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At least 1,027 records · Page 57

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NASA NTRS1982

Radiative transfer with partial frequency redistribution in inhomogeneous atmospheres - Application to the Jovian aurora

A direct finite difference numerical solution for the equation of radiative transfer by the Feautrier method is developed for use in planetary atmospheres. The procedure described here uses a plane-parallel atmosphere, and can treat partial frequency redistribution, inhomogeneity, external or internal sources, and various boundary conditions. Isotropic scattering is assumed, but in the case of no frequency redistribution, Rayleigh scattering can also be handled. A program utilizing this method is tested in a variety of situations against more powerful and elaborate methods. The case of the Lyman alpha aurora on Jupiter is then considered, where the effects of partial frequency redistribution are shown to be of great importance. New results for the detailed line profiles for Lyman alpha in the Jovian aurora are presented. The method is quite versatile, and should be especially useful in studying a wide range of problems related to auroral or dayglow emissions in planetary atmospheres.

Gladstone, G. R.↗

NASA NTRS1985

Analysis of the electromagnetic scattering from an inlet geometry with lossy walls

One of the primary goals is to develop an approximate but sufficiently accurate analysis for the problem of electromagnetic (EM) plane wave scattering by an open ended, perfectly-conducting, semi-infinite hollow circular waveguide (or duct) with a thin, uniform layer of lossy or absorbing material on its inner wall, and with a simple termination inside. The less difficult but useful problem of the EM scattering by a two-dimensional (2-D), semi-infinite parallel plate waveguide with an impedance boundary condition on the inner walls was chosen initially for analysis. The impedance boundary condition in this problem serves to model a thin layer of lossy dielectric/ferrite coating on the otherwise perfectly-conducting interior waveguide walls. An approximate but efficient and accurate ray solution was obtained recently. That solution is presently being extended to the case of a moderately thick dielectric/ferrite coating on the walls so as to be valid for situations where the impedance boundary condition may not remain sufficiently accurate.

Myung, N. H.↗

NASA NTRS1985

Aerothermodynamic heating and performance analysis of a high-lift aeromaneuvering AOTV concept

The thermal-control requirements for design-optimized aeromaneuvering performance are determined for space-based applications and low-earth orbit sorties involving large, multiple plane-inclination changes. The leading-edge heating analysis is the most advanced developed for hypersonic-rarefied flow over lifting surfaces at incidence. The effects of leading-edge bluntness, low-density viscous phenomena, and finite-rate flow-field chemistry and surface catalysis are accounted for. The predicted aerothermodynamic heating characteristics are correlated with thermal-control and flight-performance capabilities. The mission payload capability for delivery, retrieval, and combined operations is determined for round-trip sorties extending to polar orbits. Recommendations are given for future design refinements. The results help to identify technology issues required to develop prototype operational systems.

Menees, G. P.↗

NASA NTRS1985

The Influence of Low Frequency Mechanical Vibrations on the Growth of Single Crystals

The optimum conditions for crystal growth are usually achieved either by suppressing convective fluid flows (e.g., by the use of a low-gravity environment) or by over-riding thermal and solutal convection by the use of a strong stirring action. A novel stirring technique has been developed which involves subjecting a vertical crucible to a circle in a horizontal plane (without rotation). Use of an amplitude of 3 mm at a frequency of approx 6 Hz produced complete mixing of a non-uniform aqueous liquid in a few seconds. The mixing action involved the downward flow of liquid in the outer annulus of the liquid, driven by surface waves. When the downward flowing liquid reaches the bottom of the crucible, it is reflected in a central, upward flowing spiral. This flow pattern should be beneficial for crystal growth by the Bridgman method since it will sweep impurities away from the walls and produce a more convex solid-liquid interface. Initial attempts to apply the new stirring technique to CdTe crystal growth did not show significant improvement in the number of crystals nucleated, but the interface shape appeared to be close to that predicted.

Feigelson, R. S.↗

NASA NTRS1989

NASP natural environment definitions for design

Problems that emerged during the development of the natural environment definitions for the design of the National Aerospace Plane (NASP) are discussed. The NASP program objectives are reviewed. It is found that some of the data needed to determine the environmental parameters for designing the aircraft are unavailable. It is suggested that this is due to a lack of technology for making the necessary measurements.

Smith, Orvel E.↗

NASA NTRS1989

Numerical algorithms for steady and unsteady incompressible Navier-Stokes equations

The numerical analysis of the incompressible Navier-Stokes equations are becoming important tools in the understanding of some fluid flow problems which are encountered in research as well as in industry. With the advent of the supercomputers, more realistic problems can be studied with a wider choice of numerical algorithms. An alternative formulation is presented for viscous incompressible flows. The incompressible Navier-Stokes equations are cast in a velocity/vorticity formulation. This formulation consists of solving the Poisson equations for the velocity components and the vorticity transport equation. Two numerical algorithms for the steady two-dimensional laminar flows are presented. The first method is based on the actual partial differential equations. This uses a finite-difference approximation of the governing equations on a staggered grid. The second method uses a finite element discretization with the vorticity transport equation approximated using a Galerkin approximation and the Poisson equations are obtained using a least squares method. The equations are solved efficiently using Newton's method and a banded direct matrix solver (LINPACK). The method is extended to steady three-dimensional laminar flows and applied to a cubic driven cavity using finite difference schemes and a staggered grid arrangement on a Cartesian mesh. The equations are solved iteratively using a plane zebra relaxation scheme. Currently, a two-dimensional, unsteady algorithm is being developed using a generalized coordinate system. The equations are discretized using a finite-volume approach. This work will then be extended to three-dimensional flows.

Hafez, Mohammed↗

NASA NTRS1990

Flow measurements in a model of the mildly curved femoral artery of man

The effects of curvature on the flow rate near the wall in the vicinity of the mildly curved femoral artery of man, and on the pressure distributions along the curved segment, were investigated using glass and tygon flow models constructed to conform to the shape of the femoral angiogram of a human subject. The test fluid was 33 percent aqueous sucrose. Steady flow observations, made using a dye flow visualization system, revealed a flow pattern like that observed in coiled pipes. A double helical type flow was found to develop, with converging streamlines in the wall vicinity from the upper and lower plane of curvature merging asymptotically along the inner curvature in a stable manner. Pressure measurements for steady flow revealed progressively larger pressure drops with distance along the entrance region of the curved segment, relative to that for a straight lumen.

Back, L. H.↗

NASA NTRS1991

A study of hydrogen diffusion flames using PDF turbulence model

The application of probability density function (pdf) turbulence models is addressed in this work. For the purpose of accurate prediction of turbulent combustion, an algorithm that combines a conventional CFD flow solver with the Monte Carlo simulation of the pdf evolution equation has been developed. The algorithm has been validated using experimental data for a heated turbulent plane jet. The study of H2-F2 diffusion flames has been carried out using this algorithm. Numerical results compared favorably with experimental data. The computuations show that the flame center shifts as the equivalence ratio changes, and that for the same equivalence ratio, similarity solutions for flames exist.

Hsu, Andrew T.↗

NASA NTRS1992

Using the historical record to determine dust sources

We have recently completed a study of virtually all reported dust activity noted in contemporary and historical records of Mars observations. This study included the compilation of maps showing the locations of these events to the degree that they could be determined. Whenever possible, regional (major) storms were individually mapped, as well as on a composite map. Areas of local events were shown only on a composite map. The four mappable plane-encircling storms were each given separate treatment on maps that portray their development around the planet.

Martin, L. J.↗

NASA NTRS1992

Thermal-structural test facilities at NASA Dryden

The National Aero-Space Plane (NASP) has renewed interest in hypersonic flight and hot-structures technology development for both the airframe and engine. The NASA Dryden Thermostructures Research Facility is a unique national facility that was designed to conduct thermal-mechanical tests on aircraft and aircraft components by simulating the flight thermal environment in the laboratory. The layout of the facility is presented, which includes descriptions of the high-bay test area, the instrumentation laboratories, the mechanical loading systems, and the state-of-the-art closed-loop thermal control system. The hot-structures test capability of the facility is emphasized by the Mach-3 thermal simulation conducted on the YF-12 airplane. The Liquid-Hydrogen Structural Test Facility, which is presently in the design phase, will provide the capability of thermally testing structures containing hydrogen.

Deangelis, V. Michael↗

NASA NTRS1991

Aerobrake guidance law synthesis using feedback linearization

Aerobraking maneuvers are currently being considered for use in several space missions. The development of a guidance law for performing simultaneous change of both in-plane and out-of-plane orbital elements with least energy loss is described. The analysis is based on a sixth-order nonlinear point-mass vehicle model with lift, bank angle and drag modulation as the three control variables. The guidance law uses detailed vehicle aerodynamic and the atmosphere models in the feedback loop. The effects of planet oblateness, atmospheric rotation and ambient winds are included in the formulation. The guidance law is in nonlinear feedback form and is simple enough to be implemented on-board an orbit transfer vehicle.

Menon, P. K. A.↗

NASA NTRS1991

Matrix cracking in laminated composites under monotonic and cyclic loadings

An analytical model based on the internal state variable (ISV) concept and the strain energy method is proposed for characterizing the monotonic and cyclic response of laminated composites containing matrix cracks. A modified constitution is formulated for angle-ply laminates under general in-plane mechanical loading and constant temperature change. A monotonic matrix cracking criterion is developed for predicting the crack density in cross-ply laminates as a function of the applied laminate axial stress. An initial formulation for a cyclic matrix cracking criterion for cross-ply laminates is also discussed. For the monotonic loading case, a number of experimental data and well-known models are compared with the present study for validating the practical applicability of the ISV approach.

Allen, David H.↗

NASA NTRS1993

Whistlers and plasmaspheric hiss - Wave directions and three-dimensional propagation

Wave propagation directions are determined on the basis of wave data from the DE 1 satellite showing simultaneously nonducted whistlers and hiss. Hiss wave normal angles are determined as about 70 and 77 deg for f = 3.5 and 2.5 kHz, respectively, with the wave vector being almost perpendicular to the meridional plane. A novel approximate analytical formulation of 3D propagation of whistler waves is developed and used to model the drift of magnetospherically reflected whistlers in azimuth. It is shown that depending on initial parameters, the time of arrival of whistler rays at a fixed observation point can differ by 10-20 s, with signals from different magnetospherically reflected whistlers overlapping to evolve into a hisslike signal. The total azimuthal drift of whistler rays is found to not exceed about 30 deg, so that plasmaspheric hiss may be produced by nonducted whistlers at longitudes correlated with the location of thunderstorm activity.

Draganov, A. B.↗

NASA NTRS1994

Insulating Blankets Withstand Higher Temperatures

Thermal-insulation blankets withstand repeated exposure to temperatures up to 2,000 degrees C. Thin, light in weight, and flexible. Developed to protect proposed aerospacecraft like Aeroassisted Space Transfer Vehicle and National Aerospace Plane, exposed to temperatures higher than those on surface of Space Shuttle during reentry into atmosphere of Earth. Also useful as furnace insulation and in firefighters' clothing. Sewing threads made of silicon carbide yarn essential components of blankets.

Kourtides, D. A.↗

NASA NTRS1995

Analytical investigation of the hygrothermal effects and parametric study of the Edge Crack Torsion (ECT) mode 3 test lay-ups

A shear deformation theory including residual thermal and moisture effects is developed for the analysis of either symmetric or unsymmetric laminates with mid-plane edge delamination under torsion loading. The theory is based on an assumed displacement field which includes shear deformation. The governing equations and boundary conditions are obtained from the principle of virtual work. The analysis of the (90/(+/- 45)(n)/(-/+ 45)(n)/90)(s) ECT mode 3 test lay-up indicates that there are no hygrothermal effects on the mode 3 strain energy release rate because the laminate, and both sublaminates above and below the delamination, are symmetric lay-ups. A further parametric study reveals that some other lay-ups can have negligible hygrothermal effects even when the sublaminates above and below the delamination are not symmetric about their own mid-planes. However, these lay-ups may suffer from distortion after the curing process. Another Interesting set of lay-ups investigated is a class of antisymmetric laminates with (+/-(theta/(theta -90)(2)/theta))(n) lay-ups. It is observed that when n takes on even numbers (2 and 4), both hygrothermal and mode 1 effects can be neglected. From this point of view, these lay-ups provide a way to determine the mode 3 toughness between two dissimilar layers. However, when n takes on odd numbers (1 and 3), both hygrothermal and mode 1 effects may be strong in these lay-ups. In particular, when theta equals 45 deg, the lay-ups are free from both hygrothermal and mode 1 effects irrespective of n.

Li, Jian↗

NASA NTRS2001

The Material With a Need for Speed

PETI-5 is the commonly used term for phenylethynyl terminated imide oligomers, and the 5 refers to the fifth formulation out of 200 compositions. It is a chemical material that can be used as both a resin and an adhesive. Prepared from commercially available materials, PETI-5 combines superb mechanical properties and extreme durability with easy processing and environmental stability. For example, the material has temperature capabilities of 350 ?F for a lifetime of around 60,000 hours. Curing is done with the application of heat and mild pressure, which results in the formation of a strong, resistant polymer. Due to its unique combination of benefits, PETI-5 will be used in the development of the high speed civil transport (HSCT), a concept 300- passenger commercial plane that will travel at a speed of Mach 2.4. Because currently available metals are too heavy and cannot withstand the extreme high temperatures resulting from flying at such high speeds, composite materials made from graphite fibers and PETI-5 are necessary. These

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NASA NTRS1994

Disk Drives

A new material known as AlBeMet, developed by Brush Wellman for research applications in the National Aero-Space Plane (NASP) program, is now used for high performance disk drives. AlBeMet is a compression of aluminum, beryllium metal matrix composite. It reduces system weight and its high thermal conductivity can effectively remove heat and increase an electrical system's lifetime. The lighter, stiffer AlBeMet (AlBeMet 160) used in the disk drive means heads can be moved faster, improving disk performance.

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NASA NTRS2003

Nanolayered Features of Collagen-like Peptides

We have been investigating collagen-like model oligopeptides as molecular bases for complex ordered biomimetic materials. The collagen-like molecules incorporate aspects of native collagen sequence and secondary structure. Designed modifications to native primary and secondary structure have been incorporated to control the nanostructure and microstructure of the collagen-like materials produced. We find that the collagen-like molecules form a number of lyotropic rod liquid crystalline phases, which because of their strong temperature dependence in the liquid state can also be viewed as solvent intercalated thermotropic liquid crystals. The liquid crystalline phases formed by the molecules can be captured in the solid state by drying off solvent, resulting in solid nanopatterned (chemically and physically) thermally stable (to greater than 100 C) materials. Designed sequences which stabilize smectic phases have allowed a variety of nanoscale multilayered biopolymeric materials to be developed. Preliminary investigations suggest that chemical patterns running perpendicular to the smectic layer plane can be functionalized and used to localize a variety of organic, inorganic, and organometallic moieties in very simple multilayered nanocomposites. The phase behavior of collagen-like oligopeptide materials is described, emphasizing the correlation between mesophase, molecular orientation, and chemical patterning at the microscale and nanoscale. In many cases, the textures observed for smectic and hexatic phase collagens are remarkably similar to the complex (and not fully understood) helicoids observed in biological collagen-based tissues. Comparisons between biological morphologies and collagen model liquid crystalline (and solidified materials) textures may help us understand the molecular features which impart order and function to the extracellular matrix and to collagen-based mineralized tissues. Initial studies have utilized synthetic collagen-like peptides while future work will also focus on similar sequences generated via genetic engineering methods.

Valluzzi, Regina↗