Showing papers on "Convection published in 1992"

Thermal Radiative Transfer and Properties

Abstract: The Nature of Thermal Radiation. Radiative Properties and Simple Transfer. Diffuse Surface Transfer. Electromagnetic Theory Results. Classical Dispersion Theory. Monte Carlo Surface Transfer. Radiative Transfer Equation. Thermal Radiation Properties of Gases. Radiative Properties of Particles. Radiative Transfer in Nonscattering, Homogeneous Media. Nonisothermal Transfer: Radiative Equilibrium and Diffusion with Isotropic Scattering. Radiative Transfer with Anisotropic, Multiple Scattering. Radiative Transfer Coupled with Conduction and Convection. Monte Carlo in Participating Media. Appendices. Index.

A radar and electrical study of tropical hot towers

Abstract: Radar and electrical measurements for deep tropical convection are examined for both “break period” and “monsoonal” regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the “break period” regime are associated with 10–20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1°–3°C) in wet-bulb potential temperature between land and s...

Instabilities of the liquid and mushy regions during solidification of alloys

Abstract: The solidification of melts can be profoundly influenced by convection. In alloys, compositional convection can be driven by solute gradients generated as one component of the alloy is preferentially incorporated within the solid, even when the thermal field is stabilizing. In this paper, two modes of compositional convection during solidification from below are uncovered using a linear-stability analysis : one, which we shall call the ‘mushy-layer mode’, is driven by buoyant residual fluid within a mushy layer, or porous medium, of dendritic crystals; the other, which we shall call the ‘ boundary-layer mode ’ is associated with a narrow compositional boundary layer in the melt just above the mush-liquid interface. Either mode can be the first to become unstable depending on the thermodynamical and physical properties of the alloy. The marginally stable eigenfunctions suggest that the boundary-layer mode results in fine-scale convection in the melt above the mushy layer and leaves the interstitial fluid of the mushy layer virtually stagnant. In contrast, the mushy-layer mode causes perturbations to the solid fraction of the mushy layer that are indicative of a tendency to form chimneys, which are vertical channels of reduced or zero solid fraction that have been observed experimentally. Particular attention is focused on the mushy-layer mode and its dependence upon the thermodynamical properties of the alloy. The results of this analysis are used to make a number of interpretations of earlier experimental studies such as the observations that some systems are less prone to form chimneys and that the regions of melt in these systems evolve to supersaturated conditions, while the melt evolves to unsaturated conditions once chimneys have formed. In addition, good quantitative agreement is found between the results of the linear-stability analysis and the experimental results of Tait & Jaupart (1992) for the onset of the mushy-layer mode of convection.

Heat transfer in rotating serpentine passages with trips normal to the flow

Abstract: Experiments were conducted to determine the effects of buoyancy and Coriolis forces on heat transfer in turbine blade internal coolant passages. The experiments were conducted with a large scale, multipass, heat transfer model with both radially inward and outward flow. Trip strips on the leading and trailing surfaces of the radial coolant passages were used to produce the rough walls. An analysis of the governing flow equations showed that four parameters influence the heat transfer in rotating passages: coolant-to-wall temperature ratio, Rossby number, Reynolds number, and radius-to-passage hydraulic diameter ratio. The first three of these four parameters were varied over ranges which are typical of advanced gas turbine engine operating conditions. Results were correlated and compared to previous results from stationary and rotating similar models with trip strips. The heat transfer coefficients on surfaces, where the heat increased with rotation and buoyancy, varied by as much as a factor of four. Maximum values of the heat transfer coefficients with high rotation were only slightly above the highest levels obtained with the smooth wall model. The heat transfer coefficients on surfaces, where the heat transfer decreased with rotation, varied by as much as a factor of three due to rotation and buoyancy. It was concluded that both Coriolis and buoyancy effects must be considered in turbine blade cooling designs with trip strips and that the effects of rotation were markedly different depending upon the flow direction.

Compositional convection in a reactive crystalline mush and melt differentiation

Abstract: Fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them. To elucidate some physical principles of thermosolutal convection in a reactive porous medium, we describe experiments on the directional solidification of aqueous ammonium chloride solutions. The addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions, the phase diagram, and permeability. Solutions were cooled from below, and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid, released from the boundary layer at the top of the “mush.” The interstitial fluid within the mush also became unstable eventually, the onset of convection occurring when the porous medium Rayleigh number of the mush reached a critical value. This threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat. Local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature, composition, and porosity. Convective motions locally caused dissolution and precipitation, and hence fluctuations of porosity developed. Dissolution occurred preferentially in the central parts of upwellings, and the upflow gradually focused, being ultimately channelized into narrow “chimneys” devoid of crystals. All else being equal, the areal density of chimneys was greater the lower the viscosity. Chimney diameter increased with increasing solution viscosity. In the liquid above the growing mush, the convective plumes were very similar to salt fingers. Depending on solution viscosity and temperature gradient, they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities. The base plate temperature chosen was above the eutectic, and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram. The spatial distribution of crystals and the final porosity of the mush were, however, determined by the strength of compositional convection as measured by the porous medium Rayleigh number. When no convection occurred, mush thickness finally became equal to the initial layer thickness, and the system was homogeneous. When compositional convection occurred, the overlying reservoir underwent chemical evolution, and mush growth slowed dramatically. In experiments with progressively lower solution viscosity (and hence more vigorous convection), final mush thickness was progressively less and final porosity lower. Final mush thickness was found to scale with solution viscosity to the power +0.33. During fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks. We speculate that fossil chimney structures can be found in the Lower Zone of the Bushveld ultramafic complex in the form of iron-rich, platinum-bearing dunite pipes. This could explain the adcumulate nature of the Lower Zone rocks and the chemical and mineralogical similarities between the pipes and the overlying Merensky Reef; the Merensky Reef would be interpreted as a hiatus in mush growth. The results may also have applications to the flow structure in regions of partial melting and in the Earth's core.

Turbulent Heat Flux in the Upper Ocean Under Sea Ice

Abstract: Turbulence data from three Arctic drift station experiments demonstrate features of turbulent heat transfer in the oceanic boundary layer. Time series analysis of several w′T′ records shows that heat and momentum flux occur at nearly the same scales, typically by turbulent eddies of the order of 10–20 m in horizontal extent and a few meters in vertical extent. Probability distribution functions of w′T′ have large skewness and kurtosis, where the latter confirms that most of the flux occurs in intermittent “events” with positive and negative excursions an order of magnitude larger than the mean value. An estimate of the eddy heat diffusivity in the outer (Ekman) part of the boundary layer, based on measured heat flux and temperature gradient during a diurnal tidal cycle over the Yermak Plateau slope north of Fram Strait, agrees reasonably well with the eddy viscosity, with values as high as 0.15 m2 s−1. An analysis of measurements made near the ice-ocean interface at the three stations shows that heat flux increases with both temperature elevation above freezing and with friction velocity at the interface. It also reveals a surprising uniformity in parameters describing the heat and mass transfer: e.g., the thickness of the “transition sublayer” (from a modified version of the Yaglom-Kader theory) is about 10 cm at all three sites, despite nearly a fivefold difference in the under-ice roughness z0, which ranges from approximately 2 to 9 cm. A much simplified model for heat and mass transfer at the ice-ocean interface, suggested by the relative uniformity of the heat transfer coefficients at the three sites, is outlined.

Spiralling columnar convection in rapidly rotating spherical fluid shells

Abstract: It is shown that the fundamental features of both thermal instabilities and the corresponding nonlinear convection in rapidly rotating spherical systems (in the range of the Taylor number 109 < T < 1012) are determined by the fluid properties characterized by the size of the Prandtl number. Coefficients of the asymptotic power law for the onset of convection at large Taylor number are estimated in the range of the Prandtl number 0.1 ≤ Pr ≤ 100. For fluids of moderately small Prandtl number, a new type of convective instability in the form of prograde spiralling drifting columnar rolls is discovered. The linear columnar rolls extend spirally from near latitude 60° to the equatorial region, and each spans azimuthally approximately five wavelengths with the inclination angle between a spirally elongated roll and the radial direction exceeding 45°. As a consequence, the radial lengthscale of the linear roll becomes comparable with the azimuthal lengthscale. A particularly significant finding is the connection between the new instability and the predominantly axisymmetric convection. Though non-axisymmetric motions are preferred at the onset of convection, the nonlinear convection (at the Rayleigh number of the order of (R—Rc)/Rc = O(0.1)) bifurcating supercritically from the spiralling mode is primarily dominated by the component of the axisymmetric zonal flow, which contains nearly 90% of the total kinetic energy. For fluids of moderately large Prandtl numbers, thermal instabilities at the onset of convection are concentrated in a cylindrical annulus coaxial with the axis of rotation; the position of the convection cylinder is strongly dependent on the size of the Prandtl number. The associated nonlinear convection consists of predominantly non-axisymmetric columnar rolls together with a superimposed weak mean flow that contains less than 10% of the total kinetic energy at (R—Rc)/Rc = O(0.1). A double-layer structure of the temperature field (with respect to the basic state) forms as a result of strong nonlinear interactions between the nonlinear flow and the temperature field. It is also demonstrated that the aspect ratio of the spherical shell does not substantially influence the fundamental properties of convection.

Dynamo action in stratified convection with overshoot

Abstract: Results are presented from direct simulations of turbulent compressible hydromagnetic convection above a stable overshoot layer. Spontaneous dynamo action occurs followed by saturation, with most of the generated magnetic field appearing as coherent flux tubes in the vicinity of strong downdrafts, where both the generation and destruction of magnetic field is most vigorous. Whether or not this field is amplified depends on the sizes of the magnetic Reynolds and magnetic Prandtl numbers. Joule dissipation is balanced mainly by the work done against the magnetic curvature force. It is this curvature force which is also responsible for the saturation of the dynamo.

Laminar flow and convective transport processes : scaling principles and asymptotic analysis

Abstract: Basic Principles Unidirectional Flows Creeping Flows Further Results in the Creeping Flow Limit Asymptotic Approximations for Unidirectional, One-Dimensional, and Nearly Unidirectional Flows Thin Films, Lubrication, and Related Problems Weak Convection Effects Strong Convection Effects in Heat and Mass Transfer at Low Reynolds Number Laminar Boundary-Layer theory Thermal Boundary-Layer Theory at Large Reynolds Number Natural and Mixed Convection Flows.

Three-dimensional convection in a horizontal fluid layer subjected to a constant shear

Abstract: Rayleigh-Be'nard convection in the presence of a plane Couette flow is investigated by numerical computations. From earlier work it is well known that longitudinal rolls are preferred at the onset of convection and that at Prandtl numbers of the order unity or less these rolls become unstable with respect to the wavy instability which introduces wavy distortions perpendicular to the axis of the rolls. In the present analysis the three-dimensional flows arising from these distortions are studied and their stability is considered. A main result is the subcritical existence of three-dimensional flows at Rayleigh numbers far below the critical value for onset of convection.

Solar pulsational stability – I. Pulsation-mode thermodynamics

Abstract: It is not currently known what excites solar five-minute oscillations. Of the two most plausible possibilities, thermal overstability and stochastic excitation by turbulent convection, the single most important discriminating factor is the intrinsic stability of the pulsation modes. In view of this fact, the problem of the linear stability of model solar envelopes is addressed. A time-dependent, non-local mixing-length prescription is employed for convection, and the Eddington approximation to radiative transfer. The calculations reveal that low-degree acoustic modes are damped. Moreover, the theoretical damping rates compare well with measurements of solar oscillation line widths. Turbulent pressure fluctuations play a critical role in stabilizing the pulsations

New origin of a convective motion: Elastically induced convection in granular materials.

Abstract: Convection and fluidization in a vibrated bed of powder are reproduced in a numerical simulation. In the simulation, each particle of the powder, during a collision, has a viscoelastic interaction with the other colliding particle. Because of the discreteness of the particles, this elasticity causes convection. The critical values of fluidization and convection agree with experiments.

Free tropospheric ozone production following entrainment of urban plumes into deep convection

Abstract: It is shown that rapid vertical transport of air from urban plumes through deep convective clouds can cause substantial enhancement of the rate of O3 production in the free troposphere. Simulation of convective redistribution and subsequent photochemistry of an urban plume from Oklahoma City during the 1985 PRESTORM campaign shows enhancement of O3 production in the free tropospheric cloud outflow layer by a factor of almost 4. In contrast, simulation of convective transport of an urban plume from Manaus, Brazil, into a prestine free troposphere during GTE/ABLE 2B (1987), followed by a photochemical simulation, showed enhancement of O3 production by a factor of 35. The reasons for the different enhancements are (1) intensity of cloud vertical motion; (2) initial boundary layer O3 precursor concentrations; and (3) initial amount of background free tropospheric NO(x). Convective transport of ozone precursors to the middle and upper troposphere allows the resulting O3 to spread over large geographic regions, rather than being confined to the lower troposphere where loss processes are much more rapid. Conversely, as air with lower NO descends and replaces more polluted air, there is greater O3 production efficiency per molecule of NO in the boundary layer following convective transport. As a result, over 30 percent more ozone could be produced in the entire tropospheric column in the first 24 hours following convective transport of urban plumes.

Observed growth of Langmuir circulation

Abstract: Surface velocity patterns and upper ocean density profiles are presented from a period following a sudden increase in wind. A prior wind speed of 8 m/s failed to produce visible signs of Langmuir circulation. After the wind speed increased to 13 m/s, Langmuir circulation developed within 15 min. The initial scale observed was about 16 m, streak to streak, and may have been restricted by the depth scale of the measurements. This spacing is about two thirds of the dominant wavelength (4-s-period waves). The streak spacing (two cell widths) increased at roughly 40 m/h for the next hour. The density measurements indicate a depth of mixing which increased over the same period at a rate of 20 m/h; thus the vertical to horizontal aspect ratio was about 1:1. Prior stratification was weak (buoyancy frequency of about 1.5 cph), and probably did not affect the circulation initially. Mixing at the surface of the oceans is important to a wide variety of concerns, including global climate and the health of marine life. Two main causes of this mixing can be identified: (1) when surface water is cooled, it becomes denser and sinks, mixing downward either to the bottom or until limited by the preexisting stratification; and (2) the wind can mechanically stir the surface layer to some depth, usually limited by stratification. The former can result in mixing to considerable depths, but the latter occurs more often, over the majority of the world oceans. Frequently, the two driving forces occur together. What are the details of this mixing? For example, are bubbles dragged deep into the mixed layer, injecting gases at depth? Are there repeatable features which could lead to improved models of surface mixing and its effects? When wind blows over water, lines are often visible on the surface, running roughly parallel to the wind. In an exemplary series of experiments, Langmuir [1938] found these to be lines of convergence along the surface, with downwelling below each line. He found also that maxima in the downwind surface currents occur along these lines. Thus the mixing layer is organized into rolls of alternating sign, aligned with the wind, and water parcels follow helical paths downwind. This form of circulation in surface layers of water has come to be called “Langmuir circulation.” Historically, this term has not implied any particular mechanism of formation, and it generally has not been used in reference to other similar structures (e.g., roll vortices in the atmosphere or in wall-bounded convection). Langmuir [1938, p. 123] expressed his belief that “the helical vortices set up by wind apparently constitute the essential mechanism by which the epilimnion is produced.” Clearly, this form of “helical vortices” is efficient in transporting momentum, energy, and matter throughout the surface layer of water. Three mechanisms are currently considered to drive Langmuir circulation. (1) An interaction between surface waves and winddriven shear gives rise to this form of circulation as a linear instability. A review of this theory, and of observations before 1983, is given by Leibovich [1983]. (2) Even without surface waves, turbulent mixing will occur due to the breakdown of the

A second-order bulk boundary-layer model

Abstract: Bulk mass-flux models represent the large eddies that are primarily responsible for the turbulent fluxes in the planetary boundary layer as convective circulations, with an associated convective mass flux. In order for such models to be useful, it is necessary to determine the fractional area covered by rising motion in the convective circulations. This fraction can be used as an estimate of the cloud amount, under certain conditions. 'Matching' conditions have been developed that relate the convective mass flux to the ventilation and entrainment mass fluxes. These are based on conservation equations for the scalar means and variances in the entrainment and ventilation layers. Methods are presented to determine both the fractional area covered by rising motion and the convective mass flux. The requirement of variance balance is used to relax the 'well-mixed' assumption. The vertical structures of the mean state and the turbulent fluxes are determined analytically. Several aspects of this simple model's formulation are evaluated using results from large-eddy simulations.

Heat Transfer Regimes in Microstructures

Abstract: Submicron dimensions are the hallmark of integrated electronic circuits, photovoltaic cells, sensors, and actuators. The design of these devices requires heat transfer analysis. Often it is not known to the designer whether a given microstructure can be analyzed using macroscale heat transfer theory, i.e., a method not considering the size dependence of a transport property such as thermal conductivity. This study develops regime maps showing the boundary between the macroscale and microscale heat transfer regimes. The maps relate the geometric dimension separating the two regimes to temperature for conduction in solids, to temperature, pressure, and Reynolds number for convection in gases, and to the temperature of the emitting medium for radiative transfer. The material purity and defect structure strongly influence the regime boundaries. Microstructures pertaining to a given technology are marked on these maps to determine whether macroscale heat transfer theory is applicable. By marking regions on the maps for the expected future development of microtechnologies, research needs in microscale heat transfer can be anticipated.

Turbulent Convection with Overshooting: Reynolds Stress Approach

Abstract: The Reynolds stress formalism is adopted to treat turbulent convection. This methodology is reviewed, and it is suggested that it may prove very useful to treat stellar and accretion disk turbulent convection, as well as in the construction of the subgrid models needed in large eddy simulations. A set of differential equations that yield the mean and turbulent quantities such as convective fluxes, turbulent viscosity, and turbulent conductivity is presented.

Chaos in models of double convection

Abstract: In certain parameter regimes, it is possible to derive third-order sets of ordinary differential equations that are asymptotically exact descriptions of weakly nonlinear double convection and that exhibit chaotic behaviour. This paper presents a unified approach to deriving such models for two-dimensional convection in a horizontal layer of Boussinesq fluid with lateral constraints. Four situations are considered: thermosolutal convection, convection in an imposed vertical or horizontal magnetic field, and convection in a fluid layer rotating uniformly about a vertical axis. Thermosolutal convection and convection in an imposed horizontal magnetic field are shown here to be governed by the same sets of model equations, which exhibit the period-doubling cascades and chaotic solutions that are associated with the Shil'nikov bifurcation (Proctor & Weiss 1990). This establishes, for the first time, the existence of chaotic solutions of the equations governing two-dimensional magneto-convection. Moreover, in the limit of tall thin rolls, convection in an imposed vertical magnetic field and convection in a rotating fluid layer are both modelled by a new third-order set of ordinary differential equations, which is shown here to have chaotic solutions that are created in a homoclinic explosion, in the same manner as the chaotic solutions of the Lorenz equations. Unlike the Lorenz equations, however, this model provides an accurate description of convection in the parameter regime where the chaotic solutions appear.

Experiments on convective heat transfer in corrugated channels

Abstract: Experiments have been performed to study convective heat transfer in the entrance region of corrugated channels. With water as the working fluid, two channel spacings were examined for a single corrugation angle of 20°. The flow rate was varied over the range I50 ≤ Re ≤ 4000. Flow visualization under low-Reynolds-number flow conditions suggested the presence of longitudinal vortices, while at somewhat higher Reynolds numbers, the existence of spanwise vortices was clearly revealed. For Re > 1500, Nusselt numbers in the corrugated channels exceeded those in the parallel-plate channel by approximately 140% and 240% for the two channel spacings, the corresponding increases in friction factor being 130% and 280%. Performance evaluations under the criteria of equal mass flow rate, equal pumping power, and equal pressure drop per unit length established both the corrugated channels as superior to the parallel-plate channel in intensifying heat transfer.

Linear-Eddy Modeling of Turbulent Transport. Part 4. Structure of Diffusion Flames

Abstract: A simulation model of the axial structure of turbulent jet diffusion flames is formulated for the purpose of interpreting flame-structure measurements. The model, based on the linear-eddy approach, incorporates spatial and temporal variation of the air entrainment rate, reflecting buoyancy effects, and an implementation of turbulent mixing using a novel stochastic representation of convective stirring in conjunction with Fick's law governing molecular diffusion. Simulation results are compared to axial profiles of mixing-cup density measured in propane flames. The comparisons suggest that the measured Froude-number dependences reflect the combined effect of finite-rate mixing and the transition from forced to natural convection. Predictions for hydrogen flames are presented in order to assess the generality of inferences based on the propane results.

Photon bubbles - Overstability in a magnetized atmosphere

Abstract: The formation of 'photon bubbles' in a convectively stable scattering atmosphere supported against gravity entirely by radiation pressure is studied by means of linear stability theory. A simple model is developed for the 2D structure of a plasma mound formed by laminar accretion onto the magnetic poles of a neutron star, in which upward photon diffusion balances downward photon advection with the plasma. It is shown that the vertical pressure and density structure is the same as in an isothermal atmosphere. Application of the stability theory to this model suggests photon bubbles would form in a polar accretion mound under the conditions expected in accretion-powered pulsars within a few tenths of a millisecond. Because long-wavelength modes have the largest rise speeds, eventual dominance by a few large bubbles is suggested, and possible connections between bubble formation and short-time variability in accretion-powered pulsars is discussed, as well as a possible connection of the photon bubble phenomenon to the rapid time variability observed in the Rapid Burster and in quasi-period oscillator sources.

Experimental Heat Transfer Rates of Natural Convection of Molten Gallium Suppressed Under an External Magnetic Field in Either the X, Y, or Z Direction

Abstract: The heat transfer rates of natural convection of molten gallium were measured under various strengths of heating rates and three coordinate directional magnetic fields. Molten gallium (Pr = 0.024) was filled in a cubic enclosure of 30 mm {times} 30 mm {times} 30 mm whose one vertical wall was uniformly heated and an opposing wall was isothermally cooled, with otherwise insulated walls. An external magnetic field was impressed either perpendicular and horizontal to the heated wall (x direction) or in parallel and horizontal to the heated wall (y direction) of the enclosure or in a vertical direction (z direction). For the modified Grashof number, based on the heat flux, less than 4.24 {times} 10{sup 6} and the Hartmann number less than 461, the average Nusselt numbers were measured. These results proved that our previous three-dimensional numerical analyses for an isothermal hot wall boundary were in good qualitative agreement. A much higher suppression effect is given in the x- and z-directional magnetic fields than that in the y-directional one.

Ionospheric convection and the substorm cycle

Abstract: During substorms, magnetic energy is stored and released by the geomagnetic tail in cycles of growth and expansion phases, respectively. Hence substorms are inherently non-steady phenomena. On the other hand, all numerical models (and most conceptual ones) of ionospheric convection produced to date have considered only steady-state situations. In this paper, we investigate the relationship of substorms to convection. In particular, it is shown that the steady-state convection pattern represents an average over several substorm cycles and does not apply on time scales shorter than the substorm cycle period of 1-2 hours. The flows driven by the growth and expansion phases of substorms are integral (indeed dominant) part of, as opposed to a transient addition to, the overall convection pattern.

Bubble clouds and the dynamics of the upper ocean

Abstract: This is a review of turbulence in the upper ocean close to the sea surface, particularly of the information that has been obtained from sonar observations of bubble clouds produced by breaking wind waves. These clouds provide tracers of the turbulent motions and are important, especially at high wind speeds, in the process of air-sea gas transfer. the observations of bubble clouds are here related to other measurements of turbulence, particularly to direct measurements of currents and temperatures in lakes or at sea, and to laboratory studies. Some novel observations of bubble clouds and breaking waves, their frequency and relation to Langmuir circulation, are presented. There is now emerging a pattern of clues that point to the dominance of breaking surface gravity waves as a source of turbulence to a depth below the surface of 0.04 to 0.2 times the wavelength of the dominant breaking waves, although the effect of swell has yet to be clarified. the relative depth appears to increase with increasing values of W,1/c, where W10 is the wind speed and c the phase speed of the dominant waves. Below this region the generation of turbulence may be dominated by shear-stress or convection. Here, turbulence is generally similar to that in the atmospheric boundary layer. There are, however, coherent motions on the scale of the mixing layer that persist for periods of a few minutes to an hour or so, to which the transport of a large part of the momentum and heat fluxes can be attributed, and which strongly affect the dispersion of buoyant particles or bubbles. These motions deserve special study to establish their contribution to heat and momentum transport, and hence to determine if, or when, they should be specifically represented in models of the upper ocean devised, for example, to describe the dispersion of passive and non-passive tracers or the air-sea transfer of gases.

An Experimental Study of Heat Transfer in a Large-Scale Turbine Rotor Passage

Abstract: An experimental study of the heat transfer distribution in a turbine rotor passage was conducted in a large-scale, ambient temperature, rotating turbine model. Heat transfer was measured for both the full-span suction and pressure surfaces of the airfoil as well as for the hub endwall surface. The objective of this program was to document the effects of flow three-dimensionality on the heat transfer in a rotating blade row (vs a stationary cascade). Of particular interest were the effects of the hub and tip secondary flows, tip leakage and the leading-edge horseshoe vortex system. The effect of surface roughness on the passage heat transfer was also investigated. Midspan results are compared with both smooth-wall and rough-wall finite-difference two-dimensional heat transfer predictions. Contour maps of Stanton number for both the rotor airfoil and endwall surfaces revealed numerous regions of high heat transfer produced by the three-dimensional flows within the rotor passage. Of particular importance are regions of local enhancement (as much as 100 percent over midspan values) produced on the airfoil suction surface by the secondary flows and tip-leakage vortices and on the hub endwall by the leading-edge horseshoe vortex system.

Optimum Dimensions of Extended Surfaces Operating in a Convective Environment

Abstract: This article is devoted to the review of the literature on optimum dimensions of extended surfaces losing heat by pure convection to the surroundings. The review covers straight (longitudinal) fins, annular (radial) fins, and spines of different profile shapes. The optimum dimensions for each shape are given both in terms of the volume of the material as well as the heat dissipation. The effects of tip heat loss, variable heat transfer coefficient, internal heat generation, temperature dependent thermal conductivity, base convection, and primary surface thickness on the optimum dimensions are discussed. The optimization procedure is illustrated with several numerical examples. Areas of extended surface technology where further optimization studies are needed are identified. It is hoped that the article would serve the dual purpose of the state-of-the-art as well as a pedagogical review. 24 refs., 22 figs., 9 tabs.

Steady-state convection and fluctuation-driven particle transport in the H -mode transition

Abstract: Large-scale steady-state convection is found to dominate the particle transport just inside the last closed flux surface (LCFS) in Ohmic discharges in the CCT tokamak. Near the limiter radius and in the scrape-off layer, fluctuation-induced transport is prevalent. At the L-H transition, the convection pattern near the LCFS is disrupted and a more poloidally symmetric, near-sonic plasma flow develops. Convective and turbulent particle transport are reduced across the entire edge region, resulting in the formation of the H-mode edge transport barrier.