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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2017

Direct Numerical Simulation of Acoustic Noise Generation from the Nozzle Wall of a Hypersonic Wind Tunnel

The acoustic radiation from the turbulent boundary layer on the nozzle wall of a Mach 6 Ludwieg Tube is simulated using Direct Numerical Simulations (DNS), with the flow conditions falling within the operational range of the Mach 6 Hypersonic Ludwieg Tube, Braunschweig (HLB). The mean and turbulence statistics of the nozzle-wall boundary layer show good agreement with those predicted by Pate's correlation and Reynolds Averaged Navier-Stokes (RANS) computations. The rms pressure fluctuation P'(rms)/T(w) plateaus in the freestream core of the nozzle. The intensity of the freestream noise within the nozzle is approximately 20% higher than that radiated from a single at pate with a similar freestream Mach number, potentially because of the contributions to the acoustic radiation from multiple azimuthal segments of the nozzle wall.

Huang, Junji↗
NASA NTRS1984

A theoretical prediction of the acoustic pressure generated by turbulence-flame front interactions

The equations of momentum annd continuity are combined and linearized yielding the one dimensional nonhomogeneous acoustic wave equation. Three terms in the non-homogeneous equation act as acoustic sources and are taken to be forcing functions acting on the homogeneous wave equation. The three source terms are: fluctuating entropy, turbulence gradients, and turbulence-flame interactions. Each source term is discussed. The turbulence-flame interaction source is used as the basis for computing the source acoustic pressure from the Fourier transformed wave equation. Pressure fluctuations created in turbopump gas generators and turbines may act as a forcing function for turbine and propellant tube vibrations in Earth to orbit space propulsion systems and could reduce their life expectancy. A preliminary assessment of the acoustic pressure fluctuations in such systems is presented.

Huff, R. G.↗
NASA NTRS1984

A theoretical prediction of the acoustic pressure generated by turbulence-flame front interactions

The equations of momentum and continuity are combined and linearized yielding the one dimensional nonhomogeneous acoustic wave equation. Three terms in the non-homogeneous equation act as acoustic sources and are taken to be forcing functions acting on the homogeneous wave equation. The three source terms are: fluctuating entropy, turbulence gradients, and turbulence-flame interactions. Each source term is discussed. The turbulence-flame interaction source is used as the basis for computing the source acoustic pressure from the Fourier transformed wave equation. Pressure fluctuations created in turbopump gas generators and turbines may act as a forcing function for turbine and propellant tube vibrations in earth to orbit space propulsion systems and could reduce their life expectancy. A preliminary assessment of the acoustic pressure fluctuations in such systems is presented.

Huff, R. G.↗
NASA NTRS2015

Alpha-Stream Convertor

A thermo-acoustic engine and/or cooler is provided and includes an elongated tubular body, multiple regenerators disposed within the body, multiple heat exchangers disposed within the body, where at least one heat exchanger is disposed adjacent to each of the multiple regenerators, multiple transducers axially disposed at each end of the body, and an acoustic wave source generating acoustic waves. At least one of the acoustic waves is amplified by one of the regenerators and at least another acoustic wave is amplified by a second one of regenerators.

Dyson, Jr., Rodger William↗
NASA NTRS2018

Assessment of Noise Reduction Concepts for Leading-Edge Slat Noise

The leading-edge slat of a high-lift airfoil can be a significant noise contributor during aircraft landing. This paper summarizes the effects of several passive noise reduction devices on the 30P30N high-lift airfoil. Experiments are conducted on a two-dimensional multi-element high-lift airfoil with leading-edge slat extensions, gap filler, and cove filler in an anechoic wind tunnel to evaluate the effect of passive flow control on the acoustics generated by the unsteady flow field. Slat geometry modifications associated with the treatments alter the flow field in the region that dominates the generation of the acoustic field. Three angles of attack (a(k) = 8°, 10°, and 15.5°) and three different Reynolds numbers (Re(c) = 1.2e6, 1.5e6, and 1.71e6) are selected as the test conditions. Steady surface pressure measurements are conducted to assess the effect of the treatments on the lift and drag. Unsteady surface pressure measurements along with the far-field acoustic array measurements are performed to evaluate the changes in near- and far-field pressure fluctuations, respectively. Delay and Sum (DAS) beamforming method is applied to locate the noise sources on the model and provide integrated spectra. Implementation difficulties with the gap filler led to structural integration deficiencies that prevented a fair assessment of this technology. Among the other passive devices, the cove filler s the most effective noise reduction, along with a negligible change in the aerodynamic metrics.

Zhang, Yang↗
NASA NTRSDate not supplied

Slat Noise Control Using a Slat Gap Filler

The leading edge slat of a high-lift system is one of the main noise contributors on many commercial aircraft during approach. This paper continues our previous studies on the gap filler for passive noise control on the 30P30N high-lift airfoil. An improved implementation of the gap filler was applied to minimize the effects of flow leakage encountered in the previous work, which resulted in spurious noise content in the far-field acoustic spectra. To evaluate the effect of passive flow control on the acoustics generated by the unsteady flow field, anechoic wind tunnel experiments are conducted on the two-dimensional, three-element high-lift airfoil with a gap filler mounted to the slat. The slat geometry modification associated with the gap filler alters the flow field in the cove region that dominates the generation of the acoustic field. A single angle of attack (훼푘=8◦) and three flow speeds corresponding to Reynolds numbersof푅푒푐=1.2푒6,1.5푒6, and1.71푒6are selected as the test conditions. Steady surface pressure measurements are conducted to assess the effect of the treatments on the overall lift. Acoustic array measurements are used to evaluate the influence of the gap filler on the radiated noise. Delay and Sum beamforming is applied to locate the noise sources on the model and to provide the integrated spectra. The gap filler is found to eliminate the narrowband peaks in the acoustic spectra and, also, to yield a 10 dB reduction in the broadband noise in comparison with the baseline case. Time-resolved Particle Image Velocimetry results show that the flow features are significantly altered with the presence of the gap filler, which leads to a more stable slat cove shear layer and, thus, to weaker pressure and velocity fluctuations.

Yang Zhang↗
NASA NTRS1990

Non-linear generation of acoustic noise in the IAR spacecraft

The requirement to produce high level acoustic noise fields with increasing accuracy in environmental test facilities dictates that a more precise understanding is required of the factors controlling nonlinear noise generation. Details are given of various nonlinear effects found in acoustic performance data taken from the IAR Spacecraft Acoustic Chamber. This type of data has enabled the IAR to test large spacecraft to relatively tight acoustic tolerances over a wide frequency range using manually set controls. An analog random noise automatic control system was available and modified to provide automatic selection of the chamber's spectral sound pressure levels. The automatic control system when used to complete a typical qualification test appeared to equal the accuracy of the manual system and had the added advantage that parallel spectra could be easily achieved during preset tests.

Westley, R.↗
NASA NTRS1981

Acoustic noise generation by the DOE/NASA MOD-1 wind turbine

The results of a series of measurements taken over the past year of the acoustic emissions from the DOE/NASA MOD-1 Wind Turbine show the maximum acoustic energy is concentrated in the low frequency range, often below 100 Hz. The temporal as well as the frequency characteristics of the turbine sounds have been shown to be important since the MOD-1 is capable of radiating both coherent and incoherent noise. The coherent sounds are usually impulsive and are manifested in an averaged frequency domain plot as large numbers of discrete energy bands extending from the blade passage frequency to beyond 50 Hz on occasion. It is these impulsive sounds which are identified as the principal source of the annoyance to a dozen families living within 3 km of the turbine. The source of the coherent noise appears to be the rapid, unsteady blade loads encountered as the blade passes through the wake of the tower structure. Annoying levels are occasionally reached at nearby homes due to the interaction of the low frequency, high energy peaks in the acoustic impulses and the structural modes of the homes as well as by direct radiation outdoors. The peak levels of these impulses can be enhanced or subdued through complete propagation.

Kelley, N. D.↗
NASA NTRS1988

Transition region fluxes in A-F Dwarfs: Basal fluxes and dynamo activity

The transition region spectra of 87 late A and early F dwarfs and subgiants were analyzed. The emission line fluxes are uniformly strong in the early F stars, and drop off rapidly among the late A stars. The basal flux level in the F stars is consistent with an extrapolation of that observed among the G stars, while the magnetic component displays the same flux-flux relations seen among solar-like stars. Despite the steep decrease in transition region emission flux for B-V less than 0.28, C II emission is detected in alpha Aql (B-V = 0.22). The dropoff in emission is inconsistent with models of the mechanically generated acoustic flux available. It is concluded that, although the nonmagnetic basal heating is an increasingly important source of atmospheric heating among the early F stars, magnetic heating occurs in any star which has a sufficiently thick convective zone to generate acoustic heating.

Walter, Frederick M.↗
NASA NTRS1973

Line profiles and turbulence generated by acoustic waves in the solar chromosphere. II - Contours of the Ca II and Mg II K lines

Making use of the time-averaged absorption profiles derived by Oster and Ulmschneider, non-LTE line formation in the context of a two-level atom is investigated for an isothermal atmosphere and for the Ca II and Mg II K lines in the solar chromosphere as represented by the Harvard-Smithsonian Reference Atmosphere. Source functions and emergent line profiles are computed for a variety of assumptions concerning the acoustically broadened profiles and the solar velocity fields.

Shine, R. A.↗
NASA NTRS1981

The generation of acoustic energy from stellar convection zones

The heating of stellar chromospheres and coronae by the dissipation of acoustic waves is an important heating mechanism. It is only in the lower solar chromosphere that short period acoustic wave heating seems undisputed. The arguments leading to the rejection of the so called acoustic heating theory are derived from comparisons of calculated acoustic energy fluxes with observational or theoretical requirements.

Bohn, H. U.↗
NASA NTRS1988

A general-purpose computer program for studying ultrasonic beam patterns generated with acoustic lenses

A 3-D computer model of a piston radiator with lenses for focusing and defocusing is presented. To achieve high-resolution imaging, the frequency of the transmitted and received ultrasound must be as high as 10 MHz. Current ultrasonic transducers produce an extremely narrow beam at these high frequencies and thus are not appropriate for imaging schemes such as synthetic-aperture focus techniques (SAFT). Consequently, a numerical analysis program has been developed to determine field intensity patterns that are radiated from ultrasonic transducers with lenses. Lens shapes are described and the field intensities are numerically predicted and compared with experimental results.

Roberti, Dino↗
NASA NTRS1994

Analytical procedures for estimating structural response to acoustic fields generated by advanced launch systems, phase 2

This report supplements a previous report of the same title submitted in June, 1992. It summarizes additional analytical techniques which have been developed for predicting the response of linear and nonlinear structures to noise excitations generated by large propulsion power plants. The report is divided into nine chapters. The first two deal with incomplete knowledge of boundary conditions of engineering structures. The incomplete knowledge is characterized by a convex set, and its diagnosis is formulated as a multi-hypothesis discrete decision-making algorithm with attendant criteria of adaptive termination.

Elishakoff, Isaac↗