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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Tuned Chamber Core Panel Acoustic Test Results

This report documents acoustic testing of tuned chamber core panels, which can be used to supplement the low-frequency performance of conventional acoustic treatment. The tuned chamber core concept incorporates low-frequency noise control directly within the primary structure and is applicable to sandwich constructions with a directional core, including corrugated-, truss-, and fluted-core designs. These types of sandwich structures have long, hollow channels (or chambers) in the core. By adding small holes through one of the facesheets, the hollow chambers can be utilized as an array of low-frequency acoustic resonators. These resonators can then be used to attenuate low-frequency noise (below 400 Hz) inside a vehicle compartment without increasing the weight or size of the structure. The results of this test program demonstrate that the tuned chamber core concept is effective when used in isolation or combined with acoustic foam treatments. Specifically, an array of acoustic resonators integrated within the core of the panels was shown to improve both the low-frequency absorption and transmission loss of the structure in targeted one-third octave bands.

Schiller, Noah H.↗
NASA NTRS2017

A New Formulation of Time Domain Boundary Integral Equation for Acoustic Wave Scattering in the Presence of a Uniform Mean Flow

It has been well-known that under the assumption of a constant uniform mean flow, the acoustic wave propagation equation can be formulated as a boundary integral equation, in both the time domain and the frequency domain. Compared with solving partial differential equations, numerical methods based on the boundary integral equation have the advantage of a reduced spatial dimension and, hence, requiring only a surface mesh. However, the constant uniform mean flow assumption, while convenient for formulating the integral equation, does not satisfy the solid wall boundary condition wherever the body surface is not aligned with the uniform mean flow. In this paper, we argue that the proper boundary condition for the acoustic wave should not have its normal velocity be zero everywhere on the solid surfaces, as has been applied in the literature. A careful study of the acoustic energy conservation equation is presented that shows such a boundary condition in fact leads to erroneous source or sink points on solid surfaces not aligned with the mean flow. A new solid wall boundary condition is proposed that conserves the acoustic energy and a new time domain boundary integral equation is derived. In addition to conserving the acoustic energy, another significant advantage of the new equation is that it is considerably simpler than previous formulations. In particular, tangential derivatives of the solution on the solid surfaces are no longer needed in the new formulation, which greatly simplifies numerical implementation. Furthermore, stabilization of the new integral equation by Burton-Miller type reformulation is presented. The stability of the new formulation is studied theoretically as well as numerically by an eigenvalue analysis. Numerical solutions are also presented that demonstrate the stability of the new formulation.

Hu, Fang↗
NASA NTRS2018

Acoustic Directivity of the DGEN Aero-propulsion Research Turbofan at Multiple Farfield Array Locations

The NASA Glenn Research Center's DGEN Aero-propulsion Research Turbofan (DART) is based on the Price Induction DGEN380 - a small, ~500-lbf thrust class, high-bypass, geared-turbofan engine with a separate flow nozzle. The general characteristics of the DART make it an ideal candidate for utilization as a test bed for engine aeroacoustic research in a relevant performance environment. To provide a baseline acoustic profile for the DART, the system was tested in the NASA Glenn Research Center's Aero-Acoustic Propulsion Laboratory. Acoustic measurements from multiple external arrays locations were acquired over the nominal fan operating range of 50-95%. The acoustic data are evaluated and reported in terms of overall, broadband, and tonal components. The existence of interaction tones (a result of the physics of the dual-spool interactions) are noted and presented. The symmetry of the acoustic directivity was measured in preparation for potential testing in other facilities.

Turbofan↗
NASA NTRS2018

Simulation of Sound Absorption by Scattering Bodies Treated with Acoustic Liners Using a Time-Domain Boundary Element Method

Reducing aircraft noise is a major objective in the field of computational aeroacoustics. When designing next generation quiet aircraft, it is important to be able to accurately and efficiently predict the acoustic scattering by an aircraft body from a given noise source. Acoustic liners are an effective tool for aircraft noise reduction, and are characterized by a complex valued frequency-dependent impedance, Z(w). Converted into the time-domain using Fourier transforms, an impedance boundary condition can be used to simulate the acoustic wave scattering of geometric bodies treated with acoustic liners. This work uses an admittance boundary condition where the admittance, Y(w), is defined to be the inverse of impedance, i.e., Y(w) = 1/Z(w). An admittance boundary condition will be derived and coupled with a time domain boundary integral equation. The solution will be obtained iteratively using spatial and temporal basis functions and will allow for acoustic scattering problems to be modeled with geometries consisting of both unlined and soft surfaces. Stability will be demonstrated through eigenvalue analysis.

Pizzo, Michelle E.↗
NASA NTRS2019

Application of Swept-Sine Excitation for Acoustic Impedance Education

The NASA Langley Normal Incidence Tube (NIT) and Grazing Flow Impedance Tube (GFIT) are regularly employed to characterize the frequency response of acoustic liners through the eduction of their specific acoustic impedance. Both test rigs typically use an acoustic source that produces sine wave signals at discrete frequencies (Stepped-Sine) to educe the impedance. The current work details a novel approach using frequency-swept sine waveforms normalized to a constant sound pressure level for excitation. Determination of the sound pressure level and phase from microphone measurements acquired using swept-sine excitation is performed using a modified Vold-Kalman order tracking filter. Four acoustic liners are evaluated in the NIT and GFIT with both stepped-sine and swept-sine sources. Using these two methods, the educed impedance spectra are shown to compare favorably. However, the new (Swept-Sine) approach provides much greater frequency resolution in less time, allowing the acoustic liner properties to be studied in much greater detail.

Howerton, Brian M.↗
NASA NTRSDate not supplied

Quiet Spacecraft Cabin Ventilation Fan: Acoustic Measurements Results

A spacecraft cabin ventilation fan suitable for aerodynamic and acoustic ground tests was designed and two copies of the fan assembly were fabricated. Both fans were tested for aerodynamic performance and acoustic levels in the NASA Glenn Research Center Acoustical Testing Laboratory. A new test rig for small axial flow fans was designed to accommodate the instrumentation and back-pressure adjustments. Measurements acquired were – static pressures for measuring performance, a 72-channel in-duct microphone array, external microphone measurements for acoustics, and inter-stage hot wire measurements of the fan wake. This report documents the acoustic measurements as part of a series of reports.

o Small Fan Noise, Testing, Duct Modes↗
NASA NTRSDate not supplied

Quiet Spacecraft Cabin Ventilation Fan: Acoustic Measurements Results

A quiet spacecraft cabin ventilation fan suitable for aerodynamic and acoustic ground tests was designed and two copies of the fan assembly were fabricated. Both fans were tested for aerodynamic performance and acoustic levels in the NASA Glenn Research Center Acoustical Testing Laboratory. A new test rig for small axial flow fans was designed to accommodate the instrumentation and back-pressure adjustments. Measurements acquired were – static pressures for measuring performance, a 72-channel in-duct microphone array, external microphone measurements for acoustics, and inter-stage hot wire measurements of the fan wake. This report documents the acoustic measurements as part of a series of reports.

Small Fan Noise↗
NASA NTRSDate not supplied

International Space Station Acoustics – A Status Report

It is important to control acoustic noise aboard the International Space Station (ISS) to provide a satisfactory environment for voice communications, alarm audibility, and restful sleep, and to minimize the risk for hearing loss. Acoustic monitoring is an important part of the noise control process on ISS, providing critical data for trend analysis, noise exposure analysis, validation of acoustic analyses and predictions, and to provide strong evidence for ensuring crew health and safety, thus allowing Flight Certification. And since the primary noise sources on ISS include the environmental control and life support system’s air revitalization system (fans and airflow) and active thermal control system (pumps and water flow), acoustic monitoring will indicate changes in hardware noise emissions that may indicate system degradation or performance issues. This paper provides the current acoustic levels in the ISS modules and sleep stations, and is an update to the status presented in 2018. Since this last status report, noise levels have remained consistent, but issues with stalled fan noise and unexplained low frequency spectral peaks have caused some exceedances to requirements. Noise levels in the Russian Segment have either remained consistent or have been reduced slightly, except for the new Multipurpose Laboratory Module, which has some significant noise exceedances.

Christopher S Allen↗
NASA NTRS2015

Effect of Coversheet Materials on the Acoustic Performance of Melamine Foam

Melamine foam is a highly absorptive material that is often used inside the payload fairing walls of a launch vehicle. This foam reduces the acoustic excitation environment that the spacecraft experiences during launch. Often, the melamine foam is enclosed by thin coversheet materials for contamination protection, thermal protection, and electrostatic discharge control. Previous limited acoustic testing by NASA Glenn Research Center has shown that the presence of a coversheet material on the melamine foam can have a significant impact on the absorption coefficient and the transmission loss. As a result of this preliminary finding a more extensive acoustic test program using several different coversheet materials on melamine foam was performed. Those test results are summarized in this paper. Additionally, a method is provided to use the acoustic absorption and transmission loss data obtained from panel level testing to predict their combined effect for the noise reduction of a launch vehicle payload fairing.

Acoustics↗
NASA NTRS2014

Tone and Broadband Noise Separation from Acoustic Data of a Scale-Model Counter-Rotating Open Rotor

Renewed interest in contra-rotating open rotor technology for aircraft propulsion application has prompted the development of advanced diagnostic tools for better design and improved acoustical performance. In particular, the determination of tonal and broadband components of open rotor acoustic spectra is essential for properly assessing the noise control parameters and also for validating the open rotor noise simulation codes. The technique of phase averaging has been employed to separate the tone and broadband components from a single rotor, but this method does not work for the two-shaft contra-rotating open rotor. A new signal processing technique was recently developed to process the contra-rotating open rotor acoustic data. The technique was first tested using acoustic data taken of a hobby aircraft open rotor propeller, and reported previously. The intent of the present work is to verify and validate the applicability of the new technique to a realistic one-fifth scale open rotor model which has 12 forward and 10 aft contra-rotating blades operating at realistic forward flight Mach numbers and tip speeds. The results and discussions of that study are presented in this paper.

Acoustic Data↗
NASA NTRS2014

Tone and Broadband Noise Separation from Acoustic Data of a Scale-Model Contra-Rotating Open Rotor

Renewed interest in contra-rotating open rotor technology for aircraft propulsion application has prompted the development of advanced diagnostic tools for better design and improved acoustical performance. In particular, the determination of tonal and broadband components of open rotor acoustic spectra is essential for properly assessing the noise control parameters and also for validating the open rotor noise simulation codes. The technique of phase averaging has been employed to separate the tone and broadband components from a single rotor, but this method does not work for the two-shaft contra-rotating open rotor. A new signal processing technique was recently developed to process the contra-rotating open rotor acoustic data. The technique was first tested using acoustic data taken of a hobby aircraft open rotor propeller, and reported previously. The intent of the present work is to verify and validate the applicability of the new technique to a realistic one-fifth scale open rotor model which has 12 forward and 10 aft contra-rotating blades operating at realistic forward flight Mach numbers and tip speeds. The results and discussions of that study are presented in this paper.

Noise Separation↗
NASA NTRS2015

Far-Field Acoustic Power Level and Performance Analyses of F31/A31 Open Rotor Model at Simulated Scaled Takeoff, Nominal Takeoff, and Approach Conditions: Technical Report I

Far-field acoustic power level and performance analyses of open rotor model F31/A31 have been performed to determine its noise characteristics at simulated scaled takeoff, nominal takeoff, and approach flight conditions. The nonproprietary parts of the data obtained from experiments in 9- by 15-Foot Low-Speed Wind Tunnel (915 LSWT) tests were provided by NASA Glenn Research Center to perform the analyses. The tone and broadband noise components have been separated from raw test data by using a new data analysis tool. Results in terms of sound pressure levels, acoustic power levels, and their variations with rotor speed, angle of attack, thrust, and input shaft power have been presented and discussed. The effect of an upstream pylon on the noise levels of the model has been addressed. Empirical equations relating model's acoustic power level, thrust, and input shaft power have been developed. The far-field acoustic efficiency of the model is also determined for various simulated flight conditions. It is intended that the results presented in this work will serve as a database for comparison and improvement of other open rotor blade designs and also for validating open rotor noise prediction codes.

Prop-Fan Technology↗
NASA NTRS2016

9- by 15-Foot Low Speed Wind Tunnel Acoustic Improvements Expanded Overview

The 9- by 15-Foot Low Speed Wind Tunnel (9x15 LSWT) at NASA Glenn Research Center was built in 1969 in the return leg of the 8- by 6-Foot Supersonic Wind Tunnel (8x6 SWT). The 8x6 SWT was completed in 1949 and acoustically treated to mitigate community noise issues in 1950. This treatment included the addition of a large muffler downstream of the 8x6 SWT test section and diffuser. The 9x15 LSWT was designed for performance testing of V/STOL aircraft models, but with the addition of the current acoustic treatment in 1986 the tunnel been used principally for acoustic and performance testing of aircraft propulsion systems. The present document describes an anticipated acoustic upgrade to be completed in 2017.

acoustics↗
NASA NTRS2016

9x15 Low Speed Wind Tunnel Acoustic Improvements

The 9- by 15-Foot Low Speed Wind Tunnel (9x15 LSWT) at NASA Glenn Research Center was built in 1969 in the return leg of the 8- by 6-Foot Supersonic Wind Tunnel (8x6 SWT). The 8x6 SWT was completed in 1949 and acoustically treated to mitigate community noise issues in 1950. This treatment included the addition of a large muffler downstream of the 8x6 SWT test section and diffuser. The 9x15 LSWT was designed for performance testing of VSTOL aircraft models, but with the addition of the current acoustic treatment in 1986 the tunnel has been used principally for acoustic and performance testing of aircraft propulsions systems. The present document describes an anticipated acoustic upgrade to be completed in 2017.

acoustic properties↗
NASA NTRS2021

A Summary of Normal Incidence Tube Tests of NASA'S Bio-inspired Broadband Acoustic Absorber

Motivated by the need to reduce noise pollution from aircraft engines, NASA has continued to design, manufacture and test recently patented structures that mimic the geometry and the broadband and low frequency acoustic absorption of assemblies of natural reeds. This report documents a screening test, which is a precursor to more controlled modelling experiments. The purpose of this experiment was to identify variables and prototype design features that might play a dominant role in the acoustic performance of these structures. To date, twenty-four prototypes, each with an overall shape of a cube (nominally 50 mm x 50 mm x 50 mm), were designed and additively manufactured from thermoplastics using a Fused Filament Fabrication technique. The prototypes were tested in the NASA Glenn and Langley Normal Incidence Tubes to experimentally determine the acoustic absorption as a function of frequency from 400 Hz to 3000 Hz. Results indicate that a variety of structures exhibit substantial acoustic absorption in that frequency range, with an average absorption coefficient greater than 0.6, especially in the frequency range of 400 Hz to 1000 Hz. Six basic prototype designs were chosen to inform future modeling research. Results of these experiments can be used to evaluate existing physics-based models of the interaction of sound waves with these structures or to develop new models. Validated physics-based models of these structures can help engineers optimize the designs for different commercial noise control applications.

Acoustics↗
NASA NTRSDate not supplied

EVALUATION OF ACOUSTIC SNAPSHOT ARRAYS FOR ROTORCRAFT SOURCE NOISE CHARACTERIZATION

Historical development of acoustic hemispheres has required steady flight of a rotorcraft vehicle across a large linear array of microphones. The US Army, NASA, and Navy recently conducted a rotorcraft acoustics flight test in which multiple “snapshot” microphone arrays were used alongside a traditional linear microphone array. The snapshot arrays allow for a near instantaneous capture of rotorcraft acoustic emissions, without the need for steady flight. Development of the snapshot array is contained herein, and an evaluation of effectiveness of the array during adverse weather conditions. The snapshot arrays captured significant variation in acoustic emissions throughout a single run and between multiple runs of similar conditions. Hemispheres were created and modelled in land-use planning software and an investigation of A-weighted Sound Exposure Level (SEL [dBA]) was conducted. Sideline predictions of SEL compared well (within 0.1 dBA) between traditional and snapshot arrays, while centre line locations were less favourable with a difference of 1.6 dBA. Future refinement is required for the snapshot array technique, including advanced design of microphone placement and employing a semiempirical method to interpolate between measurement points, instead of the linear frequency weighting conventionally employed.

Rotorcraft↗
NASA NTRSDate not supplied

An Artificial Neural Network Approach to Predict Rotor-Airframe Acoustic Waveforms

A surrogate artificial neural network/machine learning model was developed to predict the acoustic interaction for a fixed-pitch rotor in proximity to a downstream cylindrical airframe typical of small Unmanned Aerial System (sUAS) platforms. The model was trained to predict the acoustic waveform under representative hover conditions as a function of rotational speed, airframe proximity, and observer angle. Training data were acquired in an anechoic chamber on both isolated rotors and rotor-airframe configurations. Acoustic amplitude and phase of the revolution-averaged interaction were predicted, which required up to 25 harmonics to capture the impulse event caused by the blade’s approach and departure from the airframe. Prediction performance showed, on average, that the models could estimate the acoustic amplitude and phase over the relevant harmonics for unseen conditions with 86% and 75% accuracy, respectively, enabling a time domain reconstruction of the waveform for the range of geometric and flow parameters tested.

acoustics↗
NASA NTRSDate not supplied

An Artificial Neural Network Approach to Predict Rotor-Airframe Acoustic Waveforms

A surrogate artificial neural network/machine learning model was developed to predict the acoustic interaction for a fixed-pitch rotor in proximity to a downstream cylindrical airframe typical of small Unmanned Aerial System (sUAS) platforms. The model was trained to predict the acoustic waveform under representative hover conditions as a function of rotational speed, airframe proximity, and observer angle. Training data were acquired in an anechoic chamber on both isolated rotors and rotor-airframe configurations. Acoustic amplitude and phase of the revolution-averaged interaction were predicted, which required up to 25 harmonics to capture the impulse event caused by the blade’s approach and departure from the airframe. Prediction performance showed, on average, that the models could estimate the acoustic amplitude and phase over the relevant harmonics for unseen conditions with 86% and 75% accuracy, respectively, enabling a time domain reconstruction of the waveform for the range of geometric and flow parameters tested.

acoustics↗