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At least 811 records · Page 45

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NASA NTRS1979

A comparison of high-temperature flare models with observations and implications for the low-temperature flare

EUV data from the Harvard College Observatory and Naval Research Laboratory instruments on board the Skylab Apollo telescope mount, together with SOLRAD 9 X-ray data, are analyzed in order to empirically deduce the variation of emission measure with temperature in the atmosphere of a number of solar flares. A 'mean' differential emission measure profile Q(T) for a flare is constructed which is then compared with the profile predicted by a number of theoretical models. It is found that realistic flare models must include both conductive and radiative terms in the energy equation, and that hydrodynamic terms may be important at low temperatures. The implications of the results obtained are discussed for flare models in general and it is shown that the inclusion of the conductive term into models which have hitherto neglected it can perhaps resolve some of the observational difficulties with such models.

Machado, M. E.↗

NASA NTRS1972

Methods for reducing blade passing frequency noise generated by rotor-wake - stator interaction

A theoretical model for the generation of blade-passage noise caused by the interaction of rotor wakes with the downstream stator blades in a fan stage has been compiled. This model has combined a description of the rotor wake based on existing experimental correlations with an existing theory for the response of the stator blades to those wakes. This model of the interaction-caused blade-passage noise has been used to examine some effects of blade geometry and flow conditions on the noise generated. A set of significant parameters has been identified to reduce this interaction-caused blade-passage frequency noise. A redesign of an existing fan with the use of these concepts has produced a theoretical noise reduction of more than 7 decibels.

Dittmar, J. H.↗

NASA NTRS2001

Experimental Validation of a Thermoelastic Model for SMA Hybrid Composites

This study presents results from experimental validation of a recently developed model for predicting the thermomechanical behavior of shape memory alloy hybrid composite (SMAHC) structures, composite structures with an embedded SMA constituent. The model captures the material nonlinearity of the material system with temperature and is capable of modeling constrained, restrained, or free recovery behavior from experimental measurement of fundamental engineering properties. A brief description of the model and analysis procedures is given, followed by an overview of a parallel effort to fabricate and characterize the material system of SMAHC specimens. Static and dynamic experimental configurations for the SMAHC specimens are described and experimental results for thermal post-buckling and random response are presented. Excellent agreement is achieved between the measured and predicted results, fully validating the theoretical model for constrained recovery behavior of SMAHC structures.

Turner, Travis L.↗

NASA NTRS2001

Total Electron-Impact Ionization Cross-Sections of CFx and NFx (x = 1 - 3)

The discrepancy between experimental and theoretical total electron-impact ionization cross sections for a group of fluorides, CFx, and NFx, (x = 1 - 3), is attributed to the inadequacies in previous theoretical models. Cross-sections calculated using a recently developed siBED (simulation Binary-Encounter-Dipole) model that takes into account the shielding of the long-range dipole potential between the scattering electron and target are in agreement with experimentation. The present study also carefully reanalyzed the previously reported experimental data to account for the possibility of incomplete collection of fragment ions and the presence of ion-pair formation channels. For NF3, our experimental and theoretical cross-sections compare well with the total ionization cross-sections recently reported by Haaland et al. in the region below dication formation.

Huo, Winifred M.↗

NASA NTRS1974

The internal structure of the convective envelope of the sun

An axisymmetric hydrodynamic model of the sun consisting of a core at radiant equilibrium and a convective envelope is studied with consideration of gravitational forces, rotation, turbulence effects, and electromagnetic forces. The dimensionless parameters that control the equations of plasma motion within the sun and the boundary conditions are investigated; estimates are given for these parameters on the basis of experimental data and the existing theoretical models. Stability conditions are derived for convective equilibrium, developed cellular convection, and developed turbulent convection with consideration of rotation and turbulent Reynolds stresses. These conditions characterize the possible physical state of the matter in the convective envelope. Using estimates of the dimensionless parameters and models of some of them, a method of successive approximations is proposed for solution of the problem as a whole.

Yavorskaya, I. M.↗

NASA NTRS1994

N2 Lyman-Birge-Hopfield dayglow from ATLAS 1

Spectral measurements of the far ultraviolet dayglow were made from the ATLAS 1 shuttle mission using an intensified charge coupled device (CCD) imaging spectrometer array. The instrument imaged relatively large (177 A) segments of the spectrum simultaneously and had the capability to image altitude in the second dimension of the two-dimensional focal plane detector when the entrance slit of the instrument was oriented perpendicular to the limb of the Earth. The far ultraviolet (FUV) channel of the instrument used a CsTe photocathode rather than the more conventionally used and more solar blind CsI photocathode material. As a result, the spectra were also measured with good sensitivity at FUV wavelengths longer than 1500 A. In this paper, data are selected of specific interest to a study of the N2 Lyman-Birge-Hopfield system using two shuttle attitudes: In the first the field of view was held at a fixed tangent ray height of approximately 140 km while scanning in wavelength, providing repeated FUV spectra over a range of latitudes and allowing comparison with our thermospheric airglow model. In the second observing sequence the line of sight of the instrument was scanned down through the atmosphere but covered about 75% of the full wavelength range. This sequence allows comparison of height dependence,including O2 absorption effects, with the thermospheric model. In all cases, relatively good agreement is obtained with the model (run strictly in a predictive mode) in comparisions in which the line of sight must be intergrated along a path that spans a considerable range of conditions (height, local time, latitude, solar zenith angle). In the case of the data sets examined in this study the vibrational population distributions show mixed results. In one case the distribution agrees well with a theoretical model based on direct electron impact excitation without a significant cascade contribution. A second case indicated a lower population in nu' = 3 and a higher population in nu' = 6 than would be expected from direct impact excitation alone. This supports previous indications that conditions resulting in the cascade mechanism may not always be operative.

Torr, Marsha R.↗

NASA NTRS1998

Fiber-Supported Droplet Combustion Experiment-2

A major portion of the energy produced in the world today comes from the burning of liquid hydrocarbon fuels in the form of droplets. Understanding the fundamental physical processes involved in droplet combustion is not only important in energy production but also in propulsion, in the mitigation of combustion-generated pollution, and in the control of the fire hazards associated with handling liquid combustibles. Microgravity makes spherically symmetric combustion possible, allowing investigators to easily validate their droplet models without the complicating effects of gravity. The Fiber-Supported Droplet Combustion (FSDC-2) investigation was conducted in the Microgravity Glovebox facility of the shuttles' Spacelab during the reflight of the Microgravity Science Laboratory (MSL- 1R) on STS-94 in July 1997. FSDC-2 studied fundamental phenomena related to liquid fuel droplet combustion in air. Pure fuels and mixtures of fuels were burned as isolated single and duo droplets with and without forced air convection. FSDC-2 is sponsored by the NASA Lewis Research Center, whose researchers are working in cooperation with several investigators from industry and academia. The rate at which a droplet burns is important in many commercial applications. The classical theory of droplet burning assumes that, for an isolated, spherically symmetric, single-fuel droplet, the gas-phase combustion processes are much faster than the droplet surface regression rate and that the liquid phase is at a uniform temperature equal to the boiling point. Recent, more advanced models predict that both the liquid and gas phases are unsteady during a substantial portion of the droplet's burning history, thus affecting the instantaneous and average burning rates, and that flame radiation is a dominant mechanism that can extinguish flames in a microgravity environment. FSDC-2 has provided well-defined, symmetric droplet burning data including radiative emissions to validate these theoretical models for heptane, decane, ethanol, and methanol fuels. Since most commercial combustion systems burn droplets in a convective environment, data were obtained without and with convective flow over the burning droplet (see the following photos).

Colantonio, Renato O.↗

NASA NTRS1986

Eruption rate, area, and length relationships for some Hawaiian lava flows

The relationships between the morphological parameters of lava flows and the process parameters of lava composition, eruption rate, and eruption temperature were investigated using literature data on Hawaiian lava flows. Two simple models for lava flow heat loss by Stefan-Boltzmann radiation were employed to derive eruption rate versus planimetric area relationship. For the Hawaiian basaltic flows, the eruption rate is highly correlated with the planimetric area. Moreover, this observed correlation is superior to those from other obvious combinations of eruption rate and flow dimensions. The correlations obtained on the basis of the two theoretical models, suggest that the surface of the Hawaiian flows radiates at an effective temperature much less than the inner parts of the flowing lava, which is in agreement with field observations. The data also indicate that the eruption rate versus planimetric area correlations can be markedly degraded when data from different vents, volcanoes, and epochs are combined.

Pieri, David C.↗

NASA NTRS1992

Geodynamic contributions to global climatic change

Orbital and rotational variations perturb the latitudinal and seasonal pattern of incident solar radiation, producing major climatic change on time scales of 10(exp 4)-10(exp 6) years. The orbital variations are oblivious to internal structure and processes, but the rotational variations are not. A program of investigation whose objective would be to explore and quantify three aspects of orbital, rotational, and climatic interactions is described. An important premise of this investigation is the synergism between geodynamics and paleoclimate. Better geophysical models of precessional dynamics are needed in order to accurately reconstruct the radiative input to climate models. Some of the paleoclimate proxy records contain information relevant to solid Earth processes, on time scales which are difficult to constrain otherwise. Specific mechanisms which will be addressed include: (1) climatic consequences of deglacial polar motion; and (2) precessional and climatic consequences of glacially induced perturbations in the gravitational oblateness and partial decoupling of the mantle and core. The approach entails constructing theoretical models of the rotational, deformational, radiative, and climatic response of the Earth to known orbital perturbations, and comparing these with extensive records of paleoclimate proxy data. Several of the mechanisms of interest may participate in previously unrecognized feed-back loops in the climate dynamics system. A new algorithm for estimating climatically diagnostic locations and seasons from the paleoclimate time series is proposed.

Bills, Bruce G.↗

NASA NTRS1983

Progress in rotor broadband noise research

The current status of research on various rotor broadband noise sources is reviewed. Theoretical modeling techniques which appear to have the most promise in the development of successful noise prediction capabilities are emphasized. The types of noise considered include: turbulence ingestion noise, blade self-noise due to turbulence passing the trailing edge, blade self-noise due to separated flow, and blade self-noise due to vortex shedding. Comparisons with experimental results are given, and present theoretical and experimental limitations are delineated to help identify areas of needed research.

Brooks, T. F.↗

NASA NTRS1997

Ultrasonic Nondestructive Characterization of Adhesive Bonds

Qualitative measurements of adhesion or binding forces can be accomplished, for example, by using the reflection coefficient of an ultrasound or by using thermal waves (Light and Kwun, 1989, Achenbach and Parikh, 1991, and Bostrom and wickham, 1991). However, a quantitative determination of binding forces is rather difficult. It has been observed that higher harmonics of the fundamental frequency are generated when an ultrasound passes through a nonlinear material. It seems that such non-linearity can be effectively used to characterize the bond strength. Several theories have been developed to model this nonlinear effect (Adler and Nagy, 1991; Achenbach and Parikh, 1991; Parikh and Achenbach, 1992; and Hirose and Kitahara, 1992; Anastasi and Roberts, 1992). Based on a microscopic description of the nonlinear interface binding force, a quantitative method was presented by Pangraz and Arnold (1994). Recently, Tang, Cheng and Achenbach (1997) made a comparison between the experimental and simulated results based on this theoretical model. A water immersion mode-converted shear wave through-transmission setup was used by Berndt and Green (1997) to analyze the nonlinear acoustic behavior of the adhesive bond. In this project, the nonlinear responses of an adhesive joint was investigated through transmission tests of ultrasonic wave and analyzed by the finite element simulations. The higher order harmonics were obtained in the tests. It is found that the amplitude of higher harmonics increases as the aging increases, especially the 3dorder harmonics. Results from the numerical simulation show that the material nonlinearity does indeed generate higher order harmonics. In particular, the elastic-perfect plastic behavior generates significant 3rd and 5th order harmonics.

Qu, Jianmin↗

NASA NTRS2001

Asynchronous Laser Transponders for Precise Interplanetary Ranging and Time Transfer

The feasibility of a two-way asynchronous (i.e. independently firing) interplanetary laser transponder pair, capable of decimeter ranging and subnanosecond time transfer from Earth to a spacecraft anywhere within the inner Solar System, is discussed. In the Introduction, we briefly discuss the current state-of-the-art in Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) which use single-ended range measurements to a passive optical reflector, and the limitations of this approach in ranging beyond the Moon to the planets. In Section 2 of this paper, we describe two types of transponders (echo and asynchronous), introduce the transponder link equation and the concept of "balanced" transponders, describe how range and time can be transferred between terminals, and preview the potential advantages of photon counting asynchronous transponders for interplanetary applications. In Section 3, we discuss and provide mathematical models for the various sources of noise in an interplanetary transponder link including planetary albedo, solar or lunar illumination of the local atmosphere, and laser backscatter off the local atmosphere. In Section 4, we introduce the key engineering elements of an interplanetary laser transponder and develop an operational scenario for the acquisition and tracking of the opposite terminal. In Section 5, we use the theoretical models of th~ previous sections to perform an Earth-Mars link analysis over a full synodic period of 780 days under the simplifying assumption of coaxial, coplanar, circular orbits. We demonstrate that, using slightly modified versions of existing space and ground based laser systems, an Earth-Mars transponder link is not only feasible but quite robust. We also demonstrate through analysis the advantages and feasibility of compact, low output power (<300 mW photon-counting transponders using NASA's developmental SLR2000 satellite laser ranging system as the Earth terminal. Section 6 provides a summary of the results and some concluding remarks regarding future applications.

Degnan, John J.↗

NASA NTRS2019

Radio Follow-up of a Candidate γ-Ray Transient in the Sky Localization Area of GW170608

After the identification of a candidate γ-ray transient in the error region of the binary black hole (BBH) merger GW150914 by the Fermi satellite, the question of whether BBH mergers can be associated with electromagnetic counterparts remains highly debated. Here, we present radio follow-up observations of GW170608, a BBH merger that occurred during the second observing run (O2) of the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO). Our radio follow-up focused on a specific field contained in the GW170608 sky localization area, where a candidate high-energy transient was detected by the Fermi Large Area Telescope (LAT). We make use of data collected at 1.4 GHz with the Karl G. Jansky Very Large Array (VLA), as well as with the VLA Lowband Ionosphere and Transient Experiment (VLITE). Our analysis is sensitive to potential radio afterglows with luminosity densities L(1.4GHz) ≳ 6 x 10^(28) erg/sHz. In the most optimistic theoretical models, ≈20% of BBH events occurring in massive hosts could be associated with outflows as radio luminous as this. Although we find no evidence for the presence of a radio counterpart associated with the LAT candidate in the GW170608 error region, our analysis demonstrates the feasibility of future radio follow-up observations of well-localized BBHs. Comparing our radio upper limits with theoretical expectations for the radio afterglows potentially associated with jets launched in BBH mergers, we find that for jets of energy ≈10^(49) erg seen on-axis, only jet angles θ(jet)≳40° are compatible with the observations.

Kyle Artkop↗

NASA NTRS1981

Analysis of JP-4 fire test data and development of a simple fire model

A series of field experiments involving JP-4 fires of 7.6m dia and 15.2m dia was conducted under the sponsorship of NASA. The temperature inside the fire, the specie composition and vertical axial velocity were measured at several height locations and radial positions. The peak temperature recorded is 1400-1500 K; the velocities at 6m height are 11-12 m/s. The oxygen-to-nitrogen mass ratio along the vertical axis was found to be nearly a constant at 0.05. Significant necking of the fire was observed. A simple theoretical model was developed that predicts the experimental data on the velocity and vertical temperature distribution well. The model is based on the partial combustion of entrained air and assumed top-hat distributions for various property and flow parameters inside the fire. Quantitative agreement is obtained by using a value of 0.15 for the entrainment coefficient and 0.45 for the mixing efficiency factor. The model also predicts the fire necking.

Raj, P. K.↗

NASA NTRS1984

Superthermal ionospheric outflows

In recent years observations have shown that the terrestrial ionosphere serves as a significant source of the hot plasma in the magnetosphere. These findings imply that cold plasma flows upward to great altitudes and at some point along the way gains energies much larger than typical ionospheric values. The present investigation is mainly concerned with observations of ionospheric outflows at energies higher than those predicted by ambipolar wind models but lower than those associated with keV auroral ion acceleration, giving particular attention to the energy range from a few to a few hundred electron volts. The ion flows in the energy range intermediate between polar wind and keV auroral ion beams are called superthermal ion outflows. It is shown that recent observations of superthermal ion outflows from the terrestrial ionosphere are related to theoretical models of the topside ionosphere.

Moore, T. E.↗

NASA NTRS1974

Gulf Stream model

Surface elevation deviations in the Gulf Stream region off the eastern coast of the United States between Wallops Island, Virginia and Miami, Florida were investigated. The main causes of surface elevation deviations are geoid perturbations due to the continental shelf and the geostrophic adjustment of the density field due to the Gulf Stream. Quantitative surface elevation profiles were calculated based on geophysical measurements of gravity anomalies and hydrographic data. The results are presented graphically along with contemporaneous weather data. Comparisons are made between the profiles based on hydrographic data and a mean theoretical model. The theory of geostrophic flows including some classical Gulf Stream models is also presented briefly.

Source record↗

NASA NTRS1974

Estimation of structural parameters from dynamic test data

Parameters for a lumped linear model approximating a distributed elastic structure are determined from dynamic test data comprised of several mode shapes and frequencies. Measurement errors, nonlinear response, and nonmeasurable quantities such as mode slope components are accommodated. Some mass and stiffness parameters may be known accurately, whereas the remainder are to be estimated. The method entails minimizing a quadratic function of the difference between corresponding modes and frequencies of the theoretical model and the test specimen. This technique was applied to some actual vibration test data, and the special techniques that are required to overcome convergence problems are described.

Hall, B. M.↗

NASA NTRS1977

Similarity laws of lunar and terrestrial volcanic flows

A mathematical model for the terrestrial and lunar volcanic flows is proposed. This mathematical model, which is one-dimensional, steady duct flow of a mixture of a gas and small solid particles (rock), has been analyzed in detail. The similarities and the differences of the essential features of lunar and terrestrial volcanic flows are determined. Numerical results for the equilibrium two-phase flows of lunar and terrestrial volcanics under similar conditions are presented. The main results of the theoretical model are: the lunar crater is much larger than the corresponding terrestrial crater; the exit velocity from the lunar volcanic flow may be higher than the lunar escape velocity but the exit velocity of terrestrial volcanic flow is much less than that of the linear case (this result confirms the hypothesis that Australian tektites came from the moon, as a stream of rock and gas of extremely high speed), and the thermal effects on the lunar volcanic flow are much larger than those in the terrestrial case.

Pai, S. I.↗