SEARCH · Engineering Papers

Results for “Structural anisotropy”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 361 records · Page 20

All sources NASA NTRS DOE OSTI

NASA NTRS1991

Some characteristics of small-scale turbulence in a turbulent duct flow

The fine-scale structure of turbulence in a fully developed turbulent duct flow is examined by considering the 3D velocity derivative field obtained from direct numerical simulations at two relatively small Reynolds numbers. The magnitudes of all mean-square derivatives (normalized by wall variables) increase with the Reynolds number, the increase being largest at the wall. These magnitudes are not consistent with the assumption of local isotropy except perhaps near the duct center-line. When the assumption of local isotropy is relaxed to one of local axisymmetry, or invariance with respect to rotation about a coordinate axis (here chosen in the streamwise direction), satisfactory agreement is indicated by the data outside the wall region. Support for axisymmetry is demonstrated by anisotropy invariant maps of the dissipation and vorticity tensors.

Antonia, R. A.↗

NASA NTRS1995

Anisotropic microturbulence near the Sun

Interplanetary scintillation observations which sample the spatial spectrum of electron density at scales between 10 and 100 km show power-law spectra which are flatter than the Kolmogorov spectra observed at larger scales by spacecraft and also, indirectly, by phase scintillation of coherent radio signals. Furthermore, angular broadening observations of compact radio sources have shown that the microscale density fluctuations are field-aligned and become more anisotropic as R decreases inside 10 solar radius. We present angular broadening observations taken in October of 1992 which were recorded nearly simultaneously on the VLA and VLBA arrays. The VLA samples structure at scales between 3 and 30 km, whereas the VLBA samples scales between 200 and 4000 km. The small scale VLA measurements of the south polar source 1246-075 showed lower turbulence than those of the equatorial source 1256-057, consistent with previous work showing that the density delta N(exp 2)(sub e) is a factor of 10-15 lower in coronal holes. The VLA observations inside of 10 solar radius were anisotropic, as expected. We were not able to measure the equatorial source with the VLBA inside of 10 solar radius because the scattering was too strong, however we did observe the polar source just inside this distance with both the VLA and the VLBA. Significant anisotropy was seen on the smaller scales, but the larger scales were essentially isotropic. This suggests that the process responsible for the anisotropic microturbulence is distinct from the larger scale isotropic turbulence.

Coles, William A.↗

NASA NTRS1989

Composite structure dynamics and space applicability

Guided wave modes in cladded or uncladded fiber-reinforced composite plates and tubes have been analyzed using a stiffness method in which the displacement variation through the thickness is approximated by polynomial interpolation functions. This allows for an arbitrary number of laminations and fiber orientations different from lamina to lamina. It is shown that dispersive behavior of guided modes depends significantly on the cladding, number of laminae, and interfaces between the adjacent laminae. A hybrid modeling technique is described in which an inner region containing cracks (or other defects) is discretized by finite elements, and the field in the exterior region is represented in terms of modes that are found using the stiffness method described above. It is found that the reflected and transmitted amplitudes of modes vary significantly with the size of a transverse or longitudinal (delamination) crack and frequency. We have also studied the impact response of a unidirectional fiber-reinforced plate. Received signals at the epicentral and other locations are shown. Strong longitudinal anisotropy of the graphite/epoxy plate causes the signal to be considerably different from that in an isotropic plate.

Datta, Subhendu K.↗

NASA NTRS1986

The electron-cyclotron maser instability as a source of plasma radiation

The generation of continuum bursts from the sun at dm and m wavelengths (in particular, type IV bursts) via the electron-cyclotron-maser instability is examined. The maser instability can be driven by an electron distribution with either a loss-cone anisotropy or a peak at large pitch angles. For omega(p)/Omega(e) much greater than 1, the maser emission is produced by electrons interacting through a harmonic (cyclotron) resonance and is electrostatic, being in the upper hybrid mode at frequencies approximately equal to omega(p). Coalescence processes are required to convert the electrostatic waves into transverse radiation which can escape from the source region. Whether the resultant spectrum is nearly a smooth continuum or has a zebra-stripe pattern (both of which occur in type IV bursts) depends on the form of the electron distribution, inhomogeneities in the density and magnetic field, and whether the maser reaches saturation. For at least the case of some type IV dm bursts with fine structure, comparison with observations seems to indicate that the electrons producing the emission are more likely to have a loss-cone distribution, and that the maser instability is not at saturation.

Winglee, R. M.↗

NASA NTRS1994

Anisotropic Alfven-ballooning modes in Earth's magnetosphere

We have carried out a theoretical analysis of the stability and parallel structure of coupled shear Alfven and slow magnetosonic waves in Earth's inner magnetopause (i.e., at equatorial distances between about five and ten Earth radii) including effects of finite anisotropic Grad-Shafranov equation yields an approximate self-consistent magnetohydrodynamic (MHD) equilibrium. This MHD equilibrium is used in the numerical solution of a set of eigenmode equations which describe the field line eigenfrequency, linear stability, and parallel eigenmode structure. We call these modes anisotropic Alfven-ballooning modes. The main results are: (1) The field line eigenfrequency can be significantly lowered by finite pressure effects. (2) The parallel mode structure of the transverse wave components is fairly insensitive to changes in the plasma pressure, but the compressional magnetic component can become highly peaked near the magnetic equator as a result of increased pressure, especially when P(sub perpendicular to) is greater than P(sub parallel) (here P(sub perpendicular to) and P(sub parallel) are the perpendicular and parallel plasma pressure). (3) For the isotropic (P(sub parallel) = P(sub perpendicular to) = P) case ballooning instability can occur when the ratio of the plasma presure to the magnetic pressure, beta = P/(B squared/8 pi), exceeds a critical value beta(sup B)(sub 0) is approximately equal to 3.5 at the equator. (4) Compared to the isotropic case the critical beta value is lowered by anisotropy, either due to decreased field line bending stabilization when P(sub parallel) is greater than P(sub perpendicular to) or due to increased ballooning-mirror destabilization when P(sub perpendicular to) is greater than P(sub parallel). (5) We use a beta-delta stability diagram to display the regions of instability with respect to the equatorial values of the parameters bar beta and delta, where bar beta = (1/3)(beta(sub parallel) + 2 beta(sub perpendicular to)) is an average beta value and delta = 1 - P(sub parallel)/P(sub perpendicular to) is a measure of the plasma anisotropy. The diagram is divided into regions corresponding to the firehose, mirror and ballooning instabilities. It appears that observed values of the plasma pressure are below the critical value for the isotropic ballooning instability but it may be possible to approach a ballooning-mirror instability when P(sub perpendicular to)/P(sub parallel) is greater than or approximately 2.

Chan, Anthony A.↗

NASA NTRS2003

First Year Wilkinson Microwave Anisotropy Probe (WMAP)Observations: Preliminary Maps and Basic Results

We present full sky microwave maps in five frequency bands (23 to 94 GHz) from the WMAP first year sky survey. Calibration errors are less than 0.5% and the low systematic error level is well specified. The cosmic microwave background (CMB) is separated from the foregrounds using multifrequency data. The sky maps are consistent with the 7 in. full-width at half-maximum (FWHM) Cosmic Background Explorer (COBE) maps. We report more precise, but consistent, dipole and quadrupole values. The CMB anisotropy obeys Gaussian statistics with -58 less than f(sub NL) less than 134 (95% CL). The 2 less than or = l less than or = 900 anisotropy power spectrum is cosmic variance limited for l less than 354 with a signal-to-noise ratio greater than 1 per mode to l = 658. The temperature-polarization cross-power spectrum reveals both acoustic features and a large angle correlation from reionization. The optical depth of reionization is tau = 0.17 +/- 0.04, which implies a reionization epoch of t(sub r) = 180(sup +220, sub -80) Myr (95% CL) after the Big Bang at a redshift of z(sub r) = 20(sup +10, sub -9) (95% CL) for a range of ionization scenarios. This early reionization is incompatible with the presence of a significant warm dark matter density. A best-fit cosmological model to the CMB and other measures of large scale structure works remarkably well with only a few parameters. The age of the best-fit universe is t(sub 0) = 13.7 +/- 0.2 Gyr old. Decoupling was t(sub dec) = 379(sup +8, sub -7)kyr after the Big Bang at a redshift of z(sub dec) = 1089 +/- 1. The thickness of the decoupling surface was Delta(sub z(sub dec)) = 195 +/- 2. The matter density of the universe is Omega(sub m)h(sup 2) = 0.135(sup +0.008, sub -0.009) the baryon density is Omega(sub b)h(sup 2) = 0.0224 +/- 0.0009, and the total mass-energy of the universe is Omega(sub tot) = 1.02 +/- 0.02. There is progressively less fluctuation power on smaller scales, from WMAP to fine scale CMB measurements to galaxies and finally to the Ly-alpha forest. This is accounted for with a running spectral index, significant at the approx. 2(sigma) level. The spectral index of scalar fluctuations is fit as n(sub s) = 0.93 +/-0.03 at wavenumber k(sub o) = 0.05/Mpc ((sub eff) approx. = 700), with a slope of dn(sub s)/d I(sub nk) = -0.031(sup + 0.016, sub -0.018) in the best-fit model.

Bennett, C. L.↗

NASA NTRS1995

Radio interferometer observations of solar wind turbulence from the orbit of Helios to the solar corona

We report observations of Very Long Baseline Interferometer (VLBI) phase scintillations due to turbulence in the solar wind. The observations were made at 5.00 and 8.42 GHz with the Very Long Baseline Array (VLBA) on three dates in 1991 July and August. We observed the sources 0851 + 202 and 0735 + 178 at solar elongations ranging from 2.66 deg to 13.29 deg; the closest approach of the line of sight to the Sun ranged from 10 to 49.8 solar radii. We have also included previously unpublished 5 GHz VLBI phase scintillation measurements from 1989. These measurements probe solar wind density fluctuations on spatial scales from about 200 km to 2000 km. Our measurements are in quite good agreement with the Coles & Harmon model for the radio phase structure function, which was largely determined from observations on both shorter and larger spatial scales. Departures from the Coles & Harmon functions are attributable to day-to-day variations in the solar wind conditions. Phase scintillations at the greatest solar elongations are in very good agreement with extrapolated estimates from direct measurements made with the Helios spacecraft at slightly larger heliocentric distances. Thus there is a consistency between the in-situ spacecraft and radio sensing measurements of density turbulence. All of the VLBI data are consistent with a Kolmogorov spectrum for the density fluctuations, although at the closest elongations there may be excess power at small spatial scales. An advantage of interferometric techniques over other radio propagation measurements is that they provide a measure of the anisotropy of the irregularities. Our observations at closest approach (10 solar radii) show weak evidence for anisotropic, field-aligned density irregularities with an axial ratio of order 2. This degree of anisotropy would appear to be less than that measured at similar solar elongations but on smaller spatial scales by Armstrong and colleagues. Finally, a combination of the radio propagation data and in situ Helios measurements is used to determine the heliocentric distance dependence of the normalization coefficient of the density power spectrum, C(sup 2)(sub N). Modeling C(sup2)(sub N) varies as (R/solar radii)(sup -Kappa), we find Kappa = 3.72 +/- 0.30. Subject to a number of assumptions, this result is consistent with the conclusion of D. A. Roberts that turbulence within 0.28 astronomical units adheres to a WKB formula for turbulence amplitude as a function of heliocentric distance.

Spangler, Steven R.↗

NASA NTRS1985

Large-scale spectral structure with a gap in the stably stratified atmosphere

The analysis of the large-scale structure of turbulence in a stratified atmosphere is important for the prediction and modelling of turbulence. The Monin-Obukhov similarity consideration is well suited for the modelling of small-scale turbulence. But the large-scale turbulence possesses new features and parameters, as connected with the appearance of a spectral gap in the stably stratified atmosphere, for the understanding of which an analytical investigation is needed. By a group-kinetic method, the anisotropic form of the transfer function and its governing eddy viscosity are derived. On this basis, the spectral laws k to the -5/3rd, k to the -1st, k to the -3rd are obtained for the subranges of inertia, shear, and buoyancy, respectively. A gap connects the last two subranges by the variable anisotropy, as a result of the fall of the spectrum of the vertical velocity fluctuations. The variable anisotropy causes the deviations from the spectral laws k to the -1st and k to the -3rd. An experiment supports the analytical prediction.

Larsen, S. E.↗

NASA NTRS1991

Paramagnetic resonance and susceptibility of ilmenite, FeTiO3 crystal

Large high-purity single crystals of FeTiO3 with ilmenite structure have been grown from a stoichiometric melt of Fe2O3 and TiO2 under an inert atmosphere using the modified Czochralski technique. Susceptibility and X-band paramagnetic resonance studies have been performed. Susceptibility measurements indicate a Neel temperature of about 59 K. The paramagnetic resonance spectrum for magnetic field perpendicular to the crystal c axis consists of a portion of a single, very intense approximately Lorentzian absorption line with its peak at about 600 G and half width at half maximum almost 1200 G. The absorption extends to zero magnetic field. For magnetic field approximately parallel to the c axis, the paramagnetic absorption is much smaller and may be considered a superposition of two approximately Lorentzian line shapes. The magnetic resonance measurements indicate a weak temperature dependence and large angular anisotropy.

Mcdonald, P. F.↗

NASA NTRS1978

Z-rich solar particle event characteristics 1972-1976

It is found in the reported investigation that Z-rich solar particle events usually have large and prolonged anisotropies in addition to an extremely variable charge composition that varies not only from event to event but also throughout the event. These observations suggest that one can no longer regard the event-averaged composition of solar particle events at low energies as providing an unbiased global sample of the solar atmospheric composition. The variability from event to event and among classes of events is just too great. However, the tendency for the Z-rich events to be associated with both the low-speed solar wind at or just before the onset of solar wind streams and with active regions located in the western hemisphere, indicates that charge composition studies of solar particle events can yield a better knowledge of the flare acceleration process as well as the inhomogeneous nature of magnetic field structure and particle composition in the solar atmosphere.

Zwickl, R. D.↗

NASA NTRS2012

Inner Plasma Structure of the Low-Latitude Reconnection Layer

We report a clear transition through a reconnection layer at the low-latitude magnetopause which shows a complete traversal across all reconnected field lines during northwestward interplanetary magnetic field (IMF) conditions. The associated plasma populations confirm details of the electron and ion mixing and the time history and acceleration through the current layer. This case has low magnetic shear with a strong guide field and the reconnection layer contains a single density depletion layer on the magnetosheath side which we suggest results from nearly field-aligned magnetosheath flows. Within the reconnection boundary layer, there are two plasma boundaries, close to the inferred separatrices on the magnetosphere and magnetosheath sides (Ssp and Ssh) and two boundaries associated with the Alfvén waves (or Rotational Discontinuities, RDsp and RDsh). The data are consistent with these being launched from the reconnection site and the plasma distributions are well ordered and suggestive of the time elapsed since reconnection of the field lines observed. In each sub-layer between the boundaries the plasma distribution is different and is centered around the current sheet, responsible for magnetosheath acceleration. We show evidence for a velocity dispersion effect in the electron anisotropy that is consistent with the time elapsed since reconnection. In addition, new evidence is presented for the occurrence of partial reflection of magnetosheath electrons at the magnetopause current layer.

magnetosheath↗

NASA NTRS1999

Optimal Variable-Structure Control Tracking of Spacecraft Maneuvers

An optimal control approach using variable-structure (sliding-mode) tracking for large angle spacecraft maneuvers is presented. The approach expands upon a previously derived regulation result using a quaternion parameterization for the kinematic equations of motion. This parameterization is used since it is free of singularities. The main contribution of this paper is the utilization of a simple term in the control law that produces a maneuver to the reference attitude trajectory in the shortest distance. Also, a multiplicative error quaternion between the desired and actual attitude is used to derive the control law. Sliding-mode switching surfaces are derived using an optimal-control analysis. Control laws are given using either external torque commands or reaction wheel commands. Global asymptotic stability is shown for both cases using a Lyapunov analysis. Simulation results are shown which use the new control strategy to stabilize the motion of the Microwave Anisotropy Probe spacecraft.

John L. Crassidis↗

NASA NTRS2026

Micromechanical Design of Carbon Nanotube Ribbon Reinforced Polymer Composite Materials

Lightweight materials are an important component of the design of aerospace structures. Carbon nanotube materials have been considered for this purpose due to the strength and stiffness of individual nanotubes, and the commercial availability of bulk formats such as fibers. These fibers can have a ribbon cross section which results in a different design space for their composites relative to traditional reinforcements which have a round cross section. This work applies brick-and-mortar micromechanical models and classical lamination theory with an inverse approach to investigate the design space of these composites. Using this approach, the influences of fiber geometry and axial and transverse mechanical properties are mapped. Finally, a sensitivity study is performed and the relative impacts of ±10% variations in the constituent material and geometric properties are ranked. Lamina axial moduli were found to range from a maximum of 3x to 1x minimum relative to a target quasi-isotropic laminate modulus depending on the anisotropy and shear modulus of the lamina. The fiber targets depended strongly on the fiber volume fraction in the lamina and the fiber axial modulus target was found to range from 4.8x to 3.2x the quasi-isotropic laminate target. The sensitivity analysis found that the largest driver of performance was the volume fraction, followed by the fiber axial modulus. While bio-based brick-and-mortar composites, such as nacre, can benefit from reinforcement aspect ratios above 10, for carbon nanotube ribbon (or carbon fiber)/polymer composites the sensitivity study indicated that the optimal cross-sectional aspect ratio was relatively smaller, potentially less than three.

Micromechanics↗

NASA NTRS1983

Lateral heterogeneity and azimuthal anistropy of the upper mantle: Love and Rayleigh waves 100-250 sec

The lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring Rayleigh and Love wave phase velocities in the period range 100-250 sec. Spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1=m=10. Slow regions are evident at the East Pacific rise, northeast Africa, Tibet, Tasman sea, southwestern North America and triple junctions in the Northern Atlantic and Indian oceans. Fast regions occur in Australia, western Pacific and the eastern Atlantic. Details which are not evident in previous studies include two fast regions in the central Pacific and the subduction zone in the Scotia Arc region. Inversion for azimuthal dependence showed (1) little correlation between the fast phase velocity directions and the plate motion vector in plate interiors, but (2) correlation of the fast direction with the perpendicular direction to trenches and ridges. Phase velocity is high when waves propagate perpendicular to these structures. Severe tradeoffs exist between heterogeneity and azimuthal dependence because of the yet unsatisfactory path coverage.

Tanimoto, T.↗

NASA NTRS1985

Lateral heterogeneity and azimuthal anisotropy of the upper mantle - Love and Rayleigh waves 100-250 sec

Tanimoto, T.↗

NASA NTRS1998

Far Infrared All-Sky Survey

Precise measurements of the angular power spectrum of the Cosmic Microwave Background (CMB) anisotropy will revolutionize cosmology. These measurements will discriminate between competing cosmological models and, if the standard inflationary scenario is correct, will determine each of the fundamental cosmological parameters with high precision. The astrophysics community has recognized this potential: the orbital experiments MAP and PLANCK, have been approved to measure CMB anisotropy. Balloon-borne experiments can realize much of this potential before these missions are launched. Additionally, properly designed balloon-borne experiments can complement MAP in frequency and angular resolution and can give the first realistic test of the instrumentation proposed for the high frequency instrument on PLANCK. The MAXIMA experiment is part of the MAXIMA/BOOMERANG collaboration which is doing balloon observations of the angular power spectrum of the Cosmic Microwave Background from l = 10 to l = 800. These experiments are designed to use the benefits of both North American and Antarctic long-duration ballooning to full advantage. We have developed several new technologies that together allow the power spectrum to be measured with unprecedented combination of angular resolution, beam throw, sensitivity, sky coverage and control of systematic effects. These technologies are the basis for the high frequency instrument for the PLANCK mission. Our measurements will strongly discriminate between models of the origin and evolution of structure in the universe and, for many models, will determine the value of the basic cosmological parameters to high precision.

Richards, Paul L.↗

NASA NTRS1997

Global Kinetic Modeling of Banded Electron Structures in the Plasmasphere

Significant fluxes of 10 eV to 30 keV electrons have been detected in the plasmasphere, appearing as banded structures in energy with broad spatial extents and slowly evolving over several days. It is thought that these populations are decaying plasma sheet electrons injected into the corotating region of near-Earth space. This capture can occur when the convective electric field drops rapidly and the Alfven boundary suddenly outward, trapping the inner edge of the plasma sheet along closed drift paths. Our bounce-averaged kinetic model of superthermal electron transport is able to simulate this capture and the subsequent drift, diffusion, and decay of the plasma cloud. Results of this simulation will be shown and discussed, from the initial injection during the elevated convection to the final loss of the particles. It is thought that not only Coulomb collisions but also wave-particle interactions play a significant role in altering the plasma cloud. Quasilinear diffusion is currently being incorporated into the model and the importance of this mechanism will be examined. Also, the high anisotropy of the trapped population could be unstable and generate plasma waves. These and other processes will be investigated to determine the final fate of the cloud and to quantify where, how, and when the energy of the plasma cloud is deposited. Comparisons with CRRES observations of these events are shown to verify the model and explain the data.

Liemohn, M. W.↗

NASA NTRS1995

Coronal magnetic field diagnostics via the Hanle effect of Lyman series lines

The magnetic field plays a major role in the physics of the solar corona. However, there are no direct measurements of this physical parameter. We describe a method that can provide the most direct determination of the vector magnetic field in the extended corona (i.e., at heliocentric heights between 1.2 R(solar radius) and 2.0 R(solar radius)). The method is based on polarimetric observations of UV lines of the Lyman series, that is, Lyman alpha (Ly-alpha), lambda 1216 A, Lyman beta (Ly-beta), lambda 1025 A, and Lyman gamma (Ly-gamma), lambda 972 A. These lines have a collisional and a resonantly scattered component. Linear polarization is induced in the resonant component by the anisotropy in the chromospheric radiation field that illuminates the corona. Magnetic fields can be suitably determined through the effects that they induce on this resonance polarization (Hanle effect). The Hanle effect of the Ly-alpha is sensitive to field strengths in the 10 - 100 gauss range. The resonance polarization of Ly-beta and Ly-gamma is sensitive, through the Hanle effect, to fields with strengths between 3 - 30 gauss, and 0.3 - 6 gauss, respectively. We describe a new method for separating the resonant from the collisional component of the Ly-beta and Ly-gamma; the method is based on the approximation, valid within 10%, that the collisional component of the Ly-alpha is negligible, in typical coronal conditions. From the intensity and the polarization of the resonant components of these Lyman lines, the strength and direction of coronal fields can be determined. We model the sensitivity of Hanle-effect diagnostics for different coronal structures (e.g., coronal holes and loops).

Fineschi, S.↗