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Results for “temperature gradient”

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 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Mapping the dark matter in the NGC 5044 group with ROSAT: Evidence for a nearly homogeneous cooling flow with a cooling wake

The NGC 5044 group of galaxies was observed by the ROSAT Position Sensitive Proportional Counter (PSPC) for 30 ks during its reduced pointed phase (1991 July). Due to the relatively cool gas temperature in the group (kT = 0.98 +/- 0.02 keV) and the excellent photon statistics (65,000 net counts), we are able to determine precisely a number of fundamental properties of the group within 250 kpc of the central galaxy. In particular, we present model-independent measurements of the total gravitating mass, the temperature and abundance profiles of the gas, and the mass accretion rate. Between 60 and 250 kpc, the gas is nearly isothermal with T varies as r(exp (-0.13 +/- 0.03)). The total gravitating mass of the group can be unambiguously determined from the observed density and temperature profiles of the gas using the equation of hydrostatic equilibrium. Within 250 kpc, the gravitating mass is 1.6 x 10(exp 13) solar mass, yielding a mass-to-light ratio of 130 solar mass/solar luminosity. The baryons (gas and stars) comprise 12% of the total mass within this radius. At small radii, the temperature clearly increases outward and attains a maximum value at 60 kpc. The positive temperature gradient in the center of the group confirms the existence of a cooling flow. The cooling flow region extends well beyond the temperature maximum with a cooling radius between 100 and 150 kpc. There are two distinct regions in the cooling flow separated by the temperature maximum. In the outer region, the gas is nearly isothermal with a unifor m Fe abundance of approximately 80% solar, the flow is nearly homogeneous with dot-M= 20 to 25 solar mass/year, the X-ray contours are spherically symmetric, and rho(sub gas) varies as r(exp -1.6). In the inner region, the temperature profile has a positive gradient, the mass accretion rate decreases rapidly inward, the gas density profile is steeper, and the X-ray image shows some substrucutre. NGC 5044 is offset from the centroid of the outer X-ray contours indicating that the central galaxy may have a residual velocity with respect to the center of the group potential. There is also a linear X-ray feature with an extent of approximately 30 kpc with one end coincident with NGC 5044. The X-ray emission from this feature is softer than the ambient gas. We interpret this feature as a 'cooling wake' formed by the accreting gas as it is gravitationally focused into the wake of NGC 5044. One of the most surprising results of our PSPC observation is the discovery of a nearly homogeneous cooling flow. Prior results concerning the mass accretion profile in cooling flows indicate that dot-M varies as r. This relation implies that significant mass deposition occurs at large radii which generates an inhomogeneous flow. The mass accretion rate in the NGC 5044 group is essentially a constant beyond 40 kpc (well within the cooling radius). Significant mass deposition (a declining dot-M) does not commence until the gas accretes to within 40 kpc of the group center where the radiative cooling time is approximately equals 10(exp 9) year. Th is radius also corresponds to the temperature maximum, the break in gas density profile, and the onset of structure in the X-ray image. A Hubble constant of H(sub 0) = 50 km/sec/Mpc is used throughout the paper.

David, Laurence P.↗
DOE OSTI2013

Temperature-driven motion of liquid crystals confined in a microvolume

The theoretical description of the reorientational dynamics in microsized liquid crystal (LC) cell, where the nematic sample is confined by two horizontal and two lateral surfaces, under the influence of a temperature gradient ∇T, caused by a laser beam focused on the bounding surface with and without the orientational defect, whereas the rest of the bounding surfaces of the LC cell are kept at constant temperature, has been presented. Our calculations, based on the appropriate nonlinear extension of the classical Ericksen-Leslie theory, show that due to interaction between ∇T and the gradient of the director field ∇n-circumflex in the LC sample, a thermally excited vortical fluid flow is maintained in the vicinity of the orientational defect, with the motion in the positive sense (clockwise) around the middle part of that defect. In the case of the same LC cell, but without the orientational defect on the lower hotter boundary, the heating regime can also produce the vortical flow in the vicinity of the lower boundary, but with the motion in the negative sense (anti-clockwise) around the middle part of that boundary. At that, the second vortex is characterized by a much slower speed than the vortical flow in the first case.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Advanced surrogate model for electron-scale turbulence in tokamak pedestals

We derive an advanced surrogate model for predicting turbulent transport at the edge of tokamaks driven by electron temperature gradient (ETG) modes. Our derivation is based on a recently developed sensitivity-driven sparse grid interpolation approach for uncertainty quantification and sensitivity analysis at scale, which informs the set of parameters that define the surrogate model as a scaling law. Our model reveals that ETG-driven electron heat flux is influenced by the safety factor q, electron beta β e and normalized electron Debye length λ D , in addition to well-established parameters such as the electron temperature and density gradients. To assess the trustworthiness of our model's predictions beyond training, we compute prediction intervals using bootstrapping. The surrogate model's predictive power is tested across a wide range of parameter values, including within-distribution testing parameters (to verify our model) as well as out-of-bounds and out-of-distribution testing (to validate the proposed model). Overall, validation efforts show that our model competes well with, or can even outperform, existing scaling laws in predicting ETG-driven transport.

fusion plasma↗
DOE OSTI2024

Experiments and gyrokinetic simulations of the nonlinear interaction between spinning magnetized plasma pressure filaments

A set of experiments using controlled, skin depth-sized plasma pressure filaments in close proximity have been carried out in a large linear magnetized plasma device. Two- and three-filament configurations have been used to determine the scale of cross field nonlinear interaction. When the filaments are separated by a distance of approximately five times the size of a single filament or less, a significant transfer of charge and energy occurs, leading to the generation of inter-filament electric fields. This has the effect of rotating the filaments and influencing the merging dynamics. Nonlinear gyrokinetic simulations using seeded filaments confirm the presence of unstable drift-Alfvén modes driven by the steep electron temperature gradient. When the filaments are within a few collisionless electron skin depths (separations twice the size of a single filament), the unstable perturbations drive the convective mixing of the density and temperature and rearrange the gradients such that they maximize in the region surrounding the filament bundle.

Sydora, R. D. (ORCID:0000000192543149)↗
NASA NTRS1988

The surface morphology of crystals melting under solutions of different densities

Examples of solids melting beneath liquids are described for cases where the bulk liquid volume is stabilized against convection by a positive vertical temperature gradient, either with, or without local density inversion at the melting interface. The examples include ice melting beneath brine or methanol solutions and tin or lead melting under molten Sn-20 wt pct Pb or Pb-20 wt pct Sn, respectively. Without density inversion the melting is slow, purely diffusion controlled and the interfaces are smooth; with convection assisted melting the rate increases by some two orders of magnitude and the interfaces develop a rough profile - in the case of ice both irregular and quasi-steady state features are observed. The observations are discussed in terms of prevailing temperature and concentration gradients.

Fang, Dacheng↗
DOE OSTI2013

On secondary and tertiary instability in electromagnetic plasma microturbulence

Zonal flows, widely accepted to be the secondary instability process leading to the nonlinear saturation of ion temperature gradient modes, are shown to grow at higher rates relative to the linear mode amplitude as the plasma pressure β is increased—thus, confirming that zonal flows become increasingly important in the turbulent dynamics at higher β. At the next level of nonlinear excitation, radial corrugations of the distribution function (zonal flow, zonal density, and zonal temperature) are demonstrated to modify linear growth rates moderately through perturbed-field, self-consistent gradients: on smaller scales, growth rates are reduced below the linear rate. In particular, excitation of kinetic ballooning modes well below their usual threshold is not to be expected under normal conditions. These findings strengthen the theory of the non-zonal transition [M. J. Pueschel et al., Phys. Rev. Lett. 110, 155005 (2013)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Self‐consistency tests of large‐scale dynamics parameterizations for single‐column modeling

Abstract Large‐scale dynamics parameterizations are tested numerically in cloud‐resolving simulations, including a new version of the weak‐pressure‐gradient approximation (WPG) introduced by Edman and Romps (2014), the weak‐temperature‐gradient approximation (WTG), and a prior implementation of WPG. We perform a series of self‐consistency tests with each large‐scale dynamics parameterization, in which we compare the result of a cloud‐resolving simulation coupled to WTG or WPG with an otherwise identical simulation with prescribed large‐scale convergence. In self‐consistency tests based on radiative‐convective equilibrium (RCE; i.e., no large‐scale convergence), we find that simulations either weakly coupled or strongly coupled to either WPG or WTG are self‐consistent, but WPG‐coupled simulations exhibit a nonmonotonic behavior as the strength of the coupling to WPG is varied. We also perform self‐consistency tests based on observed forcings from two observational campaigns: the Tropical Warm Pool International Cloud Experiment (TWP‐ICE) and the ARM Southern Great Plains (SGP) Summer 1995 IOP. In these tests, we show that the new version of WPG improves upon prior versions of WPG by eliminating a potentially troublesome gravity‐wave resonance.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Threshold Heat-Flux Reduction by Near-Resonant Energy Transfer

Near-resonant energy transfer to large-scale stable modes is shown to reduce transport above the linear critical gradient, contributing to the onset of transport at higher gradients. This is demonstrated for a threshold fluid theory of ion temperature gradient turbulence based on zonal-flow-catalyzed transfer. Furthermore, the heat flux is suppressed above the critical gradient by resonance in the triplet correlation time, a condition enforced by the wave numbers of the interaction of the unstable mode, zonal flow, and stable mode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Multiscale Chirping Modes Driven by Thermal Ions in a Plasma with Reactor-Relevant Ion Temperature

A thermal ion driven bursting instability with rapid frequency chirping, considered as an Alfv´enic ion temperature gradient mode, has been observed in plasmas having reactor-relevant temperature in the DIII-D tokamak. The modes are excited over a wide spatial range from macroscopic device size to micro-turbulence size and the perturbation energy propagates across multiple spatial scales. The radial mode structure is able to expand from local to global in ~ 0.1 ms and it causes magnetic topology changes in the plasma edge, which can lead to a minor disruption event. Since the mode is typically observed in the high ion temperature ≳ 10 keV and high-! plasma regime, the manifestation of the mode in future reactors should be studied with development of mitigation strategies, if needed. In conclusion, this is the first observation of destabilization of the Alfv´en continuum caused by the compressibility of ions with reactor-relevant ion temperature.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

SU-C-213-07: Fabrication and Testing of a 3D-Printed Small Animal Rectal Cooling Device to Evaluate Local Hypothermia as a Radioprotector During Prostate SBRT

Purpose: The protective effects of induced or even accidental hypothermia on the human body are widespread with several medical uses currently under active research. In vitro experiments using human cell lines have shown hypothermia provides a radioprotective effect that becomes more pronounced at large, single-fraction doses common to SBRT treatments. Relevant to prostate SBRT, this work details the fabrication and testing of a 3D-printed cooling device to facilitate the investigation of the radioprotective effect of local hypothermia on the rat rectum. Methods: A 3cm long, two-channel rectal cooling device was designed in SOLIDWORKS CAD for 3D printing. The water intake nozzle is connected to a 1mm diameter brass pipe from which water flows and circulates back around to the exit nozzle. Both nozzles are connected by plastic tubing to a water chiller pump. Following leak-proof testing, fiber optic temperature probes were used to evaluate the temperature over time when placed adjacent to the cooling device within a rat rectum. MRI thermometry characterized the relative temperature distribution in concentric ROIs surrounding the probe. CBCT images from a small-animal irradiator were evaluated for imaging artifacts which could affect Monte Carlo dose calculations during treatment planning. Results: The rectal temperature adjacent to the cooling device decreased from body temperature (37°C) to 15°C in 10–20 minutes from device insertion. Rectal temperature was maintained at 15±3°C during active cooling. MRI thermometry tests revealed a steep temperature gradient with increasing distance from the cooling device, with the desired temperature range maintained within the surrounding few millimeters. Conclusion: A 3D printed rectal cooling device was fabricated for the purpose of inducing local hypothermia in rat rectums. Rectal cooling capabilities were characterized in-vivo to facilitate an investigation of the radioprotective effect of hypothermia for late rectal toxicity following a single large dose of radiation. Funding support provided by RSNA research seed grant.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2012

Comparing Linear Microinstability of the National Compact Stellarator Expriment and a Shaped Tokamak

One metric for comparing con nement properties of di erent magnetic fusion energy con gurations is the linear critical gradient of drift wave modes. The critical gradient scale length determines the ratio of the core to pedestal temperature when a plasma is limited to marginal stability in the plasma core. The gyrokinetic turbulence code GS2 was used to calculate critical temperature gradients for the linear, collisionless ion tem- perature gradient (ITG) mode in the National Compact Stellarator Experiment (NCSX) and a prototypical shaped tokamak, based on the pro les of a JET H-mode shot and the stronger shaping of ARIES-AT. While a concern was that the narrow cross section of NCSX at some toroidal locations would result in steep gradients that drive instabilities more easily, it is found that other stabilizing e ects of the stellarator con guration o set this so that the normalized critical gradients for NCSX are competitive with or even better than for the tokamak. For the adiabatic ITG mode, NCSX and the tokamak had similar critical gradients, though beyond marginal stability, NCSX had larger growth rates. However, for the kinetic ITG mode, NCSX had a higher critical gradient and lower growth rates until a/LT ≈ 1:5 a/LT;crit, when it surpassed the tokamak's. A discussion of the results presented with respect to a/LT vs R/LT is included.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

(M4CT-18IN0707093) Investigating Electrochemical Impedance Spectroscopic (EIS) Measurement of Surrogate Oxide at High Temperatures

The objective of this work package is to develop electrochemical impedance spectroscopic (EIS) techniques for the in-pile measurement of changes in solid state chemistry of fuels and materials in coupling with model simulation and material characterization. This work package will involve the initial development of electrochemical sensing technologies for measuring spatial- and time-resolved changes in fuel and cladding chemistry. Specific attention will be paid to monitoring changes in fuel stoichiometry, monitor cladding hydride formation and deformation, and cladding corrosion. The planned work during this period was focused on theoretical investigation on high temperature impedance of material, establishing experimental capabilities, including high temperature EIS under controlled gas atmosphere, atom probe tomography (APT), Raman Spectroscopy, and Synchrotron XRD. Future work will focus on starting the electrochemical sensor development, targeting the dielectric response of oxide fuels and cladding oxides/hydrides under thermodynamic equilibria, during transients, and under irradiation. Three main achievements are summarized in this report for the FY17 carry-over. First, theoretical investigation on high temperature impedance of material were conducted and spatial distribution of defects and equivalent resistivity in stoichiometric oxides under a temperature gradient were analyzed. Second, a laboratory for EIS measurement at high temperature was established and the laboratory instruction (LI) for conducting experiments was approved; and the impedance of CeO2 at 800 C was examined and its electrical conductivities were analyzed and compared with other reported data. In addition, the capabilities of advanced characterization wer established including high temperature Ramen, atom probe tomography (APT) and SEM, and samples including Zr4 and CeO2 were characterized.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
NASA NTRS1972

An observation of the Gulf Stream surface front structure by ship, aircraft, and satellite.

Evaluation of observations of surface temperature and salinity along the western edge of the Gulf Stream made from a ship and of concurrent temperature observations obtained by instrumented aircraft at six altitudes. The major feature along a five-kilometer line normal to the Stream's edge is a temperature gradient of about 0.75 C/kilometer within which are embedded two abrupt temperature increases of about 1.5 C. Temperature variations were compensated by salinity variations, yielding nearly constant density through the frontal zone; a sharp lateral current shear was associated with the thermohaline mixing region between the steps. The attenuation of surface temperature measured by the airborne radiometer was compared with a theoretical model. The analysis supports the view that a two-part correction technique is required: one part for bulk-skin temperature differences, and another for atmospheric attenuation of sea surface emission due to the mass and temperature of interfering gases.

Maul, G. A.↗
NASA NTRS1976

Correction of laser ranging data for the effects of horizontal refractivity gradients

A formula has been developed to correct laser ranging data for the effects of horizontal refractivity gradients. The formula requires the values of the horizontal pressure and temperature gradients at the laser site. The accuracy of this technique was evaluated by comparing the formula with range corrections which were computed by ray tracing.

Gardner, C. S.↗
NASA NTRS1980

Gravity waves from thunderstorms

Gravity waves generated by severe thunderstorms in the eastern Ohio-Pennsylvania area were recorded by an array of microbarovariographs at Palisades, New York and by standard microbarographs across northeastern United States. The waves were associated with the cold mesohigh from the outflow of the thunderstorms. Along their path the waves apparently triggered new thunderstorms. The waves were observed to propagate with the velocity of the wind just below the tropopause. The long-distance propagation of the waves is explained by the presence of a dust associated with the critical level (steering level), in agreement with the derivations given by Lindzen and Tung (1976). The dust was directional and waves were absent to the west of the generating area. In the generating area wave-CISK might have been operating. Sharp vertical temperature gradients associated with the passage of the waves were observed by temperature sensors on a tower.

Balachandran, N. K.↗
NASA NTRS1956

Investigation of Heat Transfer From

The convective heat transfer from the surface of an ellipsoidal forebody of fineness ratio 3 and 20-inch maximum diameter was investigated in clear air for both stationary and rotating operation over a range of conditions including air speeds up to 240 knots, rotational speeds up to 1200 rpm, and angles of attack of 0 deg, 3 deg, and 6 deg. The results are presented in the form of heat-transfer coefficients and the correlation of Nusselt and Reynolds numbers. Both a uniform surface temperature and a uniform input heater density distribution were used. The experimental results agree well with theoretical predictions for uniform surface temperature distribution. Complete agreement was not obtained with uniform input heat density in the laminar-flow region because of conduction effects. No significant effects of rotation were obtained over the range of airstream and rotational speeds investigated. Operation at angle of attack had only minor effects on the local heat transfer. Transition from laminar to turbulent heat transfer occurred over a wide range of Reynolds numbers. The location of transition depended primarily on surface roughness and pressure and temperature gradients. Limited transient heating data indicate that the variation of surface temperature with time followed closely an exponential relation.

Lewis, James P.↗
NASA NTRS1984

High stability buffered phase comparator

A low noise RF signal phase comparator comprised of two high stability driver buffer amplifiers driving a double balanced mixer which operate to generate a beat frequency between the two RF input signals coupled to the amplifiers from the RF sources is described. The beat frequency output from the mixer is applied to a low noise zero crossing detector which is the phase difference between the two RF inputs. Temperature stability is provided by mounting the amplifiers and mixer on a common circuit board with the active circuit elements located on one side of a circuit board and the passive circuit elements located on the opposite side. A common heat sink is located adjacent the circuit board. The active circuit elements are embedded into the bores of the heat sink which slows the effect of ambient temperature changes and reduces the temperature gradients between the active circuit elements, thus improving the cancellation of temperature effects. The two amplifiers include individual voltage regulators, which increases RF isolation.

Adams, W. A.↗
NASA NTRS1984

Manufacturing in space: Fluid dynamics numerical analysis

Thermocapillary convection in an air-jet Marangoni flow suppressing technique was numerically simulated for flow under various conditions of fluid properties, gravity, and temperature gradient. Results presented include computer generated plots of streamlines, velocities, and temperatures throughout the contained fluid flow, and plots of Marangoni flow velocities over the free surface.

Robertson, S. J.↗