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Results for “Density function theory”

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
NASA NTRS1983

A mathematical characterization of vegetation effect on microwave remote sensing from the Earth

In passive microwave remote sensing of the earth, a theoretical model that utilizes the radiative transfer equations was developed to account for the volume scattering effects of the vegetation canopy. Vegetation canopies such as alfalfa, sorghum, and corn are simulated by a layer of ellipsoidal scatterers and cylindrical structures. The ellipsoidal scatterers represent the leaves of vegetation and are randomly positioned and oriented. The orientation of ellipsoids is characterized by a probability density function of Eulerian angles of rotation. The cylindrical structures represent the stalks of vegetation and their radii are assumed to be much smaller than their lengths. The underlying soil is represented by a half-space medium with a homogeneous permittivity and uniform temperature profile. The radiative transfer quations are solved by a numerical method using a Gaussian quadrature formula to compute both the vertical and horizontal polarized brightness temperature as a function of observation angle. The theory was applied to the interpretation of experimental data obtained from sorghum covered fields near College Station, Texas.

Choe, Y.↗
NASA NTRS2001

Model For Bending Actuators That Use Electrostrictive Graft Elastomers

Recently, it was reported that an electrostrictive graft elastomer exhibits large electric field-induced strain (4%). Combined with its high mechanical modulus, the elastomer can offer very promising electromechanical properties, in terms of output mechanical energy density, for an electroactive polymeric material. Therefore, it has been considered as one of the candidates that can be used in high performance, low mass actuation devices in many aerospace applications. Various bilayer- based bending actuators have been designed and fabricated. An analytic model based on beam theory in the strength of materials has been derived for the transverse deflection, or curvature, and the longitudinal strain of the bi-layer beam. The curvature and strain are functions of the applied voltage and the thickness, width, and Young s modulus of the active and passive layers. The model can be used to optimize the performance of electrostrictive graft elastomer-based actuators to meet the requirements of various applications. In this presentation, optimization and sensitivity studies are applied to the bending performance of such actuators.

Costen, Robert C.↗
NASA NTRS1995

Heavy fermion behavior explained by bosons

Conventional heavy fermion (HF) theories require existence of massive fermions. We show that heavy fermion phenomena can also be simply explained by existence of bosons with moderate mass but temperature dependent concentration below the formation temperature T(sub B), which in turn is close to room temperature. The bosons B(++) are proposed to be in chemical equilibrium with a system of holes h(+): B(++) = h(+) + h(+). This equilibrium is governed by a boson breaking function f(T), which determines the decreasing boson density and the increasing fermion density with increasing temperature. Since HF-compounds are hybridized from minimum two elements, we assume in addition existence of another fermion component h(sub s)(+) with temperature independent density. This spectator component is thought to be the main agent in binding the bosons in analogy with electronic or muonic molecules. Using a linear boson breaking function we can explain temperature dependence of the giant linear specific heat coefficient gamma(T) coming essentially from bosons. The maxima in resistivity, Hall coefficient, and susceptibility are explained by boson localization effects due to the Wigner crystallization. The antiferromagnetic transitions in turn are explained by similar localization of the pairing fermion system when their density n(sub h)(T(sub FL)) becomes lower than n(sub WC), the critical density of Wigner crystallization. The model applies irrespective whether a compound is superconducting or not. The same model explains the occurrence of low temperature antiferromagnetism also in high-T(sub c) superconductors. The double transition in UPt3 is proposed to be due to the transition of the pairing fermion liquid from spin polarized to unpolarized state.

Kallio, A.↗
NASA NTRS1997

Temperature Induced Alpha Factor

Temperature effects can be important in short pulse propagation or in CW operation of VCSELs. In those cases, the optical susceptibility function is, in general, a function of carrier density and plasma and/or lattice temperature(s). Just as there is an alpha factor induced by the density change (density alpha), there are alpha factors induced by lattice or plasma temperature change (temperature alphas). The density alpha factor has been studied quite extensively, while the temperature alpha factors have not been computed for any structure. I will report on my first calculations of alpha factors induced by the plasma and lattice temperature change for GaAs/AlGaAs quantum wells using microscopic theory. Besides, the temperature dependence of the density alpha factor is also considered.

Ning, C. Z.↗
NASA NTRS1994

The effect of wave-particle interactions on the polar winds O(+)

The escape of the polar wind plasma is an important element in the ionosphere-magnetosphere coupling. Both theory and observations indicate that the wave-particle interactions (WPI) play a significant role in the dynamics of ion outflow along open geomagnetic field lines. A Monte Carlo simulation was developed in order to include the effect of the WPI in addition to the factors that are traditionally included in the 'classical' polar wind (i.e. gravity, electrostatic field, and divergence of geomagnetic field lines). The ion distribution function (f(sub j)), as well as the profiles of its moments (density, drift velocity, temperature, etc.) were found for different levels of WPI, that is, for different values of the normalized diffusion rate in the velocity space (D(tilde) (sub j perpendicular). Although the model included O(=), H(+) and eletrons, we presented only the results related to the O(+) ion. We found that (1) both the density and drift velocity of O(+) increased with the WPI strength, and consequently, the O(+) escape flux was enhanced by a factor of up to 10(exp 5), (2) The O(+) ions could be energized up to a few electron volts; (3) for moderate and high levels of WPI D(tilde) (sub perpendicular) (O(+) greater than (tilde) 1, the distribution function f(O(+)) displayed very pronounced conic features at altitudes around 3 R(sub e). Finally, the interplay between the downward body force, the upward mirror force, and the perpendicular heating resulted in the formation of the 'pressure cooker' effect. This phenomena explained some interesting features of our solution, such as, the peak in the O(=) temperature, and the formation of 'ears' and conics for f(O(+)) around 2.5 R(sub e).

Barakat, A. R.↗
NASA NTRS1994

Recovering the real density field of galaxies from redshift space

This paper proposes a number of methods for recovering the true, real-space large-scale density distributions from redshift data sets. On the largest scales, the effects of peculiar velocities is to compress the galaxy clustering along the line of sight and consequently the density gradient in the radial direction is enhanced in comparison with the density gradient perpendicular to the line of sight. We calculate the relation between the density gradients in real and redshift space in the linear regime and show that it is a simple function of the density parameter, Omega. By considering the anisotropy of density gradients in redshift space it is possible, in principle, to obtain information about the value of Omega. Using N-body simulations we examine different methods for correcting the effects of large-scale velocities to obtain the true density distributions. We derive the peculiar velocity field from the redshift data and iterate to place each galaxy at a distance consistent with its measured redshift and its derived velocity. Linear theory, as well as second-order solutions for the peculiar velocity field are tested. The density distributions in low-density regions predicted by both linear theory and second-order theory are similar, and in good agreement with the true density field in the simulation. The use of the second-order corrections improves the prediction of the density field in high-density regions.

Gramann, Mirt↗
NASA NTRS1973

Redistribution of resonance radiation. II - The effect of magnetic fields.

Previously obtained results for scattering of radiation in the presence of collisions are restated in a density matrix formalism which employs an irreducible-tensor description of the radiation field. This formalism is particularly useful for problems associated with radiative transfer theory. The redistribution is then extended to include the effect of a weak magnetic field. By averaging over a finite bandwidth which is on the order of the Doppler width, simplified expressions of physical significance for the scattering in the Doppler core and the Lorentz wings are obtained. Expressions are also obtained for the corresponding source function of radiative transfer theory.

Omont, A.↗
NASA NTRS1986

The statistics of peaks of Gaussian random fields

A set of new mathematical results on the theory of Gaussian random fields is presented, and the application of such calculations in cosmology to treat questions of structure formation from small-amplitude initial density fluctuations is addressed. The point process equation is discussed, giving the general formula for the average number density of peaks. The problem of the proper conditional probability constraints appropriate to maxima are examined using a one-dimensional illustration. The average density of maxima of a general three-dimensional Gaussian field is calculated as a function of heights of the maxima, and the average density of 'upcrossing' points on density contour surfaces is computed. The number density of peaks subject to the constraint that the large-scale density field be fixed is determined and used to discuss the segregation of high peaks from the underlying mass distribution. The machinery to calculate n-point peak-peak correlation functions is determined, as are the shapes of the profiles about maxima.

Bardeen, J. M.↗
NASA NTRS1994

On the specta of X-ray bursters: Expansion and contraction stages

The theory of spectral formation during the explosion and contraction stages of X-ray bursters, which include the effects of Computonization and free-free absorption and emission, is described. Analytical expressions are provided for color ratios, and the spectral shape is given as a function of input parameters, elemental abundance, neutron star mass and radius, and Eddington ratio. An Eulerian calculation is used to determine the photospheric evolution accurately during the Eddington luminosity phase. The developed analytical theory for hydrodynamics of the expansion takes into account the dependence of Compton scattering opacity on electron temperature. An analytical expression is derived from the sonic point position and the value of the sonic velcoity. Using this value as a boundary condition at the sonic point, the velocity, density, and temperature profile are calculated throughout the whole photosphere. It is shown that the atmopsphere radiates spectra having a low-energy power-law shape and blackbody-like hard tail. In the expansion stage the spectra depend strongly on the temperature of the helium-burning zone at the neutron star surface. The X-ray photosheric radius increases to approximately 100 km or more, depending on the condition of the nuclear burning on the surface of the neutron star in the course of the expansion.

Titarchuk, Lev↗
NASA NTRS1999

Dark Matter in the Universe and in the Galaxy

During the past four years, Prof. Kamionkowski and collaborators have made progress in research on the nature and distribution of dark-matter in the Universe and in the Galaxy, and on related topics in astrophysics and cosmology. We have made progress on research on the cosmic microwave background, large-scale structure, issues related to particle dark matter, and the gamma-ray-burst enigma. A significant fraction of the research supported by this ATP has been on the cosmic microwave background (CMB). Prof. Kamionkowski and collaborators showed how the polarization of the CMB could be used to detect long-wavelength gravitational waves, such as those produced by inflation. With Kosowsky, Prof. Kamionkowski calculated the amplitude of a stochastic gravitational-wave background that could be detected for a satellite experiment of a given sensitivity and angular resolution. They showed that polarization should improve the sensitivity oa MAP to these gravity waves, and that the Planck Surveyor should do even better. Prof. Kamionkowski, Caldwell, and a student calculated and illustrated the CMB temperature/polarization pattern produced by a single plane-wave gravitational wave. They calculated the amplitude of such a wave that would be detectable with MAP and Planck, and compared that with the sensitivity of traditional gravitational-wave detectors like LIGO and LISA. With Lue and Wang, the PI showed how parity violation from new high-energy physics could conceivably give rise to an observable signature in the CMB polarization. With Loeb, Prof. Kamionkowski showed how measurement of the polarization of CMB photons scattered by hot gas in a cluster could be used to determine the quadrupole moment of the CMB incident on that cluster. Prof. Kamionkowski and Jaffe calculated the amplitude of secondary anisotropies produced by scattering of CMB photons from reionized regions. Research has also been carried out on probing the large-scale distribution of mass in the Universe today, and on structure-formation theories. They investigated the possibility of determining the large-scale distribution of mass in the Universe via measurement of ellipticity-ellipticity correlations in the FIRST radio survey induced by weak gravitational lensing due to mass inhomogeneities along the line of sight. Dr. Summers, Prof. Kamionkowski, and a student investigated the distribution of protogalactic masses and angular momenta in an effort to understand how the luminosity function and angular-momentum distribution of disk galaxies arises from an initial power spectrum of density perturbations. Several projects related to the distribution and possible detection of dark matter in our Galactic halo were studied. The PI was involved in several projects involving the calculation of nuclear-reaction rates needed for stellar evolution and for predictions of solar-neutrino fluxes.

Kamionkowski, Marc↗
NASA NTRS2002

Effect of surface roughness on liquid property measurements using mechanically oscillating sensors

The resonant frequency and quality factor Q of a liquid immersed magnetoelastic sensor are shown to shift linearly with the liquid viscosity and density product. Measurements using different grade oils, organic chemicals, and glycerol-water mixtures show that the surface roughness of the sensor in combination with the molecular size of the liquid play important roles in determining measurement sensitivity, which can be controlled through adjusting the surface roughness of the sensor surface. A theoretical model describing the sensor resonant frequency and quality factor Q as a function of liquid properties is developed using a novel equivalent circuit approach. Experimental results are in agreement with theory when the liquid molecule size is larger than the average surface roughness. However, when the molecular size of the liquid is small relative to the surface roughness features molecules are trapped, and the trapped molecules act both as a mass load and viscous load; the result is higher viscous damping of the sensor than expected. c2002 Elsevier Science B.V. All rights reserved.

Non-NASA Center↗
NASA NTRS1996

Heat Capacity of Superfluid (sup 4)He in the Presence of a Heat Current Near T

The thermodynamic theory of superfluid helium in the presence of a heat current is presented. We show that there is a thermodynamic relation between the heat capacity and the expression ps(W), which describes the depression of the superfluid density with the counterflow velocity W. Using this relation we show that the heat capacity of superfluid super4He in the presence of a heat current diverges at a depressed lambda transition temperature, suggesting the possibility of a new second order phase transition where the superfluid wave function is not the order parameter.

superfluid helium thermodynamic↗
NASA NTRS1972

Compressible boundary layer with normal pressure gradients: Quasi-similarity equations - Their properties at the wall and at sharp and blunt leading edges.

The failure of most viscous-inviscid interaction methods at strong interactions is attributed to the presence of a normal pressure gradient. A new theory is proposed for supersonic laminar boundary layers that can generate normal pressure gradients. The Navier-Stokes equations are reexamined by an order of magnitude analysis and all first and second order terms are retained. The approximation is found to be dependent not only on the boundary layer thickness but also on the ratio of the dimensionless viscosity and density. The equations are transformed into two quasi-similar, nonlinear, third order, ordinary integro-differential equations for the velocity and pressure as functions of a single transverse variable. The properties of the equations at the boundaries are discussed.

Seginer, A.↗
NASA NTRS1974

An analytic method to account for drag in the Vinti Satellite theory

To retain separability in the Vinti theory of earth satellite motion when a nonconservative force such as air drag is considered, a set of variational equations for the orbital elements are introduced, and expressed as functions of the transverse, radial, and normal components of the nonconservative forces acting on the system. In this approach, the Hamiltonian is preserved in form, and remains the total energy, but the initial or boundary conditions and hence the Jacobi constants of the motion advance with time through the variational equations. In particular, the atmospheric density profile is written as a fitted exponential function of the eccentric anomaly, which adheres to tabular data at all altitudes and simultaneously reduced the variational equations to indefinite integrals with closed form evaluations. The values of the limits for any arbitrary time interval are obtained from the Vinti program.

Watson, J. S.↗
NASA NTRS1983

Prediction of an axisymmetric combusting flow

A numerical model for turbulent, recirculating combusting flow is developed and applied to a research combustor. The model is based on the time-averaged Navier-Stokes equations with k-epsilon turbulence closure and the compositional fluctuations at each point are given probabilistically in terms of the mixture fraction. The probability density function is derived from transport equations for its first two moments along with an assumption regarding its shape. The resulting equations are solved using a standard line relaxation algorithm. It is found that the predictions of the model are in good agreement with data from an axisymmetric, bluff-body stabilized research combustor, while the major discrepancies are similar to those found in isothermal flow comparisons. The peculiar features of this flow which contribute to the errors are examined. The agreement between the theory and data deteriorates as the central jet velocity is increased which indicates an enhanced role for unsteady effects.

Correa, S. M.↗
NASA NTRS2006

The Dissipation Mechanism in Collisionless Magnetic Reconnection

The dissipation mechanism of magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as PIC simulations of guide-field magnetic reconnection. We will show that the thermal electron inertia-based dissipation mechanism, expressed through nongyrotropic electron pressure tensors, remains viable in three dimensions. We will demonstrate the thermal inertia effect through studies of electron distribution functions. Furthermore, we will show that the reconnection electric field provides a transient acceleration on particles traversing the inner reconnection region. This inertial effect can be described as a diffusion-like term of the current density, which matches key features of electron distribution functions.

Hesse, Michael↗
NASA NTRS2008

The Electron Diffusion Region: Forces and Currents

The dissipation mechanism of magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as PIC simulations of guide-field magnetic reconnection. We will show that the thermal electron inertia-based dissipation mechanism, expressed through nongyrotropic electron pressure tensors, remains viable in three dimensions. We will demonstrate the thermal inertia effect through studies of electron distribution functions. Furthermore, we will show that the reconnection electric field provides a transient acceleration on particles traversing the inner reconnection region. This inertia1 effect can be described as a diffusion-like term of the current density, which matches key features of electron distribution functions.

Hesse, Michael↗
NASA NTRS2009

What Turns Reconnection On and Off

The dissipation mechanism of magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as PIC simulations of guide-field magnetic reconnection. We will show that the thermal electron inertia-based dissipation mechanism, expressed through nongyrotropic electron pressure tensors, remains viable in three dimensions. We will demonstrate the thermal inertia effect through studies of electron distribution functions. Furthermore, we will show that the reconnection electric field provides a transient acceleration on particles traversing the inner reconnection region. This inertial effect can be described as a diffusion-like term of the current density. which matches key features of electron distribution functions.

Hesse, Michael↗