Search NASA⌕ Search

SEARCH · Search NASA

Results for “flat geometry”

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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Electron correlation and incipient flat bands in the Kagome superconductor CsCr 3 Sb 5

Correlated kagome materials exhibit a compelling interplay between lattice geometry, electron correlation, and topology. In particular, the flat bands near the Fermi level provide a fertile playground for novel many-body states. Here we investigate the electronic structure of CsCr 3 Sb 5 using high-resolution angle-resolved photoemission spectroscopy and ab-initio calculations. Our results suggest that Cr 3d electrons are intrinsically incoherent, showing strong electron correlation amplified by Hund’s coupling. Notably, we identify incipient flat bands close to the Fermi level, which are expected to significantly influence the electronic properties of the system. Across the density-wave-like transition at 55 K, we observe a drastic enhancement of the electron scattering rate, which aligns with the semiconducting-like property at high temperatures. These findings establish CsCr 3 Sb 5 as a strongly correlated Hund’s metal with incipient flat bands near the Fermi level, which provides an electronic basis for understanding its novel properties compared to the weakly correlated AV 3 Sb 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Atomic structure of a metal-supported two-dimensional germania film

The growth and microscopic characterization of two-dimensional germania films is presented. Germanium oxide monolayer films were grown on Ru(0001) by physical vapor deposition and subsequent annealing in oxygen. We obtain a comprehensive image of the germania film structure by combining intensity-voltage low-energy electron diffraction (I/V-LEED) and ab initio density functional theory (DFT) analysis with atomic-resolution scanning tunneling microscopy (STM) imaging. For benchmarking purposes, the bare Ru(0001) substrate and the (2×2)3O covered Ru(0001) were analyzed with I/V-LEED with respect to previous reports. STM topographic images of the germania film reveal a hexagonal network where the oxygen and germanium atom positions appear in different imaging contrasts. For quantitative LEED, the best agreement has been achieved with DFT structures where the germanium atoms are located preferentially on the top and fcc hollow sites of the Ru(0001) substrate. Moreover, in these atomically flat germania films, local site geometries, i.e., tetrahedral building blocks, ring structures, and domain boundaries, have been identified, indicating possible pathways towards two-dimensional amorphous networks.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Colloquium : Sliding and pinning in structurally lubric 2D material interfaces

A plethora of two-dimensional (2D) materials have been introduced in physics and engineering in the past two decades. Their robust, membranelike sheets permit (mostly require) deposition, giving rise to solid-solid dry interfaces whose mobility, pinning, and general tribological properties under shear stress are currently being understood and controlled, both experimentally and theoretically. Here, in this Colloquium simulated case studies of twisted graphene systems are used as a prototype workhorse tool to demonstrate and discuss the general picture of 2D material interface sliding. First highlighted is the crucial mechanical difference, often overlooked, between small and large incommensurabilities, which corresponds to, for example, small and large twist angles in graphene interfaces. In both cases, focusing on flat, structurally lubric or “superlubric” geometries, the generally separate scalings with the area of static friction in pinned states and of kinetic friction during sliding are elucidated and reviewed, tangled as they are with the effects of velocity, temperature, load, and defects. The roles of island boundaries and elasticity are also discussed, and compared when possible to results in the literature for systems other than graphene. It is proposed that the resulting picture of pinning and sliding should be applicable to interfaces in generic 2D materials that are of importance for the physics and technology of existing and future bilayer and multilayer systems.

2-dimensional systems↗
DOE OSTI2020

Field emission microscopy of carbon nanotube fibers: Evaluating and interpreting spatial emission

In this work, the authors quantify field emission properties of cathodes made from carbon nanotube (CNT) fibers. The cathodes were arranged in different configurations to determine the effect of cathode geometry on the emission properties. Various geometries were investigated including (1) flat cut fiber tip, (2) folded fiber, (3) looped fiber, and (4) fibers wound around a cylinder. The authors employ a custom field emission microscope to quantify I-V characteristics in combination with laterally resolved field-dependent electron emission area. Additionally, they look at the very early emission stages, first when a CNT fiber is turned on for the first time, which is then followed by multiple ramp-up/down runs. Upon the first turn on, all fibers demonstrated limited and discrete emission area. During ramping runs, all CNT fibers underwent multiple (minor and/or major) breakdowns, which improved emission properties in that turn-on field decreased and field enhancement factor and emission area both increased. It is proposed that breakdowns are responsible for removing initially undesirable emission sites caused by stray fibers higher than average. This initial breakdown process gives way to a larger emission area that is created when the CNT fiber subcomponents unfold and align with the electric field. Overall, the authors' results form the basis for careful evaluation of CNT fiber cathodes for dc or low frequency pulsed power systems in which large uniform area emission is required or for narrow beam high frequency applications in which high brightness is a must.

Image processing↗
DOE OSTI2018

Synchrotron X-ray Microdiffraction and Fluorescence Imaging of Mineral and Rock Samples

In this paper, we describe a detailed procedure for acquiring and processing x-ray microfluorescence (μXRF), and Laue and powder microdiffraction two-dimensional (2D) maps at beamline 12.3.2 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. Measurements can be performed on any sample that is less than 10 cm x 10 cm x 5 cm, with a flat exposed surface. The experimental geometry is calibrated using standard materials (elemental standards for XRF, and crystalline samples such as Si, quartz, or Al 2 O 3 for diffraction). Samples are aligned to the focal point of the x-ray microbeam, and raster scans are performed, where each pixel of a map corresponds to one measurement, e.g. , one XRF spectrum or one diffraction pattern. The data are then processed using the in-house developed software XMAS, which outputs text files, where each row corresponds to a pixel position. Representative data from moissanite and an olive snail shell are presented to demonstrate data quality, collection, and analysis strategies.

36 MATERIALS SCIENCE↗
NASA NTRS1976

Analysis of the gas-lubricated flat-sector-pad thrust bearing

A flat sector-shaped pad geometry for a gas-lubricated thrust bearing is analyzed considering both the pitch and roll of the pad. It is shown that maximum load capacity is achieved when the pad is tilted so as to create uniform minimum film thickness along the pad trailing edge. Performance characteristics for various geometries and operating conditions of gas thrust bearings are presented in the form of design curves, and a comparison is made with the rectangular slider approximation. It is found that this approximation is unsafe for practical design, since it always overestimates load capacity.

Etsion, I.↗
NASA NTRS1976

Stress intensity factors for long, deep surface flaws in plates under extensional fields

Using a singular solution for a part circular crack by Smith, a Taylor Series Correction Method (TSCM) was verified for extracting stress intensity factors (SIFs) from photoelastic data. Photoelastic experiments were then conducted on plates with part circular and flat bottomed cracks for flaw depth to thickness ratios of 0.25, 0.50, and 0.75 and for equivalent flaw depth to equivalent ellipse length values ranging from 0.066 to 0.319. Experimental results agreed well with the Smith theory but indicated that the use of the 'equivalent' semi-elliptical flaw for correlating the part circular flaw results with semi-elliptical flaw results was not valid for a/2c much less than 0.20. Best overall agreement for the moderate (a/t about 0.5) to deep flaws (a/t about 0.75) and a/2c greater than 0.15 was found with a semi-empirical theory due to Newman when compared on the basis of equivalent flaw depth and area. The Smith theory, when correlated on the basis of flaw depth and area, appears to yield reasonable estimates (within 10%) of the SIF for flat bottomed flaws for the geometries studied here.

Harms, A. E.↗
NASA NTRS1984

Calculations of separation bubbles using boundary-layer-type equations. I. II

Two-dimensional models of separated flows are presented with stream-function boundary equations. A patching procedure is employed to solve for unknowns in viscous-inviscid interactions simultaneously in order to derive strong coupling. The technique features simplified Navier-Stokes equations and is useful for flows around flat plates and more complex geometries such as airfoils and cascades. Sample calculations are provided for separated flows in diffusers and in the neighborhood of trailing edges. Massive bubbles and unclosed bubbles are noted to present difficulties for the models.

Halim, A.↗
NASA NTRS1991

Two-dimensional Navier-Stokes heat transfer analysis for rough turbine blades

A quasi-three-dimensional thin-layer Navier-Stokes analysis was used to predict heat transfer to rough surfaces. Comparisons are made between predicted and experimental heat transfer for turbine blades and flat plates of known roughness. The effect of surface roughness on heat transfer was modeled using a mixing length approach. The effect of near-wall grid spacing and convergence criteria on the accuracy of the heat transfer predictions are examined. An eddy viscosity mixing length model having an inner and outer layer was used. A discussion of the appropriate model for the crossover between the inner and outer layers is included. The analytic results are compared with experimental data for both flat plates and turbine blade geometries. Comparisons between predicted and experimental heat transfer showed that a modeling roughness effects using a modified mixing length approach results in good predictions of the trends in heat transfer due to roughness.

Boyle, R. J.↗
NASA NTRS1991

Two-dimensional Navier-Stokes heat transfer analysis for rough turbine blades

A quasi-three-dimensional thin-layer Navier-Stokes analysis was used to predict heat transfer to rough surfaces. Comparisons are made between predicted and experimental heat transfer for turbine blades and flat plates of known roughness. The effect of surface roughness on heat transfer was modeled using a mixing length approach. The effect of near-wall grid spacing and convergence criteria on the accuracy of the heat transfer predictions are examined. An eddy viscosity mixing length model having an inner and outer layer was used. A discussion of the appropriate model for the crossover between the inner and outer layers is included. The analytic results are compared with experimental data for both flat plates and turbine blade geometries. Comparisons between predicted and experimental heat transfer showed that a modeling roughness effects using a modified mixing length approach results in good predictions of the trends in heat transfer due to roughness.

Boyle, R. J.↗
DOE OSTI2026

Visualization of Two-phase Flow Maldistribution in Brazed Plate Heat Exchangers

Brazed plate heat exchangers (BPHEs) are widely used in refrigeration and HVAC applications, but are susceptible to two-phase flow maldistribution especially when operated as evaporators. Existing visualization approaches are either limited to idealized conditions or suffer from poor optical transparency. This paper presents a novel visualization method in which one edge of a BPHE, parallel to the refrigerant inlet or outlet port, is removed by wire electrical discharge machining and replaced with a flat, transparent plate. The planar geometry allows the use of optically and infrared (IR)-transparent materials, enabling both high-speed videography and IR thermography of the two-phase flow at the channel entrances and exits. Preliminary tests with R134a and R1234ze(Z) at saturation temperatures between 5 °C and 15 °C demonstrate that distinct two-phase flow patterns in the inlet header can be clearly identified and differentiated under realistic operating conditions. Potentials of optical flow analysis of high-speed videos are shown to provide objective, quantitative indicators for flow regime characterization and comparison. IR imaging of the outlet port reveals non-uniform temperature distributions at the channel exits, providing independent evidence of maldistribution across the channel stack. Limitations of IR temperature accuracy due to the spectral properties of the sapphire window are discussed, and directions for improvement are identified.

Hausherr, Carsten [Technical University of Berlin ↗
DOE OSTI2023

Imaging real-space flat band localization in kagome magnet FeSn

Kagome lattices host flat bands due to their frustrated lattice geometry, which leads to destructive quantum interference of electron wave functions. Here, we report imaging of the kagome flat band localization in real-space using scanning tunneling microscopy. We identify both the Fe 3 Sn kagome lattice layer and the Sn 2 honeycomb layer with atomic resolution in kagome antiferromagnet FeSn. On the Fe 3 Sn lattice, at the flat band energy determined by the angle resolved photoemission spectroscopy, tunneling spectroscopy detects an unusual state localized uniquely at the Fe kagome lattice network. We further show that the vectorial in-plane magnetic field manipulates the spatial anisotropy of the localization state within each kagome unit cell. Our results are consistent with the real-space flat band localization in the magnetic kagome lattice. We further discuss the magnetic tuning of flat band localization under the spin–orbit coupled magnetic kagome lattice model.

36 MATERIALS SCIENCE↗
NASA NTRS1978

Transitional Boundary-Layer Solutions Using a Mixing-Length and a Two-Equation Turbulence Model

Boundary-layer solutions were obtained using the conventional two-layer mixing-length turbulence model and the Wilcox-Traci two-equation model of turbulence. Both flatplate and blunt-body geometries were considered. The most significant result of the study is development of approximations for the two-equation model which permit streamwise stepsize comparable to that used in mixing-length computations. Additionally, a set of model-equation boundary conditions derived which apply equally well to both flat-plate and blunt-body geometries. Solutions obtained with the two-equations turbulence model are compared with experimental data and/or corresponding solutions obtained using the mixing-length model. Agreement is satisfactory for flat-plate boundary layers but not for blunt body boundary layers.

Anderson, E. C.↗
NASA NTRS2011

Performance Effects of Adding a Parallel Capacitor to a Pulse Inductive Plasma Accelerator Powertrain

Pulsed inductive plasma accelerators are electrodeless space propulsion devices where a capacitor is charged to an initial voltage and then discharged through a coil as a high-current pulse that inductively couples energy into the propellant. The field produced by this pulse ionizes the propellant, producing a plasma near the face of the coil. Once a plasma is formed if can be accelerated and expelled at a high exhaust velocity by the Lorentz force arising from the interaction of an induced plasma current and the magnetic field. While there are many coil geometries that can be employed to inductively accelerate a plasma, in this paper the discussion is limit to planar geometries where the coil take the shape of a flat spiral. A recent review of the developmental history of planar-geometry pulsed inductive thrusters can be found in Ref. [1]. Two concepts that have employed this geometry are the Pulsed Inductive Thruster (PIT) and the Faraday Accelerator with Radio-frequency Assisted Discharge (FARAD).

Polzin, Kurt A.↗
NASA NTRS1988

High resolution video monitoring of coating thickness during plasma spraying

A new approach to monitoring the thickness of plasma sprayed coatings during application is described. The method employs a high resolution video camera and width analyzer to accurately measure the dimensions of samples having simple geometries. This approach is best suited for cylindrical or flat substrates but it may also work for selected locations on more complex geometries. Measurement accuracy is a function of specimen dimensions and extent of magnification. Tolerances of plus or minus 0.5 mil (0.13 mm) on final coating thickness can be achieved. Additionally, the plot of cumulative coating thickess versus the number of passes has proven to be a useful diagnostic tool. While the ideal plot is linear, strong deviations from linearity - indicating the need for corrective action - may be observed.

Miller, Robert A.↗
DOE OSTI2015

Supportless, bismuth-modified palladium nanotubes with improved activity and stability for formic acid oxidation

Palladium nanotubes (PdNTs) were synthesized by templated vapor deposition and investigated for formic acid electrooxidation. Annealed PdNTs are 2.4 times more active (2.19 mA/cm 2 ) than commercial carbon-supported palladium (0.91 mA/cm 2 ) at 0.3 V vs RHE. Bismuth modification improved nanotube performance over 4 times (3.75 mA/cm 2 ) vs Pd/C and nearly 2 times vs unmodified PdNTs. A surface Bi coverage of 80% results in optimal site-specific activity by drastically reducing surface-poisoning CO generation during formic acid electrooxidation. The Bi-modified PdNTs are exceptionally stable, maintaining 2 times the area-normalized current density as Pd/C after 24 h at 0.2 V vs RHE. As a result, we attribute the enhanced activity and stability of the nanotube catalysts to the presence of highly coordinated surfaces, mimicking a flat polycrystal while retaining high surface area geometry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

A nAttractor mechanism for nAdS 2 /nCFT 1 holography

We study the nearly AdS 2 geometry of nearly extremal black holes in $\mathcal{N} = 2$ supergravity in four dimensions. In the strictly extreme limit the attractor mechanism for asymptotically flat black holes states that the horizon geometries of these black holes are independent of scalar moduli. We determine the dependence of the near extreme geometry on asymptotic moduli and express the result in simple formulae that generalize the extremal attractor mechanism to nearly extreme black holes. This is a nAttractor mechanism.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

CFD Verification and Validation of Wire-Wrapped Pin Assemblies

TerraPower is participating in a cooperative project among industry, the national lab system, and academia to perform Verification and Validation (V and V) of Computational Fluid Dynamics (CFD) methods for predicting the flow and heat transfer within liquid-metal-cooled nuclear fuel assemblies with wire-wrapped fuel pins. This project, consisting of both experimental and numerical components, uses surrogate fluids and electrically heated fuel pins to substitute for the liquid metal coolant and the nuclear heat source. The experiments include both unheated assemblies to measure the flow velocities with Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV), as well as heated assemblies to measure the temperatures of the fluid and the simulated fuel pins with thermocouples. Both sets of experiments also measure the pressure at multiple locations in the system. The numerical component of this work involves high-fidelity Large-Eddy Simulation (LES) modeling and industry-standard Reynolds-Averaged Navier-Stokes (RANS) modeling of the experiments. Furthermore, the experiments and simulations will be conducted on two duct geometries: the first is a non-deformed hexagonal duct that has flat internal duct faces that represents a beginning of life geometry. The other is a deformed duct where the internal hexagonal flow area enlarges then decreases along the axial height to represent the changes that occur to the duct geometry from pressure and irradiation effects in the core. This paper describes the methods and results of the industrial, RANS-based CFD code verification, solution verification, and solution validation. The V and V simulations are being performed for the following experiment test geometries: - Isothermal non-deformed bundle; - Heated non-deformed bundle; - Isothermal deformed bundle; - Heated deformed bundle. These flows have been computed at first with the pre-test design specifications, without the aid of any experimental results. Later, post-test calculations will be performed incorporating the actual test conditions and input from some of the experimental results. These RANS simulations of helically wire-wrapped fuel assemblies employ meshes of bare pins without the wire-wrap in the bundle. The effect of wire-wrap on the flow is accounted for by introducing a momentum source into the governing fluid equations. The momentum source is only applied to cells corresponding to the location of the wire-wrap, and its components in each cell are based on the local flow field. This methodology simplifies the geometry and mesh generation and reduces the cell count, while still capturing the effects of the wire-wrap. The momentum source approach is used for all of the assembly simulations performed in this study. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗