scispace - formally typeset
Search or ask a question

Showing papers on "Convection published in 2010"


Journal ArticleDOI
TL;DR: In this paper, the phase change problem has been formulated using pure conduction approach but the problem has moved to a different level of complexity with added convection in the melt being accounted for, which makes it difficult for comparison to be made to assess the suitability of PCMs to particular applications.
Abstract: This paper reviews the development of latent heat thermal energy storage systems studied detailing various phase change materials (PCMs) investigated over the last three decades, the heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy and the formulation of the phase change problem. It also examines the geometry and configurations of PCM containers and a series of numerical and experimental tests undertaken to assess the effects of parameters such as the inlet temperature and the mass flow rate of the heat transfer fluid (HTF). It is concluded that most of the phase change problems have been carried out at temperature ranges between 0 °C and 60 °C suitable for domestic heating applications. In terms of problem formulation, the common approach has been the use of enthalpy formulation. Heat transfer in the phase change problem was previously formulated using pure conduction approach but the problem has moved to a different level of complexity with added convection in the melt being accounted for. There is no standard method (such as British Standards or EU standards) developed to test for PCMs, making it difficult for comparison to be made to assess the suitability of PCMs to particular applications. A unified platform such as British Standards, EU standards needs to be developed to ensure same or similar procedure and analysis (performance curves) to allow comparison and knowledge gained from one test to be applied to another.

1,630 citations


Journal ArticleDOI
TL;DR: In this paper, Wang et al. used the Poincare section to analyze the fluid mixing in three-dimensional wavy microchannels with rectangular cross-sections and found that the quantity and the location of the vortices may change along the flow direction, leading to chaotic advection.

423 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present new correlations for the convective heat transfer and the friction factor developed from the experiments of nanoparticles comprised of aluminum oxide, copper oxide and silicon dioxide dispersed in 60% ethylene glycol and 40% water by mass.

378 citations


Journal ArticleDOI
TL;DR: In this paper, a three-dimensional laminar flow and heat transfer with two different nanofluids, Al 2 O 3 and CuO, in an ethylene glycol and water mixture circulating through the flat tubes of an automobile radiator have been numerically studied to evaluate their superiority over the base fluid.

343 citations


Journal ArticleDOI
TL;DR: In this paper, a two-phase Eulerian model has been implemented for the first time to study a forced convective of a nanofluid that consists of water and Al2O3 in horizontal tubes.

340 citations


Journal ArticleDOI
TL;DR: In this article, the melting of a phase-change material (PCM) in a vertical cylindrical tube is investigated by means of a numerical simulation which is compared to the previous experimental results.

325 citations


Journal ArticleDOI
TL;DR: In this article, a global magnetohydrodynamical simulation of the solar convection zone is presented, which succeeds in generating a large-scale axisymmetric magnetic component, which exhibits regular polarity reversals on decadal timescales.
Abstract: We report on a global magnetohydrodynamical simulation of the solar convection zone, which succeeds in generating a large-scale axisymmetric magnetic component, antisymmetric about the equatorial plane and undergoing regular polarity reversals on decadal timescales. We focus on a specific simulation run covering 255 years, during which 8 polarity reversals are observed, with a mean period of 30 years. Time-latitude slices of the zonally averaged toroidal magnetic component at the base of the convecting envelope show a well-organized toroidal flux system building up in each solar hemisphere, peaking at mid-latitudes and migrating toward the equator in the course of each cycle, in remarkable agreement with inferences based on the sunspot butterfly diagram. The simulation also produces a large-scale dipole moment, varying in phase with the internal toroidal component, suggesting that the simulation may be operating as what is known in mean-field theory as an αΩ dynamo.

323 citations


Journal ArticleDOI
TL;DR: Neufeld et al. as discussed by the authors presented a new analogue fluid system that reproduces the convective behavior of CO2-enriched brine and showed that convective flux scales with the Rayleigh number to the 4/5 power.
Abstract: [1] Geological carbon dioxide (CO2) storage is a means of reducing anthropogenic emissions. Dissolution of CO2 into the brine, resulting in stable stratification, increases storage security. The dissolution rate is determined by convection in the brine driven by the increase of brine density with CO2 saturation. We present a new analogue fluid system that reproduces the convective behaviour of CO2‐enriched brine. Laboratory experiments and high‐resolution numerical simulations show that the convective flux scales with the Rayleigh number to the 4/5 power, in contrast with a classical linear relationship. A scaling argument for the convective flux incorporating lateral diffusion from downwelling plumes explains this nonlinear relationship for the convective flux, provides a physical picture of high Rayleigh number convection in a porous medium, and predicts the CO2 dissolution rates in CO2 accumulations. These estimates of the dissolution rate show that convective dissolution can play an important role in enhancing storage security. Citation: Neufeld,J.A.,M.A .Hesse,A.Riaz,M. A.H allworth, H. A. Tchelepi, and H. E. Huppert (2010), Convective dissolution of carbon dioxide in saline aquifers, Geophys. Res. Lett., 37, L22404, doi:10.1029/2010GL044728.

302 citations


Journal ArticleDOI
TL;DR: In this article, a radiative magnetohydrodynamics simulation of the formation of an active region (AR) on the solar surface is presented, where the authors model the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere.
Abstract: We present a radiative magnetohydrodynamics simulation of the formation of an active region (AR) on the solar surface. The simulation models the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere. The rise of the magnetic plasma in the convection zone is accompanied by predominantly horizontal expansion. Such an expansion leads to a scaling relation between the plasma density and the magnetic field strength such that B {proportional_to} rhov{sup 1/2}. The emergence of magnetic flux into the photosphere appears as a complex magnetic pattern, which results from the interaction of the rising magnetic field with the turbulent convective flows. Small-scale magnetic elements at the surface first appear, followed by their gradual coalescence into larger magnetic concentrations, which eventually results in the formation of a pair of opposite polarity spots. Although the mean flow pattern in the vicinity of the developing spots is directed radially outward, correlations between the magnetic field and velocity field fluctuations allow the spots to accumulate flux. Such correlations result from the Lorentz-force-driven, counterstreaming motion of opposite polarity fragments. The formation of the simulated AR is accompanied by transient light bridges between umbrae andmore » umbral dots. Together with recent sunspot modeling, this work highlights the common magnetoconvective origin of umbral dots, light bridges, and penumbral filaments.« less

282 citations


Journal ArticleDOI
TL;DR: In this article, the results from direct numerical simulation (DNS) for three-dimensional Rayleigh-Benard convection in a cylindrical cell of aspect ratio 1/2 and Prandtl number Pr=0.7 are presented.
Abstract: Results from direct numerical simulation (DNS) for three-dimensional Rayleigh–Benard convection in a cylindrical cell of aspect ratio 1/2 and Prandtl number Pr=0.7 are presented. They span five decades of Rayleigh number Ra from 2 × 106 to 2 × 1011. The results are in good agreement with the experimental data of Niemela et al. (Nature, vol. 404, 2000, p. 837). Previous DNS results from Amati et al. (Phys. Fluids, vol. 17, 2005, paper no. 121701) showed a heat transfer that was up to 30% higher than the experimental values. The simulations presented in this paper are performed with a much higher resolution to properly resolve the plume dynamics. We find that in under-resolved simulations the hot (cold) plumes travel further from the bottom (top) plate than in the better-resolved ones, because of insufficient thermal dissipation mainly close to the sidewall (where the grid cells are largest), and therefore the Nusselt number in under-resolved simulations is overestimated. Furthermore, we compare the best resolved thermal boundary layer profile with the Prandtl–Blasius profile. We find that the boundary layer profile is closer to the Prandtl–Blasius profile at the cylinder axis than close to the sidewall, because of rising plumes close to the sidewall

268 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate was investigated using a CFD 1 approach.

Journal ArticleDOI
TL;DR: The steady laminar boundary layer flow over a permeable flat plate in a uniform free stream, with the bottom surface of the plate heated by convection from a hot fluid is considered and the effects of the governing parameters on the flow and thermal fields are thoroughly examined and discussed.

Journal ArticleDOI
TL;DR: In this article, a general circulation model (GCM) was developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at their laboratory.
Abstract: [1] A general circulation model (GCM) has been developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at our laboratory. Key features of this new GCM include topography, diurnal cycle, dependence of the specific heat on temperature, and a consistent radiative transfer module based on net exchange rate matrices. This allows a consistent computation of the temperature field, in contrast to previous GCMs of Venus atmosphere that used simplified temperature forcing. The circulation is analyzed after 350 Venus days (111 Earth years). Superrotation is obtained above roughly 40 km altitude. Below, the zonal wind remains very small compared to observed values, which is a major pending question. The meridional circulation consists of equator-to-pole cells, the dominant one being located within the cloud layers. The modeled temperature structure is globally consistent with observations, though discrepancies persist in the stability of the lowest layers and equator-pole temperature contrast within the clouds (10 K in the model compared to the observed 40 K). In agreement with observational data, a convective layer is found between the base of the clouds (around 47 km) and the middle of the clouds (55–60 km altitude). The transport of angular momentum is analyzed, and comparison between the reference simulation and a simulation without diurnal cycle illustrates the role played by thermal tides in the equatorial region. Without diurnal cycle, the Gierasch-Rossow-Williams mechanism controls angular momentum transport. The diurnal tides add a significant downward transport of momentum in the equatorial region, causing low latitude momentum accumulation.

Journal ArticleDOI
TL;DR: In this paper, a linear stability analysis for the onset of natural convection in a horizontal nanofluid layer is presented, which incorporates the effects of Brownian motion and thermophoresis.
Abstract: This paper presents a linear stability analysis for the onset of natural convection in a horizontal nanofluid layer. The employed model incorporates the effects of Brownian motion and thermophoresis. Both monotonic and oscillatory convection for free–free, rigid–rigid, and rigid–free boundaries are investigated. The oscillatory instability is possible when nanoparticles concentrate near the bottom of the layer, so that the density gradient caused by such a bottom-heavy nanoparticle distribution competes with the density variation caused by heating from the bottom. It is established that the instability is almost purely a phenomenon due to buoyancy coupled with the conservation of nanoparticles. It is independent of the contributions of Brownian motion and thermophoresis to the thermal energy equation. Rather, the Brownian motion and thermophoresis enter to produce their effects directly into the equation expressing the conservation of nanoparticles so that the temperature and the particle density are coupled in a particular way, and that results in the thermal and concentration buoyancy effects being coupled in the same way.

Journal ArticleDOI
TL;DR: In this paper, the CO2 solute-driven convection (CSC) was investigated in transparent Hele-Shaw cells and shown to accelerate the transfer of buoyant CO2 into the aqueous phase, where it is no longer subject to upward buoyant drive.
Abstract: Injection of carbon dioxide (CO2) into saline aquifers confined by low- permeability cap rock will result in a layer of CO2 overlying the brine. Dissolution of CO2 into the brine increases the brine density, resulting in an unstable situation in which more-dense brine overlies less-dense brine. This gravitational instability could give rise to density-driven convection of the fluid, which is a favorable process of practical interest for CO2 storage security because it accelerates the transfer of buoyant CO2 into the aqueous phase, where it is no longer subject to an upward buoyant drive. Laboratory flow visualization tests in transparent Hele-Shaw cells have been performed to elucidate the processes and rates of this CO2 solute-driven convection (CSC). Upon introduction of CO2 into the system, a layer of CO2-laden brine forms at the CO2-water interface. Subsequently, small convective fingers form, which coalesce, broaden, and penetrate into the test cell. Images and time-series data of finger lengths and wavelengths are presented. Observed CO2 uptake of the convection system indicates that the CO2 dissolution rate is approximately constant for each test and is far greater than expected for a diffusion-only scenario. Numerical simulations of our system show good agreement with the experiments for onset time of convection and advancement of convective fingers. There are differences as well, the most prominent being the absence of cell-scale convection in the numerical simulations. This cell-scale convection observed in the experiments may be an artifact of a small temperature gradient induced by the cell illumination.

Journal ArticleDOI
TL;DR: In this article, a radiative magnetohydrodynamics simulation of the formation of an active region on the solar surface is presented, where the authors model the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere.
Abstract: We present a radiative magnetohydrodynamics simulation of the formation of an Active Region on the solar surface. The simulation models the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere. The rise of the magnetic plasma in the convection zone is accompanied by predominantly horizontal expansion. Such an expansion leads to a scaling relation between the plasma density and the magnetic field strength such that $B\propto\varrho^{1/2}$. The emergence of magnetic flux into the photosphere appears as a complex magnetic pattern, which results from the interaction of the rising magnetic field with the turbulent convective flows. Small-scale magnetic elements at the surface first appear, followed by their gradual coalescence into larger magnetic concentrations, which eventually results in the formation of a pair of opposite polarity spots. Although the mean flow pattern in the vicinity of the developing spots is directed radially outward, correlations between the magnetic field and velocity field fluctuations allow the spots to accumulate flux. Such correlations result from the Lorentz-force driven, counter-streaming motion of opposite-polarity fragments. The formation of the simulated Active Region is accompanied by transient light bridges between umbrae and umbral dots. Together with recent sunspot modeling, this work highlights the common magnetoconvective origin of umbral dots, light bridges and penumbral filaments.

Journal Article
TL;DR: Kneafsey et al. as mentioned in this paper performed laboratory flow visualization tests in transparent Hele-Shaw cells to elucidate the processes and rates of this CO2 solute-driven convection (CSC).
Abstract: Laboratory Flow Experiments for Visualizing Carbon Dioxide-Induced, Density- Driven Brine Convection Timothy J. Kneafsey and Karsten Pruess Lawrence Berkeley National Laboratory Berkeley, California TJKneafsey@lbl.gov Abstract Injection of carbon dioxide (CO 2 ) into saline aquifers confined by low-permeability cap rock will result in a layer of CO 2 overlying the brine. Dissolution of CO 2 into the brine increases the brine density, resulting in an unstable situation in which more-dense brine overlies less-dense brine. This gravitational instability could give rise to density-driven convection of the fluid, which is a favorable process of practical interest for CO 2 storage security because it accelerates the transfer of buoyant CO 2 into the aqueous phase, where it is no longer subject to an upward buoyant drive. Laboratory flow visualization tests in transparent Hele-Shaw cells have been performed to elucidate the processes and rates of this CO2 solute-driven convection (CSC). Upon introduction of CO 2 into the system, a layer of CO 2 -laden brine forms at the CO 2 -water interface. Subsequently, small convective fingers form, which coalesce, broaden, and penetrate into the test cell. Images and time-series data of finger lengths and wavelengths are presented. Observed CO 2 uptake of the convection system indicates that the CO 2 dissolution rate is approximately constant for each test and is far greater than expected for a diffusion-only scenario. Numerical simulations of our system show good agreement with the experiments for onset time of convection and advancement of convective fingers. There are differences as well, the most prominent being the absence of cell-scale convection in the numerical simulations. This cell-scale convection observed in the experiments is probably initiated by a small temperature gradient induced by the cell illumination. Introduction Carbon dioxide (CO 2 ) injection into deep saline aquifers is a method being considered for sequestration of CO 2 . In such a scenario, the CO 2 would be injected into a permeable,

Journal ArticleDOI
TL;DR: In this article, the effect of different idealized low-latitude boundary conditions has on the magnetospheric configuration simulated by the Lyon-Fedder-Mobarry global magnetohydrodynamic model.
Abstract: [1] In common treatment of magnetosphere-ionosphere coupling at high latitudes, the ionosphere is represented by a thin conducting spherical shell, which closes field-aligned currents generated in the magnetosphere. In this approach, the current continuity yields a Poisson equation for the electrostatic potential associated with the ionospheric convection pattern. Solution of the Poisson equation then provides a means of self-consistently describing magnetospheric and ionospheric plasma convection with a feedback of one on the other. While the high-latitude ionospheric convection is driven by the solar wind and magnetosphere interaction, at lower latitudes atmospheric neutral winds start to dominate. The question that arises then is whether and how midlaltitude and low-latitude ionospheric convection affects high-latitude ionospheric and magnetospheric convection. In global magnetospheric models, ionospheric convection equatorward of the low-latitude boundary is excluded from the simulation domain. However, the boundary condition applied at that boundary to the electrostatic potential may be used as a proxy of this convection. In this paper, we explore effects that different idealized low-latitude boundary conditions have on the magnetospheric configuration simulated by the Lyon-Fedder-Mobarry global magnetohydrodynamic model. To this end, we perform a number of idealized simulations different only in the low-latitude ionospheric boundary condition used. We find that the behavior of the system can be influenced rather significantly by the different boundary conditions, which is expressed by changes in the evolution of the polar cap potential, global magnetospheric convection, and plasma pressure distribution in the magnetotail and on the dayside. The differences in the cross-polar cap potential can reach up to >10%, dependent on the boundary condition used. In the magnetosphere the low-latitude ionospheric boundary condition affects the strength and location of the plasma outflow from the distant tail x-line and the subsequent earthward convection. Changes in the plasma pressure distribution on the nightside are accompanied by noticeable differences in the shape of the magnetotail. We confirm that the changes in the magnetospheric and ionospheric configuration are not just temporal deviations of the system from the same average dynamical state by considering 1 h averages of the magnetospheric flow and pressure distribution. These results verify that the simulated system reaches similar but distinctly different dynamical states dependent on the low-latitude boundary condition applied.

Journal ArticleDOI
TL;DR: In this article, the effects of the wall turbulent boundary layer (i) on the structure of a hydrogen-air premixed flame, (ii) on its near-wall propagation characteristics and (iii) on spatial and temporal patterns of the convective wall heat flux are investigated.
Abstract: A turbulent flame–wall interaction (FWI) configuration is studied using three-dimensional direct numerical simulation (DNS) and detailed chemical kinetics. The simulations are used to investigate the effects of the wall turbulent boundary layer (i) on the structure of a hydrogen–air premixed flame, (ii) on its near-wall propagation characteristics and (iii) on the spatial and temporal patterns of the convective wall heat flux. Results show that the local flame thickness and propagation speed vary between the core flow and the boundary layer, resulting in a regime change from flamelet near the channel centreline to a thickened flame at the wall. This finding has strong implications for the modelling of turbulent combustion using Reynolds-averaged Navier–Stokes or large-eddy simulation techniques. Moreover, the DNS results suggest that the near-wall coherent turbulent structures play an important role on the convective wall heat transfer by pushing the hot reactive zone towards the cold solid surface. At the wall, exothermic radical recombination reactions become important, and are responsible for approximately 70% of the overall heat release rate at the wall. Spectral analysis of the convective wall heat flux provides an unambiguous picture of its spatial and temporal patterns, previously unobserved, that is directly related to the spatial and temporal characteristic scalings of the coherent near-wall turbulent structures.

Journal ArticleDOI
TL;DR: This work analyzes the reversals of the large-scale flow in Rayleigh-Bénard convection both through particle image velocimetry flow visualization and direct numerical simulations of the underlying Boussinesq equations in a (quasi-) two-dimensional, rectangular geometry of aspect ratio 1.
Abstract: We analyze the reversals of the large-scale flow in Rayleigh-Benard convection both through particle image velocimetry flow visualization and direct numerical simulations of the underlying Boussinesq equations in a (quasi-) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large-scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner-flow rolls play a crucial role for the large-scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large-scale diagonal flow, thus leading to reversal. The Rayleigh vs Prandtl number space is mapped out. The occurrence of reversals sensitively depends on these parameters

Journal ArticleDOI
TL;DR: In this article, an experiment of heat transfer to CO2, which flows upward and downward in a circular tube with an inner diameter of 6.32mm, was carried out with mass flux of 285-1200 kg/m2

Journal ArticleDOI
TL;DR: In this paper, the authors applied hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone.
Abstract: Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. We performed 2D radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for effective temperatures between 900K and 2800K, all with logg=5 assuming solar chemical composition. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in mixing material than gravity waves. The presence of clouds leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component.

Journal ArticleDOI
TL;DR: In this article, the vertical velocities in the convection were derived from Doppler radar measurements collected during several NASA field experiments from the nadir-viewing high-altitude ER-2 DOP radar (EDOP).
Abstract: This paper presents observations of deep convection characteristics in the tropics and subtropics that have been classified into four categories: tropical cyclone, oceanic, land, and sea breeze. Vertical velocities in the convection were derived from Doppler radar measurements collected during several NASA field experiments from the nadir-viewing high-altitude ER-2 Doppler radar (EDOP). Emphasis is placed on the vertical structure of the convection from the surface to cloud top (sometimes reaching 18-km altitude). This unique look at convection is not possible from other approaches such as ground-based or lower-altitude airborne scanning radars. The vertical motions from the radar measurements are derived using new relationships between radar reflectivity and hydrometeor fall speed. Various convective properties, such as the peak updraft and downdraft velocities and their corresponding altitude, heights of reflectivity levels, and widths of reflectivity cores, are estimated. The most significant findings are the following: 1) strong updrafts that mostly exceed 15 m/s, with a few exceeding 30 m/s, are found in all the deep convection cases, whether over land or ocean; 2) peak updrafts were almost always above the 10-km level and, in the case of tropical cyclones, were closer to the 12-km level; and 3) land-based and sea-breeze convection had higher reflectivities and wider convective cores than oceanic and tropical cyclone convection. In addition, the high-resolution EDOP data were used to examine the connection between reflectivity and vertical velocity, for which only weak linear relationships were found. The results are discussed in terms of dynamical and microphysical implications for numerical models and future remote sensors.

Journal ArticleDOI
TL;DR: A comprehensive review and systematic summarization of the state of the art in the research and progress in this area, which has covered numerous cavity literatures encountered in various other engineering systems, such as those in electronic cooling devices and buildings.

Journal ArticleDOI
TL;DR: In this paper, the influence of a geostrophically balanced horizontal density gradient on turbulent convection in the ocean is examined using numerical simulations and a theoretical scaling analysis, showing that a lateral density gradient, in addition to the surface forcing, can affect the stratification and the rate of growth of the surface boundary layer.
Abstract: In this study, the influence of a geostrophically balanced horizontal density gradient on turbulent convection in the ocean is examined using numerical simulations and a theoretical scaling analysis. Starting with uniform horizontal and vertical buoyancy gradients, convection is driven by imposing a heat loss or a destabilizing wind stress at the upper boundary, and a turbulent layer soon develops. For weak lateral fronts, turbulent convection results in a nearly homogeneous mixed layer (ML) whose depth grows in time. For strong fronts, a turbulent layer develops, but this layer is not an ML in the traditional sense because it is characterized by persistent horizontal and vertical gradients in density. The turbulent layer is, however, nearly homogeneous in potential vorticity (PV), with a value near zero. Using the PV budget, a scaling for the depth of the turbulent low PV layer and its time dependence is derived that compares well with numerical simulations. Two dynamical regimes are identified. In a convective layer near the surface, turbulence is generated by the buoyancy loss at the surface; below this layer, turbulence is generated by a symmetric instability of the lateral density gradient. This work extends classical scalings for the depth of turbulent boundary layers to account for the ubiquitous presence of lateral density gradients in the ocean. The new results indicate that a lateral density gradient, in addition to the surface forcing, can affect the stratification and the rate of growth of the surface boundary layer.

Journal ArticleDOI
TL;DR: In this paper, the authors use the topology optimization formulation for designing a heat dissipating structure that utilizes forced convective heat transfer, neglecting buoyancy and viscous dissipation inside fluid.
Abstract: This paper discusses the use of the topology optimization formulation for designing a heat dissipating structure that utilizes forced convective heat transfer. In addition to forced convection, there is also natural convection due to natural buoyancy forces induced by local heating inside fluid. In the present study, the temperature distribution due to forced convection, neglecting buoyancy and viscous dissipation inside fluid, was simulated and optimized. In order to analyze the heat transfer equation with forced convective heat loss and the Navier-Stokes equation, a common sequential computational procedure for this thermo/hydraulic characteristic was implemented. For topology optimization, four material properties were interpolated with respect to spatially defined density design variables: the inverse permeability in the Navier-Stokes equation, the conductivity, density, and the specific heat capacity of the heat transfer equation. From numerical examples, it was found that the balance between the conduction and convection of fluid is of central importance to the design of heat dissipating structures.

Journal ArticleDOI
TL;DR: In this paper, the authors applied hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone.
Abstract: Context. Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. Aims. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. Methods. We performed two-dimensional radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for a sequence of effective temperatures between 900 K and 2800 K, all with log g = 5 assuming solar chemical composition. Results. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in transporting and mixing material than gravity waves. The presence of clouds often leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales (presently inaccessible to observations). Conclusions. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component.

Journal ArticleDOI
TL;DR: In this article, a passive tracer was used to study entrainment in cloud-resolving simulations of deep convection in radiative-convective equilibrium, and it was found that the convective flux of undiluted parcels decays with height exponentially, indicating a constant probability per vertical distance of mixing with environmental air.
Abstract: Using a passive tracer, entrainment is studied in cloud-resolving simulations of deep convection in radiative‐convective equilibrium. It is found that the convective flux of undiluted parcels decays with height exponentially, indicating a constant probability per vertical distance of mixing with environmental air. This probability per distance is sufficiently large that undiluted updrafts are negligible above a height of 4‐5 km and virtually absent above 10 km. These results are shown to be independent of the horizontal grid size within the range of 3.2 km to 100 m. Plumes that do reach the tropopause are found to be highly diluted. An equivalent potential temperature is defined that is exactly conserved for all reversible adiabatic transformations, including those with ice. Using this conserved variable, it is shown that the latent heat of fusion (from both freezing and deposition) causes only a small increase in the level of neutral buoyancy near the tropopause. In fact, when taken to sufficiently low pressures, a parcel with an ice phase ends up colder than it would without an ice phase. Nevertheless, the contribution from fusion to a parcel’s kinetic energy is quite large. Using an ensemble of tracers, information is encoded in parcels at the cloud base and decoded where the parcel is observed in the free troposphere. Using this technique, clouds at the tropopause are diagnosed for their cloud-base temperature, specific humidity, and vertical velocity. Using these as the initial values for a Lagrangian parcel model, it is shown that fusion provides the kinetic energy required for diluted parcels to reach the tropopause.

Journal ArticleDOI
TL;DR: In this article, the heat transport and corresponding changes in the large-scale circulation in turbulent Rayleigh-Benard convection are studied by means of three-dimensional direct numerical simulations as a function of the aspect ratio Γ of a closed cylindrical cell and the Rayleigh number Ra.
Abstract: The heat transport and corresponding changes in the large-scale circulation (LSC) in turbulent Rayleigh–Benard convection are studied by means of three-dimensional direct numerical simulations as a function of the aspect ratio Γ of a closed cylindrical cell and the Rayleigh number Ra. The Prandtl number is Pr = 0.7 throughout the study. The aspect ratio Γ is varied between 0.5 and 12 for a Rayleigh number range between 107 and 109. The Nusselt number Nu is the dimensionless measure of the global turbulent heat transfer. For small and moderate aspect ratios, the global heat transfer law Nu = A × Raβ shows a power law dependence of both fit coefficients A and β on the aspect ratio. A minimum of Nu(Γ) is found at Γ ≈ 2.5 and Γ ≈ 2.25 for Ra = 107 and Ra = 108, respectively. This is the point where the LSC undergoes a transition from a single-roll to a double-roll pattern. With increasing aspect ratio, we detect complex multi-roll LSC configurations in the convection cell. For larger aspect ratios Γ ≳ 8, our data indicate that the heat transfer becomes independent of the aspect ratio of the cylindrical cell. The aspect ratio dependence of the turbulent heat transfer for small and moderate Γ is in line with a varying amount of energy contained in the LSC, as quantified by the Karhunen–Loeve or proper orthogonal decomposition (POD) analysis of the turbulent convection field. The POD analysis is conducted here by the snapshot method for at least 100 independent realizations of the turbulent fields. The primary POD mode, which replicates the time-averaged LSC patterns, transports about 50% of the global heat for Γ ≥ 1. The snapshot analysis enables a systematic disentanglement of the contributions of POD modes to the global turbulent heat transfer. Although the smallest scale – the Kolmogorov scale ηK – and the largest scale – the cell height H – are widely separated in a turbulent flow field, the LSC patterns in fully turbulent fields exhibit strikingly similar texture to those in the weakly nonlinear regime right above the onset of convection. Pentagonal or hexagonal circulation cells are observed preferentially if the aspect ratio is sufficiently large (Γ ≳ 8).

Journal ArticleDOI
TL;DR: In this paper, the authors present evidence that transition region red-shifts are naturally produced in episodically heated models where the average volumetric heating scale height lies between that of the chromospheric pressure scale height of 200 km and the coronal scale high of 50 Mm.
Abstract: We present evidence that transition region red-shifts are naturally produced in episodically heated models where the average volumetric heating scale height lies between that of the chromospheric pressure scale height of 200 km and the coronal scale height of 50 Mm. In order to do so we present results from 3d MHD models spanning the upper convection zone up to the corona, 15 Mm above the photosphere. Transition region and coronal heating in these models is due both the stressing of the magnetic field by photospheric and convection `zone dynamics, but also in some models by the injection of emerging magnetic flux.