Showing papers on "Convection published in 2009"
TL;DR: The Tropical Tropopause Layer (TTL) as discussed by the authors is a 3D model of the troposphere, and it has been shown that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp "tropopause".
Abstract: [1] Observations of temperature, winds, and atmospheric trace gases suggest that the transition from troposphere to stratosphere occurs in a layer, rather than at a sharp “tropopause.” In the tropics, this layer is often called the “tropical tropopause layer” (TTL). We present an overview of observations in the TTL and discuss the radiative, dynamical, and chemical processes that lead to its time-varying, three-dimensional structure. We present a synthesis definition with a bottom at 150 hPa, 355 K, 14 km (pressure, potential temperature, and altitude) and a top at 70 hPa, 425 K, 18.5 km. Laterally, the TTL is bounded by the position of the subtropical jets. We highlight recent progress in understanding of the TTL but emphasize that a number of processes, notably deep, possibly overshooting convection, remain not well understood. The TTL acts in many ways as a “gate” to the stratosphere, and understanding all relevant processes is of great importance for reliable predictions of future stratospheric ozone and climate.
881 citations
TL;DR: In this paper, the steady boundary-layer flow near the stagnation point on an impermeable vertical surface with slip that is embedded in a fluid-saturated porous medium is investigated.
Abstract: The steady boundary-layer flow near the stagnation point on an impermeable vertical surface with slip that is embedded in a fluid-saturated porous medium is investigated. Using appropriate similarity variables, the governing system of partial differential equations is transformed into a system of ordinary differential equations. This system is then solved numerically. The features of the flow and the heat transfer characteristics for different values of the governing parameters, namely, the Darcy–Brinkman, Γ, mixed convection, λ, and slip, γ, parameters, are analysed and discussed in detail for the cases of assisting and opposing flows. It is found that dual solutions exist for assisting flows, as well as those usually reported in the literature for opposing flows. A stability analysis of the steady flow solutions encountered for different values of the mixed convection parameter λ is performed using a linear temporal stability analysis. This analysis reveals that for γ = 0 (slip absent) and Γ = 1 the lower solution branch is unstable while the upper solution branch is stable.
507 citations
TL;DR: In this article, the effect of dispersing energy (ultrasonication) on viscosity, thermal conductivity, and the laminar convective heat transfer was studied.
472 citations
TL;DR: In this paper, the authors present a numerical study of the cooling performance of a heat source embedded on the bottom wall of an enclosure filled with nanofluids, where the top and vertical walls of the enclosure are maintained at a relatively low temperature.
Abstract: This article presents a numerical study of natural convection cooling of a heat source embedded on the bottom wall of an enclosure filled with nanofluids. The top and vertical walls of the enclosure are maintained at a relatively low temperature. The transport equations for a Newtonian fluid are solved numerically with a finite volume approach using the SIMPLE algorithm. The influence of pertinent parameters such as Rayleigh number, location and geometry of the heat source, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. The results indicate that adding nanoparticles into pure water improves its cooling performance especially at low Rayleigh numbers. The type of nanoparticles and the length and location of the heat source proved to significantly affect the heat source maximum temperature.
441 citations
TL;DR: Radiative-hydrodynamical simulations of solar surface convection can be used as 2D/3D time-dependent models of the solar atmosphere to predict the emergent spectrum, and the resulting detailed spectral line profiles agree spectacularly well with observations without invoking any micro- and macroturbulence parameters.
Abstract: We review the properties of solar convection that are directly observable at the solar surface, and discuss the relevant underlying physics, concentrating mostly on a range of depths from the temperature minimum down to about 20 Mm below the visible solar surface. The properties of convection at the main energy carrying (granular) scales are tightly constrained by observations, in particular by the detailed shapes of photospheric spectral lines and the topology (time- and length-scales, flow velocities, etc.) of the up- and downflows. Current supercomputer models match these constraints very closely, which lends credence to the models, and allows robust conclusions to be drawn from analysis of the model properties. At larger scales the properties of the convective velocity field at the solar surface are strongly influenced by constraints from mass conservation, with amplitudes of larger scale horizontal motions decreasing roughly in inverse proportion to the scale of the motion. To a large extent, the apparent presence of distinct (meso- and supergranulation) scales is a result of the folding of this spectrum with the effective “filters” corresponding to various observational techniques. Convective motions on successively larger scales advect patterns created by convection on smaller scales; this includes patterns of magnetic field, which thus have an approximately self-similar structure at scales larger than granulation. Radiative-hydrodynamical simulations of solar surface convection can be used as 2D/3D time-dependent models of the solar atmosphere to predict the emergent spectrum. In general, the resulting detailed spectral line profiles agree spectacularly well with observations without invoking any micro- and macroturbulence parameters due to the presence of convective velocities and atmosphere inhomogeneities. One of the most noteworthy results has been a significant reduction in recent years in the derived solar C, N, and O abundances with far-reaching consequences, not the least for helioseismology. Convection in the solar surface layers is also of great importance for helioseismology in other ways; excitation of the wave spectrum occurs primarily in these layers, and convection influences the size of global wave cavity and, hence, the mode frequencies. On local scales convection modulates wave propagation, and supercomputer convection simulations may thus be used to test and calibrate local helioseismic methods. We also discuss the importance of near solar surface convection for the structure and evolution of magnetic patterns: faculae, pores, and sunspots, and briefly address the question of the importance or not of local dynamo action near the solar surface. Finally, we discuss the importance of near solar surface convection as a driver for chromospheric and coronal heating.
363 citations
TL;DR: In this paper, a new method is introduced for estimating the convection velocity of individual modes in turbulent shear flows that only requires spectral information in the temporal or spatial direction over which a modal decomposition is desired, while only using local derivatives in other directions.
Abstract: A new method is introduced for estimating the convection velocity of individual modes in turbulent shear flows that, in contrast to most previous ones, only requires spectral information in the temporal or spatial direction over which a modal decomposition is desired, while only using local derivatives in other directions. If no spectral information is desired, the method provides a natural definition for the average convection velocity, as well as a way to estimate the accuracy of the frozen-turbulence approximation. Existing data from numerical turbulent channels at friction Reynolds numbers Reτ 1900 are used to validate the new method against classical ones, and to characterize the dependence of the convection velocity on the eddy wavelength and wall distance. The results indicate that the small scales in turbulent channels travel at the local mean velocity, while large ‘global’ modes travel at a more uniform speed proportional to the bulk velocity. To estimate the systematic deviations introduced in experimental spectra by the use of Taylor’s approximation with a wavelengthindependent convection velocity, a semi-empirical fit to the computed convection velocities is provided. It represents well the data throughout the Reynolds number range of the simulations. It is shown that Taylor’s approximation not only displaces the large scales near the wall to shorter apparent wavelengths but also modifies the shape of the spectrum, giving rise to spurious peaks similar to those observed in some experiments. To a lesser extent the opposite is true above the logarithmic layer. The effect increases with the Reynolds number, suggesting that some of the recent challenges to the k −1 x energy spectrum may have to be reconsidered.
348 citations
TL;DR: In this article, the authors investigated the feedback between soil moisture and precipitation over the Alpine region using two different model configurations and found that the feedback was predominantly positive (more precipitation over wet soils) over Europe.
Abstract: Moist convection is a key aspect of the extratropical summer climate and strongly affects the delicate balance of processes that determines the surface climate in response to larger-scale forcings. Previous studies using parameterized convection have found that the feedback between soil moisture and precipitation is predominantly positive (more precipitation over wet soils) over Europe. Here this feedback is investigated for one full month (July 2006) over the Alpine region using two different model configurations. The first one employs regional climate simulations performed with the Consortium for Small-Scale Modeling Model in Climate Mode (CCLM) on a grid spacing of 25 km. The second one uses the same model but integrated on a cloud-resolving grid of 2.2 km, allowing an explicit treatment of convection. Each configuration comprises one control and two sensitivity experiments. The latter start from perturbed soil moisture initial conditions. Comparison of the simulated soil moisture–precipitatio...
341 citations
TL;DR: A review of the recent experimental, analytical, and numerical work into single bubble heat transfer is presented to determine the contribution of each of the above mechanisms to the overall heat transfer as discussed by the authors.
308 citations
TL;DR: In this article, the authors systematically assessed the aerosol effects on isolated DCCs based on cloud-resolving model simulations with spectral bin cloud microphysics and found that the decreasing rate of convective strength is greater in the humid air than that in the dry air when wind shear is strong.
Abstract: [1] Aerosol-cloud interaction is recognized as one of the key factors influencing cloud properties and precipitation regimes across local, regional, and global scales and remains one of the largest uncertainties in understanding and projecting future climate changes. Deep convective clouds (DCCs) play a crucial role in the general circulation, energy balance, and hydrological cycle of our climate system. The complex aerosol-DCC interactions continue to be puzzling as more ‘‘aerosol effects’’ unfold, and systematic assessment of such effects is lacking. Here we systematically assess the aerosol effects on isolated DCCs based on cloud-resolving model simulations with spectral bin cloud microphysics. We find a dominant role of vertical wind shear in regulating aerosol effects on isolated DCCs, i.e., vertical wind shear qualitatively determines whether aerosols suppress or enhance convective strength. Increasing aerosols always suppresses convection under strong wind shear and invigorates convection under weak wind shear until this effect saturates at an optimal aerosol loading. We also found that the decreasing rate of convective strength is greater in the humid air than that in the dry air when wind shear is strong. Our findings may resolve some of the seemingly contradictory results among past studies by considering the dominant effect of wind shear. Our results can provide the insights to better parameterize aerosol effects on convection by adding the factor of wind shear to the entrainment term, which could reduce uncertainties associated with aerosol effects on climate forcing.
305 citations
TL;DR: In this paper, the development of observational understanding of the interior rotation of the Sun and its temporal variation over approximately forty years, starting with the 1960s attempts to determine the solar core rotation from oblateness and proceeding through to the detailed modern picture of the internal rotation deduced from continuous helioseismic observations during solar cycle 23.
Abstract: This article surveys the development of observational understanding of the interior rotation of the Sun and its temporal variation over approximately forty years, starting with the 1960s attempts to determine the solar core rotation from oblateness and proceeding through the development of helioseismology to the detailed modern picture of the internal rotation deduced from continuous helioseismic observations during solar cycle 23. After introducing some basic helioseismic concepts, it covers, in turn, the rotation of the core and radiative interior, the “tachocline” shear layer at the base of the convection zone, the differential rotation in the convection zone, the near-surface shear, the pattern of migrating zonal flows known as the torsional oscillation, and the possible temporal variations at the bottom of the convection zone. For each area, the article also briefly explores the relationship between observations and models.
288 citations
TL;DR: The gross moist stability relates the net lateral outflow of moist entropy or moist static energy from an atmospheric convective region to some measure of the strength of the convection in that region as discussed by the authors.
Abstract: The gross moist stability relates the net lateral outflow of moist entropy or moist static energy from an atmospheric convective region to some measure of the strength of the convection in that region. If the gross moist stability can be predicted as a function of the local environmental conditions, then it becomes the key element in understanding how convection is controlled by the large-scale flow. This paper provides a guide to the various ways in which the gross moist stability is defined and the subtleties of its calculation from observations and models. Various theories for the determination of the gross moist stability are presented and its roles in current conceptual models for the tropical atmospheric circulation are analyzed. The possible effect of negative gross moist stability on the development and dynamics of tropical disturbances is currently of great interest.
TL;DR: In this article, a combined discrete particle simulation (DPS) and computational fluid dynamics (CFD) approach is extended to study particle-particle and particle-fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale.
Abstract: The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle-fluid flow, is extended to study particle-particle and particle-fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid-particle convection, particle-particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009
TL;DR: In this paper, a simulation of the interaction between convection and large-scale flows in the tropics suggests that there are two types of convectively coupled disturbances: the moisture mode instability described above and another unstable mode dependent on fluctuations in the convective inhibition.
Abstract: Moisture mode instability is thought to occur in the tropical oceanic atmosphere when precipitation is a strongly increasing function of saturation fraction (precipitable water divided by saturated precipitable water) and when convection acts to increase the saturation fraction. A highly simplified model of the interaction between convection and large-scale flows in the tropics suggests that there are two types of convectively coupled disturbances: the moisture mode instability described above and another unstable mode dependent on fluctuations in the convective inhibition. The latter is associated with rapidly moving disturbances such as the equatorially coupled Kelvin wave. A toy aquaplanet beta-plane model with realistic sea surface temperatures produces a robust Madden–Julian oscillation–like disturbance that resembles the observed phenomenon in many ways. Convection in this model exhibits a strong dependence of precipitation on saturation fraction and does indeed act to increase this paramet...
TL;DR: In this article, an experimental and numerical study of the steady state convective losses occurring from a downward facing cylindrical cavity receiver of length 0.5m, internal diameter of 0.3m and a wind skirt diameter of0.5mm is carried out.
TL;DR: In this article, a simple theoretical argument is presented to isolate the conditions under which a tropical cyclone can rapidly develop a warm-core thermal structure and subsequently approach a steady state, based on the balanced vortex model and, in particular, on the associated transverse circulation equation and the geopotential tendency equation.
Abstract: This paper presents a simple theoretical argument to isolate the conditions under which a tropical cyclone can rapidly develop a warm-core thermal structure and subsequently approach a steady state. The theoretical argument is based on the balanced vortex model and, in particular, on the associated transverse circulation equation and the geopotential tendency equation. These second-order partial differential equations contain the diabatic forcing and three spatially varying coefficients: the static stability A, the baroclinity B, and the inertial stability C. Thus, the transverse circulation and the temperature tendency in a tropical vortex depend not only on the diabatic forcing but also on the spatial distributions of A, B, and C. Experience shows that the large radial variations of C are typically the most important effect. Under certain simplifying assumptions as to the vertical structure of the diabatic forcing and the spatial variability of A, B, and C, the transverse circulation equation and the geopotential tendency equation can be solved via separation of variables. The resultingradialstructureequationsretainthedynamicallyimportantradialvariationofCandcanbe solvedin terms of Green’s functions. These analytical solutions show that the vortex response to a delta function in the diabatic heating depends critically on whether the heating occurs in the low-inertial-stability region outside the radius of maximum wind or in the high-inertial-stability region inside the radius of maximum wind. This resultsuggeststhatrapidintensification isfavoredforstormsthathaveat leastsome oftheeyewallconvection inside the radius of maximum wind. The development of an eye partially removes diabatic heating from the high-inertial-stability region of the storm center; however, rapid intensification may continue if the eyewall heating continues to become more efficient. As the warm core matures and static stability increases over the inner core, conditions there become less favorable for deep upright convection and the storm tends to approach a steady state.
TL;DR: In this paper, a model for the long-term evolution of the geodynamo by combining core ther-modynamics with a scaling analysis of numerical dynamo simulations is presented.
Abstract: Although it is known that the geodynamo has been operating for at least 3.2 Ga, it remains difficult to infer the intensity, dipolarity and stability (occurrence of reversals) of the Precam-brian magnetic field of the Earth. In order to assist the interpretation of palaeomagnetic data, we produce models for the long-term evolution of the geodynamo by combining core ther-modynamics with a systematic scaling analysis of numerical dynamo simulations. We update earlier dynamo scaling results by exploring a parameter space, which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to Earth in the Precambrian. Our analysis highlights the central role of the convective power, which is an output of core thermodynamics and the main input of our updated scalings. As the thermal evolution of the Earth's core is not well known, two end-member models of heat flow evolution at the core–mantle boundary (CMB) are used, respectively, terminating at present heat flows of 11 TW (high-power scenario) and 3 TW (low power scenario). The resulting models predict that until the appearance of the inner core, a thermal dynamo driven only by secular cooling, and without any need for radioactive heating, can produce a dipole moment of strength comparable to that of the present field, thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence. The observed lack of strong long-term trends in palaeointensity data throughout the Earth's history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter. Specifically, the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation, but the dynamo becomes deeper-seated in the core, thus largely cancelling the increase at the core and Earth surface, and diminishing the prospect of observing this event in palaeointensity data. Our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history. In the Precambrian, we predict a dynamo with similar dipolarity and less frequent reversals than at present times, due to conditions of generally lower convective forcing. Quantifying the typical CMB heat flow variation needed for the geodynamo to cross the transition from a reversing to a non-reversing state, we find that it is unlikely that such a variation may have caused superchrons in the last 0.5 Ga without shutting down dynamo action altogether.
Book•
28 Dec 2009
TL;DR: In this article, the Navier-Stokes Equation is used to model the thermal flow in a 3D model of a single-dimensional sphere, and the dynamics of thermal flow are discussed.
Abstract: Preface . Acknowledgements . 1 Equations, General Concepts and Methods of Analysis. 1.1 Pattern Formation and Nonlinear Dynamics. 1.2 The Navier-Stokes Equations. 1.3 Energy Equality and Dissipative Structures. 1.4 Flow Stability, Bifurcations and Transition to Chaos. 1.5 Linear Stability Analysis: Principles and Methods. 1.6 Energy Stability Theory. 1.7 Numerical Integration of the Navier-Stokes Equations. 1.8 Some Universal Properties of Chaotic States. 1.9 The Maxwell Equations. 2 Classical Models, Characteristic Numbers and Scaling Arguments. 2.1 Buoyancy Convection and the Boussinesq Model. 2.2 Convection in Space. 2.3 Marangoni Flow. 2.4 Exact Solutions of the Navier-Stokes Equations for Thermal Problems. 2.5 Conductive, Transition and Boundary-layer Regimes. 3 Examples of Thermal Fluid Convection and Pattern Formation in Nature and Technology. 3.1 Technological Processes: Small-scale Laboratory and Industrial Setups. 3.2 Examples of Thermal Fluid Convection and Pattern Formation at the Mesoscale. 3.3 Planetary Structure and Dynamics: Convective Phenomena. 3.4 Atmospheric and Oceanic Phenomena. 4 Thermogravitational Convection: The Rayleigh-Benard Problem. 4.1 Nonconfined Fluid Layers and Ideal Straight Rolls. 4.2 The Busse Balloon. 4.3 Some Considerations About the Role of Dislocation Dynamics. 4.4 Tertiary and Quaternary Modes of Convection. 4.5 Spoke Pattern Convection. 4.6 Spiral Defect Chaos, Hexagons and Squares. 4.7 Convection with Lateral Walls. 4.8 Two-dimensional Models. 4.9 Three-dimensional Parallelepipedic Enclosures: Classification of Solutions and Possible Symmetries. 4.10 The Circular Cylindrical Problem. 4.11 Spirals: Genesis, Properties and Dynamics. 4.12 From Spirals to SDC: The Extensive Chaos Problem. 4.13 Three-dimensional Convection in a Spherical Shell. 5 The Dynamics of Thermal Plumes and Related Regimes of Motion. 5.1 Introduction. 5.2 Free Plume Regimes. 5.3 The Flywheel Mechanism: The 'Wind' of Turbulence. 5.4 Multiplume Configurations Originated from Discrete Sources of Buoyancy. 6 Systems Heated from the Side: The Hadley Flow. 6.1 The Infinite Horizontal Layer. 6.2 Two-dimensional Horizontal Enclosures. 6.3 The Infinite Vertical Layer: Cats-eye Patterns and Temperature Waves. 6.4 Three-dimensional Parallelepipedic Enclosures. 6.5 Cylindrical Geometries under Various Heating Conditions. 7 Thermogravitational Convection in Inclined Systems. 7.1 Inclined Layer Convection. 7.2 Inclined Side-heated Slots. 8 Thermovibrational Convection. 8.1 Equations and Relevant Parameters. 8.2 Fields Decomposition. 8.3 The TFD Distortions. 8.4 High Frequencies and the Thermovibrational Theory. 8.5 States of Quasi-equilibrium and Related Stability. 8.6 Primary and Secondary Patterns of Symmetry. 8.7 Medium and Low Frequencies: Possible Regimes and Flow Patterns. 9 Marangoni-Benard Convection. 9.1 Introduction. 9.2 High Prandtl Number Liquids: Patterns with Hexagons, Squares and Triangles. 9.3 Liquid Metals: Inverted Hexagons and High-order Solutions. 9.4 Effects of Lateral Confinement. 9.5 Temperature Gradient Inclination. 10 Thermocapillary Convection. 10.1 Basic Features of Steady Marangoni Convection. 10.2 Stationary Multicellular Flow and Hydrothermal Waves. 10.3 Annular Configurations. 10.4 The Liquid Bridge. 11 Mixed Buoyancy-Marangoni Convection. 11.1 The Canonical Problem: The Infinite Horizontal Layer. 11.2 Finite-sized Systems Filled with Liquid Metals. 11.3 Typical Terrestrial Laboratory Experiments with Transparent Liquids. 11.4 The Rectangular Liquid Layer. 11.5 Effects Originating from the Walls. 11.6 The Open Vertical Cavity. 11.7 The Annular Pool. 11.8 The Liquid Bridge on the Ground. 12 Hybrid Regimes with Vibrations. 12.1 RB Convection with Vertical Shaking. 12.2 Complex Order, Quasi-periodic Crystals and Superlattices. 12.3 RB Convection with Horizontal or Oblique Shaking. 12.4 Laterally Heated Systems and Parametric Resonances. 12.5 Control of Thermogravitational Convection. 12.6 Mixed Marangoni-Thermovibrational Convection. 12.7 Modulation of Marangoni-Benard Convection. 13 Flow Control by Magnetic Fields. 13.1 Static and Uniform Magnetic Fields. 13.2 Historical Developments and Current Status. 13.3 Rotating Magnetic Fields. 13.4 Gradients of Magnetic Fields and Virtual Microgravity. References . Index .
TL;DR: Using hourly data from a three-year simulation based on a gravity-wave resolving general circulation model, the authors inferred a global view of gravity wave sources and propagation affecting significantly the momentum balance in the mesosphere.
Abstract: [1] Using hourly data from a three-year simulation based on a gravity-wave resolving general circulation model, we have first inferred a global view of gravity wave sources and propagation affecting significantly the momentum balance in the mesosphere The meridional cross section of momentum fluxes suggests that there are a few dominant propagation paths originating from the subtropics in summer and the middle to high latitudes in winter These gravity waves are focused into the mesospheric jets in their respective seasons, acting effectively to decelerate the jets The difference in the source latitudes likely contributes to the hemispheric asymmetries of the jets The horizontal distribution of the momentum fluxes indicates that the dominant sources are steep mountains and tropospheric westerly jets in winter and vigorous monsoon convection in summer The monsoon regions are the most important window to the middle atmosphere in summer because of the easterlies associated with the monsoon circulation
TL;DR: In this paper, it was shown that the transition from rotationally dominated to non-rotating heat transfer is not determined by the global force balance, but by the relative thickness of the thermal and Ekman boundary layers.
Abstract: Turbulent rotating convection is an important dynamical process occurring on nearly all planetary and stellar bodies, influencing many observed features such as magnetic fields, atmospheric jets and emitted heat flux patterns. For decades, it has been thought that the importance of rotation's influence on convection depends on the competition between the two relevant forces in the system: buoyancy (non-rotating) and Coriolis (rotating). The force balance argument does not, however, accurately predict the transition from rotationally controlled to non-rotating heat transfer behaviour. New results from laboratory and numerical experiments suggest that the transition is in fact controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. Turbulent rotating convection controls many observed features in stars and planets, such as magnetic fields. It has been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. This paper presents results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns1,2,3,4,5,6. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force7,8,9,10,11,12. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments8,9,10,11,12,13,14,15, and is broadly applicable to natural convection systems.
TL;DR: In this article, the authors used the homotopy analysis method to detect the fin efficiency of convective straight fins with temperature-dependent thermal conductivity, compared with those of the exact solution and Adomian's decomposition method.
TL;DR: In this paper, high-resolution 3D steady RANS CFD simulations of forced convective heat transfer at the facades of a low-rise cubic (10 × 10 × 10 − 10 −10 − 10 m 3 ) building are performed to determine convective transfer coefficients (CHTC).
TL;DR: In this paper, a numerical investigation of the steady magnetohydrodynamics free convection in a rectangular cavity filled with a fluid-saturated porous medium and with internal heat generation has been performed.
TL;DR: In this article, the authors compared two optimization principles for convective heat transfer, the minimum entropy generation principle and the entransy dissipation extremum principle, and analyzed their physical implications and applicability.
TL;DR: A rash of nighttime-accentuated events in the last decade was punctuated by an unusually intense case in July 2006, which was the largest heat wave on record (1948-2006) as mentioned in this paper.
Abstract: Most of the great California–Nevada heat waves can be classified into primarily daytime or nighttime events depending on whether atmospheric conditions are dry or humid. A rash of nighttime-accentuated events in the last decade was punctuated by an unusually intense case in July 2006, which was the largest heat wave on record (1948–2006). Generally, there is a positive trend in heat wave activity over the entire region that is expressed most strongly and clearly in nighttime rather than daytime temperature extremes. This trend in nighttime heat wave activity has intensified markedly since the 1980s and especially since 2000. The two most recent nighttime heat waves were also strongly expressed in extreme daytime temperatures. Circulations associated with great regional heat waves advect hot air into the region. This air can be dry or moist, depending on whether a moisture source is available, causing heat waves to be expressed preferentially during day or night. A remote moisture source centered ...
TL;DR: In the subpolar North Atlantic Ocean, surface water sinks to depth as a distinct water mass, the characteristics of which affect the meridional overturning circulation and oceanic heat flux as mentioned in this paper.
Abstract: In the process of open-ocean convection in the subpolar North Atlantic Ocean, surface water sinks to depth as a distinct water mass, the characteristics of which affect the meridional overturning circulation and oceanic heat flux. In addition, carbon is sequestered from the atmosphere in the process. In recent years, this convection has been shallow or non-existent, which could be construed as a consequence of a warmer climate. Here we document the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008. We use profiling float data from the Argo programme to document deep mixing. Analysis of a variety of in situ, satellite and reanalysis data shows that contrary to expectations the transition to a convective state took place abruptly, without going through a phase of preconditioning. Changes in hemispheric air temperature, storm tracks, the flux of fresh water to the Labrador Sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air, making the surface water sufficiently cold and dense to initiate deep convection. Given this complexity, we conclude that it will be difficult to predict when deep mixing may occur again.
TL;DR: In this paper, the influence of heat convection and internal heat generation on structural designs is investigated. But the authors focus on the influence on the heat transfer coefficient and heat conduction on the structure surface.
TL;DR: In this paper, the authors apply Large-Eddy Simulation (LES) to simulate deep tropical convection in near equilibrium for 24 hours over an area of about 205 × 205 km2, which is comparable to that of a typical horizontal grid cell in a global climate model.
Abstract: [1] This study represents an attempt to apply Large-Eddy Simulation (LES) resolution to simulate deep tropical convection in near equilibrium for 24 hours over an area of about 205 × 205 km2, which is comparable to that of a typical horizontal grid cell in a global climate model. The simulation is driven by large-scale thermodynamic tendencies derived from mean conditions during the GATE Phase III field experiment. The LES uses 2048 × 2048 × 256 grid points with horizontal grid spacing of 100 m and vertical grid spacing ranging from 50 m in the boundary layer to 100 m in the free troposphere. The simulation reaches a near equilibrium deep convection regime in 12 hours. The simulated vertical cloud distribution exhibits a tri-modal vertical distribution of deep, middle and shallow clouds similar to that often observed in Tropics. A sensitivity experiment in which cold pools are suppressed by switching off the evaporation of precipitation results in much lower amounts of shallow and congestus clouds. Unlike the benchmark LES where the new deep clouds tend to appear along the edges of spreading cold pools, the deep clouds in the no-cold-pool experiment tend to reappear at the sites of the previous deep clouds and tend to be surrounded by extensive areas of sporadic shallow clouds. The vertical velocity statistics of updraft and downdraft cores below 6 km height are compared to aircraft observations made during GATE. The comparison shows generally good agreement, and strongly suggests that the LES simulation can be used as a benchmark to represent the dynamics of tropical deep convection on scales ranging from large turbulent eddies to mesoscale convective systems. The effect of horizontal grid resolution is examined by running the same case with progressively larger grid sizes of 200, 400, 800, and 1600 m. These runs show a reasonable agreement with the benchmark LES in statistics such as convective available potential energy, convective inhibition, cloud fraction, precipitation rates, and surface latent and sensible fluxes. All runs reveal a tri-model cloud distribution in the vertical. However, there are differences in the updraft-core cloud statistics, and convergence of statistical properties is found only between the LES benchmark and the run with 200 m grid size. The effect of vertical grid resolution is also investigated with another run that uses a typical cloud-resolving model (CRM) horizontal grid size on the order of 1 km and only 64 vertical levels. A comparison to the run with 256 vertical levels shows different vertical cloud distributions. It is concluded that representation of the often observed tri-modal vertical distribution of clouds requires a vertical grid spacing in the range of 50-100 m in mid-to-low troposphere.
TL;DR: In this paper, an anelastic general circulation model is proposed to drive the system away from an isentropic and therefore barotropic state, and the model's geometry is a full 3D sphere down to a small inner core.
TL;DR: In this paper, the authors evaluate the heat transfer coefficients between the radiant ceiling and room in typical conditions of occupancy of an office or residential building, using experimental tests in an environmental chamber.
TL;DR: In this paper, a review of how heat transfer takes place in plasma-sprayed thermal barrier coatings (TBCs) during operation of gas turbines is presented, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions.
Abstract: A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the heat flow. Contributions from convective, conductive, and radiative heat transfer are considered, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions. In fact, convective heat flow within TBCs usually makes a negligible contribution to the overall heat transfer through the coating, although what might be described as convection can be important if there are gross through-thickness defects such as segmentation cracks. Radiative heat transfer, on the other hand, can be significant within TBCs, depending on temperature and radiation scattering lengths, which in turn are sensitive to the grain structure and the pore architecture. Under most conditions of current interest, conductive heat transfer is largely predominant. However, it is not only conduction through solid ceramic that is important. Depending on the pore architecture, conduction through gas in the pores can play a significant role, particularly at the high gas pressures typically acting in gas turbines (although rarely applied in laboratory measurements of conductivity). The durability of the pore structure under service conditions is also of importance, and this review covers some recent work on how the pore architecture, and hence the conductivity, is affected by sintering phenomena. Some information is presented concerning the areas in which research and development work needs to be focussed if improvements in coating performance are to be achieved.