Tamer Elnady

Validation of an Inverse Semi-Analytical Technique to Educe Liner Impedance

In this paper, the acoustic impedance of a liner is educed by a novel semi-analytical inverse technique. The liner sample is placed flush with the solid walls in a rectangular duct with grazing flo ...

On Semi-Empirical Liner Impedance Modeling with Grazing Flow

Noise is created at the front of turbofan engines where a large-diameter fan spins at high speed to continuously compress a large volume of air. The engine is wrapped in a nacelle that has acoustic treatment to damp and absorb noise generated by the fan. This acoustic treatment is commonly of the locally reacting type. The impedance of these liners is what determines their efficiency to absorb sound waves. A lot has been published on modeling the impedance of liners, but there are still some unanswered questions. There are different criteria for defining the end correction that is added due to the finite length of the orifices and the effect of the interaction between neighboring orifices. There are several models available for the effect of grazing flow. The aim of the current paper is to contribute to answering some questions through impedance measurements on a large number of liners with grazing flow. Empirical expressions for the liner impedance were deduced. The perforate model is also of interest to the car silencers applications. A new impedance measurement technique is also proposed in this paper, which is based on liner educing methods.

An inverse analytical method for extracting liner impedance from pressure measurements

The in-situ method has been used for a long time as an easy, quick and direct method for measuring the acoustic impedance of locally reacting liners with and without flow. Several drawbacks and problems have been reported with the use of this method, which motivated the development of indirect methods to deduce the impedance from the measurement of the pressure field. A new inverse analytical technique is presented in this paper. The liner sample is placed inside a rectangular duct. The amplitude of the plane wave incident towards the lined section is measured using the two-microphone technique. The reflection coefficient at the exit plane is also measured using the same technique. These measured values are fed to an analytical model for sound propagation through the lined section, which is constructed using mode-matching. A minimization scheme is used to find the liner impedance value in the complex plane to match the calculated sound field to the measured one at the microphone positions already used for the two microphone measurements. The results show that the proposed technique can educe or measure the impedance to an acceptable accuracy.

Modeling perforates in mufflers using two-ports

One of the main sources of noise of a vehicle is the engine where its noise is usually damped by means of acoustic mufflers. A very common problem in the modeling of automotive mufflers is that of ...

Validation of an Inverse Analytical Technique to Educe Liner Impedance with Grazing Flow

This paper is based on a previously developed analytical technique to educe the acoustic impedance of a liner sample, placed inside a rectangular duct in grazing incidence. This technique uses the measurement of complex acoustic pressure at four positions inside the duct, upstream and downstream the lined section, and educing the result using the mode matching method. In the mode matching code, the flow was simplified to be uniform, which is not usually the actual case. This technique was opposed by several doubts regarding the mean flow assumption and the possible pressure discontinuity at the hard-soft and soft-hard steps. It was believed that the pressure field away from this discontinuity is representative of the real case, and the code would be capable to educe the correct liner impedance. In this paper, the code is validated against benchmark data from NASA. First the code was tested to be able to reproduce the pressure field for a given impedance. Second, it was tested to educe the correct impedance for a given pressure distribution. The results of the mode matching code were in excellent agreement with the experimental data. The effect of shear flow was investigated and found out that the differences are quite small for the chosen duct size and frequency of interest. On the other hand, the mode matching formulation was re-written based on the pressure instead of the velocity potential.

Quenching of acoustic bandgaps by flow noise

We report an experimental study of acoustic effects produced by wind impinging on noise barriers based on two-dimensional sonic crystals with square symmetry. We found that the attenuation strength of sonic-crystal bandgaps decreases for increasing values of flow speed. A quenching of the acoustic bandgap appears at a certain speed value that depends of the barrier filling ratio. For increasing values of flow speed, the data indicate that the barrier becomes a sound source because of its interaction with the wind. We conclude that flow noise should be taken into account in designing acoustic barriers based on sonic crystals.

Determination of liner impedance under high temperature and grazing flow conditions

Acoustic liners have traditionally been used to reduce fan noise from the aircraft engine intake. To increase noise reduction there are now plans to also put liners in hot stream parts of the engine. In order to test liners under as realistic conditions as possible there has been a large development in inverse techniques for determination of liner impedance under grazing flow conditions, so called impedance eduction techniques. Testing under hot stream conditions has received smaller attention. This paper discusses techniques for measuring liner impedance under hot stream conditions and present some results obtained for single degree of freedom Helmholtz resonator liners with different configurations. It could be argued that the main effect of high temperatures is a change of medium properties such as: density, viscosity and speed of sound. If this is true the high temperature impedance could be predicted by scaling from the result at cold conditions. This is investigated in the paper by comparing measured results from liner impedance models available in the literature.

Flow and Pressure Drop Calculation Using Two-Ports

Exhaust systems should be carefully designed for different applications. The main objective of an exhaust system is to reduce the engine noise. Maximum noise reduction is usually desired to the lim ...

Comparison of impedance eduction results using different methods and test rigs

This thesis is motivated by the need for noise control in aircraft engine with orifices and perforated liner. The presence of high-level acoustic excitation, different flow situations either bias flow, grazing flow or any combination in the aircraft engine, makes the acoustic behavior complex due to the interaction between sound and flow over the lined wall. Both systematic acoustic prediction of aircraft engines and liner optimization necessitate progress in impedance measurement methods by including the effect of the complex flow situations. The aim of the present thesis is to experimentally study the change in acoustic properties of orifices and perforated liners under bias or grazing flow.In order to study the effect of different combinations of bias flow and high-level acoustic excitation, an in-duct orifice has been investigated with finely controlled acoustic excitation levels and bias flow speeds. This provides a detailed study of the transition from cases when high-level acoustic excitation causes flow reversal in the orifice to cases when the bias flow maintains the flow direction. Nonlinear impedance is measured and compared, and a scattering matrix and its eigenvalues are investigated to study the potentiality of acoustic energy dissipation or production. A harmonic method is proposed for modelling the impedance, especially the resistance, which captures the change in impedance results at low frequencies compared with experimental results.The presence of grazing flow can increase the resistance of acoustic liners and shift their resonator frequency. So-called impedance eduction technology has been widely studied during the past decades, but with a limited confidence due to the interaction of grazing flow and acoustic waves. A comparison has been performed with different test rigs and methods from the German Aerospace Center (DLR). Numerical work has been performed to investigate the effect of shear flow and viscosity. Our study indicates that the impedance eduction process should be consistent with that of the code of wave propagation computation, for example with the same assumption regarding shear flow and viscosity. A systematic analysis for measurement uncertainties is proposed in order to understand the essentials for data quality assessment and model validation. The idea of using different Mach numbers for wave dispersion and in the Ingard-Myers boundary condition has been tested regarding their effect on impedance eduction. In conclusion, a local Mach number based on friction velocity is introduced and validated using both our own experimental results and those of previous studies.

HARD STRIPS IN LINED DUCTS

Acoustic liners are widely used to attenuate sound waves inside aircraft jet engines. In most cases, the existence of hard strips in the lined ducts is inevitable. Previous research has proved that segmenting the liner and the positioning of the liner segments affect the attenuation characteristics of the liner. The aim of this work is to investigate these effects, and to compare the properties of circumferentially segmented duct liners with those of uniform liners, in order to identify any potential benefits of circumferentially segmented liners. The point-matching method is used to analyze the problem. It is a straightforward numerical method based on a closed form ansatz, which fulfils the governing equations and is matched to the boundary conditions point-wise. A computer code is used to obtain the wave numbers of the different modes, from which the transmission loss for each mode can be calculated at the desired range of frequencies. An earlier paper by the authors was published on this subject. It was a step on the road towards the solution. It was intended to test the method in solving simple cases of circumferential variation of only nonlocally reacting liners. The present study includes both locally reacting and non-locally reacting liners. More realistic cases are investigated. It is found that non-locally reacting liners are less affected by the presence of hard strips than locally reacting liners, where the behavior of each mode depends on the number and width of hard splices in the duct.