Showing papers on "Convection published in 1983"

Magnetic Neutral Sheets in Evolving Fields - Part Two - Formation of the Solar Corona

Abstract: It is shown in the previous paper that whenever twisted flux tubes are bundled together, they are subject to dynamical nonequilibrium and internal neutral point reconnection, causing rapid dissipation of their torsion. In the present paper we explore the consequences of this general dynamical dissipation in the magnetic fields that produce the active corona of the Sun. The footpoints of the field are continually manipulated by the subphotospheric convection, so that the lines of force are continually wrapped and rotated about each other. The dynamical dissipation of the wrapping and rotation transfers the work done on the footpoints directly into heat in the corona, at a rate estimated to be of the order of 10/sup 7/ ergs cm/sup -2/ s/sup -1/. The effect appears to be the principal source of heat to the visible corona. This general picture implies that all magnetic fields extending outward from convecting astronomical bodies produce intense heating of the tenuous outer atmospheres of these bodies, in general agreement with the observed fact of the universal activity of stars and galaxies.

Deep convection in the World Ocean

Abstract: A brief discussion of, and a little speculation about, the relevance of the polar regions on climate is given. The main body of the paper gives a survey of the known deep convection areas of the world ocean. There are two distinct types of convection. The first is the classic sinking occurring on continental shelf slope systems, as typified by various locations around the Antarctic coast. The freezing of sea ice, and resulting brine ejection, creates dense salty water on the shelf which descends the slope under a balance of Coriolis, gravity, and frictional forces, entraining the surrounding warm deep water as it goes. The second process is the more recently observed open-ocean convection, occurring in locations such as the Mediterranean, the Labrador Sea, and two locations in the Weddell gyre, and is hypothesized to occur in the Greenland Sea. Open-ocean convection has many overall similarities in all these areas: it occurs in narrow (20–50 km) areas; it forms about 10 m³ s−l of deep water; it occurs only in regions of cyclonic mean circulation; more than one water mass in the mean circulation is involved; a preconditioning seems to be required; some surface forcing (cooling or sea ice formation) is necessary; a violent breakup of the water mass frequently occurs on time scales of 2 weeks.

The “Heatline” Visualization of Convective Heat Transfer

Abstract: Proposition d'une nouvelle methode de visualisation du transfert de chaleur dans un ecoulement de fluide. Application a la convection naturelle dans une enceinte carree chauffee lateralement

Self-potential modeling from primary flows

Abstract: This paper discusses a new method for the investigation of self potentials (SP) based on induced current sources. The induced current sources are due to divergences of the convection current which is driven, in turn, by a primary flow, either heat or fluid. As a result of using this approach there is a shift in emphasis toward the vector flow field and its interaction with current cross‐coupling structure when compared with the total potential approach of Nourbehecht (1963) which emphasized the primary flow potential and the voltage cross‐coupling. This shift in emphasis is advantageous because it is analogous to the actual physical processes. For example, fluid flow in the ground gives rise to drag (convection) currents, and the interaction of the convection currents with the electrical structure gives rise to the electrical potentials (SP). This simple physical picture should aid in developing a better intuitive understanding of the generation of SP effects. The convective current approach is easily ada...

The relationship between surface topography, gravity anomalies, and temperature structure of convection

Abstract: Consideration is given to the relationship between the temperature structure of mantle convection and the resulting surface topography and gravity anomalies, which are used in its investigation. Integral expressions relating the three variables as a function of wavelength are obtained with the use of Green's function solutions to the equations of motion for the case of constant-viscosity convection in a plane layer subject to a uniform gravitational field. The influence of the boundary conditions, particularly at large wavelengths, is pointed out, and surface topographies and gravity produced by convection are illustrated for a number of simple temperature distributions. It is shown that the upper thermal boundary layer plays an important role in determining the surface observables, while temperatures near the bottom of the layer affect mainly that boundary. This result is consistent with an explanation of geoid anomalies over mid-ocean swells in terms of convection beneath the lithosphere.

Liquid water and active resurfacing on Europa

Abstract: Arguments for recent resurfacing of Europa by H2O from a liquid layer are presented, based on new interpretations of recent spacecraft and earth-based observations and revised theoretical calculations. The heat flow in the core and shell due to tidal forces is discussed, and considerations of viscosity and convection in the interior are found to imply water retention in the outer 60 km or so of the silicates, forming a layer of water/ice many tens of km thick. The outer ice crust is considered to be too thin to support heat transport rates sufficient to freeze the underlying water. Observational evidence for the calculations would consist of an insulating layer of frosts derived from water boiling up between cracks in the surface crust. Evidence for the existence of such a frost layer, including the photometric function of Europa and the deposits of sulfur on the trailing hemisphere, is discussed.

Systematics of the equatorward diffuse auroral boundary

Abstract: DMSP/F2 and DMSP/F4 precipitating electron data are used to determine statistically the systematic variations of the equatorward diffuse auroral boundary with Kp as a function of local time. This work extends a previous study of dawn and dusk boundaries to the noon and midnight regions. The boundaries are well-ordered by Kp in the night sector but show increasingly greater scatter from dawn to postnoon. In the noon sector it is often the case that no diffuse aurora is discernible within detector sensitivity. The equatorward diffuse auroral boundary is well fit by a circle at each activity level. The center of the circle is offset from the geomagnetic pole, and the radius of the circle increases with increasing magnetic activity. The circular fits are projected to the ecliptic plane by using the Mead-Fairfield magnetic field model where they are identified with the inner edge of the plasma sheet in order to make comparison with Volland-Stern type convection electric field predictions. Previously found variations of the electric field with Kp are confirmed, including a rotation of the axis of symmetry, away from the dawn-dusk meridian. All equatorward auroral boundaries from DMSP for 1978 were compiled. Using the equations for boundary variations with Kp, each evening sector boundary was projected to a midnight boundary. The projected midnight boundary serves as an index of auroral activity and an indicator of the strength of the large-scale magnetospheric convection electric field. The auroral boundary index is given in monthly plots for 1978.

Large-Scale Response of the Tropical Atmosphere to Transient Convection

Abstract: We consider the problem of the linear response of a stratified, equatorial, β-plane model atmosphere to specified transient sources of heat and momentum. The method of solution involves transforms in all three spatial coordinates. A finite Sturm-Liouville transform is used in z, a Fourier transform in x, and a generalized Hermite transform in y. The resulting spectral equations can then be solved analytically for a specified forcing. Of particular interest is the case of a Gaussian-shaped heat source centered at latitude yo and with e-folding radius a. The heat source is transient and has time scale 1/α. Using the Parceval relation we compute how the forced energy is partitioned between Kelvin, mixed Rossby-gravity, Rossby and gravity modes as a function of a, yo, α. Model results using a heat source centered at 11°S with an e-folding radius of 750 km and a time scale of about a day indicate that many aspects of the summertime upper tropospheric circulation over South America can be explained by ...

Numerical methods in heat transfer

Abstract: Topics discussed in this book include modelling the effects of fire, ablation, heat flow in porous rock, thermal stress and dissolving coal. Alternative energy sources such as geothermal reservoirs and solar radiation are also discussed. Includes bibliographies at the end of the papers, a cited author index, and a subject index. Contents, abridged: Exact finite element solutions for linear steady state thermal problems. Steep gradient modelling in diffusion problems. Numerical solution of coupled conduction-convection problems using lumped-parameter methods. The prediction of turbulent heat transfer by the finite element methods. The influence of creep and transformation plasticity in the analysis of stresses due to heat treatment. Heat and moisture movement in wood composite materials during the pressing operation-a simplified model. Index.

The boundary correction for the Rayleigh-Darcy problem: limitations of the Brinkman equation

Abstract: The no-slip condition on rigid boundaries necessitates a correction to the critical value of the Rayleigh–Darcy number for the onset of convection in a horizontal layer of a saturated porous medium uniformly heated from below. It is shown that the use of the Brinkman equation to obtain this correction is not justified, because of the limitations of that equation. These limitations are discussed in detail. An alternative procedure, based on a model in which the porous medium is sandwiched between two fluid layers, and the Beavers–Joseph boundary condition is applied at the interfaces, is described, and an expression for the correction is obtained. It is found that the correction can be of either sign, depending on the relative magnitudes of the parameters involved.

Heat transfer and horizontally averaged temperature of convection with large viscosity variations

Abstract: It is pointed out that the understanding of convection in large-Prandtl-number Boussinesq fluids with uniform properties and contained in simple geometries is virtually complete. Present efforts are typically directed towards relaxing some of the original assumptions by going to lower Prandtl number, more complicated geometries, variable material properties, or introducing new dynamical processes such as the Lorentz forces. A description is given of experiments which are concerned with the effect on convection of relaxing the assumption of a uniform viscosity. The reported experiments were designed to measure both the horizontally averaged temperature as a function of depth and the heat transfer of convection over a range of viscosity variations up to 100,000.

Momentum Transport by a Line of Cumulonimbus

Abstract: The vertical transport of horizontal momentum normal to a line of cumulonimbus observed during GATE on 14 September 1974 is against the vertical momentum gradient, contrary to the predictions of mixing-length theory. Data from repeated aircraft passes normal to the line's axis at heights from 0.15 to 5.5 km are used to document the flux and determine its source. The flux is concentrated in roughly a 25 km wide “active zone” just behind the leading edge of the line, in kilometer-scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces. The fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic, resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical, with the clouds at the surface well ahead of those aloft. Evaluation of the terms in the momentum-flux generation equation confirms that the above process, reflected by the velocity-buoyan...

Heat transfer and intraplate deformation in the central Indian Ocean

Abstract: Nineteen new heat flow measurements made across deformed oceanic lithosphere in the Central Indian Ocean, and previously published data show that heat flow is significantly higher than predicted by models for cooling oceanic lithosphere over much of the region. Many of the temperature-depth profiles are nonlinear. Upward convection of water is the most likely explanation for the curvature of the temperature profiles, since other possible causes, including variations in bottom water temperatures, conductivity changes with depth in the sediments, and experimental error, can be eliminated. This interpretation requires water velocities of the order of 7 x 10/sup -8/ m/s, which is unusual because the lithosphere is relatively old (72-82 m.y.) and a thick sedimentary cover (1-2.5 km) is present. These observations suggest that the processes causing deformation of the plate have increased the heat flux through the sediment-water interface. We infer that extra heat is being generated at shallow depths (perhaps less than 35 km) in the plate, although the specific mechanism by which deformational energy is converted into heat is difficult to determine.

Joule heating at high latitudes

Abstract: Calculations based on simultaneous observations of the electric field magnitude, and individual measurements of ion drift velocity and particle precipitation, over the lifetime of the AE-C satellite, are used to determine high latitude Joule heating. Conductivities produced by an averaged seasonal illumination were included with those calculated from particle precipitation. It is found that high latitude Joule heating occurs in an approximately oval pattern, and consists of dayside cleft, dawn and dusk sunward convection, and night sector heating regions. On average, heating in the cleft and dawn-dusk regions contributes the largest heat input, and there is no apparent difference between hemispheres for similar seasons. Joule heat input is 50 percent greater in summer than in winter, due primarily to the greater conductivity caused by solar production.

Heat transfer in porous media heated from above with evaporation, condensation, and capillary effects

Abstract: Heat and mass transfer characteristics of a sand-water-stream system heated at the top and cooled at the bottom were studied. It was found that at steady-state conditions, the system segregated into 3 regions. The top region was conduction-dominated with the voids containing a stationary superheated steam. The middle region was convection-dominated, nearly isothermal, and exhibited an upward flow of the liquid by capillary forces and a downward flow of steam due to a slight pressure gradient. The bottom portion contained a stationary compressed liquid and was conduction dominated also. The length of the 2-phase convection zone was evaluated through the application of Darcy's equations for 2-phase flow and correlations of relative permeabilities and capillary pressure data. A stability analysis demonstrated that the superheated liquid can exist in an unconditionally stable state under conditions typical of porous systems. 17 references.

An experimental and theoretical investigation of the onset of convection in rotating spherical shells

Abstract: Convection in a rapidly rotating spherical layer with constant-temperature boundary conditions is studied in a laboratory experiment. The asymptotic theory of Busse (1970) is extended to permit a comparison with the observations of the onset of convection and its properties. It is found that the prediction of the power-law dependences of the critical buoyancy number and the critical wavenumber on the rotation rate are borne out, although discrepancies in the actual values of these quantities do exist. Calculations on the basis of equations proposed by Roberts (1968) show that a thermal wind that is present in the basic state of the model has a stabilizing influence on the onset of convection. Stewartson layers not taken into account in the asymptotic analysis for vanishing Ekman number E appear to be responsible for the remaining disagreement between theoretical predictions and observations at finite values of E.

Radiation-Natural Convection Interactions in Two-Dimensional Complex Enclosures

Abstract: A numerical finite-difference study has been carried out for the two-dimensional radiation-natural convection interaction phenomena in square enclosures with equal vertical finite-thickness partitions located at the centers of the ceiling and floor. Both participating gases (CO2 and NH3 ) and nonparticipating gas (air) are considered. In the radiation calculations, the nongray exponential wide-band models for CO2 and NH3 are used, together with a radial flux method utilizing a more realistic polar description for the radiation exchange in the enclosure. Results on the effects of both surface and gas radiation on the velocity and temperature fields and the overall heat transfer rates as functions of the partition heights at two levels of the Grashof number are presented and discussed in terms of the physical phenomena.

Numerical models for the hydrothermal field at the Galapagos Spreading Center

Abstract: The heat flow distribution at the Galapagos Spreading Center is compared to results of two-dimensional numerical models for the hydrothermal convection through oceanic crust. The model calculations are based on the equations for fluid flow through porous media adapted for the situation at spreading oceanic ridges. The temperature- and pressure-dependent thermodynamic characteristics of water were used in the fluid flow equations. Models with average permeabilities of approximately 5 x 10/sup -15/ m/sup 2/ and penetration depths between 2 and 5 km produce heat flow distributions compatible with the observations at the Galapagos Spreading Center. Because of the convective heat loss, temperatures within the hydrothermal layer are significantly lower than for conductively cooling crust. Two different types of convection cells develop. The one or two cells closest to the ridge axis are fixed in location with respect to the ridge axis. Convection there is characterized by high temperatures (>300 /sup 0/C), rapid flow rates, and low water to rock ratios (approx.1). These cells remove most of the heat associated with the intrusion process at the ridge axis. Cells farther away from the ridge axis move with the moving plate and serve to prevent the oceanic crust from reheating. Temperatures there typicallymore » are moderate to low (<200 /sup 0/C), and flow velocities are lower than those in the axial cell, but water to rock ratios can be very high in these cells.« less

A model of mean zonal flows in the major planets

Abstract: The linear theory of deep zonal flows developed by Busse (1976) for the origins of deep motions of the atmospheres of Jupiter and Saturn is extended into the nonlinear regime. Relationships for the relative magnitudes of convective heat and momentum transports are formulated. A perturbation approach is taken to the problem, with the amplitude of the convection serving as the small parameter, and the basic equations being expanded in terms of the Prandtl number. The Boussinesq approximation is employed, together with an assumption of a low Rossby number for the Jovian and Saturn atmospheres. Differences in the amplitude of the Jovian equatorial jet relative to that of Saturn are explored in terms of a low equatorial convective heat flux on Jupiter.

On Assessing Local Conditional Symmetric Instability from Atmospheric Soundings

Abstract: The standard parcel method of assessing the susceptibility of the atmosphere to moist convection using tephigrams is extended to account for the centrifugal as well as the gravitational potential energy of the displaced air parcel. This leads to a measure of the stability of the moist baroclinic atmosphere to finite slantwise reversible displacements of a two-dimensional air parcel; such a measure differs from previously derived measures of conditional symmetric instability which have considered only infinitesimal displacements in saturated atmospheres. It is demonstrated that the combined gravitational and centrifugal potential of a two-dimensional air parcel or “tube” can be assessed by displacing the tube slantwise along a surface of constant angular momentum, and that this combined potential energy can be estimated using a single atmospheric sounding. Several examples of the application of this technique are presented in the context of a case study of apparent slantwise convection. The result...

A Simulation of Bomb Tritium Entry into the Atlantic Ocean

Abstract: Tritium is used in a model calibration study that is aimed at developing three-dimensional ocean circulation and mixing models for climate and geochemical simulations. The North Atlantic tritium distribution is modeled using a three-dimensional advective field predicted by a primitive equation ocean circulation model. The effect of wintertime convection is parameterized by homogenizing the tracer to the observed March mixed-layer depth. Mixing is parameterized by horizontal and vertical Fickian diffusivities of 5 × 10−6 cm2 s−1 and 0.5 cm2 s−1, respectively. The spreading of tritium in the model is dominated by advection in the horizontal, and by wintertime convection and advection in the vertical. The horizontal and vertical mixing provided by the model have negligible effect. A comparison of the model tracer fields with observations shows that most of the basic patterns of the tritium field are reproduced. The model's mean vertical penetration of 543 m in 1972 is comparable to the 592 m penetra...

Behavior of small particles in a thermal plasma flow

Abstract: In this paper computational results are presented which reveal the effects of the Knudsen number on heat transfer and drag of small particles in a flowing thermal argon plasma. The Knudsen number is restricted to moderate values so that “temperature jump” and “velocity slip” conditions may be employed, and for the governing equations the continuum approach remains valid. It is shown that the ratio of the heat fluxes with and without the Knudsen effect is almost identical to the ratio obtained by the authors for the case of pure heat conduction. This fact is very important for modeling of the behavior of particles injected into an actual plasma reactor when the Knudsen effect has to be taken into account.

On the Evolution of the QE II Storm. II: Dynamic and Thermodynamic Structure

Abstract: The existence of convection and the hurricane-like structure in the explosively-developing cyclone studied in Part I motivates us to assess the importance heating had on this cyclogenesis. To accomplish this, a method to evaluate the three-dimensional thermodynamic and dynamic structure of the atmosphere is proposed, so that we may evaluate potential vorticity changes in the vicinity of this cyclone. Results indicate a 24 h lower tropospheric generation of from five to thirteen times the value observed at 1200 GMT 9 September 1978. An evaluation of physical effects on thickness change following the surface center shows a large mean tropospheric temperature rise to be due to bulk cumulus heating effects, which could be important in the extraordinary potential vorticity generation concurrent with this cyclone's explosive development. These vertically integrated values of heating motivate us to solve the quasi-geostrophic omega and vorticity equations forced by an idealized heating function with spe...

Double-diffusive convection caused by coupled molecular diffusion

Abstract: Double-diffusive convection is studied for the case where a large coupled diffusion (or cross-diffusion) effect is present. The Soret effect is a familiar example of this cross-diffusion where the flux of the solute depends not only on its own spatial gradient but also on the in situ temperature gradient. The linear stability analysis of double-diffusive convection has been extended to include the two cross-diffusion flux terms and it has been shown that, with a sufficiently large coupled diffusion effect, fingers can form even when the concentrations of both components make the fluid's density gradient statically stable. The conditions under which the diffusive instability can occur are compared with those for the formation of fingers and it is shown that these two types of double-diffusive convection cannot occur together in any particular set of linear property gradients. We then consider finite-amplitude, steady, infinitely long fingers and show that a sufficiently large cross-diffusion effect can again allow fingers to exist when the concentrations of both solutes increase with depth. It is also shown that the diffusion of properties from an initially sharp interface may set up vertical gradients that are favourable for the formation of fingers.