Phased microphone arrays have become a well-established tool for performing aeroacoustic measurements in wind tunnels (both open-jet and closed-section), flying aircraft, and engine test beds. This paper provides a review of the most wellknown and state-of-the-art acoustic imaging methods and recommendations on when to use them. Several exemplary results showing the performance of most methods in aeroacoustic applications are included. This manuscript provides a general introduction to aeroacoustic measurements for non-experienced microphone-array users as well as a broad overview for general aeroacoustic experts.
A method has recently been developed that employs multi-beam echo-sounder backscatter data to both obtain the number of sediment classes and discriminate between them by applying the Bayes decision rule to multiple hypotheses ͓Simons and Snellen, Appl. Acoust. 70, 1258-1268 ͑2009͔͒. In deep water, the number of scatter pixels within the beam footprint is large enough to ensure Gaussian distributions for the backscatter strengths and to increase the discriminative power between acoustic classes. In very shallow water ͑ Ͻ 10 m͒, however, this number is too small. This paper presents an extension of this high-frequency methodology for these environments, together with a demonstration of its performance using backscatter data from the river Waal, The Netherlands. The objective of this work is threefold. ͑i͒ Increasing the discriminating power of the classification method: high-resolution bathymetry data allow precise bottom slope corrections for obtaining the true incident angle, and the high-resolution backscatter data reduce the statistical fluctuations via an averaging procedure. ͑ii͒ Performing a correlation analysis: the dependence of acoustic backscatter classification on sediment physical properties is verified by observing a significant correlation of 0.75 ͑and a disattenuated correlation of 0.90͒ between the classification results and sediment mean grain size. ͑iii͒ Enhancing the statistical description of the backscatter intensities: angular evolution of the K-distribution shape parameter indicates that the riverbed is a rough surface, in agreement with the results of the core analysis.
The hydrodynamic and acoustic fields for a NACA 0018 with solid and porous trailing edge inserts are investigated. The porous inserts, covering 20% of the chord, are manufactured with metal foams with cell diameters of 450 and 800 μm and permeability values of 6 × 10 −10 and 2.7 × 10 −9 m 2. The experiments are performed at a chord-based Reynolds number of 2.63 × 10 5 and an angle of attack of 0 •. The porous trailing edge with higher permeability provides up to 11 dB noise attenuation with respect to the solid case for frequencies below a cross-over Strouhal number St = 0.26. Lower noise abatement (up to 7 dB) takes place below St = 0.3 for the insert with lower permeability. Conversely, noise increase with respect to the solid case is measured above the previously defined St value. A decrease in turbulence intensity is reported (up to 3% of the free-stream velocity), with lower intensity being measured for the insert with lower permeability. It is also observed that the permeability of the insert is linked to the increase of the anisotropy of highly energetic turbulent motions, being stretched in the streamwise direction, and the reduction of the eddy convection velocity (up to 20% with respect to the baseline case). In view of the results, the reduction of the velocity fluctuations is proposed as one of the mechanisms for low frequency noise abatement, being more relevant for the metal foam insert with lower permeability.
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