Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Low Relaxation”

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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

On Venus impact basins - Viscous relaxation of topographic relief

The viscous relaxation with time of the topographic relief of impact basins on Venus is calculated under the assumption that the surface temperature of the planet has been at or near its presently high value since the time of basin formation. The effects of a decrease in effective viscosity with depth and of the partial to complete isostatic compensation of initial topography are included in the model. An impact basin several hundred kilometers or greater in diameter and three billion years old or older should have negligible topographic relief at present. On this basis, the hypothesis that quasi-circular features of low radar backscatter but little topographic relief may be relaxed remnants of ancient impact basins cannot be excluded. However, large quasi-circular depressions on Venus, such as Atlanta Planitia, are unlikely to be remnants of impact basins, because of their current relief, and so some other process should be found to account for them.

Solomon, S. C.↗
DOE OSTI2021

Evolutionary inverse design of defects at graphene 2D lateral interfaces

Grain boundaries (GBs) in two-dimensional (2D) materials often have a profound impact on various material properties from mechanical to optical to electronic, yet predicting all possible GB formations is a challenge. Here, we introduce a workflow based on an evolutionary algorithm for exploring possible GBs formed at a lateral 2D interface. In a departure from conventional genetic algorithm based structure optimization methods, we perform genetic operations in the near interface region that allow us to be computationally efficient. We benchmark our method using graphene, which is a well-studied 2D material with a wide range of point defects. An empirical potential was used as the surrogate of the evolutionary search. More than 11.5 × 106 structures in total were evaluated for 128 GB orientations, and for each orientation, the ten best structures are recorded. A subset of low energy GBs predicted by empirical potential based search was relaxed by first-principles calculations and used to validate the energetic rank order. With the validated formation energy, we rank-ordered the best 128 GB structures and performed a detailed statistical analysis of primitive rings to find the correlation between the ring distribution and the formation energy. We found that for low energy GBs (below 0.5eV/Å), Stone–Wales defects will dominate, while structures with a higher energy (0.5–1.1eV/Å) show an increasing population of heptagons and nine-membered rings to form seven-nine defect pairs. For structures with energy higher than 1.1eV/Å, the percentage of octagons and nine-membered rings increases, which indicates that these two types of rings are not energetically favorable. Our proposed methodology is broadly applicable to explore defective low dimensional materials and represents a powerful tool that enables a systematic search of GBs of lateral interfaces for 2D materials.

Zhang, Jianan↗
DOE OSTI2021

Characterization of the strain-rate–dependent mechanical response of single cell–cell junctions

Cell–cell junctions are essential components in multicellular structures and often experience strains of different magnitudes and rates. However, their mechanical behavior is currently underexplored due to the lack of techniques to quantitatively characterize junctional stress–strain relationships. Here we developed a polymeric microstructure to strain the mutual junction of a single cell pair while simultaneously recording the junction stress and observed previously unseen strain-rate–dependent junction responses. We showed that cytoskeleton growth could relax the stress buildup and prevent junction failure at low strain rates, while high strain rates led to synchronized junction failures at remarkably large strains (over 200%). We expect this platform and our biophysical understanding to form the foundation for the rate-dependent mechanics of cell–cell junctions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Transition to Petschek Reconnection in Subrelativistic Pair Plasmas: Implications for Particle Acceleration

While relativistic magnetic reconnection in pair plasmas has emerged in recent years as a candidate for the origin of radiation from extreme astrophysical environments, the corresponding subrelativistic pair-plasma regime has remained less explored, leaving open the question of how relativistic physics affects reconnection. In this paper, we investigate the differences between these regimes by contrasting two-dimensional particle-in-cell simulations of reconnection in pair plasmas with relativistic magnetization (σ ≫ 1) and subrelativistic magnetization (σ < 1). By utilizing unprecedentedly large domain sizes and outflow boundary conditions, we demonstrate that lowering the magnetization results in a change in the reconnection geometry from a plasmoid chain to a Petschek geometry, where laminar exhausts bounded by slow-mode shocks emanate from a single diffusion region. We attribute this change to the reduced plasmoid production rate in the low-σ case: When the secondary tearing rate is sufficiently low, plasmoids are too few in number to prevent the system from relaxing into a stable Petschek configuration. This geometric change also affects particle energization: We show that while high-σ plasmoid chains generate power-law energy spectra, low-σ Petschek exhausts merely heat incoming plasma and yield negligible nonthermal acceleration. These results have implications for predicting the global current sheet geometry and the resulting energy spectra in a variety of systems.

Plasma astrophysics↗
DOE OSTI2021

Optical phonon dominated heat transport: A first-principles thermal conductivity study of BaSn 2

Acoustic phonons with long mean free paths have long been believed to control the lattice thermal conductivity κ L in solids dominantly. In this study, however, we demonstrate an optical phonon dominated κ L in BaSnS 2 . By solving the Peierls-Boltzmann transport equation, we predict a low diagonal lattice thermal conductivity κ L (D) of 0.34 W m -1 K -1 at 850 K, which is less than half the κ L (D) of SnS at the same temperature. Further calculations following the Allen-Feldman model suggest the additional off-diagonal lattice thermal conductivity κ L (OD) contributed by wavelike tunneling phonons. The κ L (OD) becomes pronounced at the high temperature ( 0.17 W m -1 K -1 at 850 K) and leads to a deviation of the temperature dependence of κ L from T -1 to T -0.76 , suggesting the potential lattice anharmonicity in BaSnS 2 . Further analyses indicate BaSnS 2 has over 68% of κ L contributed by optical phonons. We show this uncommon optical phonon dominated κ L is due to the relatively high group velocities of optical phonons in BaSnS 2 . The phonon mode visualization suggests these relatively high-velocity optical phonons correspond to the antiphase vibrations in BaSnS 2 monolayers, which is originated from the unique permutation of SnS 3 tetrahedra. Finally, by investigating the mode-resolved group velocity, relaxation time, and Grüneisen parameter, we attribute the intrinsic low κ L of BaSnS 2 to the soft lattice and the relatively high lattice anharmonicity induced by the Ba-S weak bonding and Sn(II) lone-pair electrons. Our study explicitly analyzes the microscopic mechanism of optical phonon dominated heat transport in BaSnS 2 and suggests it worthy of further experimental studies as an intrinsic low-κ L material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Slow Electron Spin Relaxation at Ambient Temperatures with Copper Coordinated by a Rigid Macrocyclic Ligand

Paramagnetic transition metal complexes can serve as quantum bits, storing phase information through unpaired electrons. Despite their promise, these systems often require low temperatures and tend to rapidly decohere. Recent efforts have sought to improve longitudinal relaxation (T 1 ), which provides an upper limit for phase coherence (T m ), by investigating existing literature compounds with reduced vibrational coupling and orbital angular momentum. However, synthetic strategies for improving T 1 through novel ligand design have remained scant. Here, we disclose the synthesis of a new modular macrocyclic ligand framework with four nitrogen donors (N 4 ) derived from phenanthroline that supports room-temperature coherent Cu(II) spin centers. The optimized complex more than doubles the T 1 over the next best Cu(II)-N 4 compound and exhibits a room temperature coherence time (T m ) of 0.28 μs, close to previously reported values. This performance enhancement arises from a tight binding site with short Cu–N distances, resulting in a stronger ligand field and reduced thermal accessibility of symmetric vibrational modes. This work demonstrates a practical approach to enabling spin coherence at room temperature, a factor critical to accessing relevant quantum bits and biological sensors, through a designer macrocyclic ligand platform.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Subterahertz Spin Relaxation Dynamics of Boron-Vacancy Centers in Hexagonal Boron Nitride

Quantum sensors based on spin defects have become powerful tools for detecting faint magnetic signals, yet their operation remains confined to low magnetic fields and gigahertz frequencies. Extending such sensors into high-field (>0.3 T) and subterahertz regimes would enable quantum metrology across a wide range of electromagnetic phenomena and scientific applications, but has proven challenging. Here, we demonstrate that negatively charged boron vacancies ($V^−_B$) in hexagonal boron nitride can function as relaxation-based quantum sensors operating up to 0.2 terahertz and 7 T fields. Their uniform spin-orientation and persistent spin-contrast at high fields enable measurement of intrinsic spin relaxation across unexplored field regimes. We reveal a crossover in relaxation behavior, initially decreasing at low fields before rising at higher fields, consistent with the emergence of single-phonon-induced resonant noise at subterahertz frequencies. These results establish $V^−_B$ centers as a versatile platform for quantum sensing in the subterahertz, high-field regime.

boron-vacancies↗
NASA NTRS1989

Multi-grid for structures analysis

In structural analysis the amount of computational time necessary for a solution is proportional to the number of degrees of freedom times the bandwidth squared. In implicit time analysis, this must be done at each discrete point in time. If, in addition, the problem is nonlinear, then this solution must be iterated at each point in time. If the bandwidth is large, the size of the problem that can be analyzed is severely limited. The multi-grid method is a possible algorithm which can make this solution much more computationally efficient. This method has been used for years in computational fluid mechanics. It works on the fact that relaxation is very efficient on the high frequency components of the solution (nearest neighbor interactions) and not very good on low frequency components of the solution (far interactions). The multi-grid method is then to relax the solution on a particular model until the residual stops changing. This indicates that the solution contains the higher frequency components. A coarse model is then generated for the lower frequency components to the solution. The model is then relaxed for the lower frequency components of the solution. These lower frequency components are then interpolated to the fine model. In computational fluid mechanics the equations are usually expressed as finite differences.

Kascak, Albert F.↗
DOE OSTI2022

Two-step annealing induced structural rejuvenation: A cause for memory effect in metallic glasses

The structural origin of the memory effect is still elusive in glassy materials. Here, in this letter, the memory effect in a series of metallic glasses (MGs) is observed by two step annealing using differential scanning calorimetry (DSC) and molecular dynamics simulations. It demonstrates that the Kovacs peak can be reflected from not only the system potential energy but also the atomic dynamics, showing the memory effect could originate from the formation of more loose packing configurations around mobile atoms by structural rejuvenation in the second step of high temperature annealing. In contrast, the high-low temperature annealing protocol can only induce the monotonic relaxation. Therefore, our results pave a pathway to better understand the origin of memory effect in MGs and other glassy materials from their dynamics and local atomic structure.

Memory effect↗
NASA NTRS1975

The Cepheid mode problem

The modal behavior of RR Lyrae stars and low mass Cepheids has been investigated using a nonlinear relaxation technique and stability analysis. The advantages of this type of numerical approach in investigations of preferred mode of pulsation are discussed. The results obtained for both classes of variable stars are quite similar: first harmonic pulsation toward the blue, fundamental pulsation toward the red, and mixed-mode behavior at the extreme red edge of the instability strip. In addition, stars near the center of the strip can pulsate in either the fundamental or the first harmonic mode. Possible implications for observational results, including the Oosterhoff dichotomy of globular clusters and the beat Cepheids, are discussed.

Stellingwerf, R. F.↗
NASA NTRS1979

Sonic-boom minimization with nose-bluntness relaxation

A procedure which provides sonic-boom-minimizing equivalent area distributions for supersonic cruise conditions is described. This work extends previous analyses to permit relaxation of the extreme bluntness required by conventional low-boom shapes and includes propagation in a real atmosphere. The procedure provides area distributions which minimize either shock strength or overpressure.

Darden, C. M.↗
NASA NTRS1989

Analysis of a parallel multigrid algorithm

This paper considers the parallel multigrid algorithm of Frederickson and McBryan (1987). This algorithm uses multiple coarse-grid problems (instead of one) in the hope of accelerating convergence, and is found to have a close relationship to traditional multigrid methods. Specifically, the parallel coarse-grid correction operator is identical to a traditional multigrid coarse-grid correction operator, except that the mixing of high and low frequencies caused by aliasing error is removed. Appropriate relaxation operators can be chosen to take advantage of this property. Comparisons between the standard multigrid and the new method are made.

Chan, Tony F.↗
NASA NTRS1989

Analysis of a parallel multigrid algorithm

The parallel multigrid algorithm of Frederickson and McBryan (1987) is considered. This algorithm uses multiple coarse-grid problems (instead of one problem) in the hope of accelerating convergence and is found to have a close relationship to traditional multigrid methods. Specifically, the parallel coarse-grid correction operator is identical to a traditional multigrid coarse-grid correction operator, except that the mixing of high and low frequencies caused by aliasing error is removed. Appropriate relaxation operators can be chosen to take advantage of this property. Comparisons between the standard multigrid and the new method are made.

Chan, Tony F.↗
NASA NTRS1995

Effects of energy release on near field flow structure of gas jets

The primary objective is to understand how buoyancy affects the structure of the shear layer, the development of fluid dynamic instabilities, and formation of the coherent structures in the near-nozzle regions of gas jets. The secondary objectives are to study the role of buoyancy in lifting and reattachment process of diffusion flames, to evaluate the scaling behavior of diffusion flames, and to aid development and/or validation of theoretical models by providing quantitative data in the absence of buoyancy. Fast reacting hydrogen or hydrogen-inert fuels are used to isolate the effects of buoyancy on fluid dynamics without masking the flame behavior by soot and radiative heat transfer. This choice of fuel also permits an evaluation of simulating low gravity in low pressure ground experiments because the similarity constraints are relaxed for the fast reacting, nonsooting diffusion flames. The diagnostics consists primarily of a color schlieren system coupled with computer generated rainbow filters, video recording, and image analysis. The project involves (1) drop tower experiments, (2) ground experiments, and (3) theoretical analysis.

Agrawal, Ajay K.↗
NASA NTRS2005

Single mode fiber array for planet detection using a visible nulling interferometer

We report the design, fabrication, and testing of a coherent large mode field diameter fiber array to be used as a spatial filter in a planet finding visible nulling interferometer. The array is a key component of a space instrument for visible-light detection and spectroscopy of Earth like extrasolar planets. In this concept, a nulling interferometer is synthesized from a pupil image of a single aperture which is then spatially filtered by a coherent array of single mode fibers to suppress the residual scattered star light. The use of the fiber array preserves spatial information between the star and planet. The fiber array uses a custom commercial large mode field or low NA step-index single mode fiber to relax alignment tolerances. A matching custom micro lens array is used to couple light into the fibers, and to recollimate the light out of the fiber array. The use of large mode field diameter fiber makes the fabrication of a large spatial filter array with 300 to 1000 elements feasible.

fiber arrays↗
NASA NTRS2013

Segmented X-Ray Optics for Future Space Telescopes

Lightweight and high resolution mirrors are needed for future space-based X-ray telescopes to achieve advances in high-energy astrophysics. The slumped glass mirror technology in development at NASA GSFC aims to build X-ray mirror modules with an area to mass ratio of approx.17 sq cm/kg at 1 keV and a resolution of 10 arc-sec Half Power Diameter (HPD) or better at an affordable cost. As the technology nears the performance requirements, additional engineering effort is needed to ensure the modules are compatible with space-flight. This paper describes Flight Mirror Assembly (FMA) designs for several X-ray astrophysics missions studied by NASA and defines generic driving requirements and subsequent verification tests necessary to advance technology readiness for mission implementation. The requirement to perform X-ray testing in a horizontal beam, based on the orientation of existing facilities, is particularly burdensome on the mirror technology, necessitating mechanical over-constraint of the mirror segments and stiffening of the modules in order to prevent self-weight deformation errors from dominating the measured performance. This requirement, in turn, drives the mass and complexity of the system while limiting the testable angular resolution. Design options for a vertical X-ray test facility alleviating these issues are explored. An alternate mirror and module design using kinematic constraint of the mirror segments, enabled by a vertical test facility, is proposed. The kinematic mounting concept has significant advantages including potential for higher angular resolution, simplified mirror integration, and relaxed thermal requirements. However, it presents new challenges including low vibration modes and imperfections in kinematic constraint. Implementation concepts overcoming these challenges are described along with preliminary test and analysis results demonstrating the feasibility of kinematically mounting slumped glass mirror segments.

McClelland, Ryan S.↗
NASA NTRS1977

Comparison of rigorous and simple vibrational models for the CO2 gasdynamic laser

The accuracy of a simple vibrational model for computing the gain in a CO2 gasdynamic laser is assessed by comparing results computed from it with results computed from a rigorous vibrational model. The simple model is that of Anderson et al. (1971), in which the vibrational kinetics are modeled by grouping the nonequilibrium vibrational degrees of freedom into two modes, to each of which there corresponds an equation describing vibrational relaxation. The two models agree fairly well in the computed gain at low temperatures, but the simple model predicts too high a gain at the higher temperatures of current interest. The sources of error contributing to the overestimation given by the simple model are determined by examining the simplified relaxation equations.

Monson, D. J.↗
NASA NTRS1981

Effect of water vapor on sound absorption in nitrogen at low frequency/pressure ratios

Sound absorption measurements were made in N2-H2O binary mixtures at 297 K over the frequency/pressure range f/P of 0.1-2500 Hz/atm to investigate the vibrational relaxation peak of N2 and its location on f/P axis as a function of humidity. At low humidities the best fit to a linear relationship between the f/P(max) and humidity yields an intercept of 0.013 Hz/atm and a slope of 20,000 Hz/atm-mole fraction. The reaction rate constants derived from this model are lower than those obtained from the extrapolation of previous high-temperature data.

Zuckerwar, A. J.↗