The usage of perforated plates in passive noise-control systems in e.g. aircraft liners and mufflers involves the standard operating conditions of grazing flow and high-amplitude acoustic incidence. This study focuses on the usage of the three-port experiment technique for the experimental passive acoustic characterisation of a perforated plate. Expanding on the previous paper about the resistance of the perforate, the influence of operating conditions on the imaginary part of the transfer impedance, i.e., the reactance is discussed here. The relationship between the end correction in the linear range, without the presence of grazing flow, and the frequency of acoustic excitation is demonstrated. The reduction in the value of the mass end correction under the individual effects of grazing flow, and high-amplitude excitation is presented. Additionally, depending on the excitation wavelength, the partial recovery of the reduced value of the end correction under simultaneous exposure is also shown. Using the experimental results, models for the end correction, and by extension the reactance are proposed. These models take into account the geometrical parameters of the perforate, and the dimensionless Mach, Shear and Strouhal numbers.

Sound reducing treatment in for instance aircraft engine and IC engine applications often involve the use of perforates. In these applications they are exposed to fluid flow and high-level acoustic excitation, and this influences the acoustic properties of the perforate, as is well known from many published papers. The acoustic properties are usually described using a transfer impedance. The transfer impedance of a perforate sample has been studied using both a conventional impedance tube (two-port) setup and an innovative three-port setup. The three-port configuration made it possible to study both the effect of grazing flow and high-level excitation effects separately as well as jointly. In the present paper these recent experimental results are presented and comparisons are made with results from previously published papers and empirical models.

Perforates are frequently used as part of sound reducing treatment in for instance aircraft engine and IC engine applications. In these applications they are exposed to fluid flow and high-level acoustic excitation, and this influences the acoustic properties of the perforate, as is well known from many published papers. The acoustic properties are usually described using a transfer impedance. The transfer impedance of a perforate sample has been studied using both a conventional impedance tube (two-port) setup and an innovative three-port setup. The three-port configuration made it possible to study both the effect of grazing flow and high-level excitation effects separately as well as jointly. The present paper concentrates on the nonlinear effects. Comparisons are made with results from previously published papers and empirical models.

Perforated plates are frequently applied in passive noise-control systems in e.g. aircraft liners and mufflers. Many of the applications of a perforated plate involve an exposure to grazing flow and high-amplitude acoustic incidence. The ongoing scientific effort of providing experimental results using multiform test setups is continued in this study. Incorporating the three-port technique, the passive acoustic response of a perforated plate is studied under acoustic excitation from three directions in presence of grazing flow and high-amplitude acoustic excitation. The applied three-port technique has an advantage of being a direct method for impedance determination and is not bound by any boundary conditions traditionally considered in presence of grazing flow. In this study, a semi-empirical model for the normalised resistance of a perforate is presented. The model takes the Mach, Strouhal, and Shear numbers into account. Additionally, for low grazing flow speeds, the non-linear effects of resistance are found to be dependent on the grazing flow speed.

Perforates are frequently used as part of sound reducing treatment in for instance aircraft engine and IC engine applications. In these applications they are exposed to fluid flow and high-level acoustic excitation, and this influences the acoustic properties of the perforate, as is well known from many published papers. The acoustic properties are usually described using a transfer impedance. In a previous part of this study the effect on the real part (resistance) of the transfer impedance was studied using both a conventional impedance tube (two-port) setup and an innovative three-port setup. The three-port configuration made it possible to study both the effect of grazing flow and high-level excitation effects separately as well as jointly. The present paper is a follow-up study where the imaginary part (reactance) of the transfer impedance is the focus. Comparisons are made with results from previously published papers and empirical models.

Varying acoustic behaviour of perforates in presence of grazing flow and under acoustic excitation from different directions has been under research since mid-1950s. Empirical and semi-empirical studies have shown differing behaviour of the perforate transfer impedance in presence of grazing flow. Studying a perforated plate in a rectangular T-Junction, this study aims to study the perforate from three acoustic incidence directions. Scattering matrix and the acoustic transfer impedance are determined experimentally under the presence of external grazing flow to characterise the perforate and the T-Junction. In accordance with previous research, an oscillating behaviour of amplification and attenuation is observed in the empty T-Junction. Results obtained of the perforate show a similar oscillating behaviour. Comparing with the rectangular T-junction, the similarity in the flow acoustic behaviour of a perforate is shown. 

Perforates, often with a backing cavity, are used in noise reducing liners and silencers for aircraft engines and internal combustion engines. Direct methods of impedance estimation can be used for the acoustic characterisation of the perforated plates standalone, without the backing cavity, in presence of grazing flow. The three-port technique is a direct method for estimating the transfer impedance of perforates in presence of grazing flow and high-amplitude acoustic excitation. In a previous study, the dependence of the experimentally determined transfer impedance on the operating conditions, namely the in-duct temperature, the grazing flow speed and microphone distances is shown. This paper is a follow up to the previous study and attempts to quantify the effect of variations in these testing conditions on the real part of the transfer impedance, i.e., the resistance. The possible sources of errors and deviations in the determination of the operating conditions are discussed. Based on the uncertainty range of the operating conditions, a Monte-Carlo simulation is performed to calculate the confidence intervals of the results. Additionally, the effect of the error distribution on the confidence intervals is displayed.

This thesis discusses the aero-acoustic characterisation of a perforate sample using a three-port technique. A rectangular T-junction with a flush mounted perforated sample at the intersection form the acoustical three-port. Under acoustic excitation from three different directions a direct method of impedance determination is incorporated to experimentally determine the passive acoustic properties of the perforate. The three-port scattering matrix and the normalised transfer impedance are calculated in the presence of grazing flow and for high-level excitation and the behaviour of these characteristics is studied. Validation of the determined results in the linear range is carried out by comparing it with existing models. Moreover, based on the experimental results for low grazing flow velocities the dependence of the real part of the transfer impedance on the grazing flow parameters as well as dimensionless numbers is described, and a semi-empirical model quantifying the behaviour is proposed.

Furthermore, the thesis explains some experimental errors pertaining to standing wave patterns and operating conditions, and corrections are suggested to reduce the errors.

Varying acoustic behaviour of perforates in presence of grazing flow and under acoustic excitation from different directions has been under research since mid-1950s. Empirical and semi-empirical studies have shown differing estimations of the perforate transfer impedance in presence of grazing flow. Studying a perforated plate in a rectangular T-Junction, this study aims to study the perforate from three-incidence directions. Scattering matrix and the acoustic transfer impedance are determined experimentally under the presence of external grazing flow to characterise the perforate and the T-Junction. In accordance with previous research, an oscillating behaviour of amplification and attenuation is observed in the empty T-Junction. Results obtained of the perforate are of a similar nature. Comparing with the rectangular T-junction, the similarity in the flow acoustic nature of a perforate is shown.

The effect of high-level excitation on the acoustic properties of perforates, and formulation of the non-linear part of the impedance is under scientific discussion. Analytical models including the non-linear properties, as well as various experimental studies give varying results for the acoustic impedance. This paper aims to provide detailed results obtained under high-level excitation with different acoustic wave incidence and flow configurations. Contrary to the well-established two-port configuration, here, a three-port measurement technique is used to observe the acoustic impedance of the perforated plate using excitation from the three different directions. Plane wave propagation is considered and physical quantities such as in-hole particle velocity, which is calculated at the perforate sample, are used as the controlling parameters. This paper is an attempt to study the effect of high-level excitation on the acoustic behavior of the perforates with grazing and normal acoustic incidence. Moreover, the non-linear behavior of the perforate determined in presence of an external grazing flow is also discussed.