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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

Measured Effects of Turbulence on the Loudness and Waveforms of Conventional and Shaped Minimized Sonic Booms

Turbulence has two distinctive effects on sonic booms: there is distortion in the form of random perturbations that appear behind the shock waves, and shock rise times are increased randomly. A first scattering theory by S.C. Crow in the late 1960s quantified the random distortions, and Crow's theory was shown to agree with available flight test data. A variety of theories for the shock thickness have been presented, all supporting the role of turbulence in increasing rise time above that of a basic molecular-relaxation structure. The net effect of these phenomena on the loudness of shaped minimized booms is of significant interest. Initial analysis suggests that there would be no change to average loudness, but this had not been experimentally investigated. The January 2004 flight test of the Shaped Sonic Boom Demonstrator (SSBD), together with a reference unmodified F-5E, included a 12500- foot linear ground sensor array with 28 digitally recorded sensor sites. This data set provides an opportunity to re-test Crow's theory for the post-shock perturbations, and to examine the net effect of turbulence on the loudness of shaped sonic booms.

Plotkin, Kenneth J.↗
NASA NTRS1991

Investigations into the damage for various types of unprotected carbon fibre composites with a variety of lightning arc attachments

Very little quantitative information exists on the extent and nature of damage caused to unprotected carbon fiber composites (CFC's) due to lightning arc attachment. An initial investigation into the arc damage to three different types and various thickness of CFC's from A and C component type lightning discharges is described. The difference in damage which the two types of waveform produced and the way the area of damage varies with different levels of action integral and charge transfer is compared. In some cases, the temperature rise at the rear of the panels was recorded for various levels of action integral and charge transfer. A comparison was made of the area of damage from visual inspection and soft x ray photography, using a suitable penetrant in the damage area. It is concluded there is a need for a more detailed analysis of the damage.

Reid, G. W.↗
NASA NTRS1983

Rotor-vortex interaction noise

A theoretical and experimental study is carried out to develop a first-principles analysis for predicting noise generated by helicopter main-rotor shed vortices interacting with the tail rotor. The generalized prediction procedure calls for a knowledge of the incident vortex velocity field, the rotor geometry, and the rotor operating conditions. The analysis encompasses compressibility effects and chordwise and spanwise noncompactness and treats oblique intersections with the blade planform. In assessing the theory, a model rotor experiment which isolates the blade-vortex interaction noise from other rotor noise mechanisms is conducted. An isolated tip vortex, generated by an upstream semispan airfoil, is convected into the model rotor, and acoustic spectra, pressure signatures, and directivity are measured. Ingestion of the vortex by the rotor is experimentally observed to generate harmonic noise and impulsive waveforms.

Schlinker, R. H.↗
NASA NTRS2001

Damage Assessment of Creep Tested and Thermally Aged Metallic Alloys Using Acousto-Ultrasonics

In recent years emphasis has been placed on the early detection of material changes experienced in turbine powerplant components. During the scheduled overhaul of a turbine, the current techniques of examination of various hot section components aim to find flaws such as cracks, wear, and erosion, as well as excessive deformations. Thus far, these localized damage modes have been detected with satisfactory results. However, the techniques used to find these flaws provide no information on life until the flaws are actually detected. Major improvements in damage assessment, safety, as well as more accurate life prediction could be achieved if nondestructive evaluation (NDE) techniques could be utilized to sense material changes that occur prior to the localized defects mentioned. Because of elevated temperatures and excessive stresses, turbine components may experience creep behavior. As a result, it is desirable to monitor and access the current condition of such components. Research at the NASA Glenn Research Center involves developing and utilizing an NDE technique that discloses distributed material changes that occur prior to the localized damage detected by the current methods of inspection. In a recent study, creep processes in a nickel-base alloy were the life-limiting condition of interest, and the NDE technique was acousto-ultrasonics (AU). AU is an NDE technique that utilizes two ultrasonic transducers to interrogate the condition of a test specimen. The sending transducer introduces an ultrasonic pulse at a point on the surface of the specimen while a receiving transducer detects the signal after it has passed through the material. The goal of the method is to correlate certain parameters of the detected waveform to characteristics of the material between the two transducers. Here, the waveform parameter of interest is the attenuation due to internal damping for which information is being garnered from the frequency domain. The parameters utilized to indirectly quantify the attenuation are the ultrasonic decay rate as well as various moments of the frequency power spectrum. A new, user-friendly, graphical interface AU system was developed at NASA Glenn. This system is an all-inclusive, multifunction system that controls the sending and receiving ultrasonic transducers as well as all posttest signal analysis. The system's postprocessing software calculates the multiple parameters used to study the material of interest.

Gyekenyesi, Andrew L.↗
NASA NTRS2011

Radar and Lidar Radar DEM

Using radar and lidar data, the aim is to improve 3D rendering of terrain, including digital elevation models (DEM) and estimates of vegetation height and biomass in a variety of forest types and terrains. The 3D mapping of vegetation structure and the analysis are useful to determine the role of forest in climate change (carbon cycle), in providing habitat and as a provider of socio-economic services. This in turn will lead to potential for development of more effective land-use management. The first part of the project was to characterize the Shuttle Radar Topography Mission DEM error with respect to ICESat/GLAS point estimates of elevation. We investigated potential trends with latitude, canopy height, signal to noise ratio (SNR), number of LiDAR waveform peaks, and maximum peak width. Scatter plots were produced for each variable and were fitted with 1st and 2nd degree polynomials. Higher order trends were visually inspected through filtering with a mean and median filter. We also assessed trends in the DEM error variance. Finally, a map showing how DEM error was geographically distributed globally was created.

remote sensing↗
NASA NTRS2021

Electron Bernstein waves and narrow band plasma waves near the electron cyclotron frequency in the near-Sun solar wind

Context. Recent studies of the solar wind sunward of 0.25 AU reveal that it contains quiescent regions, with low-amplitude plasma and magnetic field fluctuations, and a magnetic field direction similar to the Parker spiral. The quiescent regions are thought to have a more direct magnetic connection to the solar corona than other types of solar wind, suggesting that waves or instabilities in the quiescent regions are indicative of the early evolution of the solar wind as it escapes the corona. The quiescent solar wind regions are highly unstable to the formation of plasma waves near the electron cyclotron frequency (fce). Aims. We examine high time resolution observations of these waves in an effort to understand their impact on electron distribution functions of the quiescent near-Sun solar wind. Methods. High time resolution waveform captures of near-fce waves were examined to determine variations of their amplitude and frequency in time as well as their polarization properties. Results. We demonstrate that the near-fce wave intervals contain several distinct wave types, including electron Bernstein waves and extremely narrowband waves that are highly sensitive to the ambient magnetic field orientation. Using the properties of these waves, we suggest possible plasma wave mode classifications and possible instabilities that generate these waves. The results of this analysis indicate that these waves may modify the cold core of the electron distribution functions in the quiescent near-Sun solar wind.

David M. Malaspina↗
NASA NTRSDate not supplied

Developing and Testing a Physics Guided Machine Learning NeuralNetwork to Predict Tonal Noise Emitted by a Propeller

Artificial neural networks offer a highly nonlinear and adaptive model for predicting complex interactions between input-output parameters. However, these networks require large datasets which often exceed practical considerations in modeling experimental results. To alleviate the dataset size requirement, a method known as physics guided machine learning has been applied to construct several neural networks for predicting propeller tonal noise in the time domain over a broad range of flight conditions. Three space-filling designs, namely, Latin-Hypercube, Sphere-Packing, and Grid-Space, were used to distribute points throughout the input parameter space encompassing nondimensional flight conditions and observer geometry. Each neural network’s performance was validated by conditions outside of the training set and compared to the Propeller Analysis System tool from the NASA Aircraft Noise Prediction Program. Compared to the Grid-Space input design, the Latin-Hypercube and the Sphere-Packing designs provided a better representation of the domain for training. Regarding the network archetype, a fully connected perceptron was found to outperform the partially connected perceptron in their ability to predict tonal noise for small datasets. The black-box nature of these neural networks was also explored to understand how the networks constructed the waveform and understand why some network designs produce better models.

Propeller noise↗
NASA NTRS1993

A total variation diminishing finite difference algorithm for sonic boom propagation models

It is difficult to accurately model the rise phases of sonic boom waveforms with traditional finite difference algorithms because of finite difference phase dispersion. This paper introduces the concept of a total variation diminishing (TVD) finite difference method as a tool for accurately modeling the rise phases of sonic booms. A standard second order finite difference algorithm and its TVD modified counterpart are both applied to the one-way propagation of a square pulse. The TVD method clearly outperforms the non-TVD method, showing great potential as a new computational tool in the analysis of sonic boom propagation.

Sparrow, Victor W.↗
NASA NTRS1985

Signal Analysis of Gas Tungsten Arc Welds

Gas tungsten arc welding is a process in which the input parameters such as current, voltage and travel speed, can be easily controlled and/or monitored. However, weld quality is not solely a function of these parameters. An adaptive method of observing weld quality is desired to improve weld quality assurance. The use of dynamic electrical properties of the welding arc as a weld quality monitor was studied. The electrical properties of the arc are characterized by the current voltage transfer function. The hardware and software necessary to collect the data at a maximum rate of 45 kHz and to allow the off-line processing of this data are tested. The optimum input current waveform is determined. Bead-on-plate welds to observe such characteristics of the weld as the fundamental frequency of the puddle are studied. Future work is planned to observe changes of the arc response with changes in joint geometry, base metal chemistry, and shielding gas composition are discussed.

Eagar, T. W.↗
NASA NTRS2015

Link Analysis of High Throughput Spacecraft Communication Systems for Future Science Missions

NASA's plan to launch several spacecraft into low Earth Orbit (LEO) to support science missions in the next ten years and beyond requires down link throughput on the order of several terabits per day. The ability to handle such a large volume of data far exceeds the capabilities of current systems. This paper proposes two solutions, first, a high data rate link between the LEO spacecraft and ground via relay satellites in geostationary orbit (GEO). Second, a high data rate direct to ground link from LEO. Next, the paper presents results from computer simulations carried out for both types of links taking into consideration spacecraft transmitter frequency, EIRP, and waveform; elevation angle dependent path loss through Earths atmosphere, and ground station receiver GT.

Antennas↗
NASA NTRS2015

Link Analysis of High Throughput Spacecraft Communication Systems for Future Science Missions

NASA's plan to launch several spacecrafts into low Earth Orbit (LEO) to support science missions in the next ten years and beyond requires down link throughput on the order of several terabits per day. The ability to handle such a large volume of data far exceeds the capabilities of current systems. This paper proposes two solutions, first, a high data rate link between the LEO spacecraft and ground via relay satellites in geostationary orbit (GEO). Second, a high data rate direct to ground link from LEO. Next, the paper presents results from computer simulations carried out for both types of links taking into consideration spacecraft transmitter frequency, EIRP, and waveform; elevation angle dependent path loss through Earths atmosphere, and ground station receiver GT.

Amplifier↗
NASA NTRS1980

Closed loop performance of a brushless dc motor powered electromechanical actuator for flight control applications

A comprehensive digital model for the analysis and possible optimization of the closed loop dynamic (instantaneous) performance of a power conditioner fed, brushless dc motor powered, electromechanical actuator system (EMA) is presented. This model was developed for the simulation of the dynamic performance of an actual prototype EMA built for NASA-JSC as a possible alternative to hydraulic actuators for consideration in Space Shuttle Orbiter applications. Excellent correlation was achieved between numerical model simulation and experimental test results obtained from the actual hardware. These results include: various current and voltage waveforms in the machine-power conditioner (MPC) unit, flap position as well as other control loop variables in response to step commands of change of flap position. These results with consequent conclusions are detailed in the paper.

Demerdash, N. A.↗
NASA NTRS1974

Skylab altimeter applications and scientific results

Analysis of the Skylab S-193 Altimeter Experiment data has produced many significant results. The purpose of this paper is to present some of these results which illustrate the programmatic altimeter technology gained and show the scientific potential of altimetry for various applications, including correlation with existing topographic features, geoids and physical models. It will be shown that there exists a very strong correlation between subsurface topography in the broad ocean area and the shape of the ocean surface as determined from the altimeter. Modeled scattering waveforms will be correlated with Skylab Altimeter waveforms. The agreement of a variety of existing geoid models with the altimeter data will be shown. In addition the calibration, accuracy and stability of the S-193 altimeter instrumentation will be discussed.

Mcgoogan, J. T.↗
NASA NTRS2020

Analysis of the Accuracy of Actuation Electronics in the Laser Interferometer Space Antenna Pathfinder

The Laser Interferometer Space Antenna Pathfinder (LPF) main observable, labelled∆g, is the differential force per unit mass acting on the two test masses in free fall condition, after the contribution of all non-gravitational forces have been compensated. At low frequencies the differential force is compensated by an applied electrostatic actuation force,which then must be subtracted from the measured acceleration to obtain∆g. Any inaccuracy in the actuation force contaminates the residual acceleration. This study investigates the accuracy of the electrostatic actuation system and its impact on the LPF main observable. It is shown that the inaccuracy is mainly caused by the rounding errors in the waveform processing and also by the random error caused by the analog to digital converter (ADC) random noise in the control loop. Both errors are one order of magnitude smaller than the resolution of the commanded voltages. We developed a simulator based on the LPF design to compute the close-to-reality actuation voltages and consequently, the resulting actuation forces. The simulator is applied in post-processing the LPF data.

M. Armano↗
NASA NTRS1988

Computerized Analysis Of Thermal-Diffusivity Data

Improved data-acquisition and data-analysis system for thermal-diffusivity measurements using flash method incorporates digital oscilloscope and microcomputer for rapid reduction of experimental data. In thermal-diffusivity apparatus thin specimen heated on one face by pulsed xenon flashlamp, subsequent temperature rise on opposite face monitored by infrared detector. Thermal diffusivity estimated from thickness of specimen and from time after initial pulse during which temperature rise reaches half maximum value. Accuracy of estimate improved by correcting temperature measurements for radiative loss of heat from specimen and for finite duration and specific waveform of flashlamp pulse. System devised for use in high-temperature measurements of thermoelectric materials.

Chmielewski, Artur B.↗
NASA NTRS1993

An analysis of nearfield normal mode amplitude anomalies of the Landers earthquake

The 1992 Landers earthquake (M(sub w) = 7.3) occurred in the middle of the TERRAscope network. Long-period Rayleigh waves recorded at the TERRAscope stations (delta less than or = 3 deg) after traveling around the Earth show large amplitude anomalies, one order of magnitude larger than spherical Earth predictions up to a period of about 600 s. The ground motions over the epicentral region at and after the arrival of R4-5 are in phase at all stations. These observations are inconsistent with the nearly vertical strike slip mechanism of the Landers earthquake. Synthetic seismograms for a rotating, elliptic, and laterally heterogeneous Earth model calculated by the variational method agree well with the observed waveforms. Calculations for various 3D Earth models demonstrate that the amplitudes are very sensitive to the large scale aspherical structure in the crust and the mantle. The anomalies for modes shorter than 300 s period can be explained by lateral heterogeneity shallower than the upper mantle. Rotation of the Earth and lower mantle heterogeneity are required to explain mode amplitudes at longer periods. Current whole mantle seismic tomographic models can fully explain the observed amplitudes longer than 300 s. To assess the effect of the high order lateral heterogeneity in the mantle more precise estimate of the crustal correction is required.

Watada, Shingo↗
NASA NTRS1996

Detection of Delamination in Composite Beams Using Broadband Acoustic Emission Signatures

Delamination in composite structure may be caused by imperfections introduced during the manufacturing process or by impact loads by foreign objects during the operational life. There are some nondestructive evaluation methods to detect delamination in composite structures such as x-radiography, ultrasonic testing, and thermal/infrared inspection. These methods are expensive and hard to use for on line detection. Acoustic emission testing can monitor the material under test even under the presence of noise generated under load. It has been used extensively in proof-testing of fiberglass pressure vessels and beams. In the present work, experimental studies are conducted to investigate the use of broadband acoustic emission signatures to detect delaminations in composite beams. Glass/epoxy beam specimens with full width, prescribed delamination sizes of 2 inches and 4 inches are investigated. The prescribed delamination is produced by inserting Teflon film between laminae during the fabrication of composite laminate. The objectives of this research is to develop a method for predicting delamination size and location in laminated composite beams by combining smart materials concept and broadband AE analysis techniques. More specifically, a piezoceramic (PZT) patch is bonded on the surface of composite beams and used as a pulser. The piezoceramic patch simulates the AE wave source as a 3 cycles, 50KHz, burst sine wave. One broadband AE sensor is fixed near the PZT patch to measure the AE wave near the AE source. A second broadband AE sensor, which is used as a receiver, is scanned along the composite beams at 0.25 inch step to measure propagation of AE wave along the composite beams. The acquired AE waveform is digitized and processed. Signal strength, signal energy, cross-correlation of AE waveforms, and tracking of specific cycle of AE waveforms are used to detect delamination size and location.

Okafor, A. C.↗
NASA NTRS1975

Stepwise Iterative Fourier Transform: The SIFT

A program, designed specifically to study the respective effects of some common data problems on results obtained through stepwise iterative Fourier transformation of synthetic data with known waveform composition, was outlined. Included in this group were the problems of gaps in the data, different time-series lengths, periodic but nonsinusoidal waveforms, and noisy (low signal-to-noise) data. Results on sinusoidal data were also compared with results obtained on narrow band noise with similar characteristics. The findings showed that the analytic procedure under study can reliably reduce data in the nature of (1) sinusoids in noise, (2) asymmetric but periodic waves in noise, and (3) sinusoids in noise with substantial gaps in the data. The program was also able to analyze narrow-band noise well, but with increased interpretational problems. The procedure was shown to be a powerful technique for analysis of periodicities, in comparison with classical spectrum analysis techniques. However, informed use of the stepwise procedure nevertheless requires some background of knowledge concerning characteristics of the biological processes under study.

Benignus, V. A.↗