Tony L. Parrott

VALIDATION OF AN IMPEDANCE EDUCTION METHOD IN FLOW

Results are reported for validating a method for educing the normal incidence impedance of a locally reacting liner in a grazing incidence, nonprogressive acoustic wave environment with flow. The results demonstrate the ability of the method to reproduce the normal incidence admittance of a solid steel plate and normal incidence impedance of two soft test liners in a uniform flow. The selected test liners are known to be locally reacting and exhibit no amplitude-dependent impedance nonlinearities and only minimal flow effects. Baseline results for these liners are, therefore, established from measurements in a conventional normal incidence impedance tube. A key feature of the method is the expansion of the unknown impedance function as a piecewise continuous polynomial with undetermined coefficients. Stewarts adaptation (Stewart, G. W., III, A Modification of Davidons Minimization Method to Accept Difference Approximations of Derivatives, Journal of ACM, Vol. 14, No. 1, 1967, pp. 72-83) of the Davidon-Fletcher-Powell optimization algorithm is used to educe the normal incidence impedance at each Mach number by optimizing an objective function. The method very Marly reproduces the normal incidence impedance spectrum for each of the test liners; thus, its usefulness for determining the normal incidence impedance of test liners for a broad range of source frequencies and flow Mach numbers is demonstrated.

Benchmark Data for Evaluation of Aeroacoustic Propagation Codes With Grazing Flow

Increased understanding of the effects of acoustic treatment on the propagation of sound through commercial aircraft engine nacelles is a requirement for more efficient liner design. To this end, one of NASA s goals is to further the development of duct propagation and impedance reduction codes. A number of these codes have been developed over the last three decades. These codes are typically divided into two categories: (1) codes that use the measured complex acoustic pressure field to reduce the acoustic impedance of treatment that is positioned along the wall of the duct, and (2) codes that use the acoustic impedance of the treatment as input and compute the sound field throughout the duct. Clearly, the value of these codes is dependent upon the quality of the data used for their validation. Over the past two decades, data acquired in the NASA Langley Research Center Grazing Incidence Tube have been used by a number of researchers for comparison with their propagation codes. Many of these comparisons have been based upon Grazing Incidence Tube tests that were conducted to study specific liner technology components, and were incomplete for general propagation code validation. Thus, the objective of the current investigation is to provide a quality data set that can be used as a benchmark for evaluation of duct propagation and impedance reduction codes. In order to achieve this objective, two parallel efforts have been undertaken. The first of these is the development of an enhanced impedance eduction code that uses data acquired in the Grazing Incidence Tube. This enhancement is intended to place the benchmark data on as firm a foundation as possible. The second key effort is the acquisition of a comprehensive set of data selected to allow propagation code evaluations over a range of test conditions.

Evaluation of a multi-point method for determining acoustic impedance

Abstract An investigation was conducted to develop and explore potential improvements provided by a multi-point method (MPM) over the traditional standing wave method (SWM) and two-microphone method (TMM) for determining acoustic impedance. A wave propagation model, which includes wall absorption and mean flow effects, was developed to model the standing wave pattern in an impedance tube. The reflection factor and acoustic impedance of a test specimen were calculated from a “best” fit of this standing wave pattern to the point pressure measurements. Data were acquired for 30 test samples chosen to cover the reflection factor magnitude range from 0·004 to 0·999. A probe microphone was used to obtain two to six pressure measurements per half-wavelength along the centerline of the impedance tube. Three spacing distributions were investigated; uniform, random and selective. The calculated standing wave patterns based on the propagation model were found to match the point pressure measurements with good to excellent agreement for all three spacing distributions and over the entire reflection factor magnitude range investigated. Because of extremely consistent and repeatable results, as well as ease of implementation, the uniform spacing distribution is preferable. Comparisons of the acoustic impedance determined by using 2, 3, 6, and 18 measurement points showed that, while two points are generally sufficient for repeatable measurements, random measurement error is insignificant when using at least six pressure measurements at points evenly spaced over a distance of one half-wavelength. The effects of turbulent mean flow noise contamination on results obtained with the MPM were simulated by adding a broadband noise source to the discrete frequency source. Data were acquired at uniformly distributed points and impedances were calculated using the MPM and TMM algorithms. The results from these two algorithms were then compared with a reference set of MPM results obtained with only the discrete frequency source activated. The MPM results were found to converge to the reference results (i.e., the random measurement error was minimised) with fewer averages than was the case with the TMM results. This observation indicates that the MPM will be less subject to flow induced random error than the TMM in the presence of significant broadband noise levels associated with mean flow.

Comparison of Acoustic Impedance Eduction Techniques for Locally-Reacting Liners

Typical acoustic liners used in current aircraft inlets and aft-fan ducts consist of some type of perforated facesheet bonded to a honeycomb core. A number of techniques for determining the acoustic impedance of these locallyreacting liners have been developed over the last five decades. In addition, a number of models have been developed to predict the acoustic impedance of locallyreacting liners in the presence of grazing flow, and to use that information together with aeroacoustic propagation codes to assess the noise absorption provided by these liners. These prediction models have incorporated the results from databases acquired with specific impedance eduction techniques. Thus, while these prediction models are acceptable for liners that are similar to those tested in these databases, their application to new liner configurations must be viewed with caution. The primary purpose of this paper is to provide a comparison of impedance eduction techniques that have been implemented at various aerospace research laboratories in the United States (NASA Langley Research Center, General Electric Aircraft Engines, B. F. Goodrich and Boeing). A secondary purpose is to provide data for liner configurations that extend the porosity range beyond that which has been previously used in common aircraft engine nacelles. Two sets of liners were designed to study the effects of three parameters: perforate hole diameter, facesheet thickness and porosity. These two sets of liners were constructed for testing in each of the laboratories listed above. The first set of liners was designed to fit into the NASA Langley and Boeing test facilities. The second set was designed to fit into the General Electric Aircraft Engines and B. F. Goodrich test facilities. By using the same parent material, both sets of liners were identical to within the limits of material and fabrication variability. Baseline data were obtained in the normal incidence impedance tubes at NASA Langley and B. F. Goodrich. The results were found to compare extremely well. The

Comparison of Two Waveguide Methods for Educing Liner Impedance in Grazing Flow

Acoustic measurements taken with several liners in a flow impedance tube are used to assess two waveguide methods, the single mode method (SMM) and the finite element method (FEM), for impedance eduction in the presence of uniform grazing flow. Both methods use complex acoustic pressure data acquired over the liner length to educe the liner impedance. The SMM is based on the assumption that the sound pressure level and phase decay rates of a single progressive mode can be extracted from the measured complex acoustic pressures. No a priori assumptions are made in the FEM regarding the measured data. For no-flow conditions, the accuracy of each method is demonstrated by the excellent agreement between no-flow impedances educed in a grazing incidence tube and those acquired in a normal incidence tube. For grazing flow conditions (Mach numbers up to 0.5), the relative accuracy of the two waveguide methods is demonstrated by comparing the impedances educed with the FEM to the corresponding results for the SMM. Significant discrepancies occur for both methods for tests conducted at 0.5 kHz. Possible explanations for these discrepancies are explored with, as yet, no clear answer. Above 0.5 kHz, the results indicate the SMM can be used when the acoustic pressure profile is dominated by a single progressive mode, whereas the FEM can be used for all cases.

Design and Evaluation of Modifications to the NASA Langley Flow Impedance Tube

The need to minimize fan noise radiation from commercial aircraft engine nacelles continues to provide an impetus for developing new acoustic liner concepts. If the full value of such concepts is to be attained, an understanding of grazing flow effects is crucial. Because of this need for improved understanding of grazing flow effects, the NASA Langley Research Center Liner Physics Group has invested a large effort over the past decade into the development of a 2-D finite element method that characterizes wave propagation through a lined duct. The original test section in the Langley Grazing IncidenceTube was used to acquire data needed for implementation of this finite element method. This test section employed a stepper motor-driven axial-traversing bar, embedded in the wall opposite the test liner, to position a flush-mounted microphone at pre-selected locations. Complex acoustic pressure data acquired with this traversing microphone were used to educe the acoustic impedance of test liners using this 2-D finite element method and a local optimization technique. Results acquired in this facility have been extensively reported, and were compared with corresponding results from various U.S. aeroacoustics laboratories in the late 1990 s. Impedance data comparisons acquired from this multi-laboratory study suggested that it would be valuable to incorporate more realistic 3-D aeroacoustic effects into the impedance eduction methodology. This paper provides a description of modifications that have been implemented to facilitate studies of 3-D effects. The two key features of the modified test section are (1) the replacement of the traversing bar and its flush-mounted microphone with an array of 95 fixed-location microphones that are flush-mounted in all four walls of the duct, and (2) the inclusion of a suction device to modify the boundary layer upstream of the lined portion of the duct. The initial results achieved with the modified test section are provided in this report, and a comparison of these results with those achieved using the original test section is used to demonstrate that the data acquisition and analysis with the new test section can be confidently used for impedance eduction.

Assessment of Soft Vane and Metal Foam Engine Noise Reduction Concepts

Two innovative fan-noise reduction concepts developed by NASA are presented - soft vanes and over-the-rotor metal foam liners. Design methodologies are described for each concept. Soft vanes are outlet guide vanes with internal, resonant chambers that communicate with the exterior aeroacoustic environment via a porous surface. They provide acoustic absorption via viscous losses generated by interaction of unsteady flows with the internal solid structure. Over-the-rotor metal foam liners installed at or near the fan rotor axial plane provide rotor noise absorption. Both concepts also provide pressure-release surfaces that potentially inhibit noise generation. Several configurations for both concepts are evaluated with a normal incidence tube, and the results are used to guide designs for implementation in two NASA fan rigs. For soft vanes, approximately 1 to 2 dB of broadband inlet and aft-radiated fan noise reduction is achieved. For over-the-rotor metal foam liners, up to 3 dB of fan noise reduction is measured in the low-speed fan rig, but minimal reduction is measured in the high-speed fan rig. These metal foam liner results are compared with a static engine test, in which inlet sound power level reductions up to 5 dB were measured. Brief plans for further development are also provided.

Optimization Method for Educing Variable-Impedance Liner Properties

A new approach to educing normal incidence acoustic impedance of nonuniform test specimens in grazing incidence and grazing-flow environments is described. An optimization algorithm is shown to provide an efficient means for searching out an impedance distribution to match the measured acoustic field along the upper wall of a duct opposite the test specimen. The flow duct is allowed to transmit multimodal, nonprogressive acoustic waves in a flow environment; however, only a no-flow environment is discussed. A key feature of the method is the expansion of the impedance function as a plecewise continuous polynomial with undetermined coefficients. The upper wall acoustic pressure is computed numerically as a function of these coefficients by using a finite element method. The Davidon-Fletcher-Powell optimization algorithm is used to educe the normal incidence impedance by determining the values of the undetermined coefficients that minimize the difference between the measured and the numerically computed upper wall pressure. Results show that this more robust method reduces by a factor of 30 the time required to make impedance determinations as compared with the contour deformation method and is better suited for liners with spatially varying impedance.

Design and Attenuation Properties of Periodic Checkerboard Liners

A finite element methodology is coupled to both a parallel equation solver and a parallel genetic optimization algorithm to assess the merits of a 4and 8-segment checkerboard liner. This assessment is carried out in a three-dimensional, zero flow, square cross-sectional, rectangular duct with the liner located in the lower wall. A 7-kHz acoustic pressure distribution that contains six cut-on modes with equal mode amplitudes is used as the sound source. The principal conclusion of the study is that the optimum attenuation of the 4-segment checkerboard liner is 50% greater than that of the uniform liner. When the impedance distribution of the 4-segment checkerboard liner is arranged periodically across the span of the duct to create an 8-segment checkerboard liner, the optimum attenuation is 214% greater than that of the uniform liner. Thus, the periodic checkerboard liner configurations have apparent advantages over the uniform liner for this limited geometry.

Uncertainty and Sensitivity Analyses of a Two-Parameter Impedance Prediction Model

This paper presents comparisons of predicted impedance uncertainty limits derived from Monte-Carlo-type simulations with a Two-Parameter (TP) impedance prediction model and measured impedance uncertainty limits based on multiple tests acquired in NASA Langley test rigs. These predicted and measured impedance uncertainty limits are used to evaluate the effects of simultaneous randomization of each input parameter for the impedance prediction and measurement processes. A sensitivity analysis is then used to further evaluate the TP prediction model by varying its input parameters on an individual basis. The variation imposed on the input parameters is based on measurements conducted with multiple tests in the NASA Langley normal incidence and grazing incidence impedance tubes; thus, the input parameters are assigned uncertainties commensurate with those of the measured data. These same measured data are used with the NASA Langley impedance measurement (eduction) processes to determine the corresponding measured impedance uncertainty limits, such that the predicted and measured impedance uncertainty limits (95% confidence intervals) can be compared. The measured reactance 95% confidence intervals encompass the corresponding predicted reactance confidence intervals over the frequency range of interest. The same is true for the confidence intervals of the measured and predicted resistance at near-resonance frequencies, but the predicted resistance confidence intervals are lower than the measured resistance confidence intervals (no overlap) at frequencies away from resonance. A sensitivity analysis indicates the discharge coefficient uncertainty is the major contributor to uncertainty in the predicted impedances for the perforate-over-honeycomb liner used in this study. This insight regarding the relative importance of each input parameter will be used to guide the design of experiments with test rigs currently being brought on-line at NASA Langley.