Reduction of multi-path effects in a low frequency spherical near-field test facility

2006 ◽  
Author(s):  
Matthew Giles ◽  
Shantnu Mishra
Keyword(s):  
Field Test ◽  
Near Field ◽  
Low Frequency ◽  
Test Facility

Flow and Noise Investigations of a Separate Flow Exhaust System

2005 ◽  
Author(s):  
Mihai Mihaˇescu ◽  
Ro´bert-Zolta´n Sza´sz ◽  
Laszlo Fuchs ◽  
Ephraim Gutmark
Keyword(s):  
Flow Field ◽  
Numerical Study ◽  
Near Field ◽  
Low Frequency ◽  
Exhaust System ◽  
Separate Flow ◽  
Test Facility ◽  
Frequency Noise ◽  
Frequency Spectra ◽  
Field Source

A major component of aircraft noise is the jet noise created by the high velocity hot stream exhausting from a jet engine, interacting with itself and with the surrounding cold air. In the present paper the flow and acoustic fields that are generated by two coaxial jets are considered. Numerically, the problem is divided into a flow related part (Navier-Stokes system of equation) and an acoustic part (an inhomogeneous wave equation). The flow field is handled by well resolved Large Eddy Simulation (LES). The acoustical sources can then be computed from the flow field calculations, on the near-field “source” grid. The acoustic field is solved, on the same or even on a larger separate grid, by using an acoustic approximation with appropriate acoustic boundary conditions. The computed flow and acoustical fields are compared to those measured on the separate flow nozzle test facility. The comparisons in terms of velocity and sound pressure levels are shown to validate the used approach. Frequency spectra of the acoustic density fluctuation are presented in order to indicate the locations where the high- or low- frequency noise dominates. The numerical study is focused as well on the Reynolds number effects on the flow and acoustics.

scholarly journals Occluded Grape Cluster Detection and Vine Canopy Visualisation Using an Ultrasonic Phased Array

Sensors ◽  
2021 ◽  
Vol 21 (6) ◽  
pp. 2182
Author(s):  
Baden Parr ◽  
Mathew Legg ◽  
Stuart Bradley ◽  
Fakhrul Alam
Keyword(s):  
Near Field ◽  
Low Frequency ◽  
Volume Estimation ◽  
Yield Estimation ◽  
3D Scan ◽  
Management Processes ◽  
Canopy Volume ◽  
Ultrasonic Phased Array ◽  
Added Benefit ◽  
Grape Yield

Grape yield estimation has traditionally been performed using manual techniques. However, these tend to be labour intensive and can be inaccurate. Computer vision techniques have therefore been developed for automated grape yield estimation. However, errors occur when grapes are occluded by leaves, other bunches, etc. Synthetic aperture radar has been investigated to allow imaging through leaves to detect occluded grapes. However, such equipment can be expensive. This paper investigates the potential for using ultrasound to image through leaves and identify occluded grapes. A highly directional low frequency ultrasonic array composed of ultrasonic air-coupled transducers and microphones is used to image grapes through leaves. A fan is used to help differentiate between ultrasonic reflections from grapes and leaves. Improved resolution and detail are achieved with chirp excitation waveforms and near-field focusing of the array. The overestimation in grape volume estimation using ultrasound reduced from 222% to 112% compared to the 3D scan obtained using photogrammetry or from 56% to 2.5% compared to a convex hull of this 3D scan. This also has the added benefit of producing more accurate canopy volume estimations which are important for common precision viticulture management processes such as variable rate applications.

Hydrodynamic Noise and Radiated Mechanism of 3D Cavity

2012 ◽  
Vol 217-219 ◽  
pp. 2590-2593 ◽  
Author(s):  
Yu Wang ◽  
Bai Zhou Li
Keyword(s):  
Sound Pressure ◽  
Near Field ◽  
Low Frequency ◽  
Dipole Source ◽  
Fluctuating Pressure ◽  
Common Structure ◽  
Hydrodynamic Noise ◽  
Near Field Sound ◽  
Equivalent Sources ◽  
Field Sound

The flow past 3D rigid cavity is a common structure on the surface of the underwater vehicle. The hydrodynamic noise generated by the structure has attracted considerable attention in recent years. Based on LES-Lighthill equivalent sources method, a 3D cavity is analyzed in this paper, when the Mach number is 0.0048. The hydrodynamic noise and the radiated mechanism of 3D cavity are investigated from the correlation between fluctuating pressure and frequency, the near-field sound pressure intensity, and the propagation directivity. It is found that the hydrodynamic noise is supported by the low frequency range, and fluctuating pressure of the trailing-edge is the largest, which is the main dipole source.

Ground Attenuation Factor Based on Measurements

2021 ◽  
Vol 263 (3) ◽  
pp. 3436-3447
Author(s):  
Dan Lin ◽  
Andrew Eng
Keyword(s):  
Near Field ◽  
Attenuation Factor ◽  
Measurement Data ◽  
Low Frequency ◽  
Sound Sources ◽  
Frequency Sound ◽  
Sound Levels ◽  
Different Types ◽  
Noise Measurements ◽  
Field Sound

Assumptions made on the ground types between sound sources and receivers can significantly impact the accuracy of environmental outdoor noise prediction. A guideline is provided in ISO 9613-2 and the value of ground factor ranges from 0 to 1, depending on the coverage of porous ground. For example, a ground absorption factor of 1 is suggested for grass ground covers. However, it is unclear if the suggested values are validated. The purpose of this study is to determine the sound absorption of different types of ground by measurements. Field noise measurements were made using an omnidirectional loudspeaker and two microphones on three different types of ground in a quiet neighborhood. One microphone was located 3ft from the loudspeaker to record near field sound levels in 1/3 and 1 octave bands every second. The other microphone was located a few hundred feet away to record far field sound in the same fashion as the near field microphone. The types of ground tested were concrete, grass, and grass with trees. Based on the measurement data, it was found that grass and trees absorb high frequency sound well and a ground factor of 1 may be used for 500Hz and up when using ISO 9613-2 methodology. However, at lower frequencies (125 Hz octave band and below), grassy ground reflects sound the same as concrete surfaces. Trees absorb more low frequency sound than grass, but less than ISO 9613-2 suggested.

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