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Experimental Acoustic Characterisation of Perforates

Application of multi-port techniques

Time: Fri 2024-12-20 09.00

Location: Sal Kolllegiesalen Brinellvägen 6 & 8

Video link: https://kth-se.zoom.us/j/64185873928

Language: English

Subject area: Vehicle and Maritime Engineering

Doctoral student: Shail A. Shah , Fordonsteknik och akustik, MWL Sound and Vibration Research

Opponent: Prof. Dr. Ing. Anita Schulz, HTW Berlin

Supervisor: Prof. Susann Boij, Fordonsteknik och akustik; Prof. Em. Hans Bodén, Fordonsteknik och akustik

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QC 241202

Abstract

Perforated plates are widely used in noise reduction systems with applications ranging from mufflers and aircraft liners, to burners. A shift from propeller to jet engines has accelerated the usage of aircraft liners over the past decades. To optimally design the liner, the acoustic characterisation of perforated plates under standard operating conditions has garnered a lot of interest in the scientific community.

This thesis aims to contribute to the research by providing novel multi-port techniques and experimental results of the acoustic behaviour of a perforated plate. These multi-port techniques allow for the acoustic characterisation of perforates in the presence of grazing flow, bias flow, andhigh-amplitude acoustic incidence. The transfer impedance of the perforate is determined underplane wave acoustic incidence from different directions with respect to the air flow. The multi-port techniques used here, namely the three-port and the four-port technique, are direct methods of impedance determination, i.e., the transfer impedance is calculated by measuring the acoustic far-field in the ducts. Correlation between the transfer impedance and the coefficients of the multi-port scattering matrix is explored to minimise the experimental errors due to strong termination reflections. Based on the results the behavioural dependence of the transfer impedance on the operating conditions is studied. To study the individual, as well as the combined effects of the operating conditions, dimensionless numbers such as Strouhal-, Shear-, and Mach number are used to normalise the transfer impedance. A comparison of the results against previously proposed models,and their respective hypotheses is also shown. Lastly, using global sensitivity analysis, the relative contribution of the uncertainty in determining each operating condition, on the overall uncertainty of the experimentally determined transfer impedance is presented.

One of the major findings of the thesis suggests a flow speed based Strouhal number defining theresistive as well as the inertial behaviour of the perforate. Moreover, a particle velocity based Strouhal number governs the transfer impedance in the transition state from linear to highly non-linear regimes. However, based on the comparison between perforated plates of differing geometries, observations suggest that the Strouhal number determined with respect to thickness of the plate, rather than the perforation diameter shows a better agreement in the characterisation. The error analysis shows the transfer impedance to be more sensitive to errors in the determination of the acoustic field parameters, than the flow field parameters.

Lastly, upon a simultaneous exposure, the results suggest a non-linear coupling between the individual effects of the operating conditions, where the transfer impedance is characterised with respect to the dimensionless numbers, and an additional dependence on the incident wavelength of the acoustic waves is observed.

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-357028