A multi-chamber micro-perforated panel absorber is designed and optimized for broadband attenuation in grazing flow acoustic incidence conditions. Mach number dependent analytical equations for impedance are input into a graph-theory-based two-point impedance method (TpIM) so that the total impedance of these highly complex multi-degree of freedom absorbers can be modeled and calculated. A novel Cremer impedance-based cost-function is presented which attempts to optimize the impedance of the absorber within a broadband target frequency range of interest. The optimization routine converges on an optimal impedance and also provides the geometric parameters of the absorber, such as individual sub-chamber depths, hole diameters and porosities per uniquely perforated chamber top, or internal side-wall. This allows the absorber to be 3D printed and numerically modeled using COMSOL. Liner samples 200 mm in length were 3D printed and tested in a grazing flow facility as a function of Mach number - up to M=0.25, sound pressure level - up to 140 dB, and where the acoustic source can be located upstream or downstream of the liner. Experimental results compare extremely well with those predicted validating the analytical design and optimization methodology. TL levels of 25 dB were achieved at 140 dB at M=0.25.

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.

This paper is focused on analysis of the acoustic properties of one of the liners defined under the framework of the International Forum for Aviation Research (IFAR), challenge four. Challenge four had the topic of exploring the effects of high sound pressure amplitudes on the measured impedance of liners. The effect of different source fields including: broadband random, tonal and multi-tone excitation was to be studied. Originally four liner samples of varying geometric properties were defined by NASA and should be produced by additive manufacturing. Since the study focuses on high level but no flow conditions measurements can be made in normal incidence impedance tubes. The present paper is only concerned with one of the four liner designs defined for the challenge, since this liner has already been shown to exhibit stronger nonlinear effects compared to the other three, which makes it the most interesting for the purpose of the study. Less focus is also on benchmarking with results from the other partners participating but some comparisons is made with published results from NASA and DLR. Results is presented investigating nonlinear interaction effects mostly for multi-tone excitation conditions but also a combination of broadband random and tonal excitation.

In a previous paper the effect of changing the downstream termination conditions on the nonlinear acoustic properties of a perforate mounted in a plane wave two-port test rig was studied. It is often assumed that the nonlinear acoustic properties of perforates, which are found for high level acoustic excitation, are local and can be determined separately for the perforate. A model for a backing cavity can be added to give the complete acoustic properties in terms of an impedance for a liner configuration. It was found that there were indications showing that the nonlinear acoustic properties in terms of a transfer impedance were changed depending on the acoustic properties of the downstream passive termination. The results were, however, not completely conclusive. The aim of the present study is therefore to extend the previous study to a wider frequency range and to higher acoustic excitation levels. A related question is if a transfer impedance is sufficient to describe the acoustic reflection and transmission properties of perforates under high level nonlinear excitation conditions.

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.

The flow over circular cylinders or tubes arranged in different configurations is widely employed in various engineering applications, ranging from combustion systems to HVAC systems. The Reynolds number serves as the governing parameter defining the flow around the cylinders, determining the separation of the flow from the cylinder surface, and establishing a separation point. This separation point, in turn, dictates the formation of a jet with parameters such as jet height, jet velocity, and jet length. The jet formation, influenced by varying Reynolds numbers, introduces the possibility of various characteristic lengths, including the diameter of the cylinders, height of the duct, gap between adjacent cylinders, jet height, and jet length. This study aims to explore the profound influence of these characteristic lengths on the aeroacoustic properties and the flow acoustic coupling at cylinders in cross flow. The jet parameters are derived from the analysis of experimental and analytical modeling data. Through a systematic investigation, we examine the interdependencies between these characteristic lengths and the resulting aeroacoustic properties.

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.