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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Scattering and collapse of Langmuir waves driven by a weak electron beam

Wave-wave interactions are examined, particularly for significance in relation to Langmuir turbulence associated with type III solar radio emissions. The Zakharov equations are considered as two coupled nonlinear partial differential equations for the low-frequency density and the Langmuir envelope, and the electron and ion temperatures are regarded as equal. The Zhakharov equations are solved in one dimension, and display no collapse of a driven broadband pump, indicating that scattering instabilities are important in the saturation of a beam-plasma instability. A two-dimensional numerical simulation is developed to interpret the role of parametric instabilities, and it is concluded that the distortion of a wave packet is due to scattering instability, and not a change in the physics of direct collapse due to an artificial numerical damping. Further discussions are presented for comparisons of models of Langmuir turbulence in type III bursts.

Hafizi, B.↗
NASA NTRS1989

Pluto's interaction with the solar wind

If Pluto's atmospheric escape rate is significantly greater than 1.5 x 10 to the 27th molecules/s then the interaction with the tenuous solar wind at 30 A.U. will be like that of a comet. There will be extensive ion pick-up upstream and the size of the interaction region will vary directly with variations in the solar wind flux. If the escape flux is much less, then one expects that the solar wind will be deflected around Pluto's ionosphere in a Venus-like interaction. In either case, the weak interplanetary magnetic field at 30 A.U. results in very large gyroradii for the picked-up ions and a thick bow shock, necessitating a kinetic treatment of the interaction. Strong variations in the size of the interaction region are expected on time scales of days due to changes in the solar wind.

Bagenal, Fran↗
NASA NTRS1985

On the relation between 'mixing length' and 'direct interaction approximation' theories of turbulence

The capabilities of direct interaction approximations (DIA) and weak-coupling approximations (WCA) in modeling turbulence in plasma physics studies are examined. DIA and WCA formalisms are reviewed and a DIA expression for homogeneous incompressible hydrodynamics is simplified to derive an energy spectrum in an arbitrary wavenumber space. A solution is defined for the expression when applied to long wavelength phenomena generated by an instability. The equation is further generalized to yield a transport equation for modeling the spectrum generated by homogeneous turbulence in a linearly unstable system undergoing nonlinear interaction. Finally, the technique is applied to two-dimensional turbulence in a low pressure, weakly ionized plasma embedded in a homogeneous magnetic field and experiencing instabilities caused by electron density and electric potential gradients, such as found in the ionospheric E region.

Sudan, R. N.↗
NASA NTRS1972

Observation of a quadrupole interaction for cubic imperfections exhibiting a dynamic Jahn-Teller effect.

The observation and interpretation of weak EPR transitions, identified as 'forbidden' transitions, establish the existence of a new type of quadrupole interaction for cubic-symmetry imperfections. This interaction is simply a consequence of the ground-vibronic-state degeneracy. The signs as well as the magnitudes of the quadrupole-coupling coefficients are determined experimentally. These data agree well with the predictions of crystal field theory modified to account for a weak-to-moderate vibronic interaction (i.e., a dynamic Jahn-Teller effect).

Herrington, J. R.↗
NASA NTRS1987

The emission of narrow-band Jovian kilometric radiation

A model based on the nonlinear coupling of electrostatic plasma waves is proposed to explain the emission of the narrow-band Jovian kilometric radiation (nKOM) observed by the Voyager spacecraft. It is shown that upper-hybrid branch electrostatic waves propagating through the inhomogeneities in the outer periphery of the Io plasma torus can attain the proper geometry for localized upconversion interactions leading to pump depletion. Plasma waves propagating into a weak density gradient and reflected at the critical layer interact with the incident waves leading to the electromagnetic emission, which is beamed at large angles with respect to the background magnetic field. In general, both L-O and R-X mode waves can be generated. The observed power and net polarization (L-O) are consistent with pump depletion of electrostatic waves at a level of about 10 mV/m. A possible excitation mechanism for the electrostatic waves is also discussed.

Fung, S. F.↗
NASA NTRS2007

Assessment of Turbulent Shock-Boundary Layer Interaction Computations Using the OVERFLOW Code

The performance of two popular turbulence models, the Spalart-Allmaras model and Menter s SST model, and one relatively new model, Olsen & Coakley s Lag model, are evaluated using the OVERFLOWcode. Turbulent shock-boundary layer interaction predictions are evaluated with three different experimental datasets: a series of 2D compression ramps at Mach 2.87, a series of 2D compression ramps at Mach 2.94, and an axisymmetric coneflare at Mach 11. The experimental datasets include flows with no separation, moderate separation, and significant separation, and use several different experimental measurement techniques (including laser doppler velocimetry (LDV), pitot-probe measurement, inclined hot-wire probe measurement, preston tube skin friction measurement, and surface pressure measurement). Additionally, the OVERFLOW solutions are compared to the solutions of a second CFD code, DPLR. The predictions for weak shock-boundary layer interactions are in reasonable agreement with the experimental data. For strong shock-boundary layer interactions, all of the turbulence models overpredict the separation size and fail to predict the correct skin friction recovery distribution. In most cases, surface pressure predictions show too much upstream influence, however including the tunnel side-wall boundary layers in the computation improves the separation predictions.

Oliver, A. B.↗
NASA NTRS1970

Lagrangian description of warm plasmas

Efforts are described to extend the averaged Lagrangian method of describing small signal wave propagation and nonlinear wave interaction, developed by earlier workers for cold plasmas, to the more general conditions of warm collisionless plasmas, and to demonstrate particularly the effectiveness of the method in analyzing wave-wave interactions. The theory is developed for both the microscopic description and the hydrodynamic approximation to plasma behavior. First, a microscopic Lagrangian is formulated rigorously, and expanded in terms of perturbations about equilibrium. Two methods are then described for deriving a hydrodynamic Lagrangian. In the first of these, the Lagrangian is obtained by velocity integration of the exact microscopic Lagrangian. In the second, the expanded hydrodynamic Lagrangian is obtained directly from the expanded microscopic Lagrangian. As applications of the microscopic Lagrangian, the small-signal dispersion relations and the coupled mode equations are derived for all possible waves in a warm infinite, weakly inhomogeneous magnetoplasma, and their interactions are examined.

Kim, H.↗
NASA NTRS1984

Simulation of blunt-fin-induced shock wave and turbulent boundary-layer interaction

A supersonic flow over a blunt fin mounted on a flat plate is numerically simulated. The fin shock causes the boundary layer to separate and results in a complicated, three-dimensional shockwave and boundary-layer interaction. The computed result is in good agreement with the measured pressure on the fin and the flat plate. The main features, such as peak pressure on the fin leading edge and a double peak pressure on the plate, are closely predicted. The role of the horseshoe vortex is discussed. The vortex leads to the development of high-speed flow, and, hence, low-pressure regions on the fin and the plate. Different thicknesses of the incoming boundary layer have been studied. Varying the thicknesses by an order of magnitude shows that the size of the horseshoe vortex and therefore the spatial extent of the interaction are inviscid-dominated, and are weakly dependent on the Reynolds number. Colored graphics are used to show details of the interaction flow field.

Hung, C.-M.↗
NASA NTRS2019

Fully-Coupled Fluid-Structure Interaction Simulations of a Supersonic Parachute

A validated computational fluid-structure interaction method for simulating the complex interaction between the large deformation of very thin, highly deformable structures and compressible flows is extended to consider large-scale problems in supersonic flows using parallel computing. The coupled fluid-structure interaction system is solved in a partitioned, or weakly-coupled, manner. The foundations of the applied fluid-structure interaction method are a higher-order, block-structured Cartesian, sharp immersed boundary method for the compressible Navier-Stokes equations and a computational structural dynamics solver employing a geometrically nonlinear 3-node shell element based on the mixed interpolation of tensorial components formulation. The method is applied to large deformation fluid-structure interaction validation cases before being applied to the inflation of a supersonic parachute in the upper Martian atmosphere where the goal is to demonstrate the capabilities of the solver when considering large-scale problems in supersonic flows.

Boustani, Jonathan↗
NASA NTRS1984

Relationships of a growing magnetic flux region to flares

The evolution of flare sites at the boundaries of major new and growing magnetic flux regions within complexes of active regions has been analyzed using H-alpha images. A spectrum of possible relationships of growing flux regions to flares is described. An 'intimate' interaction between old and new flux and flare sites occurs at the boundaries of their regions. Forced or 'intimidated' interaction involves new flux pushing older, lower flux density fields toward a neighboring old polarity inversion line, followed by the occurrence of a flare. In 'influential' interaction, magnetic lines of force over an old polarity inversion line reconnect to new emerging flux, and a flare occurs when the magnetic field overlying the filament becomes too weak to prevent its eruption. 'Inconsequential' interaction occurs when a new flux region is too small or has the wrong orientation for creating flare conditions. 'Incidental' interaction involves a flare occurring without any significant relationship to new flux regions.

Martin, S. F.↗
NASA NTRS1985

A possible mechanism for the observed streaming of O(+) and H(+) ions at nearly equal speeds in the distant magnetotail

In recent years, O(+) and H(+) ions streaming away from the earth along geomagnetic field lines have been observed in the distant magnetotail region. In the present paper, it is suggested that the transverse acceleration of the ions occurs on auroral field lines at altitudes above the field-aligned potential drops, where ion beams and electrostatic hydrogen cyclotron (EHC) waves have been simultaneously observed. It is pointed out that the preferential acceleration of O(+) relative to H(+) occurs through the interaction of O(+) ions with weak EHC waves, as suggested by Singh et al. (1983). A quantitative explanation is provided for the observed relationship between the energies of O(+) and H(+) ions.

Singh, N.↗
NASA NTRS1989

On the bonding in Mn2(+)

Qualitative calculations are presented on the Mn2(+) ion. These yield a 12Sigma(g)(+) ground state, in agreement with experiment, in which the 3d electrons are ferromagnetically coupled. The 2Sigma(u)(+) state of Mu2(+) is found to lie 0.44 eV higher in energy; the 3d electrons on each center are high-spin coupled, but the interatomic coupling is antiferromagnetic. The large splitting between the 12Sigma(g)(+) and 2Sigma(u)(+) states is shown to be due to the intraatomic 4s-3d exchange energy, which is an order of magnitude larger than the weak interatomic 3d-3d interaction. The Mn2(+) ion is compared and contrasted with the Mn2 neutral species, where the analogous 11Sigma(u)(+) and 1Sigma(g)(+) states are reversed in order, but almost degenerate.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1992

Recent advances in carbon-carbon substrate technology at NASA. Langley Research Center

A comparison of specific strengths of candidate high-temperature materials as a function of temperature is shown. From this comparison, it is apparent why there is an interest in carbon-carbon composites for applications as a strong, light-weight thermal protection system (TPS), or as hot structure, for applications above 2500 F. The lower bound of the carbon-carbon band is representative of the tensile strength of cross-ply Advanced Carbon-Carbon (ACC). The upper bound represents capabilities of various experimental carbon-carbon composites. Thin carbon-carbon composites, such as would be used as TPS panels or hot aero-structure, are usually constructed of layups of 2-D fabrics of carbon-fiber yarns (tows). Although the in-plane strengths of these composites can be very attractive, a major problem area is low interlaminar strength. The low interlaminar strength is the result of a relatively weak carbon matrix and poor interaction between the fibers and matrix. The purpose of this paper is to discuss strategies being employed to improve the interlaminar strengths of the materials at the upper bound of the carbon-carbon band, and to present some recent encouraging results. The emphasis of these strategies is to improve interlaminar shear and tensile strengths while maintaining, or even improving, the inplane properties.

Ransone, Philip O.↗
NASA NTRS1980

Transonic Flow Past a Symmetrical Airfoil-- Inviscid and Turbulent Flow Properties

Flowfield measurements are presented for a symmetrical NACA 64A010 airfoil section at transonic conditions. Measurements were obtained for three angles of attack with the freestream Mach number fixed at O.S. The cases studied included a weak shock-wave/boundary-layer interaction, an interaction of medium strength with mild separation, and an interaction of sufficient strength to produce a shock-induced stall situation. Two nonintrusive optical techniques, lower velocimetry and holographic interferometry, were used to characterize the flows. The results include Mach number contours and flow angle distributions in the inviscid flow regions, and turbulent flow properties, including the turbulent Reynolds stresses, of the upper surface viscous layers, and of the near-wake. The turbulent flow measurements reveal that the turbulence fluctuations attain equilibrium with the local mean flow much faster than previously expected.

Johnson D. A.↗
NASA NTRS2002

Nanostructured Composites: Effective Mechanical Property Determination of Nanotube Bundles

Carbon nanotubes naturally tend to form crystals in the form of hexagonally packed bundles or ropes that should exhibit a transversely isotropic constitutive behavior. Although the intratube axial stiffness is on the order of 1 TPa due to a strong network of delocalized bonds, the intertube cohesive strength is orders of magnitude less controlled by weak, nonbonding van der Waals interactions. An accurate determination of the effective mechanical properties of nanotube bundles is important to assess potential structural applications such as reinforcement in future composite material systems. A direct method for calculating effective material constants is developed in the present study. The Lennard-Jones potential is used to model the nonbonding cohesive forces. A complete set of transverse moduli are obtained and compared with existing data.

Saether, E.↗
NASA NTRS2006

The Solar Wind and Geomagnetic Activity as a Function of Time Relative to Corotating Interaction Regions

Corotating interaction regions during the declining phase of the solar cycle are the cause of recurrent geomagnetic storms and are responsible for the generation of high fluxes of relativistic electrons. These regions are produced by the collision of a high-speed stream of solar wind with a slow-speed stream. The interface between the two streams is easily identified with plasma and field data from a solar wind monitor upstream of the Earth. The properties of the solar wind and interplanetary magnetic field are systematic functions of time relative to the stream interface. Consequently the coupling of the solar wind to the Earth's magnetosphere produces a predictable sequence of events. Because the streams persist for many solar rotations it should be possible to use terrestrial observations of past magnetic activity to predict future activity. Also the high-speed streams are produced by large unipolar magnetic regions on the Sun so that empirical models can be used to predict the velocity profile of a stream expected at the Earth. In either case knowledge of the statistical properties of the solar wind and geomagnetic activity as a function of time relative to a stream interface provides the basis for medium term forecasting of geomagnetic activity. In this report we use lists of stream interfaces identified in solar wind data during the years 1995 and 2004 to develop probability distribution functions for a variety of different variables as a function of time relative to the interface. The results are presented as temporal profiles of the quartiles of the cumulative probability distributions of these variables. We demonstrate that the storms produced by these interaction regions are generally very weak. Despite this the fluxes of relativistic electrons produced during those storms are the highest seen in the solar cycle. We attribute this to the specific sequence of events produced by the organization of the solar wind relative to the stream interfaces. We also show that there are large quantitative differences in various parameters between the two cycles.

McPherron, Robert L.↗
NASA NTRS2021

A lingering local exopshere created by a gas plume of a lunar lander

We model the expected local exosphere created by outgassing from a lunar lander plume water deposit located adjacent to a lunar lander. We assume the landing is near dawn within Lacus Mortis at 45oN. We consider a set of water desorption scenarios especially focusing on the water-surface desorption activation energy that can possess a distribution of energy states. Our initial simulation starts with a deposition surface having 1020 water molecules per square meter in a 2 km × 2 km region where 90% of the water is retained by weak water-water sorption interactions and 10% are bound by water-regolith sorption interactions. We find that the loosely bound water molecules in water-water interactions quickly desorb within hours after landing. However, water molecules bound to surface sites having a desorption activation greater than 0.7 eV were found to be retained until the source region rotated to later local times (i.e., surface temperature becomes relatively high). We found such time-delayed desorption leads to a tenuous but lingering exosphere emitted from the lander plume deposit region for many 10's of hours after landing. We also adjusted the initial condition by considering different landing local times and considered a case where 100% of the water is bound by water-regolith interactions (mimicking a porous regolith).

W.M. Farrell↗