Showing papers on "Convection published in 1993"

Effects of an endothermic phase transition at 670 km depth in a spherical model of convection in the Earth's mantle

Abstract: Numerical modelling of mantle convection in a spherical shell with an endothermic phase change at 670 km depth reveals an inherently three-dimensional flow pattern, containing cylindrical plumes and linear sheets which behave differently in their ability to penetrate the phase change. The dynamics are dominated by accumulation of downwelling cold material above 670 km depth, resulting in frequent avalanches of upper-mantle material into the lower mantle. This process generates long-wavelength lateral heterogeneity, helping to resolve the contradiction between seismic tomographic observations and expectations from mantle convection simulations.

Bénard cells and Taylor vortices

Abstract: Part I. Benard Convection and Rayleigh-Benard Convection: 1. Benard's experiments 2. Linear theory of Rayleigh-Benard convection 3. Theory of surface tension driven Benard convection 4. Surface tension driven Benard convection experiments 5. Linear Rayleigh-Benard convection experiments 6. Supercritical Rayleigh-Benard convection experiments 7. Nonlinear theory of Rayleigh-Benard convection 8. Miscellaneous topics Part II. Taylor Vortex Flow: 9. Circular Couette flow 10. Rayleigh's stability criterion 11. G. I. Taylor's work 12. Other early experiments 13. Supercritical Taylor vortex experiments 14. Experiments with two independently rotating cylinders 15. Nonlinear theory of Taylor vortices 16. Miscellaneous topics.

A solar dynamo surface wave at the interface between convection and nonuniform rotation

Abstract: A simple dynamo surface wave is presented to illustrate the basic principles of a dynamo operating in the thin layer of shear and suppressed eddy diffusion beneath the cyclonic convection in the convection zone of the sun. It is shown that the restriction of the shear delta(Omega)/delta(r) to a region below the convective zone provides the basic mode with a greatly reduced turbulent diffusion coefficient in the region of strong azimuthal field. The dynamo takes on the character of a surface wave tied to the lower surface z = 0 of the convective zone. There is a substantial body of evidence suggesting a fibril state for the principal flux bundles beneath the surface of the sun, with fundamental implications for the solar dynamo.

Vibration-induced size separation in granular media: The convection connection.

Abstract: We have investigated the rise of a single large glass bead through a vibrated cylindrical column of smaller particles and found that vibration-induced size separation in this geometry arises from convective processes rather than from local rearrangements as had been proposed previously. A convection cycle, rising in the middle and dropping in a thin stream along the walls of the cell, is responsible for all particle movement. Particles larger than the width of the thin downward convection zone are carried to the top of the column and then trapped, resulting in size segregation. For a variety of accelerations the position of the rising particle can be scaled onto a single curve.

Stream temperature dynamics: Measurements and modeling

Abstract: A numerical model based on a finite difference solution of the unsteady heat advection-dispersion equation is formulated to predict water temperatures in streams at time increments of 1 hour. An energy balance accounts for the effects of air temperature, solar radiation, relative humidity, cloud cover, and wind speed on the net rate of heat exchange through the water surface, and heat conduction between water and streambed. Continuous stream temperature recordings in shallow streams show strong dynamic behavior including diurnal variations of several degrees Celsius which are lost in the standard daily records. These measured water temperatures are used to calibrate the model for the optimum percentages of Sun shading and wind sheltering. Stream exposure to solar radiation is shown to vary from 30 to 100% and wind exposure from 10 to 30% depending on the character of the stream. Values are related to stream width and season because of variable leaf cover of trees on stream banks. After calibration, accuracies of hourly and daily water temperature predictions over periods of several weeks are of the order of 0.2° to 1°C. Solar (shortwave) radiation is shown to be the most important component of the heat flux across the stream water surface, but none of the other components, i.e., long-wave radiation, evaporation, and convection to the atmosphere, are negligible. Conductive heat exchange between the streambed and the water is a significant heat balance component in shallow streams.

An Analysis of the Conditional Instability of the Tropical Atmosphere

Abstract: The ice phase is included in thermodynamic calculations of convective available potential energy (CAPE) for a large number of soundings in the tropical atmosphere, at both land and ocean stations. It is found that the positive-buoyancy contribution to CAPE resulting from the latent heat of fusion more than offsets the negative-buoyancy contribution due to water loading in the reversible thermodynamic process. The departure from moist neutrality in much of the tropical atmosphere exhibits a threshold in boundary-layer wet-bulb potential temperature of 22°–23°C. The corresponding sea surface temperature is approximately 26°C, close to the empirical threshold for hurricane formation, which suggests that conditional instability plays an important role in the latter phenomenon. The simultaneous presence of finite CAPE and infrequent deep convection in the tropics is tentatively attributed to the convective inhibition energy (CINE) and to the mixing process that destroys positive buoyancy in incipient ...

Solar ponds for space heating

Abstract: Solar ponds are shallow bodies of water in which an artificially maintained salt concentration gradient prevents convection. They combine heat collection with long-term storage and can provide sufficient heat for the entire year. We consider the absorption of radiation as it passes through the water, and we derive equations for the resulting temperature range of the pond during year round operation, taking into account the heat that can be stored in the ground underneath the pond. Assuming a heating demand of 25000 Btu/degree day (Fahrenheit), characteristic of a 2000 ft2 house with fair insulation, and using records of the U.S. Weather Bureau, we carry out detailed calculations for several different locations and climates. We find that solar ponds can supply adequate heating, even in regions near the arctic circle. In midlatitudes the pond should be, roughly speaking, comparable in surface area and volume to the space it is to heat. Under some circumstances, the most economical system will employ a heat pump in conjunction with the solar pond. Cost estimates based on present technology and construction methods indicate that solar ponds may be competitive with conventional heating.

Radiative-convective equilibrium with explicit two-dimensional moist convection

Abstract: Radiative-convective statistical equilibria are obtained using a two-dimensional model in which radiative transfer is interactive with the predicted moisture and cloud fields. The domain is periodic in x, with a width of 640 km, and extends from the ground to 26 km. The lower boundary is a fixed-temperature water-saturated surface. The model produces a temperature profile resembling the mean profile observed in the tropics. A number of integrations of several months' duration are described in this preliminary examination of the model's qualitative behavior. The model generates a QBO-like oscillation in the x-averaged winds with an apparent period of ∼60 days. This oscillation extends into the troposphere and influences the convective organization. In order to avoid the associated large vertical wind shears, calculations are also performed in which the x-averaged winds are constrained to vanish. The convection then evolves into a pattern in which rain falls only within a small part of the domain. ...

Convection with Rotation in a Neutral Ocean: A Study of Open-Ocean Deep Convection

Abstract: The intensity and scale of the geostrophically adjusted end state of the convective overturning of a homogeneous rotating ocean of depth H at a latitude where the Coriolis parameter is f, induced by surface buoyancy loss of magnitude B0, are studied by numerical experiment. The experiments are related to observations and laboratory studies of open-ocean deep convection. A numerical model based on the nonhydrostatic Boussinesq equations is used. The grid spacing of the model is small enough that gross aspects of convective plumes themselves can be resolved, yet the domain of integration is sufficiently large to permit study of the influence of plumes on the large scale and geostrophic adjustment of the convected water. Numerical simulations suggest that cooling at the sea surface is offset by buoyancy drawn from depth through the agency of convective plumes. These plumes efficiently mix the water column to generate a dense chimney of fluid, which subsequently breaks up through the mechanism of bar...

On the generation of the large-scale and turbulent magnetic fields in solar-type stars

Abstract: It is thought that the large-scale solar-cycle magnetic field is generated in a thin region at the interface of the radiative core (RC) and solar convection zone (SCZ). We show that the bulk of the SCZ virogoursly generates a small-scale turbulent magnetic field. Rotation, while not essential, increases the generation rate of this field.

Large-Scale Variability of Atmospheric Deep Convection in Relation to Sea Surface Temperature in the Tropics

Abstract: Empirical relationships between tropical sea surface temperature (SST) and atmospheric deep convection are examined. Large-scale features of tropical deep convection are estimated from two independent satellite datasets: monthly mean outgoing longwave radiation of 15 years and high-resolution pentad (5 day) fractional coverage of infrared radiation histograms of 5 years. Results based on the two datasets lead to the same conclusions. The relationships are addressed from two aspects: how deep convection varies with changing SST and how it varies at constant SST. Deep convection remains weak and rarely observed for SST 27°C, situations of no deep convection and vigorous deep convection can both be observed; the areas coverage of convectively related high clouds is always dominated by that ...

Convective cloud systems and warm-pool sea surface temperatures : coupled interactions and self-regulation

Abstract: Questions regarding the upper limits on tropical sea surface temperatures and the processes determining those limits have recently come under renewed interest and debate. We present results from an analysis of the relationship between observed sea surface temperature (SST) and organized deep convection in the tropics that has produced new and important findings relevant to this issue. First, the analysis reveals that the highest observed tropical SSTs are generally associated with diminished convection. Second, the maximum convective activity occurs, on average, at an SST of about 29.5°C. Third, at SSTs of about 29°C and greater, intense deep convection is associated with ocean surface cooling of approximately 0.1°C per month, while suppressed deep convection is associated with a similar degree of ocean surface warming. These three findings, together with results from simplified model analyses, emphasize the importance of the cooling mechanisms associated with deep convection in determining the observed upper limits on tropical SST. Implications of the observed relationship between deep convection and SST on the temporal correlations between these fields is discussed, as is the convective cloud system's relative influence on the solar and evaporative heat flux components of the surface energy budget.

The diurnal cycle of tropical convection

Abstract: The diurnal cycle of tropical convection is investigated with global cloud imagery constructed from 11μm radiance measurements taken aboard six satellites. Four harmonics of the diurnal cycle are resolved in the cloud imagery with about 50 km horizontal resolution. To isolate deep convective activity from other processes which cause diurnal fluctuations in longwave radiance, an index of deep convective activity is constructed by thresholding to brightness temperatures less than 230 K. Significant diurnal amplitude of deep convection is found only over tropical landmasses. Over the tropical oceans the diurnal cycle is weak and is barely discernible from the background red spectrum of convective variance. Nonetheless, oceanic convection exhibits a systematic diurnal fluctuation with maximum intensity in the early morning. Nocturnal subsidence along the cloud-free equator is postulated to play a role in forcing diurnal variation in the intertropical convergence zones. Other mechanisms are also implied to contribute as a similar early morning maximum in deep convection is seen even where no adjoining cloud-free regions occur. The diurnal cycle of deep convection is found to be organized on planetary scales predominantly in nonmigrating modes (i.e., modes with phase speeds not equal to that of the Sun). The nonmigrating modes are postulated to be produced by the nonuniform distribution of convective centers. The nonmigrating, zonally symmetric component is found to be especially large. While the role of this mode in forcing diurnal variations of the tropospheric circulation (e.g., the Hadley cell) is questionable, its role in modulating the ionospheric electric potential is well established. Discrepancies with the traditional notion that cloud-to-ground lightning is the dominant mechanism by which the Earth is negatively charged are discussed. In particular, the probable role of nonlightning-producing clouds (e.g., the stratiform portion of convective complexes) in diurnally modulating the negative charge on Earth is discussed.

High Rayleigh number β-convection

Abstract: High resolution numerical simulations of thermal convection in a rapidly rotating channel with gravity perpendicular to the rotation vector are described. The convecting columns are subject to a β-effect resulting from topographic vortex stretching. The symmetries of the problem allow many invariant wavenumber sets, and this property is associated with the existence of stable multiple-equilibria at modest supercriticality. The transition to chaotic behavior involves the production of intermittent unstable orbits off a two-torus in energy space. At very high Rayleigh number (of order 106 to 107) the motion can be turbulent, depending on the size of β. However, the turbulence is usually characterized by an almost-periodic formation of patches of small scale convection that cause regular pulsations in the accompanying strong zonal jets. The processes maintaining these flows may be related to those responsible for the zonal currents on Jupiter and for cyclic variability on the Sun.

A model of solar wind fluctuations with two components: Alfvén waves and convective structures

Abstract: A two-component incompressible fluctuation model is presented to explain the radial evolution of the solar wind fluctuations. The basic idea is to consider the small-scale fluctuations in the solar wind as being composed of Alfven waves and convective structures. The major Alfven waves are believed to be created near the coronal base and to propagate outward along the magnetic field lines. The convective structures are defined as the small-scale variations perpendicular to the local magnetic field direction. They are either quasi-static or turbulent and slowly evolving in the plasma frame of reference. The small-scale perpendicular variations are connected, in the parallel direction, with large-scale magnetic field variations, which are convected by the solar wind as quasi-static structures during the wind expansion time. The decomposition of the original fluctuations can be done by using special space and time averages, which are defined by space averaging along the directions parallel and perpendicular to the local magnetic field vector and by time averaging in the plasma frame of reference. The equations of motion of the fluctuations and of the correlation functions for both Alfven waves and convective structures have been derived from the one-fluid MHD equations. A combination of the correlation functions of these two components is then used for a comparison with observational results. The influence of the angle between the sampling direction and the magnetic field vector on the final results has also been considered. As a first step to apply these equations, a simple model has been suggested that is based on the assumption that the fluctuations are only composed of outward propagating Alfven waves and static magnetic structures. For comparison with the observations, new statistical results from data obtained by Helios 1 during days 1–95, 1975, and Helios 2 during days 19–109, 1976, are presented. The numerical solutions are shown to describe well the basic evolution trend of the fluctuation energy, the normalized cross helicity, and the Alfven ratio. It is also shown that the basic physical process of the evolution of the convective structures is the convection of the fluctuating velocity vortex lines and the magnetic field lines by the expanding solar wind.

A Review of Thunderstorm Electrification Processes

Abstract: Recent developments in the area of thunderstorm electrification processes are reviewed. These processes have two main divisions; (a) convective, in which particles charged by ion capture are moved by convection currents to strengthen the electric field in the cloud, and (b) processes involving charge transfer during particle interactions, following which oppositely charged particles move apart in the updraft to form the observed charge centers. Type-b processes are further subdivided into inductive (relying on the preexistence of an electric field) and noninductive charge-transfer mechanisms. Field and laboratory evidence points to the importance of interactions between particles of the ice phase, in the presence of liquid water droplets, in separating electric charge in thunderstorms. Recent experimental studies have investigated the dependence of charge transfer on the size and relative velocity of the interacting particles and have determined the dependence of the sign of the charge transfer o...

Thermohaline Oscillations Induced by Strong Steady Salinity Forcing of Ocean General Circulation Models

Abstract: A series of numerical experiments is conducted with a three-dimensional ocean general circulation model and a two-dimensional counterpart both designed for efficient integration over diffusive (millennial) time scales. With strong steady salinity fluxes (salting at low latitudes and freshening at high), basin mean temperature and several other diagnostics show a series of self-sustaining oscillations. The oscillations termed deep decoupling oscillations, exhibit halocline catastrophes at regular intervals, followed by warming deep decoupled phases (when the deep overturning is weak), cooling flushes, and in the lower range of salinity forcing, a coupled phase when the deep ocean advective/diffusive heat balance is almost, but not quite, met. It is suggested that oscillations arise when a steady overturning circulation encounters a contradiction: the poleward salt and heat transport needed to maintain convection in the polar ocean requires more overturning than is consistent with the reduced therm...

Observations of an enhanced convection channel in the cusp ionosphere

Abstract: Transient or patchy magnetic field line merging on the dayside magnetopause, giving rise to flux transfer events (FTEs), is thought to play a significant role in energizing high-latitude ionospheric convection during periods of southward interplanetary magnetic field. Several transient velocity patterns in the cusp ionosphere have been presented as candidate FTE signatures. Instrument limitations, combined with uncertainties about the magnetopause processes causing individual velocity transients, mean that definitive observations of the ionospheric signature of FTEs have yet to be presented. This paper describes combined observations by the PACE HF backscatter radar and the DMSP F9 polar-orbiting satellite of a transient velocity signature in the southern hemisphere ionospheric cusp. The prevailing solar wind conditions suggest that it is the result of enhanced magnetic merging at the magnetopause. The satellite particle precipitation data associated with the transient are typically cusplike in nature. The presence of spatially discrete patches of accelerated ions at the equatorward edge of the cusp is consistent with the ion acceleration that could occur with merging. The combined radar line-of-sight velocity data and the satellite transverse plasma drift data are consistent with a channel of enhanced convection superposed on the ambient cusp plasma flow. This channel is at least 900 km in longitudinal extent but only 100 km wide. It is zonally aligned for most of its extent, except at the western limit where it rotates sharply poleward. Weak return flow is observed outside the channel. These observations are compared with and contrasted to similar events seen by the EISCAT radar and by optical instruments.

Distribution of Tropical Tropospheric Water Vapor

Abstract: Utilizing a conceptual model for tropical convection and observational data for water vapor, the maintenance of the vertical distribution of the tropical tropospheric water vapor is discussed. While deep convection induces large-scale subsidence that constrains the turbulent downgradient mixing to within the convective boundary layer and effectively dries the troposphere through downward advection, it also pumps hydrometeors into the upper troposphere, whose subsequent evaporation appears to be the major source of moisture for the large-scale subsiding motion. The development of upper-level clouds and precipitation from these clouds may also act to dry the outflow, thus explaining the low relative humidity near the tropopause. A one-dimensional model is developed to simulate the mean vertical structure of water vapor in the tropical troposphere. It is also shown that the horizontal variation of water vapor in the tropical troposphere above the trade-wind boundary layer can be explained by the variation of a moisture source that is proportional to the amount of upper-level clouds. Implications for the nature of water vapor feedback in global warming are discussed.

Observations of vertical currents and convection in the central greenland sea during the winter of 1988-1989

Abstract: During the winter of 1988–1989 five acoustic Doppler current profilers (ADCPs) were moored in the central Greenland Sea to measure vertical currents that might occur in conjunction with deep mixing and convection. Two ADCPs were looking up from about 300 m and combined with thermistor strings in the depth range 60–260 m, two were looking downward from 200 m, and one was looking upward from 1400 m. First maxima of vertical velocity variance occurred at two events of strong cold winds in October and November when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline. Beginning in late November the marginal ice zone expanded eastward over the central Greenland Sea, reaching its maximum extent in late December. In mid-January a bay of ice-free water opened over the central Greenland Sea, leaving a wedge of ice, the “is odden,” curled around it along the axis of the Jan Mayen Current and then northeastward and existing well into April 1989. Below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover, corresponding to an average heat loss of about 40 W m−2. Through brine rejection, mixed-layer salinity increased steadily, reducing stability to underlying weakly stratified layers (Roach et al., 1993). During the ice cover period, vertical currents were at a minimum. After the opening of the ice-free bay, successive mixed-layer deepening to >350 m occurred in conjunction with cooling events around February 1 and 15, accompanied by strong small-scale vertical velocity variations. Upward mixing of more saline waters of Atlantic origin during this phase reduced the stability further, generating a pool of homogeneous water of >50 km horizontal extent in the central Greenland Sea, preconditioned for subsequent convection to greater depths. Individual convection events were observed during March 6–16, associated with downward velocities at the 1400-m level of about 3 cm s−l. One event was identified as a plume of about 300-m horizontal scale, in agreement with recently advanced scaling arguments and model results, and with earlier similar observations in the Gulf of Lions, western Mediterranean. The deep convection occurred in the center of the ice-free bay; hence brine rejection did not seem necessary for its generation. Plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool, suggesting entrainment of surrounding warmer waters on the way down. Mean vertical velocity over a period of convection events was indistinguishable from zero, suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses. However, different from the surface pool that was governed by mixed-layer physics, the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes. Overall, and compared to results from the Gulf of Lions, convection activity in the central Greenland Sea was weak and limited to intermediate depths in winter 1988–1989.

Suspension in convective layers and style of differentiation of a terrestrial magma ocean

Abstract: Recent physical theories for the formation of the Earth suggest that about 4.5 b.y. ago the mantle of the Earth was partially or completely molten. Fractional crystallization of this hypothetical magma ocean would result in a strong chemical stratification of the Earth's mantle. Such a scenario is controversial from the geochemical point of view. However, it has been noted that the simple scenario of fractional crystallization could be avoidable in a convective magma ocean if crystals remain suspended. In this paper, the problem of suspension is developed with the help of an energetic approach: convection must do some work against gravitational settling. We distinguish three regimes of convective suspensions. Absolute or complete sedimentation occurs when the energy dissipation due to the settling exceeds the heat loss from the convective layer. This is possible only in large-scale systems like magma oceans and implies that cooling can proceed only together with sedimentation, crystallization, and a decrease in the liquidus temperature at a constant pressure. A regime of partial differentiation occurs when the energy dissipation due to the settling is less than the total heat loss but larger than the power which can be spent by convection on the crystal reentrainment process. The differentiation is not complete, and a competition between the rate of cooling, the rate of sedimentation, and the rate of turbulent diffusion determines the degree of differentiation. The third regime is an absolute suspension which could be sustained for an indefinitely long time. In this case, sedimentation starts only when the crystal fraction reaches the maximum packing value: when the viscosity of the magma rapidly increases. The power which can be spent by convection on reentrainment is equal to eαgd/cp of the total energy supply to the convective layer, where e 15 GPa) or 10^(−3) – 10^(−1) cm during crystallization of shallow layers, the first regime (“fractional crystallization”) is unavoidable. The estimates depend on various poorly constrained parameters and processes, such as heat flux, viscosity, thermodynamical disequilibrium and highly variable viscosity convection. For absolute suspension the crystal size must be at least e^(½) times less, or 10^(−3) – 10^(−1) cm and 10^(−4) – 10^(−2) cm, respectively, if e ∼ 0.01. The partial differentiation occurs in a narrow (one decade) range between these two regimes. The radius of about 1 cm must be considered as an absolute upper bound above which fractional differentiation is guaranteed. These estimates for the critical crystal size are orders of magnitude lower than suggested previously, and thus the problem of crystal sizes becomes a central one for magma oceans. A necessary condition for reentrainment is the existence of local mechanisms. The absence of such mechanisms to reentrain the particles from the bottom would mean that an absolute suspension is impossible even if the energetics allows it. Turbulence is considered as a possible important factor. A simple model of convection predicts a strong turbulence, provided the viscosity is less than 10^9 – 10^(10) P. Rotation reduces this critical viscosity to 10^5 – 10^8 P but this is still sufficiently large and is reached only near the maximum packing crystal fraction. Power law or Bingham rheology of partial melts can exclude any turbulence already at 20 – 30% of crystal fraction. We also show that the energetic criterion for the absolute suspension with e ∼ 1 coincides with the condition that the particle concentration gradient suppresses the turbulence.

Unglazed transpired solar collectors: Heat loss theory

Abstract: Unglazed transpired solar collectors offer a potentially low cost, high-efficiency option for once-through applications such as preheating air for ventilation, crop drying, and desiccant regeneration. This paper examines the major heat loss mechanisms associated with this concept. Radiation heat loss is determined by considering losses to both the sky and the ground. Convective heat losses are obtained by integrating the product of the temperature and velocity profiles in the boundary layer at the downwind edge of the collector. This convective heat loss is then expressed in terms of the thermal equivalent length of irradiated absorber, and analysis shows that this loss can be very low for large collectors event under windy conditions. These results are incorporated into a simple computer model which predicts collector efficiency as a function of suction velocity, wind speed, ambient temperature, and radiation. Remaining research issues are discussed.

The Betts-Miller Scheme

Abstract: The impetus for the development of this simple lagged convective adjustment scheme came from the series of tropical field experiments in the decade 1969–79 [VIMHEX, the Venezuela International Meteorological the Hydrological Experiment in 1969 and 1972; GATE, the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment in 1974, and MONEX, the Monsoon Experiment in 1979]. Deep convection is the dominant vertical transport process in the tropics. In conjunction with the radiation field and the subsiding branches of the tropical circulations, convective processes maintain a vertical thermal structure, which is quite close to the moist adiabat through the equivalent potential temperature θ e of the subcloud layer in the regions of deep convection. This was the basis of early cumulus parameterization schemes. Manabe (1965) proposed adjustment toward a moist-adiabatic structure to remove conditional instability in large-scale models. Kuo (1965, 1974) proposed a simple cloud model for deep convection that adjusted the atmosphere toward the saturated moist pseudoadiabat in the presence of grid-scale moist convergence. However, the mean tropical atmosphere is always cooler by several degrees in the middle troposphere than this reference moist adiabat, even in regions of vigorous convection (see Figs. 9.5–9.7). At the same time, the deep convective transports also maintain the water vapor and cloud distributions in the tropics, which in turn play a crucial role in the radiative fluxes.

The thermal conductivity of foams. I. Models for heat conduction

Abstract: The transfer of heat through a cellular material is considered, taking into account conduction in the solid and gas phases, but excluding radiation and convection Formulae are derived which relate the apparent conductivity of the material to the conductivities of the component phases and to the density Some published methods of reaching such a conclusion are critically reviewed and compared It is shown by binomial expansion that the various formulae tend to the same approximate result for foams of low density

Convective Heat Transfer Enhancement Due to Intermittency in an Impinging Jet

Abstract: An experimental investigation has been performed to study the effect of flow intermittency on convective heat transfer to a planar water jet impinging on a constant heat flux surface. Enhanced heat transfer was achieved by periodically restarting an impinging flow and thereby forcing renewal of the hydrodynamic and thermal boundary layers. Although convective heat transfer was less effective during a short period when flow was interrupted, high heat transfer rates, which immediately follow initial wetting, prevailed above a threshold frequency, and a net enhancement occurred. Experiments with intermittent flows yielded enhancements in convective heat transfer coefficients of nearly a factor of two, and theoretical considerations suggest that higher enhancements can be achieved by increasing the frequency of the intermittency. Enhancements need not result in an increased pressure drop within a flow system, since flow interruptions can be induced beyond a nozzle exit. Experimental results are presented for both the steady and intermittent impinging jets at distances up to seven jet widths from the stagnation line. A theoretical model of the transient boundary layer response is used to reveal parameters that govern the measured enhancements. A useful correlation is also provided of local heat transfer results for steadily impinging jets.

A Boundary-Layer Model for Mars: Comparison with Viking Lander and Entry Data

Abstract: A 1D boundary-layer model of Mars based on a momentum equation that describes friction, pressure gradient, and Coriolis forces is presented. Frictional forces and convective heating are computed using the level-2 turbulence closure theory of Mellor and Yamada (1974). The model takes into account the radiative effects of CO2 gas and suspended dust particles. Both radiation and convection depend on surface temperatures which are computed from a surface heat budget. Model predictions are compared with available observations from Viking landers. It is concluded that, in general, the model reproduces the basic features of the temperature data. The agreement is particularly good at entry time for the V L-2 site, where the model and observations are within several degrees at all levels for which data are available.