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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Stochastic modal velocity field in rough-wall turbulence

Stochastically generated instantaneous velocity profiles are used to reproduce the outer region of rough-wall turbulent boundary layers in a range of Reynolds numbers extending from the wind tunnel to field conditions. Each profile consists in a sequence of steps, defined by the modal velocities and representing uniform momentum zones (UMZs), separated by velocity jumps representing the internal shear layers. Height-dependent UMZ is described by a minimal set of attributes: thickness, mid-height elevation, and streamwise (modal) and vertical velocities. These are informed by experimental observations and reproducing the statistical behaviour of rough-wall turbulence and attached eddy scaling, consistent with the corresponding experimental datasets. Sets of independently generated profiles are reorganized in the streamwise direction to form a spatially consistent modal velocity field, starting from any randomly selected profile. The operation allows one to stretch or compress the velocity field in space, increases the size of the domain and adjusts the size of the largest emerging structures to the Reynolds number of the simulated flow. By imposing the autocorrelation function of the modal velocity field to be anchored on the experimental measurements, we obtain a physically based spatial resolution, which is employed in the computation of the velocity spectrum, and second-order structure functions. The results reproduce the Kolmogorov inertial range extending from the UMZ and their attached-eddy vertical organization to the very-large-scale motions (VLSMs) introduced with the reordering process. The dynamic role of VLSM is confirmed in the –u'w' co-spectra and in their vertical derivative, representing a scale-dependent pressure gradient contribution.

42 ENGINEERING↗
NASA NTRS1982

Stationarity of magnetohydrodynamic fluctuations in the solar wind

Solar wind research and studies of charged particle propagation often assume that the interplanetary magnetic field represents a stationary random process. The extent to which ensemble averages of the solar wind magnetic fields follow the asymptotic behavior predicted by the ergodic theorem was investigated. Several time periods, including a span of nearly two years, are analyzed. Data intervals which span many solar rotations satisfy the conditions of weak stationarity if the effects of solar rotation are included in the asymptotic analysis. Shorter intervals which include a small integral number of interplanetary sectors also satisfy weak stationarity. The results are illustrated using magnetometer data from the ISEE-3, Voyager and IMP spacecraft.

Matthaeus, W. H.↗
NASA NTRS1972

Random response of rectangular panels to the pressure field beneath a turbulent boundary layer in subsonic flows

The response of a rectangular panel under the excitation of a turbulent boundary layer with a zero longitudinal mean pressure gradient in a subsonic flow was studied in detail. The method of normal mode was used together with the technique of spectral analysis. Both simply supported and clamped edge conditions of a panel were considered, and the displacement power spectral density of the panel response was computed. The results for the clamped edge panel compare favorably with existing data. Charts of structural acceptance, which provide a framework for estimating the response of other rectangular panels, are presented, and the physical significances of structural acceptances discussed.

Chyu, W. J.↗
NASA NTRS1980

Limits on the possible intrinsic magnetic field of Venus

Magnetic field measurements obtained by the Pioneer Venus orbiter at low altitudes in the solar wind wake region are examined for possible surface-correlated features and any possible intrinsic magnetic moment. The field variations observed in the wake do not resemble those expected for a solar wind interaction with even a weak intrinsic magnetic field. Little orbit-to-orbit persistence of features is found in the magnetic records. The magnetic field measurements in the wake are averaged in 10 deg x 10 deg bins to minimize the effects of external field sources. In these 37 bins, the average fields appear to be randomly oriented and consistent with zero mean in the region mapped. Using these 37 averaged vector fields, a maximum intrinsic magnetic dipole moment is obtained of 4.3 + or - 2.0 x 10 to the 21st G cu cm, approximately an order of magnitude less than previous estimates. It is noted that a more conservative estimate of the probable error of the mean is 5.5 x 10 to the 21st G cu cm. The Pioneer Venus measurements are thus consistent with zero planetary moment. The present measurements are found to be far below estimates made on the basis of angular momentum, the so-called magnetic Bode's law, and far below the dynamo scaling law of Busse.

Russell, C. T.↗
NASA NTRS1984

Scattering of electromagnetic waves from a randomly perturbed quasiperiodic surface

Electromagnetic-wave scattering by a quasi-periodic surface with random perturbations (as in the remote sensing of plowed fields) is investigated analytically, applying the Kirchhoff approximation and modeling the plowed fields by means of Gaussian random variation, sinusoidal variation, and Gaussian random variation about the spatial frequency. Coherent and incoherent bistatic scattering coefficients are derived in closed form by evaluating the physical-optics integral and shown to be proportional, in the geometric-optics limit, to the occurrence probability of slopes which reflect the incident wave specularly in the direction of the scattered wave. Backscattering cross sections are plotted as functions of incidence angle for a number of cases, demonstrating the strong effect of row direction.

Shin, R. T.↗
NASA NTRS1987

Small-scale energy storage and release as the cause of the stellar X-ray corona

It has yet to be established why ordinary stars possess X-ray coronas. Detailed observations of the Sun make it doubtful that the X-ray corona is produced by the dissipation of Alfven waves. The uniform X-ray brightness over all scales from 10,000 to 100,000 km simply does not look like a resonance phenomenon. It is demonstrated that the arbitrary winding patterns introduced into the bipolar magnetic fields of X-ray corona produce discontinuities within the field. It is suggested that the random motions of the footprints of the field cause the accumulation of internal strains in the field, which dissipate through neutral reconnection across the associated discontinuities to provide the primary heat source for the X-ray corona. It is emphasized, however, that the essential high resolution observations of the Sun have yet to be carried out, and until the theory is firmly established for the Sun, it cannot be known how to interpret the X-ray emission of the Sun or of the other stars.

Parker, E. N.↗
DOE OSTI2024

Long-term Evolution of Relativistic Unmagnetized Collisionless Shocks

Abstract We study a relativistic collisionless electron–positron shock propagating into an unmagnetized ambient medium using 2D particle-in-cell simulations of unprecedented duration and size. The shock generates intermittent magnetic structures of increasingly larger size as the simulation progresses. Toward the end of our simulation, at around 26,000 plasma times, the magnetic coherence scale approaches λ ∼ 100 plasma skin depths, both ahead and behind the shock front. We anticipate a continued growth of λ beyond the time span of our simulation, as long as the shock accelerates particles to increasingly higher energies. The post-shock field is concentrated in localized patches, which maintain a local magnetic energy fraction ε B ∼ 0.1. Particles randomly sampling the downstream fields spend most of their time in low field regions ( ε B ≪ 0.1) but emit a large fraction of the synchrotron power in the localized patches with strong fields ( ε B ∼ 0.1). Our results have important implications for models of gamma-ray burst afterglows.

Astronomy & Astrophysics↗
DOE OSTI2020

Sparse and Random Sampling Techniques for High-Resolution, Full-Field, BSS-Based Structural Dynamics Identification from Video

Video-based techniques for identification of structural dynamics have the advantage that they are very inexpensive to deploy compared to conventional accelerometer or strain gauge techniques. When structural dynamics from video is accomplished using full-field, high-resolution analysis techniques utilizing algorithms on the pixel time series such as principal components analysis and solutions to blind source separation the added benefit of high-resolution, full-field modal identification is achieved. An important property of video of vibrating structures is that it is particularly sparse. Typically video of vibrating structures has a dimensionality consisting of many thousands or even millions of pixels and hundreds to thousands of frames. However the motion of the vibrating structure can be described using only a few mode shapes and their associated time series. As a result, emerging techniques for sparse and random sampling such as compressive sensing should be applicable to performing modal identification on video. This work presents how full-field, high-resolution, structural dynamics identification frameworks can be coupled with compressive sampling. The techniques described in this work are demonstrated to be able to recover mode shapes from experimental video of vibrating structures when 70% to 90% of the frames from a video captured in the conventional manner are removed.

47 OTHER INSTRUMENTATION↗
NASA NTRS1975

Interplanetary magnetic fields, their fluctuations, and cosmic ray variations

The cause of Forbush decreases is examined using neutron monitor data and measurements of the interplanetary magnetic field. It is found that for the period examined (Dec. 15, 1965 to April 23, 1966) large enhancements of the interplanetary magnetic field correlate well with decreases in cosmic ray intensity, while various parameters connected with the fluctuations in the field do not display such good correlation. The inference is drawn that Forbush decreases are not related to the turbulence or random motions in the field but to the large scale features of the field.

Barouch, E.↗
DOE OSTI2020

Dynamically controlled random lasing with colloidal titanium carbide MXene

Control of lasing properties through tailorable and dynamically tunable materials and reconfigurable compositions can augment the performance of random lasers for a wide range of applications. Here, a colloid of randomly dispersed weakly scattering single-layer titanium carbide (Ti 3 C 2 T x ) MXene flakes embedded within rhodamine 101 gain medium is experimentally shown to provide feedback for random lasing. Additionally, in contrast to previously reported random laser systems where the optical properties of scatterers are static, the relative permittivity of Ti 3 C 2 T x MXene flakes can be varied under optical pumping due to the saturable absorption properties. Numerical simulations indicate that the observed nonlinear response of Ti 3 C 2 T x MXene flakes enables dynamically tunable random lasing. Thus, pumping the Ti 3 C 2 T x MXene flakes with a second optical source decreases the gain threshold required to obtain random lasing. Also, using numerical simulations, it is shown that the control over the intensity of the second pump enables tuning the field distribution of the random lasing modes. Considering the diversity of the MXenes family, the proposed MXene colloidal metamaterial design opens up a new avenue to advanced control of lasing properties for photonic applications.

36 MATERIALS SCIENCE↗
NASA NTRS2019

Electromagnetic Scattering by Discrete Random Media. I: The Dispersion Equation and the Configuration-Averaged Exciting Field

We consider the scattering of a plane electromagnetic wave obliquely incident on a plane-parallel layer of discrete random medium with non-scattering boundaries. We solve the Lax integral equation for the conditional configuration-averaged exciting field coefficients by assuming a special-form solution, that is, by representing the conditional configuration-averaged exciting field coefficients as a linear combination of the coefficients corresponding to an up-going and a down-going wave. This solution representation is supposed to be valid within the whole domain occupied by the particles, even in the close proximity of the boundaries. By balancing the waves with different propagation directions and wavenumbers we derive two homogeneous systems of equations corresponding to the generalized Lorenz–Lorentz law and two inhomogeneous systems of equations corresponding to the generalized Ewald–Oseen extinction theorem. It is shown that (i) the two homogeneous systems of equations of the generalized Lorenz–Lorentz law reduce to a single homogeneous system of equations corresponding to a semi-infinite discrete random medium at normal incidence; (ii) the dispersion equation is direction and polarization independent; and (iii) the two inhomogeneous systems of equations of the generalized Ewald–Oseen extinction theorem can be reduced to two scalar equations by means of the addition theorem for vector spherical harmonics. It is also shown that the same dispersion equation can be obtained without assuming a special-form solution representation in the proximity of the boundaries.

Adrian Doicu↗
NASA NTRS1982

Crop classification using airborne radar and Landsat data

NASA 13.3 GHz airborne radar data from a soil moisture measurement analysis is used to investigate the statistical nature of the radar backscattering coefficient for bare ground and three different crop types, and to evaluate the crop classification rates using Landsat data alone or combined with the airborne survey. The scatterometer was a fan-beam Doppler system, VV polarized, and is considered only for 50 deg angles of incidence. A total of 36 fields were covered a week apart by the aircraft and Landsat, and Rayleigh statistics were used in the frequency averaging to eliminate fluctuations due to random fluctuations. Within-field variances were calculated for the Landsat and the radar data and used to design optimum crop classification procedures. The Landsat Band 4 readings were 67% accurate, and an increase in accuracy of 10% was achieved by the addition of the radar data.

Ulaby, F. T.↗
NASA NTRS1985

Channeled propagation of solar particles

Bartley (1966) and McCracken and Ness (1966) identified bundles of interplanetary magnetic field (IMF) lines that differed in direction from the interplanetary field lines in which they were imbedded. These bundles, called filaments differed in direction by as much as several tens of degrees from the surrounding field. The filaments werre first noticed due to the large and sudden change in flow direction of highly anisotropic solar flare protons in the energy range 1 to 13 MeV. Passage of the filaments over the spacecraft required a few hours, implying a diameter for the filaments of approximately 3 x 10 to the 6th power km at a distance of 1 AU from the Sun. In 1968, Jakipii and Parker used Leighton's hypothesis of random walk of magnetic field lines associated with granules and supergranules (1964) to develop a picture of an interplanetary medium composed of a tangle of field lines frozen into the solar wind, but whose feet were carried about by the random motions at the solar surface. Jakipii and Parker noted that using a correlation length of 15,000 km - about the radius of a supergranule - the magnetic structure would be 3 x 10 to the 6th power km in size of the filaments as determined by Bartley and McCracken and Ness. These workers did not find changes in the solar particle intensity, anisotropy ratio or energy spectrum as the spacecraft entered the filament.

Anderson, K. A.↗
NASA NTRS2011

Direct Simulation of Multiple Scattering by Discrete Random Media Illuminated by Gaussian Beams

The conventional orientation-averaging procedure developed in the framework of the superposition T-matrix approach is generalized to include the case of illumination by a Gaussian beam (GB). The resulting computer code is parallelized and used to perform extensive numerically exact calculations of electromagnetic scattering by volumes of discrete random medium consisting of monodisperse spherical particles. The size parameters of the scattering volumes are 40, 50, and 60, while their packing density is fixed at 5%. We demonstrate that all scattering patterns observed in the far-field zone of a random multisphere target and their evolution with decreasing width of the incident GB can be interpreted in terms of idealized theoretical concepts such as forward-scattering interference, coherent backscattering (CB), and diffuse multiple scattering. It is shown that the increasing violation of electromagnetic reciprocity with decreasing GB width suppresses and eventually eradicates all observable manifestations of CB. This result supplements the previous demonstration of the effects of broken reciprocity in the case of magneto-optically active particles subjected to an external magnetic field.

Mackowski, Daniel W.↗
NASA NTRS2017

Micromechanics of Composite Materials Governed by Vector Constitutive Laws

The high-fidelity generalized method of cells micromechanics theory has been extended for the prediction of the effective property tensor and the corresponding local field distributions for composites whose constituents are governed by vector constitutive laws. As shown, the shear analogy, which can predict effective transverse properties, is not valid in the general three-dimensional case. Consequently, a general derivation is presented that is applicable to both continuously and discontinuously reinforced composites with arbitrary vector constitutive laws and periodic microstructures. Results are given for thermal and electric problems, effective properties and local field distributions, ordered and random microstructures, as well as complex geometries including woven composites. Comparisons of the theory's predictions are made to test data, numerical analysis, and classical expressions from the literature. Further, classical methods cannot provide the local field distributions in the composite, and it is demonstrated that, as the percolation threshold is approached, their predictions are increasingly unreliable. XXXX It has been observed that the bonding between the fibers and matrix in composite materials can be imperfect. In the context of thermal conductivity, such imperfect interfaces have been investigated in micromechanical models by Dunn and Taya (1993), Duan and Karihaloo (2007), Nan et al. (1997) and Hashin (2001). The present HFGMC micromechanical method, derived for perfectly bonded composite materials governed by vector constitutive laws, can be easily generalized to include the effects of weak bonding between the constituents. Such generalizations, in the context of the mechanical micromechanics problem, involve introduction of a traction-separation law at the fiber/matrix interface and have been presented by Aboudi (1987), Bednarcyk and Arnold (2002), Bednarcyk et al. (2004) and Aboudi et al. (2013) and will be addressed in the future.

micromechanics↗
NASA NTRS1996

Chain Dynamics in a Dilute Magnetorheological Fluid

The structure, formation, and dynamics of dilute, mono-dispersive ferrofluid emulsions in an external magnetic field have been investigated using dynamic light scattering techniques. In the absence of the magnetic field, the emulsion particles are randomly distributed and behave like hard spheres in Brownian motion. An applied magnetic field induces a magnetic dipole moment in each particle. Dipolar interactions between particles align them into chains where correlation functions show two decay processes. The short-time decay shows the motion of straight chains as a whole where the apparent chain length increases with the applied magnetic field and the particle volume fraction. Good scaling results are obtained showing that the apparent chain length grows with time following a power law with exponent of 0.6 and depends on the applied field, particle volume fraction, and diffusion constant of the particles. The long-time decay in the correlation function shows oscillation when the chains reach a certain length with time and stiffness with threshold field This result shows that chains not only fluctuate, but move in a periodic motion with a frequency of 364 Hz at lambda = 15. It may suggest the existence of phonons. This work is the first step in the understanding of the structure formation, especially chain coarsening mechanism, of magnetorheological (MR) fluids at higher volume fractions.

Liu, Jing↗