Showing papers on "Convection published in 1976"

The dynamics and simulation of tropical cumulonimbus and squall lines

Abstract: Techniques of theoretical analysis and numerical simulation are combined to produce a dynamical model of tropical cumulonimbus convection which features a close cooperation between the updraught and downdraught circulations. The cloud-scale dynamics determine the structure and transfer properties. Sub-cloud-scale transfer is unimportant. A steady-state dynamical model shows that the upshear or down-shear propagation speed, c, of a cumulonimbus cell relative to the mid-level flow is determined as a function of the convective available potential energy, CAPE, and weakly influenced by the windshear through a non-dimensional number, R, of the large-scale flow. This propagation speed is almost constant for a wide range of R, with c≃ 0.3CAPE, but only possible if R ≳ 2.8 (small shear). This contrasts with a previous result for another regime of convection, obtained by Moncrieff and Green (1972), if R ⩽ 1 (high shear). The transfer of momentum is distinctive and of large magnitude. The initiation and growth of the convective circulation represent an essentially nonlinear, finite-amplitude process, whose properties are closely related to the wind profile in the tropical atmosphere. The numerical simulations attain a quasi-steady state of a complex, three dimensional nature, basic features of which can be represented in terms of the steady-state analysis. Moreover, the outflow of the downdraught air is closely related to the attainment of a steady circulation and suggests a mechanism both for the maintenance and the eventual breakdown of the convective regime.

A model for the formation of ignimbrite by gravitational column collapse

Abstract: Eruption columns consist of two components. The lower gas thrust component results from decompression of the gas phase, and decelerates rapidly to near zero velocity at heights of 1.5–4.5 km for initial gas velocities of 400–600 m/s. The upper convective thrust component is due to the column having a lower density than the atmosphere, and can transport the column to heights of 30–40 km. At the base, the effective density of a column is considerably greater than that of the atmosphere and is very sensitive to changes of gas content. Fall out of clasts and incorporation and heating of air reduce the density substantially during the gas thrust part. It is shown that columns formed from magmas with high water contents (5%) are likely to show convective motion. Magmas with low water contents ( 1 2 %) or high proportions of CO 2 will form a column with an effective density greater than the atmosphere, and gravitational column collapse can occur to generate ignimbrite-forming pyroclastic flows. In magmas with intermediate gas contents, the occurrence of convection (plinian case) or collapse (ignimbrite-forming) depends on vent radius, proportion of ash and gas content. The model presented here can explain: the sharp transition from plinian to ignimbrite-forming activity; the increase of temperature with time shown by some ignimbrites; the common association of low temperature ignimbrites with preceding plinian eruptions, and the apparent mobility of pyroclastic flows.

Effects of electric fields and other processes upon the nighttime high-latitude F layer

Abstract: The dynamics of the nighttime high-latitude F region is studied with special emphasis on the formation of the electron-density trough region which lies equatorward of the auroral oval. It is found that the absence of photoionization together with ordinary ionic recombination and slow plasma convection velocity can give a deep trough over a period of many hours. However, the normal global pattern of electric fields has regions of plasma convection sufficiently rapid to affect the rate of O(+) + N2 reactions and to speed the rate of ionospheric decay. In addition, the escape of thermal plasma via the polar winds as well as N2 vibrational excitation and enhanced N2 densities act to deplete the ionosphere. In combination, these destructive processes can readily account for the great variety of troughs found by experimentation. Thus, it appears that there is no single cause for the observed troughs, but that at various times, different processes act together to create density depressions of substantial magnitude.

A Comprehensive Model for Nucleate Pool Boiling Heat Transfer Including Microlayer Evaporation

Abstract: The results of an experimental investigation are presented in which dichloromethane (methylene chloride) boiling on a glass surface was studied using laser interferometry and high-speed photography. New data for active site density, frequency of bubble emission, and bubble departure radius were obtained in conjunction with measurements of the volume of microlayer evaporated from the film underlying the base of each bubble for various combinations of heat flux and subcooling. These results were used to support a model for predicting boiling heat flux incorporating microlayer evaporation, natural convection, and nucleate boiling mechanisms. Microlayer evaporation heat transfer is shown to represent a significant proportion of the total heat transfer for the range of heat flux and subcooling investigated.

Some Uses of High-Resolution GOES Imagery in the Mesoscale Forecasting of Convection and Its Behavior

Abstract: The high-resolution satellite imagery presently available from the Geostationary Operational Environmental Satellite (GOES) gives us a unique view of convective activity. This paper addresses some of the mesoscale phenomena, revealed in both pictures from GOES and in movies made from those pictures, which are important in the initiation and maintenance of convection. Specific attention is given to the organization of convection into lines and the importance of those convective lines in subsequent thunderstorm formation. The place where two convective lines merge almost invariably marks the location of intense convective development; under the right conditions, that activity will be severe.

Natural Convection in Enclosed Spaces—A Review of Application to Solar Energy Collection

Abstract: A useful solar-thermal converter requires effective control of heat losses from the hot absorber to the cooler surroundings. Based upon the theory and some experimental measurements it is shown that the spacing between the tilted hot solar absorber and successive glass covers should be in the range 4 to 8 cm to assure minimum gap conductance. Poor choice of spacing can significantly affect thermal conversion efficiency, particularly when the efficiency is low or when selective black absorbers are used. Recommended data for gap Nusselt number are presented as a function of the Rayleigh number for the high aspect ratios of interest in solar collector designs. It is also shown that a rectangular cell structure placed over a solar absorber is an effective device to suppress natural convection, if designed with the proper cell spacing d, height to spacing ratio L/d and width to spacing ratio W/d needed to give a cell Rayleigh number less than the critical value.

Studies of finite amplitude non‐Newtonian thermal convection with application to convection in the Earth's mantle

Abstract: Studies of non-Newtonian thermal convection have been made to determine the importance of non-Newtonian viscosity on flow in the earth's mantle. Finite difference solutions have been obtained with a viscosity law representing the sum of deformation rates due to diffusion and dislocation creep. Non-Newtonian flow structures differ only slightly from corresponding Newtonian flows. The principal effect of the non-Newtonian viscosity is to increase the effective Rayleigh number. An effective Rayleigh number based on a strain rate squared averaged viscosity provides a good correlation between Newtonian and non-Newtonian flows over a wide range of Rayleigh numbers.

The relation between heat flow, sediment thickness, and age in the eastern Pacific

Abstract: Heat flow measurements in the eastern Pacific now total over 800, a sufficient number to permit the analysis of their distribution within a wide range of age zones. Major results are that (1) the mid-ocean ridge crestal regions have lower heat flow than that expected from conductive cooling models of the lithospheric plate, (2) an increase in heat flow is observed some distance away from the crest in a zone which appears to delineate a transition from dominantly convective to conductive heat transfer, and (3) the heat flow in crust older than this transition zone closely approximates that predicted by conductive cooling models for oceanic lithospheric plates. The transition from low to high heat flow occurs when 150-200 m of sediment is deposited over basement, apparently preventing convective transfer of heat from the oceanic crust to seawater. Thus the general shape of the empirical heat flow versus age curve for the East Pacific Rise (EPR) agrees with that from the Galapagos Spreading Center, but the age of the transition zone occurs in 5- to 6-m.y.-old crust on the Galapagos Spreading Center, whereas it occurs in 10- to 15-m.y.-old sea floor on the EPR. Proximity to the equatorial high sedimentation regionmore » causes the deposition of a thick layer of sediment much more quickly on the Galapagos Spreading Center than on the East Pacific Rise. Quantitatively, the degree of heat transfer by convection appears to correlate inversely with the ratio of sediment thickness to topographic relief.« less

The Effect of Heater Size, Location, Aspect Ratio, and Boundary Conditions on Two-Dimensional, Laminar, Natural Convection in Rectangular Channels

Abstract: The effect of localized heating in rectangular channels was studied by solving the partial differential equations for the conservation of mass, momentum, and energy numerically using an unsteady state formulation and the alternating-direction-implicit method. The heating element was a long, horizontal, isothermal strip located in one, otherwise-insulated vertical wall. The opposing wall was maintained at a lower uniform temperature and the upper and lower surfaces were insulated or maintained at the lower temperature. Computations were carried out for Pr = 0.7, 0 less than or equal to Ra less than or equal to 10/sup 5/, a complete range of heater widths and locations and a wide range of aspect ratios. Flow visualization studies and comparison with prior computed results for a limiting case confirm the validity of the computed values. The computed rates of heat transfer and circulation provide guidance for locating heaters or coolers.

Radiative Transfer Within the Earth's Troposphere and Stratosphere: A Simplified Radiative-Convective Model

Abstract: A radiative transfer model of the troposphere and stratosphere is presented which includes both long-wave cooling and solar heating due to H2O, CO2 and O3 and has a simplified formulation which facilitates the inclusion of Doppler broadening, H2O continuum bands, hot and minor isotopic bands of CO2, and overlap of H2O bands with CO2 and O3 bands. The radiative model is used to develop an accurate radiative-convective model for studying the global surface temperature, stratospheric thermal structure and the net outgoing long-wave flux.

On the reliability of oceanic heat flow averages

Abstract: One of the major problems in the use of heat flow data in a quantitative manner has been the variability of closely spaced measurements. It is suggested that this variability is directly related to hydrothermal circulation in an ''effectively permeable'' oceanic crust. As a consequence, only where this crust is sealed from the seawater by an extensive and thick layer of impermeable sediment can reliable estimates be made of the flux at depth. Heat flow data from such areas are analyzed. It is shown that the scatter in the data is low and that in most areas the mean heat flow through a province of known age is close to that predicted by plate models which account for the increase in depth with increasing age of the ocean floor. From this analysis it is argued that these heat flow means are a reliable estimate of the flux at depth. In order to be able to use heat flow measurements to investigate the age of the crust, the thickness of the lithospheric plate, upper mantle convection, and local anomalies, it is necessary to substantiate this analysis and to find an adequate explanation of the low mean heat flow observed at themore » center of the sediment bulge in the equatorial Pacific.« less

Internal Gravity Waves Generated by Penetrative Convection

Abstract: Penetrative convection is modeled by a series of idealized disturbances at the base of an atmospheric temperature inversion. Internal gravity waves excited in the inversion by the disturbances are theoretically found to drain away a fraction of the initial energy of the disturbances. The portion of energy lost upward is significant when the temperature inversion is weak. Vertical energy fluxes due to internal gravity waves are mathematically described using wave group approaches.

Thermoconvective instabilities in a porous medium bounded by two concentric horizontal cylinders

Abstract: The study of natural convection in a saturated porous medium bounded by two concentric, horizontal, isothermal cylinders reveals different types of evolution according to the experimental conditions and the geometrical configuration of the model. At small Rayleigh numbers the state of the system corresponds to a regime of pseudo-conduction. The isotherms are coaxial with the cylinders. At larger Rayleigh numbers a regime of steady two-dimensional convection sets in between the two cylinders. Finally, for Rayleigh numbers above the critical Rayleigh number Ra*c the phenomena become three-dimensional and fluctuating. The appearance of these different regimes depends, moreover, on the geometry considered and, in particular, on two numbers: R, the ratio of the radii of the cylinders, and A, the ratio of the length of the cylinders to the radius of the inner one. In order to approach these experimental observations and to obtain realistic theoretical models, several methods of solving the equations have been used.The perturbation method yields information about the thermal field and the heat transfer between the cylinders under conditions close to the equilibrium state.A numerical two-dimensional model enables us to extend the range of investigation and to represent properly the phenomena when steady convection appreciably modifies the temperature distribution and the velocities within the porous layer.Neither of these models allows account to be taken of the instabilities observed experimentally above a critical Rayleigh number Ra*c. For this reason, a study of stability has been carried out using a Galerkin method based on equations corresponding to an initial state of steady convection. The results obtained show the importance of three-dimensional effects for the onset of fluctuating convection. The critical transition Rayleigh number Ra*c is thus determined in terms of the ratio of the radii R by solving an eigenvalue problem.A numerical three-dimensional model based on the method of finite elements has thus been developed in order to point out the different types of evolution with time. Steady two-dimensional convection and fluctuating three-dimensional convection have been actually found by calculation. The solution of the system of equations by the method of finite elements is briefly described.The experimental and theoretical results are then compared and a general physical interpretation is given.

Laboratory simulation of thermal convection in rotating planets and stars.

Abstract: Because of dynamical constraints in a rotating system, the component of gravity perpendicular to the axis of rotation is the dominant driving force of convection in liquid planetary cores and in stars. Except for the sign, the centrifugal force closely resembles the perpendicular component of gravity. Convection processes in stars and planets can therefore be modeled in laboratory experiments by using the centrifugal force with a reversed temperature gradient.

Onset of Convection in a Porous Medium With Internal Heat Generation

Abstract: The conditions leading to the onset of thermal convection in a horizontal porous layer are determined analytically using the method of linear stability of small disturbances. The lower boundary is treated as a rigid surface and the upper boundary as a free surface. The critical internal and external Rayleigh numbers are determined for both stabilizing and destabilizing boundary temperatures. The predicted critical external Rayleigh number in the limit of no heat generation is in agreement with the critical number predicted for a porous medium heated from below. (16 refs.)

Natural Convection From a Vertical Surface to a Thermally Stratified Fluid

Abstract: Results are presented of a study of natural convection from an isothermal finite plate immersed in a stable thermally stratified fluid. An analytical solution to the problem is obtained by using the local nonsimilarity method. Theoretical local and overall heat transfer coefficients are given for Pr = 0.7 and 6.0. Velocity and temperature profiles are given for Pr = 6.0. The actual experimental configuration was a vertical copper cylinder enclosed in a cube with rigid walls. Heat transfer data are correlated with the measured ambient thermal gradient. Visual studies of the flow field are also discussed. Excellent agreement is achieved between analysis and experiment.

Transient combined laminar free convection and radiation in a rectangular enclosure

Abstract: The mass, momentum and energy-transfer equations are solved to determine the response of a rectangular enclosure to a fire or other high-temperature heat source. The effects of non-participating radiation, wall heat conduction, and laminar natural convection are examined. The results indicate that radiation dominates the heat transfer in the enclosure and alters the convective flow patterns significantly. At a dimensionless time of 5·0 the surface of the wall opposite a vertical heated wall has achieved over 99% of the hot-wall temperature when radiation is included but has yet to change from the initial temperature for pure convection in the enclosure. At the same time the air at the centre of the enclosure achieves 33% and 13% of the hot-wall temperature with and without radiation, respectively. For a hot upper wall the convection velocities are not only opposite in direction but an order of magnitude larger when radiation transfer between the walls is included.

Influence of the interplanetary magnetic field on the occurrence and thickness of the plasma mantle

Abstract: The response of the plasma mantle to the orientation of the interplanetary magnetic field (IMF) has been studied by correlating Heos 2 plasma and Imp 6 magnetic field data. The mantle is nearly always present when the IMF has a southward component and often also when the field has a weak northward component. In addition, the mantle appears increasingly thicker with greater southward components. On the other hand, the mantle is thin or missing (from the region where it is normally found) when the average IMF has a strong northward component. This result supports the idea that polar cap convection plays a dominant role in the formation of the plasma mantle: mantle plasma originates in the magnetosheath, enters the magnetosphere through the day side polar cusps, and is transported across the cusp to the night side by means of a convection electric field whose magnitude is controlled by the orientation of the IMF.

Numerical models for hydrothermal circulation in the oceanic crust

Abstract: Two-dimensional numerical calculations of hydrothermal circulation in permeable oceanic crust have been carried out. The effects of Rayleigh number, impermeable and permeable upper boundaries, and permeability variations with depth have been investigated. Flow and temperature fields as well as surface heat flux distributions are presented. Spatial distributions of surface heat flux are compared with observations at the Galapagos spreading center. The hydrothermal circulation alters the spatial distribution of surface heat flow but not its mean value. It is concluded that the oceanic crust has a permeability of about 4.5×10−12 cm2. With a permeable upper boundary the mass of seawater circulating through the crust equals the mass of the crust in about 2.1 m.y. A corresponding typical circulation time is estimated to be about 4 years.

Axisymmetric convection in a cylinder

Abstract: In three-dimensional BBnard convection regions of rising and sinking fluid are dissimilar. This geometrical effect is studied for axisymmetric convection in a Boussinesq fluid contained in a cylindrical cell with free boundaries. Near the critical Rayleigh number R, the solution is obtained from a perturbation expansion, valid only if both the Reynolds number and the PBclet number are small. For values of the Nusselt number N 1 there is a viscous regime with N M 2(R/Rc)i; when R/R, 2 pb, N increases more rapidly, approximately as R0.4. At high Rayleigh numbers a large isothermal region develops, in which the ratio of vorticity to distance from the axis is nearly constant.

Convective heat transfer in a liquid saturated porous layer

Abstract: The convective heat transfer in fluid-saturated porous beds either heated from below or heated by distributed sources is investigated for several bed thicknesses and permeabilities. For the case of heating from below, Rayleigh numbers range from about 10 to 10,000. For distributed heat sources, Rayleigh numbers range from about 10 to 1,000. Critical Rayleigh numbers for the onset of convection are estimated as 38 for heating from below and 31.8 for distributed heat sources. Heat transfer results for convection induced by heating from below are in good agreement with analytical upper bound estimates obtained by Gupta and Joseph. (12 refs.)