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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Vertical Structure of Radiation-pressure-dominated Thin Disks: Link between Vertical Advection and Convective Stability

In the classic picture of standard thin accretion disks, viscous heating is balanced by radiative cooling through the diffusion process, and the radiation-pressure-dominated inner disk suffers convective instability. However, recent simulations have shown that, owing to the magnetic buoyancy, the vertical advection process can significantly contribute to energy transport. In addition, in comparing the simulation results with the local convective stability criterion, no convective instability has been found. In this work, following on from simulations, we revisit the vertical structure of radiation-pressure-dominated thin disks and include the vertical advection process. Our study indicates a link between the additional energy transport and the convectively stable property. Thus, the vertical advection not only significantly contributes to the energy transport, but it also plays an important role in making the disk convectively stable. Our analyses may help to explain the discrepancy between classic theory and simulations on standard thin disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Experimental study of forced convection heat transfer during upward and downward flow of helium at high pressure and high temperature

Fundamental high pressure/high temperature forced convection experiments have been conducted in support of the development of a Very High Temperature Reactor (VHTR) with a prismatic core. The experiments utilize a high temperature/high pressure gas flow test facility constructed for forced convection and natural circulation experiments. The test section has a single 16.8 mm ID flow channel in a 2.7 m long, 108 mm OD graphite column with four 2.3kW electric heater rods placed symmetrically around the flow channel. This experimental study presents the role of buoyancy forces in enhancing or reducing convection heat transfer for helium at high pressures up to 70 bar and high temperatures up to 873 degrees K. Wall temperatures have been compared among 10 cases covering the inlet Re numbers ranging from 500 to 3,000. Downward flows display higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases due to the influence of buoyancy forces. In the entrance region, convection heat transfer is reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancyinduced mixing causes higher convection heat transfer and lower wall temperatures. However, their influences are reduced as the Reynolds number increases. This experimental study is of specific interest to VHTR design and validation of safety analysis codes.

Buoyancy↗
DOE OSTI2012

Reactor Engineering: Experimental Investigation of Alpha Convection

This report summarizes the progress on DE-FG07-07ID14806 project funded by the US Department of Energy under Nuclear Engineering Education Research (NEER) program. During the project a test apparatus was designed and manufactured for experimental investigation of Alpha Convection. Benchmark testing of the new apparatus without the neutron radiation was performed and numerical simulations were conducted to validate the experimental results on natural convection. These results were reported in a journal publication. Subsequently, we conducted the experiment at Missouri S&T research reactor in the beam port to investigate the phenomenon of Alpha Convection. Unfortunately, these experiments were inconclusive. No observable effect of neutron beam was noticed. Insufficient neutron flux as it passes through the borated water is responsible for the unsuccessful experiments. A number of new experiments are designed and manufactured to investigate the phenomenon. A new experiment using shadowgraphy is designed and manufactured to investigate and visualize the natural convection and the impact of neutron on the phenomenon. Moreover, to continue with the original and newly designed experiments a neutron generator was needed to supply a vertical beam of neutrons. This new source of neutron is acquired using DOE NEUP General Scientific Infrastructure number DE-FOA-0000321. The new facility is installed and tested but is yet to be licensed by NRC for regular use and continuation of the experiments.

Conduction convection transition.↗
DOE OSTI2017

Relationships between radiation, clouds, and convection during DYNAMO

In this paper, the relationships between radiation, clouds, and convection on an intraseasonal time scale are examined with data taken during the Dynamics of the Madden-Julian Oscillation (MJO) field campaign. Specifically, column-net, as well as vertical profiles of radiative heating rates, computed over Gan Island in the central Indian Ocean (IO) are used along with an objective analysis of large-scale fields to examine three MJO events that occurred during the 3 month period (October to December 2011) over this region. Longwave (LW) and shortwave radiative heating rates exhibit tilted structures, reflecting radiative effects associated with the prevalence of shallow cumulus during the dry, suppressed MJO phase followed by increasing deep convection leading into the active phase. As the convection builds going into the MJO active phase, there are increasingly top-heavy anomalous radiative heating rates while the column-net radiative cooling rate Q r progressively decreases. Temporal fluctuations in the cloud radiative forcing, being quite sensitive to changes in high cloudiness, are dominated by LW effects with an intraseasonal variation of ~0.4–0.6 K/d. While both the water vapor and cloud fields are inextricably linked, it appears that the tilted radiative structures are more related to water vapor effects. The intraseasonal variation of column-net radiative heating Q r enhances the convective signal in the mean by ~20% with a minimum in this enhancement ~10 days prior to peak MJO rainfall and maximum ~7 days after. Finally, this suggests that as MJO convective envelope weakens over the central IO, cloud-radiative feedbacks help maintain the mature MJO as it moves eastward.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Sensitivity of the summertime tropical Atlantic precipitation distribution to convective parameterization and model resolution in ECHAM6

A realistic simulation of the tropical Atlantic precipitation distribution remains a challenge for atmospheric general circulation models, owing to their too coarse resolution that makes it necessary to parameterize convection. During boreal summer, models tend to underestimate the northward shift of the tropical Atlantic rain belt, leading to deficient precipitation over land and an anomalous precipitation maximum over the west Atlantic ocean. In this study, the model ECHAM6 is used to test the sensitivity of the precipitation biases to convective parameterization and horizontal resolution. Two sets of sensitivity experiments are performed. In the first set of experiments, modifications are applied to the convection scheme in order to investigate the relative roles of the trigger, entrainment, and closure formulations. In the second set, the model is run at high resolution with low-resolution boundary conditions in order to identify the relative contributions of a high-resolution atmosphere, orography, and surface. Results show that the dry bias over land in the model can be reduced by weakening the entrainment rate over land. Over ocean, it is found that the anomalous precipitation maximum occurs because of model choices that decrease the sensitivity of convection to the monsoon circulation in the east Atlantic. A reduction of the west Atlantic precipitation bias can be achieved by (i) using a moisture convergence closure, (ii) increasing the resolution of orography, or (iii) enhancing the production of deep convection in the east Atlantic. As a result, the biases over land and over ocean do not impact each other.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Dependence of Lamellar Eutectic Growth with Convection on Boundary Conditions and Geometric Confinement: A Phase-Field Lattice-Boltzmann Study

Simulating eutectic growth with convection is challenging because of the enormous computing demand resulting from the required domain size compared with the eutectic scale. In this study, a phase-field lattice-Boltzmann approach was employed to reproduce the eutectic growth with convection (both natural and forced). To enable and accelerate large-scale simulations, a parallel-adaptive mesh refinement algorithm was developed to combine with this numerical approach. The effects of the boundary conditions and the geometric confinement on the eutectic growth with convection were systematically investigated in the Al-Cu eutectic alloy. Results showed that both boundary conditions and the geometric confinement altered the eutectic evolution by affecting the distributions of the solute and velocity ahead of the eutectic solid/liquid interface. As the first attempt to explore the two factors on the eutectic growth with convection, our thorough investigation lays a foundation for modeling the coupled convection–solute eutectic evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Thermovelocimetric characterization of liquid metal convection in a rotating slender cylinder

Rotating turbulent convection occurs ubiquitously in natural convective systems encompassing planetary cores, oceans and atmospheres, as well as in many industrial applications. While the global heat and mass transfer of water-like rotating Rayleigh–Bénard convection is well-documented, the dynamics in low-Prandtl-number liquid metals remain less understood. In this study, we experimentally investigate rotating Rayleigh-Bénard convection in liquid gallium (Prandtl number Pr ≈ 0.027) within a slender cylinder (diameter-to-height aspect ratio Γ = D/H =1/2) using novel thermovelocimetric diagnostic techniques that integrate simultaneous multi-point thermometry and ultrasonic Doppler velocity measurements. Our results reveal the formation of a stable, global-scale azimuthal wavenumber m = 2 quadrupolar vortex at low supercriticality. We propose that enhanced wall modes facilitated by the slender cylinder geometry interact with the bulk flow to create these large-scale axialized vortices. Furthermore, our findings imply a distinct scaling behavior for the wall-mode precession frequency in liquid metals, extending previous results obtained for moderate-Pr fluids. In conclusion, this provides new insights into wall-bulk coupling mechanisms of low-Pr rotating convective turbulence.

Magnetohydrodynamics↗
DOE OSTI2012

Potential Aerosol Indirect Effects on Atmospheric Circulation and Radiative Forcing through Deep Convection

Aerosol indirect effects, i.e., the interactions of aerosols with clouds by serving as cloud condensation nuclei (CCN) or ice nuclei (IN), constitute the largest uncertainty in climate forcing and projection. Previous IPCC reported aerosol indirect forcing is negative, which does not account for aerosol-convective cloud interactions because the complex processes involved are poorly understood and represented in climate models. Here we report that aerosol indirect effect on deep convective cloud systems can lead to enhanced regional convergence and a strong top-of atmosphere (TOA) warming. Aerosol invigoration effect on convection can result in a strong radiative warming in the atmosphere (+5.6 W m-2) due to strong night-time warming, a lofted latent heating, and a reduced diurnal temperature difference, all of which could remarkably impact regional circulation and modify weather systems. We further elucidated how aerosols change convective intensity, diabatic heating, and regional circulation under different environmental conditions and concluded that wind shear and cloud base temperature play key roles in determining the significance of aerosol invigoration effect for convective systems.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Do We Need to Parameterize Mesoscale Convective Organization to Mitigate the MJO-Mean State Trade-Off?

Modifications in cumulus parameterizations that improve Madden-Julian oscillation (MJO) simulation tend to degrade the mean state—known as the MJO-mean state trade-off. The impacts of parameterizing mesoscale convective organization on the relationship between the MJO and the mean state simulation fidelity are examined. A series of experiments are made with a general circulation model that parameterizes the mesoscale convective organization and simulates well both the MJO and the mean state. In the control simulation, a prognostic nondimensional variable (Ω) represents the degree of convective organization. In order to examine the effect of the parameterized convective organization, fixed Ω values are imposed in a series of constrained experiments. The fixed-Ω simulations show a negative relationship between the MJO and the mean state simulation fidelity. The control simulation is deviated from the negative relationship, suggesting the parameterized mesoscale convective organization helps general circulation models to mitigate the MJO-mean state trade-off.

58 GEOSCIENCES↗
DOE OSTI2019

The Interaction Between Boundary Layer and Convection Schemes in a WRF Simulation of Post Cold Frontal Clouds Over the ARM East North Atlantic Site

Abstract The correct representation of low‐level midlatitude clouds found in the wake of cold fronts strongly relies on the representation of planetary boundary layer (PBL) and convection processes, which are typically parameterized separately in numerical models. Using the Weather Research and Forecasting Model (WRF), this study investigates how distinct pairs of PBL and convection parameterization schemes represent cloud fraction in the post cold frontal region. The simulations focus on the region of the Department of Energy Atmospheric Radiation Measurement (ARM) Eastern North Atlantic observation site in the Azores Islands in the wake of a cold front that passed on 25 December 2015. Different PBL and convection schemes are combined to create 12 distinct configurations. The main differences between the selected physical parameterizations are the strength of vertical mixing and the entrainment. The simulations produce a wide range of cloud fractions, where some configurations significantly underestimate while others clearly overestimate satellite and surface‐based estimates of cloud fraction. A skill score is used to quantitatively assess the performance of each configuration with respect to ground‐based radar data. The key processes that are found to significantly impact the cloud fraction distribution are the strength of the PBL decoupling, the vertical wind shear, entrainment and detrainment rates in shallow convection, and the occurrence of drizzle. This indicates that to successfully simulate post cold frontal clouds, modeled physics must balance strong internal vertical mixing and weak exchange with the free troposphere. For this case study, cloud fraction was more sensitive to the choice of convection scheme than PBL scheme.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Hurricane‐Like Vortices in Conditionally Unstable Moist Convection

Abstract This study investigates the emergence of hurricane‐like vortices in idealized simulations of rotating moist convection. A Boussinesq atmosphere with simplified thermodynamics for phase transitions is forced by prescribing the temperature and humidity at the upper and lower boundaries. The governing equations are solved numerically using a variable‐density incompressible Navier‐Stokes solver with adaptive mesh refinement to explore the behavior of moist convection under a broad range of conditions. In the absence of rotation, convection aggregates into active patches separated by large unsaturated regions. Rotation modulates this statistical equilibrium state so that the self‐aggregated convection organizes hurricane‐like vortices. The warm and saturated air converges to the center of the vortices, and the latent heat released through the upwelling, forms the warm core structure. These hurricane‐like vortices share characteristics similar to tropical cyclones in the earth's atmosphere. The hurricane‐like vortices occur under conditionally unstable conditions where the potential energy given at the boundaries is large enough, corresponding to a moderate rate of rotation. This regime shares many similar characteristics to the tropical atmosphere indicating that the formation of intense meso‐scale vortices is a general characteristic of rotating moist convection. The model used here does not include any interactions with radiation, wind‐evaporation feedback, or cloud microphysics, indicating that, while these processes may be relevant for tropical cyclogenesis in the Earth atmosphere, they are not its primary cause. Instead, our results confirm that the formation and maintenance of hurricane‐like vortices involve a combination of atmospheric dynamics under the presence of rotation and of phase transitions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Sensitivity of deep convection and cross-tropopause water transport to microphysical parameterizations in WRF

Deep convective storms can overshoot the tropopause, thus altering the composition of the stratosphere by vertically transporting tropospheric air. The transport of water vapor and ice particles into a sub-saturated environment can hydrate the stratosphere, with implications for radiative forcing and ozone chemistry. Cloud-resolved models, if employed at high spatial resolutions, are used to probe process-level questions about cross-tropopause deep convective hydration and its controls. There is considerable diversity in model representations of processes associated with water transport and transformation, and the choice of a microphysics scheme affects model skill in simulating deep convective events. This motivates our evaluation of state-of-the-art, as well as widely used standard schemes, in a high spatial- and temporal-resolution framework. Six bulk microphysics schemes were employed in a WRF-LES setup, initialized with a sounding profile representative of a tropopause-overshooting storm. Here, we used an idealized framework to isolate the effect of microphysics on the dominant processes that control the reach of deep convection and stratospheric hydration. All schemes produced the highest reaching updrafts 8-12 hours into the simulation but the strength and persistence of updrafts varied across the schemes; maximum storm heights ranged 9.1-12.6 km across the schemes. Varying microphysics produced large differences in the vertical extent and horizontal aggregation of convection, and an order of magnitude spread in above-tropopause water vapor concentrations.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

A Decentralized Approach for Modeling Organized Convection Based on Thermal Populations on Microgrids

Abstract In this study, a spectral model for convective transport is coupled to a thermal population model on a two‐dimensional horizontal “microgrid,” covering the typical gridbox size of general circulation models. The goal is to explore new ways of representing impacts of spatial organization in cumulus cloud fields. The thermals are considered the smallest building block of convection, with thermal life cycle and movement represented through binomial functions. Thermals interact through two simple rules, reflecting pulsating growth and environmental deformation. Long‐lived thermal clusters thus form on the microgrid, exhibiting scale growth and spacing that represent simple forms of spatial organization and memory. Size distributions of cluster number are diagnosed from the microgrid through an online clustering algorithm, and provided as input to a spectral multiplume eddy‐diffusivity mass flux scheme. This yields a decentralized transport system, in that the thermal clusters acting as independent but interacting nodes that carry information about spatial structure. The main objectives of this study are (a) to seek proof of concept of this approach, and (b) to gain insight into impacts of spatial organization on convective transport. Single‐column model experiments demonstrate satisfactory skill in reproducing two observed cases of continental shallow convection. Metrics expressing self‐organization and spatial organization match well with large‐eddy simulation results. We find that in this coupled system, spatial organization impacts convective transport primarily through the scale break in the size distribution of cluster number. The rooting of saturated plumes in the subcloud mixed layer plays a key role in this process.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2013

Collective phase description of oscillatory convection

We formulate a theory for the collective phase description of oscillatory convection in Hele-Shaw cells. It enables us to describe the dynamics of the oscillatory convection by a single degree of freedom which we call the collective phase. The theory can be considered as a phase reduction method for limit-cycle solutions in infinite-dimensional dynamical systems, namely, stable time-periodic solutions to partial differential equations, representing the oscillatory convection. We derive the phase sensitivity function, which quantifies the phase response of the oscillatory convection to weak perturbations applied at each spatial point, and analyze the phase synchronization between two weakly coupled Hele-Shaw cells exhibiting oscillatory convection on the basis of the derived phase equations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Barriers and variable spacing enhance convective cooling and increase power output in solar PV plants

When the temperature of solar photovoltaic (PV) modules rises, efficiency drops and module degradation accelerates. Thus, it is beneficial to reduce module operating temperatures. Previous studies of solar power plants have illustrated that incoming flow characteristics, turbulent mixing, and array geometry can strongly impact convective cooling, as measured by the convective heat transfer coefficient h. In the fields of heat transfer and plant canopy flow, previous work has shown that system-scale arrangement modifications—e.g., variable spacing, barriers, or windbreaks—can passively alter the flow, enhance turbulent mixing, and influence convection. However, researchers have not yet explored how variable spacing or barriers might enhance convective cooling in solar power plants. Here, high-resolution large-eddy simulations model the air flow and heat transfer through solar power plant arrangements modified with missing modules and barrier walls. We then perform a control volume analysis to evaluate the net heat flux and compute h, which quantifies the influence of these spatial modifications on convective cooling and, thus, module temperature and power output. Installing barrier walls yields the greatest improvements, increasing h by 3.4%, reducing module temperature by an estimated 2.5 °C, and boosting power output by an estimated 1.4% on average. These findings indicate that incorporating variable spacing or barrier-type elements into PV plant designs can reduce module temperature and, thus, improve PV performance and service life.

14 SOLAR ENERGY↗
DOE OSTI2026

Numerical Simulation of a Natural Convection–Driven Air-Cooled Reactor Cavity Cooling System Experiment

Ensuring the efficient removal of decay heat from the reactor vessel is essential for the safety of advanced reactor technologies. Several Generation-IV concepts incorporate variations in the reactor vessel cooling systems to achieve this objective. High-temperature gas-cooled reactors utilize a reactor cavity cooling system (RCCS), a passive ex-vessel system designed to operate without active components or external power during accident conditions. The RCCS removes decay heat primarily through radiative and convective heat transfer mechanisms. Here, this study presents a comprehensive validation of a computational fluid dynamics Reynolds-averaged Navier-Stokes model for the University of Wisconsin-Madison air-cooled RCCS facility. Validation was conducted for both high- and low-power natural convection cases under a uniform heating profile. Near-wall resolution was found to be critical for accurately modeling natural convection in the RCCS; employing an all-𝑦 + wall treatment resulted in wall temperature discrepancies exceeding 50 °⁢𝐶 compared to a wall-resolved mesh. Thermal-hydraulic behaviors under natural and forced convection conditions were compared within the heated cavity and RCCS. A turbulence model sensitivity analysis indicated that low-Reynolds number k-ɛ, k-ω shear stress transport (SST), and Reynolds stress transport models produce similar wall temperature predictions. A buoyancy modeling sensitivity study revealed that the Boussinesq approximation significantly underpredicted thermal-hydraulic behavior in the RCCS. Based on these findings, modeling recommendations are provided. The validated data set along with identified sensitivities refine the modeling of natural convection in the RCCS. The information produced by this study supports RCCS design, optimization, and safety evaluations, enabling the calibration and verification of reduced-order thermal-hydraulic models.

22 - GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2012

CONVECTIVE BURSTS AND THE COUPLING OF SATURN'S EQUATORIAL STORMS AND INTERIOR ROTATION

Temporal variations of Saturn's equatorial jet and magnetic field hint at rich dynamics coupling the atmosphere and the deep interior. However, it has been assumed that rotation of the interior dynamo must be steady over tens of years of modern observations. Here we use a numerical convection model and scaling estimates to show how equatorial convective bursts can transfer angular momentum to the deeper interior. The numerical model allows angular momentum transfer between a fluid outer spherical shell and a rigid inner sphere. Convection drives a prograde equatorial jet exhibiting quasiperiodic bursts that fill the equatorial volume outside the tangent cylinder. For each burst strong changes in the equatorial surface velocity are associated with retrograde torque on the inner sphere. Our results suggest that Saturn's Great White Spot, a giant storm that was observed to fill the equatorial region in 1990, could mobilize a volume of fluid carrying roughly 15% of Saturn's moment of inertia. Conservation of angular momentum then implies that a 20% change in the equatorial jet angular velocity could change the average interior rotation rate by about 0.1%-roughly an order of magnitude less than the apparent rotation rate changes associated with Saturn's kilometric radio (SKR) signal. However, if the SKR signal originates outside the liquid metal core in a 'planetary tachocline' that separates the layer of fast zonal flow from the magnetically controlled and slowly convecting deep interior, then convective bursts can provide a possible mechanism for the observed {approx}1% SKR changes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

LOW MACH NUMBER MODELING OF CORE CONVECTION IN MASSIVE STARS

This work presents three-dimensional simulations of core convection in a 15 M{sub Sun} star halfway through its main sequence lifetime. To perform the necessary long-time calculations, we use the low Mach number code MAESTRO, with initial conditions taken from a one-dimensional stellar model. We first identify several key factors that the one-dimensional initial model must satisfy to ensure efficient simulation of the convection process. We then use the three-dimensional simulations to examine the effects of two common modeling choices on the resulting convective flow: using a fixed composition approximation and using a reduced domain size. We find that using a fixed composition model actually increases the computational cost relative to using the full multi-species model because the fixed composition system takes longer to reach convection that is in a quasi-static state. Using a reduced (octant rather than full sphere) simulation domain yields flow with statistical properties that are within a factor of two of the full sphere simulation values. Both the octant and full sphere simulations show similar mixing across the convection zone boundary that is consistent with the turbulent entrainment model. However, the global character of the flow is distinctly different in the octant simulation, showing more rapid changes in the large-scale structure of the flow and thus a more isotropic flow on average.

79 ASTRONOMY AND ASTROPHYSICS↗