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At least 253 records · Page 14

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NASA NTRS1986

The poleward edge of the mid-latitude trough - Its formation, orientation and dynamics

Data from the Advanced Ionospheric Sounder (AIS) deployed at Halley, Antarctica (76-deg S, 27-deg W; L = 4.2) and the Dynamics Explorer-2 spacecraft (DE-2) are used to investigate several aspects of the formation processes and dynamics of the poleward edge of the midlatitude electron density trough. These include a study of the flux and energy of charged particles precipitating into the F-region as a function of Magnetic Local Time. It is found that local energetic electron precipitation is a major source of ionization of the poleward edge in the evening sector, but only after magnetic midnight transport processes become more important. Occasionally a significant increase in the flux of conjugate photoelectrons is colocated with the poleward edge of the trough in the morning sector. The combination of AIS and DE-2 data has allowed identification of significant longitudinal structure on the poleward edge of the trough that may be the result of substorm activity. It is found that the orientation of the poleward edge of the trough and the locus of the plasmapause predicted from the 'tear-drop' model vary in rather a similar manner with local time, though no close physical link between the two features is inferred from this coincidence.

Rodger, A. S.↗

NASA NTRS1990

An experimental study of the nonlinear dynamic phenomenon known as wing rock

An experimental investigation into the physical phenomena associated with limit cycle wing rock on slender delta wings has been conducted. The model used was a slender flat plate delta wing with 80-deg leading edge sweep. The investigation concentrated on three main areas: motion characteristics obtained from time history plots, static and dynamic flow visualization of vortex position, and static and dynamic flow visualization of vortex breakdown. The flow visualization studies are correlated with model motion to determine the relationship between vortex position and vortex breakdown with the dynamic rolling moments. Dynamic roll moment coefficient curves reveal rate-dependent hysteresis, which drives the motion. Vortex position correlated with time and model motion show a time lag in the normal position of the upward moving wing vortex. This time lag may be the mechanism responsible for the hysteresis. Vortex breakdown is shown to have a damping effect on the motion.

Arena, A. S., Jr.↗

NASA NTRS1990

Conical Euler simulation and active suppression of delta wing rocking motion

A conical Euler code was developed to study unsteady vortex-dominated flows about rolling highly-swept delta wings, undergoing either forced or free-to-roll motions including active roll suppression. The flow solver of the code involves a multistage Runge-Kutta time-stepping scheme which uses a finite volume spatial discretization of the Euler equations on an unstructured grid of triangles. The code allows for the additional analysis of the free-to-roll case, by including the rigid-body equation of motion for its simultaneous time integration with the governing flow equations. Results are presented for a 75 deg swept sharp leading edge delta wing at a freestream Mach number of 1.2 and at alpha equal to 10 and 30 deg angle of attack. A forced harmonic analysis indicates that the rolling moment coefficient provides: (1) a positive damping at the lower angle of attack equal to 10 deg, which is verified in a free-to-roll calculation; (2) a negative damping at the higher angle of attack equal to 30 deg at the small roll amplitudes. A free-to-roll calculation for the latter case produces an initially divergent response, but as the amplitude of motion grows with time, the response transitions to a wing-rock type of limit cycle oscillation. The wing rocking motion may be actively suppressed, however, through the use of a rate-feedback control law and antisymmetrically deflected leading edge flaps. The descriptions of the conical Euler flow solver and the free-to-roll analysis are presented. Results are also presented which give insight into the flow physics associated with unsteady vortical flows about forced and free-to-roll delta wings, including the active roll suppression of this wing-rock phenomenon.

Lee, Elizabeth M.↗

NASA NTRS1993

Thermal control paints on LDEF: Results of M0003 sub-experiment 18

Several thermal control paints were flown on the Long Duration Exposure Facility (LDEF), including the white paints Chemglaze A276, S13GLO, and YB-71, and the black paint D-111. The effects of low earth orbit, which includes those induced by UV radiation and atomic oxygen, varied significantly with each paint and its location on LDEF. For example, samples of Chemglaze A276 located on the trailing edge of LDEF darkened significantly due to UV-induced degradation of the paint's binder, while leading edge samples remained white but exhibited severe atomic oxygen erosion of the binder. Although the response of S13GLO to low earth orbit is much more complicated, it also exhibited greater darkening on trailing edge samples as compared to leading edge samples. In contrast, YB-71 and D-111 remained relatively stable and showed minimal degradation. The performance of these paints as determined by changes in their optical and physical properties, including solar absorptance as well as surface chemical changes and changes in surface morphology is examined. It will also provide a correlation of these optical and physical property changes to the physical phenomena that occurred in these materials during the LDEF mission.

Jaggers, C. H.↗

NASA NTRS1993

Effects of inclination angle on the spectra of X-ray binaries

Iron emission and absorption features from nine X-ray binaries are compared with a model that predicts characteristics of iron K-shell features corresponding to different viewing angles for the accretion disk. In this model, emission and absorption features arise from an accretion disk corona whose structure is computed under the assumptions of ionization, thermal, and hydrostatic balance of gas illuminated by the central continuum source. Hence the equivalent widths of the emission and absorption features are predicted without the introduction of any free parameters to control the physical conditions in the disk corona. Our data/model comparison suggests an explanation for the equivalent widths of the iron K-alpha emission and the depth of the absorption edge in terms of (1) the angle of inclination between the observer's line of sight and the plane of the accretion disk and (2) the shape of the incident source spectrum.

Vrtilek, S. D.↗

NASA NTRS1994

The Origins of Polycyclic Aromatic Hydrocarbons: Are They Everywhere?

During the past 15 years considerable progress in observational techniques has been achieved in the middle-infrared region (5000-500 per centimeter, 2-20 micron), the region where most diagnostic molecular vibrations occur. Spectra of many different astronomical infrared sources, some deeply embedded in dark molecular clouds and others at their edges, are now available. These spectra provide a powerful probe, not only for the identification of interstellar molecules in both the gas and solid phases, but also of the physical and chemical conditions which prevail in these two very different domains. The two lectures will focus on the evidence that polycyclic aromatic hydrocarbons (PAHs) are an important, ubiquitous and abundant interstellar species. PAHs are. extremely stable species which can range in size from a few angstroms across to several hundred angstroms (PAHs are also the building blocks of amorphous carbon particles). This identification rests on the suggestive agreement between the laboratory spectra of PAHs with a set of IR emission bands which emanate from many different sources where ultraviolet starlight impinges on a "dusty" region. The picture is that individual PAHs are first pumped into highly vibrationally excited states and relax by fluorescence at their fundamental vibrational frequencies. That PAHs are a ubiquitous interstellar component has serious ramifications in other spectral regions as well, including the strong extinction in the ultraviolet, and the classic visible diffuse interstellar bands discovered more than 50 years ago (but unexplained to this day) The first part of the course will focus on the interpretation of astronomical spectra. The second lecture will start by showing how recent laboratory data on PAHs taken under realistic interstellar conditions has con borated the PAH hypothesis and led to great insight into the conditions in the PAH containing regions. This lecture will end by reviewing the ever-increasing evidence for interstellar PAHs in meteorites and interplanetary dust particles. This in conjunction with the recent suggestion that PAHs are abundant in Jupiter's atmosphere will make the point that "PAHs are Everywhere".

Allamandola, L. J.↗

NASA NTRS1979

Development of a realistic stress analysis for fatigue analysis of notched composite laminates

A finite element stress analysis which consists of a membrane and interlaminar shear spring analysis was developed. This approach was utilized in order to model physically realistic failure mechanisms while maintaining a high degree of computational economy. The accuracy of the stress analysis predictions is verified through comparisons with other solutions to the composite laminate edge effect problem. The stress analysis model was incorporated into an existing fatigue analysis methodology and the entire procedure computerized. A fatigue analysis is performed upon a square laminated composite plate with a circular central hole. A complete description and users guide for the computer code FLAC (Fatigue of Laminated Composites) is included as an appendix.

Humphreys, E. A.↗

NASA NTRS2012

PDS 144: The First Confirmed Herbig Ae-Herbig Ae Wide Binary

PDS 144 is a pair of Herbig Ae stars that are separated by 5.35" on the sky. It has previously been shown to have an A2Ve Herbig Ae star viewed at 83 deg inclination as its northern member and an A5Ve Herbig Ae star as its southern member. Direct imagery revealed a disk occulting PDS 144 N - the first edge-on disk observed around a Herbig Ae star. The lack of an obvious disk in direct imagery suggested PDS 144 S might be viewed face-on or not physically associated with PDS 144 N. Multi-epoch HST imagery of PDS 144 with a 5 yr baseline demonstrates PDS 144 N & S are comoving and have a common proper motion with TYC 6782-878-1. TYC 6782-878-1 has previously been identified as a member of Upper Sco sub-association A at d = 145 +/- 2 pc with an age of 5 - 10 Myr. Ground-based imagery reveals jets and a string of HH knots extending 13' (possibly further) which are aligned to within 7 deg +/- 6 deg on the sky. By combining proper motion data and the absence of a dark mid-plane with radial velocity data, we measure the inclination of PDS 144 S to be i = 73 deg +/- 7 deg. The radial velocity of the jets from PDS 144 N & S indicates they, and therefore their disks, are misaligned by 25 deg +/- 9 deg.. This degree of misalignment is similar to that seen in T-Tauri wide binaries.

Hornbeck, J. B.↗

NASA NTRS1988

Research at the earth's edge

The Space Shuttle Orbiter-deployed Tethered Satellite System (TSS) could allow an Orbiter at a 200 km orbital altitude to reach down to atmospheric altitudes of 90 km, in order to study weather phenomena, pollutant transport, 'nuclear winter' smoke transport, atmospheric physics and dynamics, sun-earth interactions, ecosystem interactions, and radio communications. The TSS satellite, a 1.5-m diameter sphere, would carry scientific instrumentation which could initially be dedicated to the investigation of energy and momentum transfer between a tethered system and the upper atmosphere.

Anderson, John L.↗

NASA NTRS2015

Utilizing Direct Numerical Simulations of Transition and Turbulence in Design Optimization

Design optimization methods that use the Reynolds-averaged Navier-Stokes equations with the associated turbulence and transition models, or other model-based forms of the governing equations, may result in aerodynamic designs with actual performance levels that are noticeably different from the expected values because of the complexity of modeling turbulence/transition accurately in certain flows. Flow phenomena such as wake-blade interaction and trailing edge vortex shedding in turbines and compressors (examples of such flows) may require a computational approach that is free of transition/turbulence models, such as direct numerical simulations (DNS), for the underlying physics to be computed accurately. Here we explore the possibility of utilizing DNS data in designing a turbine blade section. The ultimate objective is to substantially reduce differences between predicted performance metrics and those obtained in reality. The redesign of a typical low-pressure turbine blade section with the goal of reducing total pressure loss in the row is provided as an example. The basic ideas presented here are of course just as applicable elsewhere in aerodynamic shape optimization as long as the computational costs are not excessive.

Direct numerical simulations of transition and tur↗

NASA NTRS2021

Interrelationships of Sea Surface Salinity, Chlorophyll-α Concentration, and Sea Surface Temperature near the Antarctic Ice Edge

Satellite data can now provide a coherent picture of sea surface salinity (SSS), chlorophyll-α concentration (Chlα), sea surface temperature (SST), and sea ice cover across the Southern Ocean. The availability of these data at the basin scale enables novel insight into the physical and biological processes in an area that has historically been difficult to gather in situ data from. The analysis shows large regional and interannual variability of these parameters but also strong coherence across the Southern Ocean. The covariability of the parameters near the marginal ice zone shows a generally negative relationship between SSS and Chlα (r = −0.87). This may in part be attributed to the large seasonality of the variables, but analysis of data within the spring period (from November to December) shows similarly high correlation (r = −0.81). This is the first time that a large-scale robust connection between low salinity and high phytoplankton concentration during ice melt period has been quantified. Chlorophyll-α concentration is also well correlated with SST (r = 0.79) providing a potential indicator of the strength of the temperature limitation on primary productivity in the region. The observed correlation also varied regionally due to differences in ice melt patterns during spring and summer. Overall, this study provides new insights into the physical characteristics of the Southern Ocean as observed from space. In a continually warming and freshening Southern Ocean, the relationships observed here provide a key data source for testing ocean biogeochemical models and assessing the effect of sea ice–ocean processes on primary production.

Cynthia Garcia-Eidell↗

NASA NTRS1998

Prediction of Unsteady Transitional Layers in Turbomachinery Using Navier Stokes Equations

The objective of the research reported in this presentation is to develop computational techniques for the prediction of unsteady transitional flows associated with the rotor stator interaction in turbomachinery. Three low-Reynolds number turbulence models are incorporated in two unsteady Navier-Stokes codes (one is pressure based and the other is time marching with Runge-Kutta time stepping) and evaluated for accuracy in predicting the onset and the end of unsteady transitional patches due to wake passing. The best model is then used for modification and improvement for the leading edge effect. An existing steady Navier-Stokes code was modified to include pseudo-time stepping, which provided acceleration from 5 to 25 times that of the original code. A systematic validation procedure was implemented to assess the effects of the grid, artificial dissipation, physical, and the pseudo-time step for an accurate prediction of transitional flows resulting from the rotor-stator interaction. The ability of the Navier-Stokes code to predict the unsteady transitional flow on a turbomachinery blade is demonstrated. The unsteady pressure and velocity fields are in good agreement with the experimental data and the prediction from the Euler/boundary layer approach. The numerical solver was able to capture all zones (wake induced transitional strip, wake induced turbulent strip, calmed region, etc.) associated with wake induced transition in a compressor cascade. Another significant step is the assessment of k-epsilon turbulence models, including the leading edge modifications. Best results were obtained from the FLB model. The LB model predicted earlier inception of the transition and shorter transition length. Modification of the k-epsilon model was found to be essential for an accurate prediction of the unsteady transitional flow in a compressor cascade. The CH model failed to predict the unsteady transitional flow. Predicted boundary layer was turbulent from the leading edge, even with the modification of the k-epsilon model near the stagnation point. A comparison of the instantaneous shape factor, the skin friction coefficient, and the momentum thickness indicates that the Navier-Stokes predictions are reasonably good. Interaction between the upstream wake and stator wake results in shedding of unsteady vortices from the trailing edge and increased dissipation in the stator wake and, as a consequence, increased rate of decay of the stator wake. The procedure developed at Penn State should aid the designers of turbomachinery in allowing for the unsteady transitional flows due to rotor-stator interaction.

Lakshminarayana, B.↗

NASA NTRS1979

Stellar occultation studies of the solar system

The paper covers the principles, observational procedures, and results relating to occultations of stars by solar system bodies other than the moon. Physical processes involved in occultations are presented including (1) extinction by ring material, (2) differential refraction by a planetary atmosphere, (3) extinction by a planetary atmosphere, and (4) Fresnel diffraction by sharp edges. It is noted that from a sufficient number of immersion and emersion timings of a stellar occultation, the radius and ellipticity of the occulting body can be accurately determined. From an occultation by a planet having an atmosphere, temperature, pressure, and number density profiles can be obtained along with information about the composition of the atmosphere and the extinction.

Elliot, J. L.↗

NASA NTRS1981

Computational vision

The range of fundamental computational principles underlying human vision that equally apply to artificial and natural systems is surveyed. There emerges from research a view of the structuring of vision systems as a sequence of levels of representation, with the initial levels being primarily iconic (edges, regions, gradients) and the highest symbolic (surfaces, objects, scenes). Intermediate levels are constrained by information made available by preceding levels and information required by subsequent levels. In particular, it appears that physical and three-dimensional surface characteristics provide a critical transition from iconic to symbolic representations. A plausible vision system design incorporating these principles is outlined, and its key computational processes are elaborated.

Barrow, H. G.↗

NASA NTRS1990

Inflection point caustic problems and solutions for high-gain dual-shaped reflectors

The singular nature of the uniform geometrical theory of diffraction (UTD) subreflector scattered field at the vicinity of the main reflector edge (for a high-gain antenna design) is investigated. It is shown that the singularity in the UTD edge-diffracted and slope-diffracted fields is due to the reflection distance parameter approaching infinity in the transition functions. While the geometrical optics (GO) and UTD edge-diffracted fields exhibit singularities of the same order, the edge slope-diffracted field singularity is more significant and is substantial for greater subreflector edge tapers. The diffraction analysis of such a subreflector in the vicinity of the main reflector edge has been carried out efficiently and accurately by a stationary phase evaluation of the phi-integral, whereas the theta-integral is carried out numerically. Computational results from UTD and physical optics (PO) analysis of a 34-m ground station dual-shaped reflector confirm the analytical formulations for both circularly symmetric and offset asymmetric subreflectors. It is concluded that the proposed PO(theta)GO(phi) technique can be used to study the spillover or noise temperature characteristics of a high-gain reflector antenna efficiently and accurately.

Galindo-Israel, Victor↗

NASA NTRS2007

On the Magnitude of the Electric Field Near Thunderstorm-Associated Clouds

Electric field measurements made in and near clouds during two airborne field mill programs are presented. Aircraft equipped with multiple electric field mills and cloud physics sensors were flown near active convection and into thunderstorm anvil and debris clouds. The magnitude of the electric field was measured as a function of position with respect to the cloud edge in order to provide an observational basis for modifications to the lightning launch commit criteria (LLCC) used by the U.S. space program. These LLCC are used to reduce the risk that an ascending launch vehicle will trigger a lightning strike that could cause the loss of the mission or vehicle. The results suggest that even with fields of tens of kV/m inside electrically active convective clouds, the fields external to these clouds decay to less than 3 kV/m within fifteen kilometers of cloud edge. Fields exceeding 3 kV/m were not found external to anvil and debris clouds.

Merceret, Francis J.↗

NASA NTRS2013

Poisson-Spot Intensity Reduction with a Partially-Transparent Petal-Shaped Optical Mask

The presence of Poisson's spot, also known as the spot of Arago, formed along the optical axis in the geometrical shadow behind an obstruction, has been known since the 18th century. The presence of this spot can best be described as the consequence of constructive interference of light waves diffracted on the edge of the obstruction where its central position can··be determined by the symmetry of the object More recently, the elimination of this spot has received attention in the fields of particle physics, high-energy lasers, astronomy and lithography. In this paper, we introduce a novel, partially transparent petaled mask shape that suppresses the bright spot by up to 10 orders of magnitude in intensity, with powerful applications to many of the above fields. The optimization technique formulated in this design can identify mask shapes having partial transparency only near the petal tips.

Shiri, Shahram↗

NASA NTRSDate not supplied

Linking Strategic Astrophysics Missions, Technology Gaps, and Technology Maturation Investments

The National Aeronautics and Space Administration’s (NASA) Astrophysics Division (APD) funds the development of cutting-edge technologies through multiple programs to enable its strategic missions to achieve ambitious and groundbreaking science goals. These technology development efforts are managed by the Cosmic Origins (COR), Exoplanet Exploration (ExE), and Physics of the Cosmos (PhysCOS) programs. The 2020 Astronomy and Astrophysics Decadal Survey, “Pathways to Discovery in Astronomy and Astrophysics for the 2020s,” (Astro2020) recommended a pan-chromatic set of missions, including three Great Observatories (GO) and three Probe-class missions that could collect unprecedented data over the coming decades. We show the correlation between these strategic missions and the current Astrophysics technology gaps, as well as recent and current technology maturation projects funded to help close these technology gaps. We also cover how these investments advanced their Technology Readiness Levels (TRLs) [1] and where they have been infused into missions and projects.

SAT↗