Origin Of Convection Research Articles

The role of planetary geodynamics in the creation of the asthenosphere and convective and transtensional structures of the continental lithosphere

In this work, we consider the general problems of the Earth's planetary geodynamics, starting with its origin as a result of the separation of the spiral-shaped cosmic vortex into parts, which created the double planet ‒ the Earth and the Moon. The fundamentals of the origin of planetary tectonic asymmetry, its role in the origin of convection and the formation of a heterogeneous oceanic and continental asthenosphere overlaying the mantle have been studied. We characterize the original model of the geodynamics of the fractal structure of the Earth, the place in it of cellular convection of different scales, subduction, the combination of convective structures with transtension structures due to planetary rotational geodynamics and the lateral distribution of deep masses under the oceans.

A Synoptic-Scale Cold-Reservoir Hypothesis on the Origin of the Mature-Stage Super Cloud Cluster: A Case Study with a Global Nonhydrostatic Model

Abstract This chapter proposes a working assumption as a way of conceptual simplification of the origin of Madden–Julian oscillation (MJO)-associated convection, or super cloud cluster (SCC). To develop the simplification, the importance of the synoptic-scale cold reservoir underlying the convection and its interaction with the accompanying zonal–vertical circulation is highlighted. The position of the convection with respect to that of climatological warm pool is postulated to determine the effectiveness of this framework. The authors introduce a prototype hypothesis to illustrate the usefulness of the above assumption based on a numerical simulation experiment with a global nonhydrostatic model for the boreal summer season. Premises for the hypothesis include 1) that the cloud cluster (CC) is a basic building block of tropical convection accompanying the precipitation-generated cold reservoir in its subcloud layer and 2) that a warm-pool-induced quasi-persistent zonal circulation is key for the upscale organization of CCs. The theory of squall-line structure by Rotunno, Klemp, and Weisman (hereafter RKW) is employed for the interpretation. No account is taken regarding the influences of equatorial waves as a first-order approximation. Given the premises, an SCC of O(1000) km scale is interpretable as a gigantic analog of a multicellular squall line embedded in the quasi-stationary westerly shear branch of the zonal circulation east of the warm water pool. A CC corresponds to the “cell,” and its successive formation to the east and westward movement represents an upshear-tilting core of intense updraft. The upshear-tilted SCC is favorably maintained with the precipitating area being separated from the gust front boundary between the cold reservoir and a low-level easterly, which is supported in the realm of the RKW theory where two horizontal vortices associated with the cold reservoir and vertical shear are opposite in sign but cold reservoir’s vorticity can be inferred to be larger, leading to upshear-tilted and multicellular behavior. As a counterexample, CCs to the west of the warm pool (Indian Ocean and Arabian Sea) are embedded in the easterly shear and organized into a less coherent cloud cluster complex (CCC) given the situation of RKW where two horizontal vortices associated with the cold reservoir and vertical shear are still opposite in sign, but the smaller vertical shear west of the warm pool causes even more suboptimal vorticity imbalance in the western flank of cold reservoir, leading to larger tilt with height and intermittent, less viable storm situations. A cold pool or cold reservoir, having been prevalent in mesoscale convection research, is argued to be important for the MJO as pointed out by the emerging evidence in the international field campaign for the MJO called Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/DYNAMO. The simplified and idealistic hypothesis proposed here does not cover all aspects of MJO and its validation awaits further modeling and observational studies, but it can offer a framework for characterizing a fundamental aspect of the origin of MJO-associated convection.

Intraplate volcanic-hydrothermal systems of the Clarion-Clipperton zone, Pacific Ocean

The results of recent studies of specific craterlike structures in sediments of the Clarion-Clipperton zone (CCZ) of the Pacific Ocean are summarized. It was established from the example of a crater located in the bottom area with age of ∼40 Ma that such craters from several hundreds of meters to a few kilometers in diameter and from 15 to 100 m deep are a result of evolution of intraplate volcanic-hydrothermal systems mostly active in the CCZ area beginning from the Middle Miocene. Vertical chimneylike channels from a few centimeters to 0.5 m across in pre-Middle Miocene carbonate sediments in the areas of subvolcanic basaltic stocks with an age of 16–18 Ma and less are the structural elements of these systems. The channels are filled with nontronite clays and Fe-Mn hydroxides, which are the products of discharge of low-temperature hydrotherms. New data have allowed us to reach a conclusion on the origin of thermal convection of volcanichydrothermal systems, which dissolved the host carbonate sediments and formed the craterlike structures at the final stage of evolution.

The threshold origin of solutocapillary Marangoni convection on a bounded free surface

The Marangoni convection is the key mechanism responsible for the motion of non-isothermal fluid with a free surface under microgravity conditions. It is generally assumed that the surface of a Newtonian fluid begins to move at any arbitrary small shear stress. However, the experiments show that under actual conditions the capillary fluid flow can be initiated by a certain (threshold) stress. For subthreshold shear stresses generated by the surface forces or volumetric flows, the fluid surface remains immovable. Such a behavior of the surface is caused by the existence of an adsorption layer formed by uncontrolled surface-active impurities contained in the fluid. The composition and amount of impurities absorbed on the surface are dependent on the degree of purification of the fluid and the value of its surface tension. The condition of a threshold initiation of the capillary motion has been determined experimentally for the case of development of the solutal Marangoni convection after placing a droplet of the surfactant liquor on the interface of water, aqueous surfactant solutions of lower concentrations and some organic fluids. A group of monohydric alcohols and carboxylic acids were used as the surfactants. It has been found that the value of the threshold difference in surfactant concentration rapidly increases with a decrease in fluid initial concentration and the characteristic dimension of the free surface. Due to the latter fact the threshold effects are of crucial importance for a variety of problems in microfluidics. It has been found that a threshold origin of a capillary motion is also peculiar to the liquid–liquid interface. The relationships between the threshold Marangoni number and the surface activity of a surfactant have been determined.

Review of some Aspects of Single Crystal Growth Using Czochralski Crystal Growth Technique

Abstract Single crystal that semiconductor industry thrive on, are grown using Czochralski (CZ) crystal growth technique. Over the years many researchers have investigated the problems related to CZ crystal growth system and number of studies related to different aspects of CZ crystal growth is available in open literature. Some of the important aspects related to Czochralski (CZ) crystal growth system used for growing single crystal are discussed. Origin of melt convection due to buoyant forces, forced convection due to crystal and crucible rotation and Marangoni convection due to thermo-capillary forces, and their effect on heat and species transport phenomena in the melt pool has been reviewed. Flow in present day large size crystal growth using CZ crystal growth system becomes oscillatory. Oscillations in melt can be damped using externally applied magnetic field. Effect of axial, transverse and inhomogeneous (CUSP) magnetic field on different melt convection has been a subject of interest for various research efforts. Oxygen release from silica crucible during growth of silicon crystal and mechanism for its incorporation into the growing crystal has also been reviewed. Conditions leading to growth of low oxygen concentration silicon crystal are elaborated.

Vertical velocity observed by Doppler lidar during cops - A case study with a convective rain event

Convective and Orographically-Induced Precipitation Study (COPS), conducted in the Black Forest region in Southern Germany and Eastern France during the summer of 2007. From the 13 June to the 16 August 2007, the National Centre for Atmospheric Science (NCAS), Facility for Ground-based Atmospheric Measurement (FGAM) 1.5 lm scanning Doppler lidar was deployed at Super Site R, Achern, in the Rhine Valley, in order to contribute to the extensive COPS observation campaign. The FGAM Doppler lidar system provides measurements of radial wind and aerosol backscatter in the layer 100–1500 m. Profiles of horizontal wind velocity are presented, these being derived from performing azimuth scans. Profiles of vertical velocity, its variance and skewness derived from the vertical scans are also presented and discussed in the paper. Knowledge of vertical velocity skewness is important for the understanding of the structure and origin of turbulent convection in the atmospheric boundary layer (ABL). The skewness of vertical velocity can provide a measure of the asymmetry in the distribution of vertical velocity perturbations within the ABL and can be estimated using the Doppler lidar. In addition, we investigate the behaviour of the boundary layer using data from the FGAM Doppler lidar and Automatic Weather Station (AWS), the University of Basilicata Raman lidar (BASIL) and the DLR’s Poldirad C-band radar. A case study event on the 6 th August 2007 is selected and investigations of possible causes of layers with positive and negative skewness are presented, along with comparisons with output from the National Center for Atmospheric Research (NCAR) Weather Research and Forecasting (WRF) model to assess the accuracy of the model output, including location and timing of rainfall onset.

The second continent: the origin of mantle convection

The second continent: the origin of mantle convection

The origin of granular convection in vertically vibrated particle beds: The differential shear flow field

This paper investigates the particle scale dynamics of granular convection in vertically vibrated granular beds. The onset of the convection is found to coincide with the noticeable particle transverse migrations from the side walls towards the centre of the bed, which only take place in the wake of the gravity wave front dividing the upward moving particles and the falling ones. The mechanism driving the particle inward flows and thus sustaining the complete convection rolls can be understood in light of a convection model based on void penetration. This stochastic convection model reveals that the underlying driving force is a distinctive differential shear flow field arising from the combined effect of frictional holdback by the walls and the downward pull of gravity. The changes of the convection pattern with inceasing acceleration amplitude, in terms of the convection strength and the thickness of the bottom of the convection rolls, can be accounted for by this model.

Influence of natural convection on microstructure evolution during the initial solidification transient: comparison of phase-field modeling with in situ synchrotron X-ray monitoring data

The influence of natural convection on the evolution of the solid-liquid (s/l) interface during the initial transient of upward directional solidification was studied on Al-4 wt.% Cu alloy by coupling the two dimensional quantitative phase-field model with the Navier-Stokes equations. The simulations were compared with in situ and real-time synchrotron X-ray monitoring data. The origin of natural convection in experiment was the presence of a small unavoidable horizontal temperature gradient. Due to the stringent requirement on the phase-field interface width parameters, the simulated domain could not be chosen as large as the size of the experimental sample. As the calculated fluid flow strength would be weakened by using a smaller domain, a horizontal temperature gradient ten times larger than the estimated experimental value was applied in simulation to recover a fluid flow washing the s/l interface similarly to experiments. Direct comparison to experimental measurements demonstrated that the phase-field simulations with convection qualitatively reproduced the evolution of all the characteristic parameters measured in experiments. Based on these results, the effects of natural convection on the growth dynamics of the s/l interface during directional solidification of alloy were further clarified.

Thermocapillary Convection during Subcooled Boiling in Reduced Gravity Environments

Complete understanding of the origin of thermocapillary convection during subcooled boiling remains elusive. Some recent studies have suggested that the presence of dissolved gas inside the vapor bubble leads to the formation of a localized concentration and temperature gradient along the liquid-vapor interface and thus to the onset of thermocapillary convection. These studies reported that the dissolved gas content determines the onset of thermocapillary convection and that no thermocapillary motion was observed for subcooled boiling with pure liquid. However, other experiments performed in reduced gravity suggest that the strength of the thermocapillary convection around the primary bubble in a gas-saturated fluid is much weaker than that in a degassed fluid. This paper presents a qualitative study of the effects of dissolved gas content, bubble shape and size, and heat transfer coefficient on the strength of thermocapillary convection and offers possible explanations for the existing confusion. Owing to the presence of different complex and interrelated mechanisms, we decoupled the individual mechanisms to study their overall effect on thermocapillary convection. The results indicate that variations in the heat transfer coefficient and in the shape and the size of the bubble play a major role in the development of thermocapillary motion.

Supersonic convection in stellar interiors

According to the result of the numerical simulation on stellar convective envelopes by the traditional mixing length theory, supersonic convection would occur on the top of the convective regions of the yellow-and-red giants and supergiants. This, however, is not self-consistent with the local convection theory itself. This paper is devoted to analyze in details the origin of supersonic convection, and at the same time to present a plot of the supersonic convection region on the H-R diagram based on the evolutionary track of population I stars. The main results of this paper are as follows: in the process of evolution, (1) no supersonic convection occurs in low-mass stars; (2) for intermediate-mass stars, because of the ionization of hydrogen, the supersonic convection occurs chiefly in the region of 3.6< lgT e <4.0, tilting from the lower left to the upper right on the H-R diagram and extending to the region of red giants for relatively massive stars; (3) for massive stars, the supersonic convection occurs after they deviate from ZAMS, the greater the stellar mass is, the earlier the supersonic convection emerges. For blue supergiants, the supersonic convection occurs at the absorbtion peak (lg T ∼5.3) of Fe and in the secondary ionization region of helium, but for the red- and yellow-giants and supergiants, it occurs in the ionization region of hydrogen.

The origin of thermocapillary convection in subcooled nucleate pool boiling

The significance of noncondensibles as the origin of thermocapillary convection in subcooled pool boiling is demonstrated in this paper. The temperature gradients along the bubble interface inducing thermocapillary convection are caused by saturation pressure gradients due to a nonuniform accumulation of noncondensable gas along the interface. On the vapour side, the noncondensable gas inhibits the condensation of vapour. The corresponding increase of the saturation pressure gradient and temperature gradient, respectively, forces thermocapillary convection on the liquid side of the bubble. A bubble in subcooled liquid can grow or shrink according to the heat and mass transfer at its top, and even a steady-state mass flow through the bubble can be maintained. By a combined analytical and numerical approach the various parameters of this complex system are studied. Furthermore, it is demonstrated that even small amounts of noncondensable gas are sufficient to induce thermocapillary flow. This flow, in turn, inhibits bubble detachment.

The thermo-acoustic response of a premixed swirl burner

Application of the theory of thermo-acoustic excitation of flames requires a detailed knowledge of the flame response to plane-wave velocity fluctuations of a given frequency through the burner. A model of the type of swirl burner used in some designs of industrial gas turbine is investigated, chemiluminescent emission being used as a marker of the hear release from both methane and propane premixed flames. The effects of the equivalence ratio and the withdrawal of the swirler so as to allow an annulus of unswirled premixture are also examined. It is found that there are two distinct modes of self-excitation. The response of different burner configurations in the mode with the higher frequency is analysed in terms of four parameters. These characterize the position and spread of the axial distribution of heat release fluctuations, and the origin and effective velocity of convection of disturbances. From these parameters, the Rayleigh integral can be calculated and this defines the frequency range over which excitation is possible. If the acoustic conditions require that the flame is lengthened, retracting the swirler, reducing the equivalence ratio or changing from propane to methane are all shown to be possible control measures for this particular burner.

Convective instability of air in snow

Convective instability of air in the pores between ice crystals in snow is considered. In the Boussinesq approximation, a system of equations that describes the origin of thermal convection within the snow thickness is derived. It is shown that for snow, as for a liquid, there is a criterial number analogous to the Rayleigh number that determines the origin of air instability in snow. The contribution of natural convection of air to the heat- and mass-transfer processes in snow is estimated and possible reasons for the considerable scatter of reported experimental data on the thermal conductivity and diffusivity of snow are discussed.

Plasma flow reversals at the dayside magnetopause and the origin of asymmetric polar cap convection

A number of events have been observed in the Los Alamos/Garching fast plasma experiment data from ISEE 2 within ±3 hours of noon wherein the y component of the plasma flow within the low latitude boundary layer and magnetopause current layer is oppositely directed to that in the adjacent magnetosheath. When the y component, By, of the local magnetosheath magnetic field is positive, events of this nature are found preferentially in the northern dusk and southern dawn quadrants, and when By is negative, they are found preferentially in the opposite two quadrants. Plasma flows in these events are always also poleward, that is they are always directed away from the GSE equatorial plane. The observations are qualitatively and quantitatively consistent with previous observations of accelerated flows at the magnetopause and with models of magnetic reconnection, with the reconnection occurring at low latitudes near the GSE XY plane independent of the magnitude or the sign of the y component of the local magnetosheath magnetic field. Local magnetic shears at the magnetopause for these events range from 60 to 180 degs, which, together with the occurrence of these events at low latitudes, does not tend to support the antiparallel merging hypothesis. These observations of By‐dependent flow reversals provide a graphic demonstration of how asymmetric polar cap convection and related phenomena, such as the Svalgaard‐Mansurov effect, originate in magnetic reconnection at the dayside magnetopause.

Air motion and precipitation growth in frontal systems

In recent years there has been much progress in describing and understanding the detailed dynamical and microphysical structure of frontal precipitation systems. In this article we consider phenomena on a hierarchy of scales: the broad synoptic-scale flows in mid-latitude depressions, the mesoscale patterns of precipitation, the nature and origin of convection at the surface and aloft, and the microphysical properties of the resulting precipitation. The influence of orography is also briefly discussed.

Origin of oscillatory convection in a porous medium heated from below

In order to investigate the significance of cyclic interaction in the evolution of fluctuating convective flow in a rectangular cell of porous material, the behavior of confined and unconfined flows are compared. In the latter case the constant pressure boundary condition at the top surface does not force the fluid to recirculate around the cell. However, regular oscillatory flow similar to that which occurs in an enclosed cell is still observed. This indicates that the oscillatory flow arises from the instability of the thermal boundary layer on the heated bottom surface. In both cases the frequency of the oscillatory flow is proportional to the Rayleigh number to the power of 3/2.

Origin of convection in horizontal layers of a non-newtonian fluid

Origin of convection in horizontal layers of a non-newtonian fluid

A JUSTIFICATION OF THE AVERAGING METHOD FOR A PROBLEM OF CONVECTION IN A FIELD OF RAPIDLY OSCILLATING FORCES AND FOR OTHER PARABOLIC EQUATIONS

Results of a previous paper of the author (Math. USSR Sb. 10 (1970), 51—59; MR 41 #2244) are applied to parabolic equations and the Navier-Stokes equation. Of central importance is an investigation of the equation of convection in a vibrating container, which justifies the already completed formal application of the averaging method to deduce the stabilizing influence of a vibration (cf. S. M. Zen'kovskaja and I. V. Simonenko, On the influence of a high frequency vibration on the origin of convection, Izv. Akad. Nauk SSSR Ser. Meh. Židk. Gaza 1966, no. 5, 51—55; S. M. Zen'kovskaja, An analysis of convection in a layer of fluid with the presence of a vibrational force, ibid. 1968, no. 1, 55-58). Bibliography: 16 items.

On the origin of convection

The method of investigating bifurcations developed in [1 and 2] is applicable to many hydrodynamic problems. In the present paper it is applied to investigate the origin of convection in a horizontal fluid layer heated from below. Secondary stationary flows are of particular interest in the convection problem since the loss of stability is associated with these flows: “the principle of the change in stability” is not only valid here but has been proved rigorously [3]. It has also been proved that secondary stationary flows are generated by branching off from the state of rest [4 and 5]. The problem under consideration is invariant relative to the group of motions of a horizontal plane. The single solution invariant relative to this whole group is the rest solution. When this solution is unstable, it is natural to expect the occurrence of solutions invariant relative to some subgroup of the group of motions. If the mentioned subgroup is generated by a pair of translations (in perpendicular directions), we arrive at doubly-periodic solutions (Section 1), and if invariance relative to rotation through a certain angle is required in addition, we arrive at solutions of hexagonal type (Section 2). As is known, precisely these latter are realized in convection experiments [6]. Deductions on the existence of doubly-pertodic convection flows are elucidated in Theorem 1.1, and the existence of solutions of hexagonal convection type is asserted in Theorem 1.2. The applied method has slight connection with the boundary conditions. Only for definiteness is it assumed that the boundaries of the layer are solid walls on each of which the temperature is specified.