Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “time dependent behavior”

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.

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

At least 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Secondary frequencies in the wake of a circular cylinder with vortex shedding

A detailed numerical study of two-dimensional flow past a circular cylinder at moderately low Reynolds numbers was conducted using three different numerical algorithms for solving the time-dependent compressible Navier-Stokes equations. It was found that if the algorithm and associated boundary conditions were consistent and stable, then the major features of the unsteady wake were well-predicted. However, it was also found that even stable and consistent boundary conditions could introduce additional periodic phenomena reminiscent of the type seen in previous wind-tunnel experiments. However, these additional frequencies were eliminated by formulating the boundary conditions in terms of the characteristic variables. An analysis based on a simplified model provides an explanation for this behavior.

Abarbanel, Saul S.↗
NASA NTRS1991

Breaking of the first adiabatic invariants of charged particles in time-dependent magnetic fields - Computer simulations and theory

The mechanics of the first adiabatic invariant mu of nonrelativistic charged particles in time-dependent magnetic inductions B (t) are studied by means of computer simulations and analytic theory. Linear-ramp magnetic-induction profiles are utilized, as well as hyperbolic-tangent ramps and sine half-wave ramps. The change in mu that results from an induction change Delta B that occurs over a time Delta t is quantified for all values of Delta B and Delta t, as well as for all values of the particle position. It is found that the cases fall into two categories with very different mu behavior: cases in which the change in the magnetic induction occurs over a time Delta t that is exactly equal to an integer number of gyroperiods (textbook case) or cases in which the change in the induction occurs over a time Delta t that is not equal to an integer number of gyroperiods (more general case). In both categories mu is an adiabatic invariant, although the conservation of mu is much poorer in the latter category.

Borovsky, Joseph E.↗
NASA NTRS1991

Secondary frequencies in the wake of a circular cylinder with vortex shedding

A detailed numerical study of two-dimensional flow past a circular cylinder at moderately low Reynolds numbers was conducted using three different numerical algorithms for solving the time-dependent compressible Navier-Stokes equations. It was found that if the algorithm and associated boundary conditions were consistent and stable, then the major features of the unsteady wake were well-predicted. However, it was also found that even stable and consistent boundary conditions could introduce additional periodic phenomena reminiscent of the type seen in previous wind-tunnel experiments. However, these additional frequencies were eliminated by formulating the boundary conditions in terms of the characteristic variables. An analysis based on a simplified model provides an explanation for this behavior.

Abarbanel, Saul S.↗
NASA NTRS1992

Postexplosion hydrodynamics of SN 1987A

A comprehensive study is presented of the postexplosion hydrodynamics of the ejecta of SN 1987A encompassing the Rayleigh-Taylor instabilities at the shell interfaces and the dynamical effects of the energy input due to the radioactive decay of Ni-56 and Co-56. The dependence of the hydrodynamical behavior on progenitor structure, initial perturbations, explosion energy, and dimensionality of the numerical simulation is investigated in order to explain the observed high-velocity wings of the iron line of SN 1987A. It is shown that none of these factors leads to the required high-velocity tail of iron. When nickel is artificially premixed in the initial model, it is found that this early mixing has to affect the inner 1.5 solar mass of the ejecta to reproduce the high-velocity wings at a time of 90 d. It is argued that the high-entropy bubble associated with the delayed mechanism is responsible for this premixing by driving convective motions early in the explosion.

Herant, Marc↗
NASA NTRS2000

On the Relationship of Multiple Solutions in Tropospheric Photochemistry to Observable Phenomena

Multiple steady-state solutions are a fairly robust feature of simplified models of tropospheric photochemistry and have been reported for a range of different modeling assumptions. Multiple solutions occur through a bifurcation as changes in a control parameter induce a transition from low to high NO(x) conditions. The usual control parameters are the sources of NO and of CO, CH4 or non-methane hydrocarbons. Typically, with increasing NO source, bifurcations occur at NO(x). concentrations that are higher than would be expected of even heavily polluted conditions. However, there are other ways of inducing a low NO(x) - high NO(x) transition. In this paper the primary control parameter is solar zenith angle. This is varied throughout the year by computing noontime steady states on successive days. Background NO(x), varied by assuming different NO(x) sources values, is used as a secondary control parameter. It is found that bifurcations can occur from high to low NO(x) conditions, for reasonable background NO(x) levels, as the model progresses through spring and then from low to high NO,, during the progression through fall. A time dependent version of this model has been run for the same parameter values as in the steady state runs. This shows rapid spring and fall transitions between high and low NO(x) states. H2O2, for example, rises from sub-ppb levels to about 2 ppb over a five day period in spring and declines quickly, but less precipitously, in fall. This study supports the suggestions that the rapid change in peroxide concentrations between summer and winter conditions may be understood as a manifestation of different underlying steady-state behavior.

Stewart, Richard W.↗
DOE OSTI2019

Typical occupancy profiles and behaviors in residential buildings in the United States

The energy performance of a residential building is highly dependent on occupant's presence or non-presence in a building and their interactions with energy-consuming appliances. Typical occupancy schedules for residential buildings must be defined for applications such as building energy modeling as well as for assessing energy savings associated with the use of occupancy sensing technologies and occupancy-dependent controls. Currently, commonly used simulation programs assume a typical occupancy schedule, however, there is significant opportunity for improvement to these schedules as this is generally based on engineering judgement. This research uses 12 years of the American Time Use Survey (ATUS) data to develop typical occupancy schedules for a range of household types and occupant age ranges. This is compared to currently utilized residential occupancy schedules. In many cases the developed schedules exhibit similar patterns, however, differences are also found to be as high as 41% for certain periods of time. For occupancy sensing applications, the spatial-temporal distribution of occupants in residential buildings is also evaluated. These locations vary based on temporal factors as well as demographic factors such as age and number of occupants. Finally, the results of this research work towards improved occupancy schedule development can benefit both industry professionals and researchers.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2022

Universality aspects of quantum corrections to transverse momentum broadening in QCD media

We study non-linear quantum corrections to transverse momentum broadening (TMB) of a fast parton propagating in dense QCD matter in the leading logarithmic approximation. These non-local corrections yield an anomalous super-diffusive behavior characterized by a heavy tailed distribution which is associated with Lévy random walks. Using a formal analogy with the physics of traveling waves, we show that at late times the transverse momentum distribution tends to a universal scaling regime. We derive analytic solutions in terms of an asymptotic expansion around the scaling limit for both fixed and running coupling. We note that our analytic approach yields a good agreement with the exact numerical solutions down to realistic values of medium length. Finally, we discuss the interplay between system size and energy dependence of the diffusion coefficient $\hat{q}$ and its connection with the gluon distribution function that is manifest at large transverse momentum transfer.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Interactions of clathrate hydrate promoters sodium dodecyl sulfate and tetrahydrofuran investigated using 1H diffusion nuclear magnetic resonance at hydrate-forming conditions

Thermodynamic hydrate promoters and kinetic hydrate promoters can be used to reduce the P–T conditions for clathrate hydrate synthesis to decrease the nucleation induction time while increasing growth rates. Two commonly used promoters for hydrate research are tetrahydrofuran (THF) and sodium dodecyl sulfate (SDS), which can increase the overall hydrate promotion when used in tandem as compared to individually. There are several molecular theories regarding how SDS promotes hydrate growth. This study explores the micellular theory, for which hydrate formation depends on surfactant aggregates (micelles) at a critical micelle concentration (CMC) to increase the interfacial surface area. The micellular theory is the most investigated and criticized surfactant hydrate promotion theory. To address questions related to micellar behavior, this study investigates the intermolecular behavior between SDS and THF for the identification of micelles at hydrate-forming conditions. The systems explored contained THF at 3 and 5 wt. % with varying concentrations of SDS below and above the CMC. Several methods including a qualitative visual method, conductivity, interfacial tensiometry, 13C Liquid-state Nuclear Magnetic Resonance (NMR) spectroscopy, and 1H diffusion NMR spectroscopy were evaluated at temperatures below the Krafft point of SDS and above 0 °C. The presence of THF at low concentrations decreased the critical temperature for the formation of SDS micelles, where SDS is solubilized in THF/water solution at hydrate-forming temperatures without precipitation. The CMC of SDS was decreased significantly even at hydrate-forming conditions. Mixed surfactant–cosolvent micellular behavior of SDS in the presence of low concentrations of THF was confirmed at hydrate-forming conditions above 0 °C.

Chemistry↗
DOE OSTI2023

Effect of dielectric target properties on plasma surface ionization wave propagation

Surface ionization waves (SIWs) propagating along dielectric covered, grounded surfaces have been studied for various dielectric bulk and surface conditions; a dependence on the propagation velocity with respect to dielectric electrical thickness and near surface permittivity profiles are observed. SIWs generated by an atmospheric pressure plasma source are imaged interacting with planar dielectric surface. Surface wave velocity is obtained by tracking emission intensity as a function of time. Target dielectric thickness is varied from $d = 0.15-10$ mm and dielectric constant is varied from $\epsilon_r = 6.21 - 9.4$. The propagation of SIWs can be generally predicted by relating their velocity to the RC time constant of the circuit generated between the plasma and the dielectric surface, but it is found that this approximation breaks down for dielectric substrates of sufficient thickness and wave velocity becomes constant. The results show that wave velocity is stable and predictable for target thicknesses beyond a certain point determined by the permittivity of the target material. It is also shown that SIW propagation is strongly driven by the dielectric material near to the surface of the target in addition to the bulk material. The possible mechanisms driving these thickness dependent behaviors is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1990

Spectral behavior of Hercules X-1 - Its long-term variability and pulse phase spectroscopy

Results are presented of the analysis of the broad-energy-band Her X-1 spectra obtained by HEAO 1. Data from 13 pointed observations covered three 35-day cycles in February, August, and September of 1978. The pulse phase-averaged spectrum did not show any apparent variation during the main-on state, except for the overall intensity. The pulse phase-resolved spectrum, on the other hand, showed a moderately strong phase dependence in each of the main-on state pointings. This may imply that the orientation of the emission region is not changing on a monthly time frame, but the geometry of that region may evolve with time.

Soong, Y.↗
NASA NTRS2022

Effect of Sulfur on siderophile element partitioning between olivine and martian mantle primary melt

Ni and Co variations in primary martian magmas exhibit anomalous incompatible behavior, which has remained an unexplained conundrum. Because martian magmas are S-rich, and some trace metals are reported to have enhanced solubility in S-bearing magmas, we have carried out a series of experiments to evaluate the effect of high-S melts on the olivine/melt partitioning of Ni, Co, Mn, V, and Cr. Near-liquidus experiments on a synthetic primary martian mantle melt (Yamato-980459 [Y98]) were completed in a piston-cylinder apparatus at 0.75 GPa. Previous studies in S-free systems illustrate that the partition coefficients for these elements are dependent chiefly on D(Mg(Ol/melt)) (the partition coefficient defined as wt% Mg in olivine/wt% Mg in melt, a proxy for temperature), and were used to calibrate a predictive expression that includes the effects of temperature [i.e., D(Mg(Ol/melt))], melt composition, and oxygen fugacity. These predictive expressions are then used to isolate any effect in DM olivine/melt due to dissolved sulfur. The results show that S might have a small effect for Co, but not enough to change Co partitioning from compatible to incompatible in our experiments. The addition of a sulfur term to the D(Co) predictive expressions shows that nearly 8000 ppm of sulfur would be required in the melt (at liquidus temperature of Y98) for D(Co) to become <1. These S contents are two times higher than those of a sulfide-saturated melt at the P-T conditions of a martian mantle source region. Therefore, the anomalous incompatible behavior observed in these primary magma suites must be due to another mechanism. High temperature, oxygen fugacity, and diffusion are not viable mechanisms, but magma mixing, assimilation, or kinetic crystallization effects remain possibilities.

shergottite↗
DOE OSTI2023

High-fidelity simulations of the run-in process for a pebble-bed reactor

Pebble-bed reactors (PBRs) rely on a continual feed of fuel pebbles being cycled through the core. As a result, they require a “run-in” period in order to reach an equilibrium state. The run-in period for a PBR is a complex, time-dependent problem that requires the injection of new fuel, different types of fuel, and power increases. This complexity in the run-in makes it important to capture the physical processes in order to generate an accurate representation. The present work details the creation of a high-fidelity Monte Carlo methodology for analyzing the run-in and subsequent approach to equilibrium for PBRs. The methodology entails a Python module wrapped around Serpent so as to perform neutronics calculations, move pebbles, refuel the core, and discharge pebbles, thereby modeling the explicit behavior of the PBR run-in. Further, three run-in simulations (a constant temperature profile, a linear temperature profile, and a constant temperature profile using control rods) were examined in order to identify the key physical phenomena present in the run-in process. Utilizing kugelpy, we found the inclusion of a temperature profile to be important for accurately capturing a discharge burnup (around 141 MWd/kg), a consistent k-eff (around 1.005), and an average pebble power (around 2.5 kW/pebble) that all fall within acceptable limits.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2024

Defect-Limited Mobility and Defect Thermochemistry in Mixed A-Cation Tin Perovskites: (CH3NH3)1-xCsxSnBr3

Hybrid organic-inorganic semiconductors crystallizing in the perovskite structure present a significant opportunity for realizing defect-tolerant semiconductors. In this work, we examine the solid solution, (CH3NH3)1-xCsxSnBr3, and identify the thermochemistry dictating the intrinsic carrier concentrations and how local structural distortions influences this electronic behavior. This family of compounds exhibits the expected systematic trend in decreasing optical gap with the cesium to methylammonium A-site mixing ratio, x, in the visible region. However, the carrier mobility, as determined from time-resolved microwave conductivity measurements, trends opposite to that expected from first-principles calculations combined with Boltzmann scattering theory calculations of the carrier mobility. We propose that this is a result of increasing carrier scattering with x in (CH3NH3)1-xCsxSnBr3 due to a significant increase in the carrier density with x. By examining the dependence of the carrier density as a function of x, we infer the compositional-dependence of the average enthalpy and nonconfigurational entropy per defect. While diffraction reveals a cubic aristotypic perovskite structure as a function of x at room temperature, the pair distribution functions obtained from synchrotron X-ray total scattering from these materials are better described by symmetry-adapted displacement modes of the Pm3m crystal structure, which we attribute to a large degree of anharmonic dynamics of the atom positions. This analysis shows that CH3NH3+-rich compositions retain mostly linear Sn-Br-Sn bonding environments through the displacements, while Cs+-rich compositions lead to more significantly bent Sn-Br-Sn environments. We propose that these bent bonding arrangements in Cs-rich compositions yield a higher propensity for defect formation. This also provides a rationale for carrier trapping that gives rise to anomalous microwave transients. Together, these results provide insight into the structure-dynamics-properties relationships in this highly anharmonic system with high amplitude atomic motions and low defect formation energies.

defect thermochemistry↗
DOE OSTI2025

Experimental and theoretical evidence of universality in superfluid vortex reconnections

The minimum separation between reconnecting vortices in fluids and superfluids obeys a universal scaling law with respect to time. The prereconnection and the postreconnection prefactors of this scaling law are different, a property related to irreversibility and to energy transfer and dissipation mechanisms. In the present work, we determine the temperature dependence of these prefactors in superfluid helium from experiments and a numeric model which fully accounts for the coupled dynamics of the superfluid vortex lines and the thermal normal fluid component. At all temperatures, we observe a pre- and postreconnection asymmetry similar to that observed in other superfluids and in classical viscous fluids, indicating that vortex reconnections display a universal behavior independent of the small-scale regularizing dynamics. We also numerically show that each vortex reconnection event represents a sudden injection of energy in the normal fluid. Finally we argue that in a turbulent flow, these punctuated energy injections can sustain the normal fluid in a perturbed state, provided that the density of superfluid vortices is large enough.

Reconnections↗
DOE OSTI2021

Mechanical Response of Castlegate Sandstone under Hydrostatic Cyclic Loading

The stress history of rocks in the subsurface affects their mechanical and petrophysical properties. Rocks can often experience repeated cycles of loading and unloading due to fluid pressure fluctuations, which will lead to different mechanical behavior from static conditions. This is of importance for several geophysical and industrial applications, for example, wastewater injection and reservoir storage wells, which generate repeated stress perturbations. Laboratory experiments were conducted with Castlegate sandstone to observe the effects of different cyclic pressure loading conditions on a common reservoir analogue. Each sample was hydrostatically loaded in a triaxial cell to a low effective confining pressure, and either pore pressure or confining pressure was cycled at different rates over the course of a few weeks. Fluid permeability was measured during initial loading and periodically between stress cycles. Samples that undergo cyclic loading experience significantly more inelastic (nonrecoverable) strain compared to samples tested without cyclic hydrostatic loading. Permeability decreases rapidly for all tests during the first few days of testing, but the decrease and variability of permeability after this depend upon the loading conditions of each test. Cycling conditions do affect the mechanical behavior; the elastic moduli decrease with the increasing loading rate and stress cycling. The degree of volumetric strain induced by stress cycles is the major control on permeability change in the sandstones, with less compaction leading to more variation from measurement to measurement. The data indicate that cyclic loading degrades permeability and porosity more than static conditions over a similar period, but the petrophysical properties are dictated more by the hydrostatic loading rate rather than the total length of time stress cycling is imposed.

58 GEOSCIENCES↗
NASA NTRS1974

Quantum graviton creation in a model universe

Consideration of the mechanism of production of gravitons in the empty, anisotropic, spatially inhomogeneous Gowdy three-torus cosmology. The Gowdy cosmology is an exact solution of the vacuum Einstein equations and is obtained as a generalization of the homogeneous empty Bianchi Type I (Kasner) cosmology by permitting the metric components to depend on one of the space variables in addition to time. The Hamiltonian methods of Arnowitt, Deser, and Misner are employed to identify the dynamical variables which are to be quantized. The WKB regime solution is identical to that found by Doroshkevich, Zel'dovich, and Novikov (DZN) for a universe containing collisionless anisotropic radiation. Using a procedure similar to that of Parker (1971) or Zel'dovich and Starobinskii (1971) for defining quantum number, it is found that the DZN large-time radiation consists of quanta (gravitons) created from an initial vacuum. The quantum behavior is much like the semiclassical enhancement of quantum number with the added feature of creation of quanta from vacuum fluctuations.

Berger, B. K.↗
NASA NTRS1990

Consistency and convergence for numerical radiation conditions

The problem of imposing radiation conditions at artificial boundaries for the numerical simulation of wave propagation is considered. Emphasis is on the behavior and analysis of the error which results from the restriction of the domain. The theory of error estimation is briefly outlined for boundary conditions. Use is made of the asymptotic analysis of propagating wave groups to derive and analyze boundary operators. For dissipative problems this leads to local, accurate conditions, but falls short in the hyperbolic case. A numerical experiment on the solution of the wave equation with cylindrical symmetry is described. A unified presentation of a number of conditions which have been proposed in the literature is given and the time dependence of the error which results from their use is displayed. The results are in qualitative agreement with theoretical considerations. It was found, however, that for this model problem it is particularly difficult to force the error to decay rapidly in time.

Hagstrom, Thomas↗
NASA NTRS1992

Numerical models of mantle convection

An overview of numerical methods describing the structure and dynamics of the mantle is presented with attention given to novel 3D modeling techniques. The paper reviews 3D spherical and Cartesian models for constant viscosity emphasizing the assumptions regarding style of convection, time dependence, and implications for the mantle. Similarly treated are 3D Cartesian models with temperature-dependent viscosities, and briefly examined are models that are based on compressibility, nonlinear viscosity, or plates. Extensive illustrations are presented detailing: (1) temperature variations from models of 3D thermal convection in spherical shells; (2) thermal anomalies in equatorial cross sections; and (3) temperature variations in a spherical shell heated from within. The discussion relates the numerical results of the models with real mantle-convection events, and the simulations are shown to yield increasingly realistic representations of material behavior.

Schubert, G.↗