Grazing Flow Impedance Tube Research Articles

Experimental investigations on the acoustical performance of open-celled metal foams under grazing flow

Open-celled metal foams offer a possible solution for implementing acoustic liners in non-traditional nacelle locations, such as directly over the turbofan rotor. Previously, we have shown that the normal incidence absorption performance of metal foams can be further improved by compressing them to reduce their effective pore size. In this paper, we focus on the acoustical performance of compressed metal foams under grazing flow conditions. Experimental tests were conducted using a grazing flow impedance tube set up within Spirit AeroSystems' acoustics lab. Metal foams with four different compression ratios were tested under various flow rates and incident sound pressure levels. The sound pressure measurements were then utilized to educe their effective impedance under grazing flow conditions. Our results show that a uniform through-thickness compression can improve the acoustic performance of open-celled metal foams under grazing flow conditions and can aid the further development of acoustic liners for over the rotor applications.

On the acoustic performance of double degree of freedom helmholtz resonator based acoustic liners

This study investigates the acoustic performance of double degree of freedom Helmholtz resonators and multi cell acoustic liners made of these resonators, both experimentally and through finite element analysis. The dimensions of the resonator internal cavities were varied to assess the effect of the volume ratio on the acoustic performance of the system. Experiments were performed using a grazing flow impedance tube and instrumented 3D printed modular resonators, which can be switched between a single and double degree of freedom resonating cavity configuration. An aluminium acoustic liner with 16 modular resonating cavities was also used in experiments. The effect of the volume ratio is investigated by presenting the transmission loss and transmission coefficient induced and the changes in sound pressure level within the resonator. The underlying sound attenuation mechanisms are further elaborated through the finite element analysis results of the acoustic pressure and velocity fields in the different resonator configurations. The results showed that as the volume ratio is increased, the difference between the first and second resonance frequencies tends to have a convex behaviour. This may indicate the presence of an optimum volume ratio of the two internal cavities for a broad bandwidth of sound absorption. Moreover, the finite element analysis results showed that increasing the volume ratio increased both the transmission loss induced and the bandwidth of frequencies attenuated at the first resonance frequency due to an in-phase velocity magnitude relation, for both the neck and septum of the resonator. Conversely, for the second resonance frequency, there is an out-of-phase relation between the neck and septum velocity magnitudes. In addition to the single resonator analysis, the acoustic performance of liner configurations with 16 resonators was also investigated. The results showed an increase in the bandwidth of attenuated frequencies when each resonating cavity had a different volume ratio, as compared to a fixed volume ratio. In addition, the order of the resonators of different volume ratio in the liner arrangement did not seem to have a significant effect on the overall sound attenuation performance.

Impedance eduction for uniform and multizone acoustic liners

This paper presents results for five uniform and two multizone liners based on data acquired in the NASA Langley Grazing Flow Impedance Tube. Two methods, Prony and CHE, are used to educe the impedance spectra for each of these liners for many test conditions. The Prony method is efficient and generally provides accurate results for uniform liners, but is not well suited for multizone liners. The CHE method supports assessment of both uniform and multizone liners, but is much more computationally expensive. The results from these liners demonstrate the efficacy of both eduction methods, but also clearly demonstrate that sufficient attenuation is required to support accurate impedance eduction. For the liners considered in this study, the data indicate approximately 3 dB attenuation is needed for each zone of a multizone liner in order to ensure quality impedance eduction results. This study was conducted in response to two acoustic liner research challenges in support of a collaboration of multiple national laboratories under the International Forum for Aviation Research.

Effects of Mean Flow Assumption and Harmonic Distortion on Impedance Eduction Methods

This investigation uses methods based on the Pridmore-Brown and convected Helmholtz equations to study the acoustic behavior of a single-layer, conventional liner fabricated by DLR, German Aerospace Center and tested in the NASA Langley Grazing Flow Impedance Tube. Two key assumptions are explored in this investigation. First, a comparison of results achieved with uniform-flow and shear-flow impedance eduction methods is considered. Second, an approach based on the Prony method is used to extend these methods from single-mode to multimode implementations. In addition, a detailed study into the effects of harmonic distortion on the educed impedance is performed, and the results are used to develop guidelines regarding acceptable levels of harmonic distortion.

Experimental validation of numerical simulations for an acoustic liner in grazing flow: Self-noise and added drag

Abstract A coordinated experimental and numerical simulation effort is carried out to improve our understanding of the physics of acoustic liners in a grazing flow as well our computational aeroacoustics (CAA) method prediction capability. A numerical simulation code based on advanced CAA methods is developed. In a parallel effort, experiments are performed using the Grazing Flow Impedance Tube at the NASA Langley Research Center. In the experiment, a liner is installed in the upper wall of a rectangular flow duct with a 2 in. by 2.5 in. cross section. Spatial distribution of sound pressure levels and relative phases are measured on the wall opposite the liner in the presence of a Mach 0.3 grazing flow. The computer code is validated by comparing computed results with experimental measurements. Good agreements are found. The numerical simulation code is then used to investigate the physical properties of the acoustic liner. It is shown that an acoustic liner can produce self-noise in the presence of a grazing flow and that a feedback acoustic resonance mechanism is responsible for the generation of this liner self-noise. In addition, the same mechanism also creates additional liner drag. An estimate, based on numerical simulation data, indicates that for a resonant liner with a 10 percent open area ratio, the drag increase would be about 4 percent of the turbulent boundary layer drag over a flat wall.

Validation of an Improved Experimental Method for Use in Impedance Eduction

Results from impedance eduction methods developed by NASA Langley Research Center are used throughout the acoustic liner community. Occasional anomalies persist with these methods at frequencies where the liner produces minimal attenuation. An approach to educe impedance spectra with increased confidence is demonstrated, by combining results from successive tests with different cavity depths. First, a raylometer is used to measure the DC flow resistance of three wire-mesh facesheets. These facesheets are then mounted onto frames and a normal incidence tube is used to determine their respective acoustic impedance spectra. Next, each facesheet is successively mounted onto three frames with different cavity depths, and a grazing flow impedance tube is used to educe their respective acoustic impedance spectra with and without mean flow. Since the resonance frequency varies with cavity depth, each sample provides robust results over a different frequency range. Hence, a combination of results can be used to determine the facesheet acoustic resistance. When combined with the acoustic reactance (weakly dependent on the source sound pressure level and grazing flow Mach number), the acoustic impedance can be educed with increased confidence. Representative test results are discussed, and the complete database is available in electronic format upon request.

New Numerical Procedure for Impedance Eduction in Ducts Containing Mean Flow

A new impedance eduction method is presented and validated against a benchmark method, and the effects of measurement uncertainty errors on the impedances educedwith this newmethod are assessed.Unique features of the newmethod include the following: 1) the upstream and downstream boundary conditions contain higher-order duct modes, 2) the impedance spectra of unique nonuniform test liners on opposite walls may be educed simultaneously, and 3) the measured data for the impedance eduction are acquired only at the source and duct termination planes. The validation exercise is performedwith a rigid-wall insert and a conventional liner over a range of frequencies and flowMach numbers in the NASALangleyResearchCenter’s grazing flow impedance tube. The primary conclusions of the study are that the impedance spectra of the rigid-wall insert and the conventional liner that were educed from the newmethod are in very good agreement with those that were educed by using the benchmarkmethod. However, the effects of measurement uncertainty on the educed impedance are greater at the lower frequencies and the higher Mach numbers for the newmethod. All indications are that this occurs because the newmethod 1) uses significantly less data to perform the impedance eduction than the benchmark and 2) is currently based on a rather crude approximation to the measured pressure gradient, which is more sensitive to the refractive effects of the boundary layer than the measured lower-wall pressure that is required in the benchmark method.

Efficient Implementation of Tam and Auriault'sTime-Domain Impedance Boundary Condition

A new and efficient time-domain implementation is proposed for the impedance model originally introduced by Tam and Auriault (Tam, C. K. W., and Auriault, L., Time-Domain Impedance Boundary Conditions for Computational Aeroacoustics, AIAA Journal, Vol. 34, No. 5, May 1996, pp. 917-923). In the frequency domain, the pressure can be calculated from the product of the impedance with the normal velocity. In the time domain, a convolution is needed, which may become a costly operation. Present solutions for this problem involve either a full convolution and storage of a time history of data or a translation of the frequency-domain models to time-domain formulations involving time derivatives. The former approach is very costly in terms of memory and CPU time; the latter makes the implementation discretization specific and vulnerable to stability issues. A new formulation, based on recursive convolution, results in an efficient implementation that stores only one accumulator, which can be updated at each time step with minimal addition and multiplication. It allows for the exact representation of the impedance at a given frequency. The formulation is validated by a comparison with the measurement data from the NASA Langley Grazing Flow Impedance Tube. The simulations are performed by introducing a plane wave at a single frequency in the tube, and this is done for six frequencies and five different mean flow speeds.

Extended reacting impedance characteristics of polyurethane foam in a duct with flow

Extensive use at the Boeing Company of Scottfelt, Grade 900, polyurethane foam as a relatively inexpensive test material for jet-engine nacelle acoustic studies prompted the development of a polyurethane-foam impedance math mode. A math model for an extended reacting foam liner in a duct with flow has been formulated, based on results of previous normal incidence impedance tube tests and math model studies reported at the 86th Meeting of the Acoustical Society of America. Using this model, predictions of the complex axial wavenumber are compared to measurements of firmness 4 Scottfelt in the Boeing/Wichita Grazing Flow Impedance Tube. Results for thicknesses of 1/2 and 1 in. at duct Mach numbers of 0, 0.35, and 0.5 are reported. The extended reacting impedance model has also been incorporated into a duct attenuation prediction program. Predicted attenuations are compared with polyurethane-foam flow duct test results as a further check on the math model.