A proper framework for studying noise from jets with non-compact sources

Abstract

A quantitative assessment of the acoustic source field produced by a laboratory-scale heated jet with a gas dynamic Mach number of 1.55 and an acoustic Mach number of 2.41 is performed using arrays of microphones that are traversed across the axial and radial plane of the jet's acoustic field. The nozzle contour comprises a method of characteristics shape so that shock-related noise is minimal and the dominant sound production mechanism is from Mach waves. The spatial topography of the overall sound pressure level is shown to be dominated by a distinct lobe residing on the principal acoustic emission path, which is expected from flows of this kind with supersonic convective acoustic Mach numbers. The sound field is then analysed on a per-frequency basis in order to identify the location, strength, convection velocity and propagation angle of the various axially distributed noise sources. The analysis reveals a collection of unique data-informed polar patterns of the sound intensity for each frequency. It is shown how these polar patterns can be propagated to any point in the far field with extreme accuracy using the inverse square law. Doing so allows one to gauge the kinds of errors that are encountered using a nozzle-centred source to calculate sound pressure spectrum levels and acoustic power. It is proposed that the measurement strategy described here be used for situations where measurements are being used to compare different facilities, for extrapolating measurements to different geometric scales, for model validation or for developing noise control strategies.

A workflow for designing contoured axisymmetric nozzles for enhancing additively manufactured cold spray deposits

“…Further guidance on the use of CONTUR is provided in Shope [35]. According to Wang and Jiang [36], 'the Sivells method is still one of the internationally-recognized design methods for high-velocity nozzles that feature high accuracy' and has been applied with success in various engineering projects [37][38][39][40][41][42][43][44][45][46][47][48].…”

Jet noise predictions by time marching of single-snapshot tomographic PIV fields

“…For example, the pressure wave-packet characteristics can be inferred by measuring the pressure fluctuations in the acoustic near field with arrays of microphones (i.e., Breaky et al 2017). Additionally, array of microphones can be used to map the spatial topography of the sound pressure of the jet along the emission path, as demonstrated by Baars et al (2021). However, capturing the hydrodynamic velocity and pressure fields with sufficient spatial dynamic range is very challenging, especially when needed to tune or validate analytical methods for far-field noise prediction.…”

A study of Mach wave coalescence using spark sources and large-eddy simulation

“…As a component of turbulence mixing noise, Mach waves form when the convective speed of the large-scale turbulent structures is greater than the sound speed of the neighboring gas. Mach waves produce significant levels of noise for high Mach number supersonic jets, thus making them a source of interest for various jet noise phenomena [1]. Cumulative nonlinear waveform distortion causes Mach waves of sufficient amplitude to undergo steepening, even to the point of forming shocks.…”