Buoyancy

About: Buoyancy is a research topic. Over the lifetime, 14652 publications have been published within this topic receiving 273183 citations. The topic is also known as: upthrust.

A Scheme for Representing Cumulus Convection in Large-Scale Models

Abstract: Observations of individual convective clouds reveal an extraordinary degree of inhomogeneity, with much of the vertical transport accomplished by subcloud-scale drafts. In view of these observations, a representation of moist convective transports for use in large-scale models is constructed, in which the fundamental entities are these subcloud-scale drafts rather than the clouds themselves. The transport by these small-scale drafts is idealized as follows. Air from the subcloud layer is lifted to each level i between cloud base and the level of neutral buoyancy for undilute air. A fraction (ϵi) of the condensed water is then converted to precipitation, which falls and partially or completely evaporates in an unsaturated downdraft. The remaining cloudy air is then assumed to form a uniform spectrum of mixtures with environmental air at level i; these mixtures ascend or descend according to their buoyancy. The updraft mass fluxes Mi are represented as vertical velocities determined by the amount o...

The dispersion of marked fluid in turbulent shear flow

Abstract: The analysis used by Taylor (1954) and based on the Reynolds analogy has been extended to describe the diffusion of marked fluid in the turbulent flow in an open channel The coefficient of longitudinal diffusion arising from the combined action of turbulent lateral diffusion and convection by the mean flow is computed to be 5·9uτh, where h is the depth of fluid and uτ the friction velocity This is in agreement with experiments described herein The laterla diffusion coefficient is found by experiment to be 0·23uτh, which is three times larger than the value obtained by the assumption of isotropy The same analysis can be used to describe the longitudinal dispersion of discrete particles, both of zero buoyancy and of finite buoyancy, and comparison is made with observations by Batchelor, Binnie & Phillips (1955) and Binnie & Phillips (1958)

Tidal straining, density currents, and stirring in the control of estuarine stratification

Abstract: Buoyancy input as fresh water exerts a stratifying influence in estuaries and adjacent coastal waters. Predicting the development and breakdown of such stratification is an inherently more difficult problem than that involved in the analogous case of stratification induced by surface heating because the buoyancy input originates at the lateral boundaries. In the approach adopted here, we have adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring. Freshwater input induces horizontal gradients which drive the estuarine circulation in which lighter fluid at the surface is moved seaward over heavier fluid moving landward below. This contribution to stratification is expected to vary in time as the level of turbulence varies over the tidal cycle. The density gradient also interacts directly with the vertical shear in the tidal current to induce a periodic input to stratification which is positive on the ebb phase of the tide. Comparison of this input with the available stirring energy leads to a simple criterion for the existence of strain-induced stratification. Observations in a region of Liverpool Bay satisfying this criterion confirm the occurrence of a strong semidiurnal variation in stratification with complete vertical mixing apparent around high water except at neap tides when more permanent stratification may develop. A simulation of the monthly cycle based on a model including straining, stirring, and the estuarine circulation is in qualitative agreement with the main features of the observations.

Fluid‐mechanical models of crack propagation and their application to magma transport in dykes

Abstract: The ubiquity of dykes in the Earth's crust is evidence that the transport of magma by fluid-induced fracture of the lithosphere is an important phenomenon. Magma fracture transports melt vertically from regions of production in the mantle to surface eruptions or near-surface magma chambers and then laterally from the magma chambers in dykes and sills. In order to investigate the mechanics of magma fracture, the driving and resisting pressures in a propagating dyke are estimated and the dominant physical balances between these pressures are described. It is shown that the transport of magma in feeder dykes is characterized by a local balance between buoyancy forces and viscous pressure drop, that elastic forces play a secondary role except near the dyke tip and that the influence of the fracture resistance of crustal rocks on dyke propagation is negligible. The local nature of the force balance implies that the local density difference controls the height of magma ascent rather than the total hydrostatic head and hence that magma is emplaced at its level of neutral buoyancy (LNB) in the crust. There is a small overshoot beyond this level which is calculated to be typically a few kilometres. Magma accumulating at the LNB will be intruded in lateral dykes and sills which are directed along the LNB by buoyancy forces since the magma is in gravitational equilibrium at this level. Laboratory analogue experiments demonstrate the physical principle of buoyancy-controlled propagation to and along the LNB. The equations governing the dynamics of magma fracture are solved for the cases of lithospheric ascent and of lateral intrusion. Volatiles are predicted to be exsolved from the melt at the tips of extending fractures due to the generation of low pressures by viscous flow into the tip. Chilling of magma at the edges of a dyke inhibits cross-stream propagation and concentrates the downstream flow into a wider dyke. The family of theoretical solutions in different geometries provides simple models which describe the relation between the elastic and fluid-mechanical phenomena and from which the lengths, widths and rates of propagation can be calculated. The predicted dimensions are in broad agreement with geological observations.

Simulations of Solar Granulation. I. General Properties

Abstract: Numerical simulations provide information on solar convection not available by direct observation. We present results of simulations of near surface solar convection with realistic physics: an equation of state including ionization and three-dimensional, LTE radiative transfer using a four-bin opacity distribution function. Solar convection is driven by radiative cooling in the surface thermal boundary layer, producing the familiar granulation pattern. In the interior of granules, warm plasma ascends with ≈ 10% ionized hydrogen. As it approaches and passes through the optical surface, the plasma cools, recombines, and loses entropy. It then turns over and converges into the dark intergranular lanes and further into the vertices between granulation cells. These vertices feed turbulent downdrafts below the solar surface, which are the sites of buoyancy work that drives the convection. Only a tiny fraction of the fluid ascending at depth reaches the surface to cool, lose entropy, and form the cores of these downdrafts. Granules evolve by pushing out against and being pushed in by their neighboring granules, and by being split by overlying fluid that cools and is pulled down by gravity. Convective energy transport properties that are closely related to integral constraints such as conservation of energy and mass are exceedingly robust. Other properties, which are less tightly constrained and/or involve higher order moments or derivatives, are found to depend more sensitively on the numerical resolution. At the highest numerical resolution, excellent agreement between simulated convection properties and observations is found. In interpreting observations it is crucial to remember that surfaces of constant optical depth are corrugated. The surface of unit optical depth in the continuum is higher above granules and lower in the intergranular lanes, while the surface of optical depth unity in a spectral line is corrugated in ways that are influenced by both thermal and Doppler effects.