise in WindTunnel Testing

In this werk, various microphone phased array data processing techniques are applied to two existing datasets from aeroacoustic wind tunnel tests. The first of these is from a !arge closed-wall facility, DLRs Kryo-Kanal Koln (DNW·KKK), and is a measurement cf the high-lift noise cf a semispan model. The second is from a small-scale open-jet facility, the NASA Langley Quiet Flow Facility (QFF), and is a measurement cf a clean airfoil selfnoise. The data had been made publicly available in 2015, and were analyzed by several research groups using multiple analysis techniques. This procedure allows the assessment of the variability of individual methods across various organizational implementations, as weil as the variability cf results produced by different array analysis methods. This paper summarizes the results presented at panel sessions held at AIAA conferences in 2015 and 2016. Results show that with appropriate handling of background noise, all advanced methods can identify dominant acoustic sources for a broad range cf frequencies. Lowerlevel sources may be masked er underpredicted. lntegrated levels are more robust and in closer agreement between methods than narrowband maps for individual frequencies. Overall there is no obvious best method, though multiple methods may be used to bound expected behavior.

Characterisation and modelling of axial fan noise

This paper presents experimental data concerning the noise produced by an axial fan. Acoustic measurements have been taken with point microphones and a microphone array both with and without a grid at the entrance to the fan to examine the e↵ect of incoming turbulence levels on noise. The acoustic data are used to evaluate a strip-wise turbulent in-flow analytical broadband noise model. Two propagation models are employed in the analytical model to predict far-field noise. The first is based on the spherical spreading of acoustic waves and the second uses an experimentally measured transfer function. The best results are obtained with the experimental transfer function employed in the strip-wise noise model as it can recreate some of the finer spectral features of the recorded noise.