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At least 1,261 records · Page 70
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DOE OSTI2013

MAGNETIC ENERGY CASCADE IN SPHERICAL GEOMETRY. I. THE STELLAR CONVECTIVE DYNAMO CASE

We present a method to characterize the spectral transfers of magnetic energy between scales in simulations of stellar convective dynamos. The full triadic transfer functions are computed thanks to analytical coupling relations of spherical harmonics based on the Clebsch-Gordan coefficients. The method is applied to mean field {alpha}{Omega} dynamo models as benchmark tests. From a physical standpoint, the decomposition of the dynamo field into primary and secondary dynamo families proves very instructive in the {alpha}{Omega} case. The same method is then applied to a fully turbulent dynamo in a solar convection zone, modeled with the three-dimensional MHD Anelastic Spherical Harmonics code. The initial growth of the magnetic energy spectrum is shown to be non-local. It mainly reproduces the kinetic energy spectrum of convection at intermediate scales. During the saturation phase, two kinds of direct magnetic energy cascades are observed in regions encompassing the smallest scales involved in the simulation. The first cascade is obtained through the shearing of the magnetic field by the large-scale differential rotation that effectively cascades magnetic energy. The second is a generalized cascade that involves a range of local magnetic and velocity scales. Non-local transfers appear to be significant, such that the net transfers cannot be reduced to the dynamics of a small set of modes. The saturation of the large-scale axisymmetric dipole and quadrupole is detailed. In particular, the dipole is saturated by a non-local interaction involving the most energetic scale of the magnetic energy spectrum, which points to the importance of the magnetic Prandtl number for large-scale dynamos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CHEMICAL TRANSPORT AND SPONTANEOUS LAYER FORMATION IN FINGERING CONVECTION IN ASTROPHYSICS

A region of a star that is stable to convection according to the Ledoux criterion may nevertheless undergo additional mixing if the mean molecular weight increases with radius. This process is called fingering (thermohaline) convection and may account for some of the unexplained mixing in stars such as those that have been polluted by planetary infall and those burning {sup 3}He. We propose a new model for mixing by fingering convection in the parameter regime relevant for stellar (and planetary) interiors. Our theory is based on physical principles and supported by three-dimensional direct numerical simulations. We also discuss the possibility of formation of thermocompositional staircases in fingering regions, and their role in enhancing mixing. Finally, we provide a simple algorithm to implement this theory in one-dimensional stellar codes, such as KEPLER and MESA.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

FLUX EMERGENCE IN A MAGNETIZED CONVECTION ZONE

We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux ropes and their influence on the global external magnetic field. We ran three-dimensional MHD numerical simulations using the ASH code (anelastic spherical harmonics) and analyzed the dynamical evolution of such buoyant flux ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model can only very crudely and inaccurately represent the local dynamics of the buoyant rise of the implanted magnetic structure, but nonetheless allows us to study the influence of global effects, such as self-consistently generated differential rotation and meridional circulation, and of Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun, where the local dynamics are completely different. The properties of initial phases of the buoyant rise are determined essentially by the flux-rope's properties and the convective flows and consequently are in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increasingly strong as the flux ropes evolve. During the buoyant rise across the convection zone, the flux-rope's magnetic field strength scales as B{proportional_to}{rho}{sup {alpha}}, with {alpha} {approx}< 1. An increase of radial velocity, density, and current density is observed to precede flux emergence at all longitudes. The geometry, latitude, and relative orientation of the flux ropes with respect to the background magnetic field influences the resulting rise speeds, zonal flow amplitudes (which develop within the flux ropes), and the corresponding surface signatures. This influences the morphology, duration and amplitude of the surface shearing, and the Poynting flux associated with magnetic flux-rope emergence. The emerged magnetic flux influences the system's global polarity, leading in some cases to a polarity reversal while inhibiting the background dynamo from doing so in others. The emerged magnetic flux is slowly advected poleward while being diffused and assimilated by the background dynamo field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TOMOGRAPHY OF PLASMA FLOWS IN THE UPPER SOLAR CONVECTION ZONE USING TIME-DISTANCE INVERSION COMBINING RIDGE AND PHASE-SPEED FILTERING

The consistency of time-distance inversions for horizontal components of the plasma flow on supergranular scales in the upper solar convection zone is checked by comparing the results derived using two k-{omega} filtering procedures-ridge filtering and phase-speed filtering-commonly used in time-distance helioseismology. I show that both approaches result in similar flow estimates when finite-frequency sensitivity kernels are used. I further demonstrate that the performance of the inversion improves (in terms of a simultaneously better averaging kernel and a lower noise level) when the two approaches are combined together in one inversion. Using the combined inversion, I invert for horizontal flows in the upper 10 Mm of the solar convection zone. The flows connected with supergranulation seem to be coherent only for the top {approx}5 Mm; deeper down there is a hint of change of the convection scales toward structures larger than supergranules.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Helium-abundance and other composition effects on the properties of stellar surface convection in solar-like main-sequence stars

We investigate the effect of helium abundance and α-element enhancement on the properties of convection in envelopes of solar-like main-sequence stars using a grid of three-dimensional radiation hydrodynamic simulations. Helium abundance increases the mean molecular weight of the gas and alters opacity by displacing hydrogen. Since the scale of the effect of helium may depend on the metallicity, the grid consists of simulations with three helium abundances (Y = 0.1, 0.2, 0.3), each with two metallicities (Z = 0.001, 0.020). We find that changing the helium mass fraction generally affects structure and convective dynamics in a way opposite to that of metallicity. Furthermore, the effect is considerably smaller than that of metallicity. The signature of helium differs from that of metallicity in the manner in which the photospheric velocity distribution is affected. We also find that helium abundance and surface gravity behave largely in similar ways, but differ in the way they affect the mean molecular weight. A simple model for spectral line formation suggests that the bisectors and absolute Doppler shifts of spectral lines depend on the helium abundance. We look at the effect of α-element enhancement and find that it has a considerably smaller effect on the convective dynamics in the superadiabatic layer compared to that of helium abundance.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Spoke-like differential rotation in a convective dynamo with a coronal envelope

We report on the results of four convective dynamo simulations with an outer coronal layer. The magnetic field is self-consistently generated by the convective motions beneath the surface. Above the convection zone, we include a polytropic layer that extends to 1.6 solar radii. The temperature increases in this region to ≈8 times the value at the surface, corresponding to ≈1.2 times the value at the bottom of the spherical shell. We associate this region with the solar corona. We find solar-like differential rotation with radial contours of constant rotation rate, together with a near-surface shear layer. This non-cylindrical rotation profile is caused by a non-zero latitudinal entropy gradient that offsets the Taylor-Proudman balance through the baroclinic term. The meridional circulation is multi-cellular with a solar-like poleward flow near the surface at low latitudes. In most of the cases, the mean magnetic field is oscillatory with equatorward migration in two cases. In other cases, the equatorward migration is overlaid by stationary or even poleward migrating mean fields.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

RADIAL STELLAR PULSATION AND THREE-DIMENSIONAL CONVECTION. IV. FULL AMPLITUDE THREE-DIMENSIONAL SOLUTIONS

Three-dimensional hydrodynamic simulations of full amplitude RR Lyrae stars have been computed for several models across the instability strip. The three-dimensional nature of the calculations allows convection to be treated without reference to a phenomenological approach such as the local mixing length theory. Specifically, the time-dependent interaction of large-scale eddies and radial pulsation is controlled by conservation laws, while the effects of smaller convective eddies are simulated by an eddy viscosity model. The light amplitudes for these calculations are quite similar to those of our previous two-dimensional calculations in the middle of the instability strip, but somewhat lower near the red edge, the fundamental blue edge, and for the one first overtone model we computed. The time-dependent interaction between the radial pulsation and the convective energy transport is essentially the same in three dimensions as it is in two dimensions. There are some differences between the light curves of the two- and three-dimensional simulations, particularly during decreasing light. Reasons for the differences, both numerical and physical, are explored.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

GRAND MINIMA AND EQUATORWARD PROPAGATION IN A CYCLING STELLAR CONVECTIVE DYNAMO

The 3D MHD Anelastic Spherical Harmonic code, using slope-limited diffusion, is employed to capture convective and dynamo processes achieved in a global-scale stellar convection simulation for a model solar-mass star rotating at three times the solar rate. The dynamo-generated magnetic fields possesses many timescales, with a prominent polarity cycle occurring roughly every 6.2 years. The magnetic field forms large-scale toroidal wreaths, whose formation is tied to the low Rossby number of the convection in this simulation. The polarity reversals are linked to the weakened differential rotation and a resistive collapse of the large-scale magnetic field. An equatorial migration of the magnetic field is seen, which is due to the strong modulation of the differential rotation rather than a dynamo wave. A poleward migration of magnetic flux from the equator eventually leads to the reversal of the polarity of the high-latitude magnetic field. This simulation also enters an interval with reduced magnetic energy at low latitudes lasting roughly 16 years (about 2.5 polarity cycles), during which the polarity cycles are disrupted and after which the dynamo recovers its regular polarity cycles. An analysis of this grand minimum reveals that it likely arises through the interplay of symmetric and antisymmetric dynamo families. This intermittent dynamo state potentially results from the simulation’s relatively low magnetic Prandtl number. A mean-field-based analysis of this dynamo simulation demonstrates that it is of the α-Ω type. The timescales that appear to be relevant to the magnetic polarity reversal are also identified.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SUPERGRANULES AS PROBES OF SOLAR CONVECTION ZONE DYNAMICS

Supergranules are convection cells seen at the Sun's surface as a space filling pattern of horizontal flows. While typical supergranules have diameters of about 35 Mm, they exhibit a broad spectrum of sizes from {approx}10 Mm to {approx}100 Mm. Here we show that supergranules of different sizes can be used to probe the rotation rate in the Sun's outer convection zone. We find that the equatorial rotation rate as a function of depth as measured by global helioseismology matches the equatorial rotation as a function of wavelength for the supergranules. This suggests that supergranules are advected by flows at depths equal to their wavelengths and thus can be used to probe flows at those depths. The supergranule rotation profiles show that the surface shear layer, through which the rotation rate increases inward, extends to depths of {approx}50 Mm and to latitudes of at least 70 Degree-Sign . Typical supergranules are well observed at high latitudes and have a range of sizes that extend to greater depths than those typically available for measuring subsurface flows with local helioseismology. These characteristics indicate that probing the solar convection zone dynamics with supergranules can complement the results of helioseismology.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CYCLIC THERMAL SIGNATURE IN A GLOBAL MHD SIMULATION OF SOLAR CONVECTION

Global magnetohydrodynamical simulations of the solar convection zone have recently achieved cyclic large-scale axisymmetric magnetic fields undergoing polarity reversals on a decadal time scale. In this Letter, we show that these simulations also display a thermal convective luminosity that varies in-phase with the magnetic cycle, and trace this modulation to deep-seated magnetically mediated changes in convective flow patterns. Within the context of the ongoing debate on the physical origin of the observed 11 yr variations in total solar irradiance, such a signature supports the thesis according to which all, or part, of the variations on decadal time scales and longer could be attributed to a global modulation of the Sun's internal thermal structure by magnetic activity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

IDENTIFYING POTENTIAL MARKERS OF THE SUN'S GIANT CONVECTIVE SCALE

Line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory (SDO) are analyzed using a diagnostic known as the magnetic range of influence (MRoI). The MRoI is a measure of the length over which a photospheric magnetogram is balanced and so its application gives the user a sense of the connective length scales in the outer solar atmosphere. The MRoI maps and histograms inferred from the SDO/HMI magnetograms primarily exhibit four scales: a scale of a few megameters that can be associated with granulation, a scale of a few tens of megameters that can be associated with super-granulation, a scale of many hundreds to thousands of megameters that can be associated with coronal holes and active regions, and a hitherto unnoticed scale that ranges from 100 to 250 Mm. We infer that this final scale is an imprint of the (rotationally driven) giant convective scale on photospheric magnetism. This scale appears in MRoI maps as well-defined, spatially distributed concentrations that we have dubbed ''g-nodes''. Furthermore, using coronal observations from the Atmospheric Imaging Assembly on SDO, we see that the vicinity of these g-nodes appears to be a preferred location for the formation of extreme-ultraviolet (and likely X-Ray) brightpoints. These observations and straightforward diagnostics offer the potential of a near real-time mapping of the Sun's largest convective scale, a scale that possibly reaches to the very bottom of the convective zone.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

MEAN-FIELD MODELING OF AN α{sup 2} DYNAMO COUPLED WITH DIRECT NUMERICAL SIMULATIONS OF RIGIDLY ROTATING CONVECTION

The mechanism of large-scale dynamos in rigidly rotating stratified convection is explored by direct numerical simulations (DNS) in Cartesian geometry. A mean-field dynamo model is also constructed using turbulent velocity profiles consistently extracted from the corresponding DNS results. By quantitative comparison between the DNS and our mean-field model, it is demonstrated that the oscillatory α{sup 2} dynamo wave, excited and sustained in the convection zone, is responsible for large-scale magnetic activities such as cyclic polarity reversal and spatiotemporal migration. The results provide strong evidence that a nonuniformity of the α-effect, which is a natural outcome of rotating stratified convection, can be an important prerequisite for large-scale stellar dynamos, even without the Ω-effect.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

EXPLAINING THE COEXISTENCE OF LARGE-SCALE AND SMALL-SCALE MAGNETIC FIELDS IN FULLY CONVECTIVE STARS

Despite the lack of a shear-rich tachocline region, low-mass fully convective (FC) stars are capable of generating strong magnetic fields, indicating that a dynamo mechanism fundamentally different from the solar dynamo is at work in these objects. We present a self-consistent three-dimensional model of magnetic field generation in low-mass FC stars. The model utilizes the anelastic magnetohydrodynamic equations to simulate compressible convection in a rotating sphere. A distributed dynamo working in the model spontaneously produces a dipole-dominated surface magnetic field of the observed strength. The interaction of this field with the turbulent convection in outer layers shreds it, producing small-scale fields that carry most of the magnetic flux. The Zeeman–Doppler-Imaging technique applied to synthetic spectropolarimetric data based on our model recovers most of the large-scale field. Our model simultaneously reproduces the morphology and magnitude of the large-scale field as well as the magnitude of the small-scale field observed on low-mass FC stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Measurement of Convective Heat Transfer Coefficients With Supercritical CO 2 Using the Wilson-Plot Technique

This paper describes the measurement of convective heat transfer coefficients and friction factors for sCO 2 flowing in a smooth tube and compares the results with published correlations for validation. The paper also describes the Heat Exchange and Experimental Testing (HEET) rig recently designed and built at the U.S. Department of Energy’s (DoE’s) National Energy Technology Laboratory (NETL) in Morgantown, WV. Here, the Wilson-plot technique used for measuring the heat transfer coefficients is described along with the data reduction process. The Wilson-plot technique was chosen as the basis for the design of NETL’s HEET rig. Advantages of the Wilson-plot technique include the (1) ability to measure high convective heat transfer coefficients accurately, (2) ability to measure average heat transfer coefficient for complicated heat exchange geometries like those produced using additive manufacturing, (3) ability to measure heat transfer coefficients on both sides of a heat exchanger independently, and (4) simplicity of experimental setup. Capabilities of the HEET rig include pressure to 24 MPa (3500 psig), temperature to 538 °C (1000 °F), mass flow rate to 1.5 kg/s (3 lb/s), and Re to 500,000. The rig is designed to operate with pure CO 2 or a mixture of CO 2 and up to 10% N2 by volume to study the impact of gas mixtures typical of direct-fired sCO 2 power cycles on the convective heat transfer and pressure drop. Preliminary tests in the HEET rig were performed with smooth stainless-steel tube and pure CO 2 , and the results were compared with published correlations for Nusselt number (Nu) and friction factor. Over a Reynolds number (Re) range from 58,000 to 228,000, measured Nu was compared to predictions using the Dittus and Boelter equation (Kreith and Bohn, 1993, “Principles of Heat Transfer, West Publishing Company”) within 5% and measured friction factors were compared to predictions using the McAdams correlation (“McAdams, 1954, “Heat Transmission,” 3rd ed., McGraw Hill, New York)” for smooth tube to be within 5%.

42 ENGINEERING↗
DOE OSTI2017

Rayleigh convective instability in a cloud medium

The problem of convective instability of an atmospheric layer containing a horizontally finite region filled with a cloud medium is considered. Solutions exponentially growing with time, i.e., solitary cloud rolls or spatially localized systems of cloud rolls, have been constructed. In the case of axial symmetry, their analogs are convective vortices with both ascending and descending motions on the axis and cloud clusters with ring-shaped convective structures. Depending on the anisotropy of turbulent exchange, the scale of vortices changes from the tornado scale to the scale of tropical cyclones. The solutions with descending motions on the axis can correspond to the formation of a tornado funnel or a hurricane eye in tropical cyclones.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Influence of Sedimentation Length on the Convective Stability of a Colloidal Suspension

The convective stability of a colloidal suspension is studied in the case when the vertical dimension of the cavity is less than or comparable with the sedimentation length of nanoparticles. The analysis is carried out within the Boussinesq approximation on the basis of a modified model that takes into account the dependence of a thermodiffusion flow on the local value of impurity concentration. A new parameter of the problem is the ratio of the sedimentation length to the vertical dimension of the cavity. For a quiescent colloidal suspension, exact and approximate (in the case of small concentrations) solutions are obtained that describe the distributions of nanoparticles. A transformation is obtained that allows one to investigate the convective stability of a colloidal suspension stratified in the gravitational field by the Galerkin method with a set of simple trial functions. Instability boundaries and the characteristics of critical perturbations are determined. It is shown that, in the case of negative thermodiffusion, a decrease in the sedimentation length leads to a decrease in the convection threshold and the frequency of neutral oscillations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Effect of natural convection on the current-voltage characteristic of a DC discharge in neon at intermediate pressures

The parameters of the positive column of a glow discharge in neon are calculated with allowance for the induced hydrodynamic motion. It is shown that natural convection in the pressure range of {approx}0.1 atm significantly affects the profiles of the parameters of the positive column and its current-voltage characteristic. The convection arising at large deposited energies improves heat removal, due to which the temperature in the central region of the discharge becomes lower than that calculated from the heat conduction equation. As a result, the current-voltage characteristic is shifted. With allowance for convection, the current-voltage characteristic changes at currents much lower than the critical current at which a transition into the constricted state is observed. This change is uniquely related to the Rayleigh number in the discharge. Thus, a simplified analysis of thermal conduction and diffusion, even with detailed account of kinetic processes occurring in the positive column, does not allow one to accurately calculate the current-voltage characteristic and other discharge parameters at intermediate gas pressures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Forecasting for ESCAPE: A Multi-Institution Hybrid Forecasting and Nowcasting Operation for Sea-Breeze Convection Supporting a Ground-Based and Airborne Field Campaign

The Experiment of Sea-Breeze Convection, Aerosols, Precipitation and Environment (ESCAPE) field project deployed two aircraft and ground-based assets in the vicinity of Houston, Texas, between 27 May and 2 July 2022, examining how meteorological conditions, dynamics, and aerosols control the initiation, early growth stage, and evolution of coastal convective clouds. To ensure that airborne- and ground-based assets were deployed appropriately, a forecasting and nowcasting team was formed. Daily forecasts guided real-time decision-making by assessing synoptic weather conditions, environmental aerosol, and a variety of atmospheric modeling data to assign a probability for meeting specific ESCAPE campaign objectives. During the research flights, a small team of forecasters provided “nowcasting” support by analyzing radar, satellite, and new model data in real time. The nowcasting team proved invaluable to the campaign operation, as sometimes changing environmental conditions affected, for example, the timing of convective initiation. In addition to the success of the forecasting and nowcasting teams, the ESCAPE campaign offered a unique “testbed” opportunity where in-person and virtual support both contributed to campaign objectives. The forecasting and nowcasting teams were each composed of new and experienced forecasters alike, where new forecasters were given invaluable experience that would otherwise be difficult to attain. Both teams received training on forecast models, map analysis, Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), and thermodynamic sounding analysis before the beginning of the campaign. In this article, the ESCAPE forecasting and nowcasting teams reflect on these experiences, providing potentially useful advice for future field campaigns requiring forecasting and nowcasting support in a hybrid virtual/in-person framework.

54 ENVIRONMENTAL SCIENCES↗