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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Design of an Acoustic Shielding Flap Concept with Prediction and Validation

As part of a NATO task group, NASA has designed a noise reduction technology called a Shielding Flap for application to a hybrid wing body aircraft concept. The design drivers and objectives for this technology are outlined in this paper. Design parameters are defined, and recently-developed NASA software for prediction of acoustic scattering is used to investigate the expected performance of the technology for a range of parameter values. Detailed scattering computational results are presented for several simulated noise sources and for many variations of the shielding flap concept. A selection of computational results is compared with experimental data collected in the NASA Langley Quiet Flow Facility. Overall, the results indicate that the Shielding Flap has strong potential to reduce aft-radiated noise, which was a primary design objective, by shielding sound from the simulated sources while also shielding sound that is scattered from the main airframe.

Propulsion Airframe Aeroacoustics↗
NASA NTRS2006

Fast Scattering Code (FSC) User's Manual: Version 2

The Fast Scattering Code (version 2.0) is a computer program for predicting the three-dimensional scattered acoustic field produced by the interaction of known, time-harmonic, incident sound with aerostructures in the presence of potential background flow. The FSC has been developed for use as an aeroacoustic analysis tool for assessing global effects on noise radiation and scattering caused by changes in configuration (geometry, component placement) and operating conditions (background flow, excitation frequency).

Tinetti, Ana F.↗
NASA NTRS2011

Efficient Solution of Three-Dimensional Problems of Acoustic and Electromagnetic Scattering by Open Surfaces

We present a computational methodology (a novel Nystrom approach based on use of a non-overlapping patch technique and Chebyshev discretizations) for efficient solution of problems of acoustic and electromagnetic scattering by open surfaces. Our integral equation formulations (1) Incorporate, as ansatz, the singular nature of open-surface integral-equation solutions, and (2) For the Electric Field Integral Equation (EFIE), use analytical regularizes that effectively reduce the number of iterations required by iterative linear-algebra solution based on Krylov-subspace iterative solvers.

sound-soft acoustic scattering↗
NASA NTRS2013

Open Rotor Aeroacoustic Installation Effects for Conventional and Unconventional Airframes

As extensive experimental campaign was performed to study the aeroacoustic installation effects of an open rotor with respect to both a conventional tube and wing type airframe and an unconventional hybrid wing body airframe. The open rotor rig had two counter rotating rows of blades each with eight blades of a design originally flight tested in the 1980s. The aeroacoustic installation effects measured in an aeroacoustic wind tunnel included those from flow effects due to inflow distortion or wake interaction and acoustic propagation effects such as shielding and reflection. The objective of the test campaign was to quantify the installation effects for a wide range of parameters and configurations derived from the two airframe types. For the conventional airframe, the open rotor was positioned in increments in front of and then over the main wing and then in positions representative of tail mounted aircraft with a conventional tail, a T-tail and a U-tail. The interaction of the wake of the open rotor as well as acoustic scattering results in an increase of about 10 dB when the rotor is positioned in front of the main wing. When positioned over the main wing a substantial amount of noise reduction is obtained and this is also observed for tail-mounted installations with a large U-tail. For the hybrid wing body airframe, the open rotor was positioned over the airframe along the centerline as well as off-center representing a twin engine location. A primary result was the documentation of the noise reduction from shielding as a function of the location of the open rotor upstream of the trailing edge of the hybrid wing body. The effects from vertical surfaces and elevon deflection were also measured. Acoustic lining was specially designed and inserted flush with the elevon and airframe surface, the result was an additional reduction in open rotor noise propagating to the far field microphones. Even with the older blade design used, the experiment provided quantification of the aeroacoustic installation effects for a wide range of open rotor and airframe configurations and can be used with data processing methods to evaluate the aeroacoustic installation effects for open rotors with modern blade designs.

Czech, Michael J.↗
NASA NTRS1977

The generation of sound by vorticity waves in swirling duct flows

Swirling flow in an axisymmetric duct can support vorticity waves propagating parallel to the axis of the duct. When the cross-sectional area of the duct changes a portion of the wave energy is scattered into secondary vorticity and sound waves. Thus the swirling flow in the jet pipe of an aeroengine provides a mechanism whereby disturbances produced by unsteady combustion or turbine blading can be propagated along the pipe and subsequently scattered into aerodynamic sound. In this paper a linearized model of this process is examined for low Mach number swirling flow in a duct of infinite extent. It is shown that the amplitude of the scattered acoustic pressure waves is proportional to the product of the characteristic swirl velocity and the perturbation velocity of the vorticity wave. The sound produced in this way may therefore be of more significance than that generated by vorticity fluctuations in the absence of swirl, for which the acoustic pressure is proportional to the square of the perturbation velocity. The results of the analysis are discussed in relation to the problem of excess jet noise.

Howe, M. S.↗
NASA NTRS1989

The normal modes of a resonant cavity containing discrete inhomogeneities - The influence of fibril magnetic fields on the solar acoustic oscillations

Motivated by considerations of the interaction between fibril magnetic fields and solar p-modes, the acoustic spectrum of a cylindrical cavity filled with ideal gas in which a number of magnetic flux tubes are embedded is studied. A formalism, based on the T-matrix approach to acoustic scattering, is developed which can be used to determine the eigenfrequencies and eigenfunctions for any arbitrary distribution of flux tubes. For weak scatterers, the frequency shifts and velocity eigenfunctions are calculated using perturbation theory for the cases of a single flux tube and a random distribution of up to 100 flux tubes. The results of this 'exact' approach are used to give a critical appraisal of the predictions of theories based on some form of averaging, such as the one discussed recently by Bogdan and Zweibel (1987).

Bogdan, Thomas J.↗
NASA NTRS1992

Air-sea interaction and remote sensing

The first part of the proposed research was a joint effort between our group and the Applied Physics Laboratory (APL), University of Washington. Our own research goal is to investigate the relation between the air-sea exchange processes and the sea state over the open ocean and to compare these findings with our previous results obtained over a small body of water namely, Lake Washington. The goals of the APL researchers are to study (1) the infrared sea surface temperature (SST) signature of breaking waves and surface slicks, and (2) microwave and acoustic scattering from water surface. The task of our group in this joint effort is to conduct measurements of surface fluxes (of momentum, sensible heat, and water vapor) and atmospheric radiation (longwave and shortwave) to achieve our research goal as well as to provide crucial complementary data for the APL studies. The progress of the project is summarized.

Katsaros, Kristina B.↗
NASA NTRS1997

Numerical Boundary Conditions for Computational Aeroacoustics Benchmark Problems

Category 1, Problems 1 and 2, Category 2, Problem 2, and Category 3, Problem 2 are solved computationally using the Dispersion-Relation-Preserving (DRP) scheme. All these problems are governed by the linearized Euler equations. The resolution requirements of the DRP scheme for maintaining low numerical dispersion and dissipation as well as accurate wave speeds in solving the linearized Euler equations are now well understood. As long as 8 or more mesh points per wavelength is employed in the numerical computation, high quality results are assured. For the first three categories of benchmark problems, therefore, the real challenge is to develop high quality numerical boundary conditions. For Category 1, Problems 1 and 2, it is the curved wall boundary conditions. For Category 2, Problem 2, it is the internal radiation boundary conditions inside the duct. For Category 3, Problem 2, they are the inflow and outflow boundary conditions upstream and downstream of the blade row. These are the foci of the present investigation. Special nonhomogeneous radiation boundary conditions that generate the incoming disturbances and at the same time allow the outgoing reflected or scattered acoustic disturbances to leave the computation domain without significant reflection are developed. Numerical results based on these boundary conditions are provided.

Tam, Chritsopher K. W.↗
NASA NTRS1998

Evaluation of Boundary Conditions for the Gust-Cascade Problem

Using a high-order accuracy finite-difference time-domain algorithm, the acoustic scattering from a flat-plate cascade is computed. Keeping the grid and time step fixed, the effect of four different boundary conditions on the accuracy and stability of the computed solution is compared.

Hixon, R.↗
NASA NTRS1999

A High-Order Method Using Unstructured Grids for the Aeroacoustic Analysis of Realistic Aircraft Configurations

A method for the prediction of acoustic scatter from complex geometries is presented. The discontinuous Galerkin method provides a framework for the development of a high-order method using unstructured grids. The method's compact form contributes to its accuracy and efficiency, and makes the method well suited for distributed memory parallel computing platforms. Mesh refinement studies are presented to validate the expected convergence properties of the method, and to establish the absolute levels of a error one can expect at a given level of resolution. For a two-dimensional shear layer instability wave and for three-dimensional wave propagation, the method is demonstrated to be insensitive to mesh smoothness. Simulations of scatter from a two-dimensional slat configuration and a three-dimensional blended-wing-body demonstrate the capability of the method to efficiently treat realistic geometries.

Atkins, Harold L.↗
NASA NTRS2013

Peter Waterman and T-Matrix Methods

This paper summarizes the scientific legacy of Peter C. Waterman (1928-2012) who introduced concepts and theoretical techniques that have had a major impact on the fields of scattering by particles and particle groups, optical particletcharacterization, radiative transfer, and remote sensing. A biographical sketch is also included.

electromagnetic scattering↗
NASA NTRSDate not supplied

Propulsion Airframe Aeroacoustics and Aircraft System Noise Flight Research Test: NASA Overview

Motivations and the formulation of the objectives are described for an ambitious flight research test conducted in collaboration between NASA and The Boeing Company. The Propulsion Airframe Aeroacoustics and Aircraft System Noise Flight Research Test was executed by the Boeing ecoDemonstrator Program with an Etihad Airways Boeing 787-10 aircraft. Five key technical approaches are described that were used to accomplish the more unconventional and challenging objectives of the research. In addition to the modern technology of the 787, these include evaluating multiple acoustic shielding and reflection effects using both the wing and fuselage in straight flight and banking flight paths, hardwalling the aft duct liner, and utilizing an extensive instrumentation array both on the ground and on the aircraft. The research level version of NASA’s aircraft system noise prediction capability including a new acoustic scattering method were used to confirm the technical approaches and provide guidance for the detailed design of instrumentation and flight test plans. Initial comparisons are shown of flight test data to predictions using the research level of NASA’s Aircraft Noise Prediction Program. Significant prediction challenges are revealed when compared to these high-quality data, while major progress is shown both in the measurement and the prediction of propulsion airframe aeroacoustic effects in flight. This comparison marks the first rigorous validation with modern flight test data and establishes a basis to further understand and quantify the state of NASA’s current capability and to develop improvements in the fidelity of aircraft system noise predictions. The context of multiple coordinated companion research papers is described as well as future plans for the continued analysis of this flight research data.

Propulsion Airframe Aeroacoustics↗
NASA NTRSDate not supplied

Geometric Acoustics for Aircraft Noise Scattering

This paper discusses aircraft noise scattering by geometric acoustics, which consists of the basic features of sound propagation and reflection in rays or ray tubes, diffraction by smooth geometry in terms of surface creeping waves, and diffraction by abrupt geometry features such as the wing trailing edges. Based on the classic theories of these features, prediction methodologies can be constructed for aircraft noise scattering. The methodologies, however, require important modifications and extensions to account for unique features in aircraft noise applications, for both current conventional aircraft and future unconventional designs. These modifications and extensions include the derivation of a general reflection coefficient that contains the effects of surface geometry curvature, surface impedance, and mean flow. Corrections to the basic formulation of diffraction, both smooth geometry and sharp edges, are formulated to account for the finite dimensions of practical applications. For smooth geometry diffraction, a second order correction is used to continuously transition from insolified to shadow zones and an approach is presented to compute the properties of the geodesic path of surface wave propagation. A model is developed to make use of the ray propagation formulation for incoherent and partially coherent propagation and scattering, which is an important phenomenon in aircraft noise. Mean flow effect is also included in the methodology for low Mach number flows. Examples of calculations based on these theoretical developments are presented to illustrate the unique features in aircraft noise applications.

Yueping Guo↗
NASA NTRS1990

A ray-acoustics approach to fuselage scattering of rotor noise

A ray acoustics approach to fuselage scattering of rotor noise is considered. The method is based on a combination of classical geometrical acoustics and the paraxial ray approximation. The method can handle scattering by objects of arbitrary shapes and can be applied in an inhomogeneous moving medium. Applications to aeroacoustics include the scattering of blade vortex interaction (BVI) pulses by rigid scattering objects. The BVI is modeled by a rotating impulsive point force. It has been found that scattering effects of rotating sources cannot be ignored. Flow has also been found to cause a modification and displacement of the directivity pattern and the shadow zones behind scatterers.

Atalla, Noureddine↗
NASA NTRS1990

Scattering measurements on natural and model trees

The acoustical back scattering from a simple scale model of a tree has been experimentally measured. The model consisted of a trunk and six limbs, each with 4 branches; no foliage or twigs were included. The data from the anechoic chamber measurements were then mathematically combined to construct the effective back scattering from groups of trees. Also, initial measurements have been conducted out-of-doors on a single tree in an open field in order to characterize its acoustic scattering as a function of azimuth angle. These measurements were performed in the spring, prior to leaf development. The data support a statistical model of forest scattering; the scattered signal spectrum is highly irregular but with a remarkable general resemblance to the incident signal spectrum. Also, the scattered signal's spectra showed little dependence upon scattering angle.

Rogers, James C.↗