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.

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.

In sound reducing treatment for aircraft engine and IC engine applications it is common to use perforates. They can then be exposed to fluid flow and high level acoustic excitation which will change their acoustic properties. To study the nonlinear high level excitation effects separately it is possible to use plane wave impedance tubes where the perforate is mounted either at an open end or in the middle of the duct with microphones on both sides. The normalized transfer impedance defined as the acoustic pressure difference over the sample divided by the particle velocity through the perforate can then be determined. The transfer impedance can for the case that the sample is mounted at the end of the tube be estimated as the difference between the impedance at the sample cross section with or without the perforate in place. When the sample is sitting in the middle of the impedance tube the acoustic pressure and particle velocity can be measured at both sides of the sample which can then be used to estimate the transfer impedance. It is then assumed that the downstream (passive termination) does not change the measured nonlinear transfer impedance meaning that the nonlinear effects are occurring only locally at the perforate because of the flow separation effects caused by the high acoustic particle velocities in the holes. Some preliminary experimental results indicates that this may not be the case. The aim of the present study is therefore to clarify this using an experimental campaign backed up by numerical simulations.

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. 

In boilers and heat exchangers devices, tube banks are used to transfer heat. One of the commonly used arrangement in heat exchangers are staggered tube banks, which consists of tubes arranged in triangular array form. The tube banks are characterized by their longitudinal and transverse pitch ratios, which is the pitch-to- diameter ratio. Based on the pitch ratio the tube bank is considered either a compact or a widely spaced tube bank arrangement. The tube bank may cause acoustic instabilities in the system. In this study, an investigation was performed to analyse the aero-acoustic properties of tube bank samples. Experiments were conducted for three staggered tube bank samples with pitch ratio varying from 1.2 (compact arrangement) to 2 (widely spaced arrangement). Passive properties determine the sound propagation through the system, while the active acoustic properties describe the acoustic source in the system. The attention is in this paper on the passive acoustic properties of staggered tube banks. The experimental results consist of scattering coefficients for samples with different pitch ratios and power balance for all three samples which are compared to see the pitch ratio effect on the acoustic properties.

This paper presents a significant advancement in micro-perforated panel absorber technologies. Two distinct types of sub-wavelength multi-chamber micro-perforated panel absorbers (MC-MPPA) for low-frequency broadband noise absorption under both normal and grazing acoustic incidence conditions are developed. Micro-perforated panel absorbers (MPPAs) exhibit versatility, eco-friendliness, and a straightforward, robust construction, making them ideal acoustic solutions. A graph-theory-based two-point impedance method (TpIM) and the Cremer impedance method are used for system modeling, enabling the design of the MC-MPPA to be optimized for maximum sound absorption in a chosen frequency range for both normal and grazing acoustic incidence. This graph theory approach represents a breakthrough, as an equivalent circuit model approach could not be applied to such complex models. Under normal incidence conditions, the experimental overall absorption coefficient is measured to be 0.8273 for a 22 mmthick MC-MPPA in the frequency range of [660 2000] Hz, and 0.8284 for a 52 mm thick MC-MPPA in the frequency range of [400 2000] Hz. Under grazing incidence conditions, an optimized MC-MPPA with a mere 30 mm sub-chamber depth achieves a transmission loss of 66 dB at 1180 Hz. Additionally, a 50 mm sub-chamber depth yields a transmission loss greater than 10 dB over an 880 Hz wide frequency range: [820 1700] Hz. The developed MC-MPPA technologies hold great promise for various duct noise attenuation applications including aeroengine acoustic liners.

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.

Tube banks are common design elements in heat exchangers. The most common tube bank patterns are in-line tube banks which consists of tubes spaced parallel and equally. Tube banks are characterized by their longitudinal and transverse pitch ratios (pitch to diameter). The tube bank with a pitch ratio of less than 1.25 is considered a compact tube bank and a pitch ratio equal to or greater than 2 is considered as widely spaced. In this paper, the attention is focused on the effects of pitch ratio in tube banks on its passive acoustics properties which are experimentally investigated. The flow duct experimentation is performed with flow to analyse the passive acoustic properties of tube banks. The experiment is conducted for three In-line tube banks with pitch ratios 1.2,1.4 and 2. The Experimental results consist of scattering matrix coefficients and power balance. The results are compared to see the pitch ratio effect. Transmission and reflection coefficients were found to increase with frequency for higher flow speeds and shows more waviness for compact tube bank. The power balance is calculated to see the amplification and attenuation effects of the tube bank. It is observed that compact tube banks give amplification for higher frequencies and wide spaced tube bank give moderate attenuation for all frequencies.