Source Of Buoyancy Research Articles

Dry and Moist Idealized Experiments with a Two-Dimensional Spectral Element Model

Abstract A nonhydrostatic, fully compressible spectral element (SE) model is evaluated in a series of two-dimensional idealized simulations. A dry formulation of the model is evaluated for a linear hydrostatic mountain-wave case, and a version with moisture is tested for a squall line. In the SE method, two setup parameters control the spatial resolution: the number of elements (h) and the polynomial order (p) of the basis functions. In this paper, the h−p parameter space is systematically explored, with the average horizontal resolution (Δx) varying from 0.2 to 10 km in 91 simulations. The dry experiments are evaluated using an analytic solution. The ratio of Δx/a < 0.2, where a is the mountain half-width, is sufficient to accurately resolve the mountain wave. Accuracy, computational cost, and convergence to the analytic solution are evaluated and compared to a second-order finite-difference (FD) model. The increase in computational cost by refining the spatial resolution yields a significant accuracy gain for the SE, with only a marginal improvement for the FD model. The squall line is evaluated across the control parameter space by assessing three integrated quantities: total precipitation accumulation, maximum vertical velocity, and maximum precipitation rate. The squall line is adequately resolved with Δx < 2 km and p > 5. There is little variation in metrics due to the varying nodal spacing within an element at the same average Δx. When the spatial resolution is refined, the analyzed metrics no longer converge. The nonlinear nature of moist convection is responsible for this resolution dependence as a result of localized buoyancy sources, evident in the vertical velocity spectrum.

Powering Ganymede's dynamo

It is generally believed that Ganymede's core is composed of an Fe‐FeS alloy and that convective motions inside it are responsible for generating the satellite's magnetic field. Analysis of the melting behavior of Fe‐FeS alloys at Ganymede's core pressures suggests that, besides the growth of a solid inner core, convection can be driven by two novel mechanisms: Fe snow and FeS flotation. To advance our understanding of magnetic field generation in Ganymede, we construct dynamo models in which deep inner core growth, Fe‐snow and FeS flotation drive convection. Although a dynamo can be found in each of these cases, the dynamos have different characteristics. For example, some dynamos are dipole dominant and others are not. It is found that multipole‐dominant magnetic fields are generated in all Fe‐snow cases, while dipole dominant dynamos are found in FeS flotation cases and in inner core growth cases. Ganymede's present dipole‐dominant magnetic field suggests that the Fe‐snow process does not play a primary role in driving Ganymede's core convection. The reason that Fe‐snow driven convection does not produce a dipole‐dominant dynamo can be related to the buoyancy flux. In Fe‐snow cases, the buoyancy source is located at the core‐mantle boundary (CMB), and the buoyancy flux peaks there, while in the other two cases, the buoyancy source is located at the inner core boundary where the buoyancy flux peaks.

Quasi-steady states in natural displacement ventilation driven by periodic gusting of wind

AbstractWe investigate the natural displacement ventilation of a space connected to a body of warm fluid through high- and low-level vents. The space is subject to discrete periodic gusts of wind entering at high level from a cold exterior. The cold exterior air entering the space produces buoyancy differences between the space and the body of warm fluid, driving a ventilation flow. Initially we examine the case of a series of identical gusts of wind modelled as turbulent buoyant thermals. New laboratory experiments show that an approximately two-layer stratification is established and the height of the interface is quasi-steady if the period between thermals is much less than the draining time of the space but longer than the fall time of individual thermals. Experiments also show that the interface height depends on the average buoyancy flux associated with the wind gusts, the time between thermals as well as the geometric properties of the vents. This contrasts with the case of a continuous source of buoyancy where the interface height depends only on the geometric properties of the vents and is independent of the buoyancy flux. We develop a quasi-steady two-layer model of the flow based on the classical theory of turbulent thermals and show that it is consistent with our new experimental data. We generalize the model to explore the sensitivity of the results to temporal variations in the size of thermals. We then extend the model to explore the effects of longer interval times between successive thermals and find a two-layer stratification still develops but that the interface height now varies cyclically in time. We then discuss the implications of these results for the ventilation of a shopping mall subject to gusts of wind.

Numerical simulations of convection in crystal‐bearing magmas: A case study of the magmatic system at Erebus, Antarctica

The sustained heat and gas output from Erebus volcano reflects a regime of magma convection that we investigate here using a bi‐phase (melt and crystals), fluid dynamical model. Following validity and verification tests of the model, we carried out four single‐phase and three bi‐phase numerical 30‐year‐ simulations, in an idealized 2D geometry representing a lava lake cooled from above and a reservoir heated from below that are linked by a 4‐to‐10–m‐diameter conduit. We tested the effects of crystals on convection while changing conduit size and the system boundaries from closed to open. Neglecting crystal settling yields only a limited number of features, i.e., (i) the formation of a central instability, (ii) the average temperature evolution, and (iii) the average velocity range of the surface flow motion. Bi‐phase simulations show that while crystals are quite efficiently transported by the liquid phase a small decoupling reflecting their large size (5 cm) results in settling. This leads to more complex circulation patterns and enhances the vigor of fluid motion. A sufficiently large conduit sustains convection and retains 6 and 20% of crystals in suspension, for a closed and open system, respectively. Model outputs do not yet correspond well with field observations of Erebus lava lake (e.g., real surface velocities are much faster than those modeled), suggesting that exsolved volatiles are an important source of buoyancy.

Turbulent buoyant convection from a maintained source of buoyancy in a narrow vertical tank

AbstractWe describe new experiments to examine the buoyancy-induced mixing which results from the injection of a small constant volume flux of fluid of density ${\rho }_{s} $ at the top of a long narrow vertical tank with square cross-section which is filled with fluid of density ${\rho }_{0} \lt {\rho }_{s} $. The injected fluid vigorously mixes with the less dense fluid which initially occupies the tank, such that a dense mixed region of turbulent fluid propagates downwards during the initial mixing phase of the experiment. For an ideal source of constant buoyancy flux ${B}_{s} $, we show that the height of the mixed region grows as $h\ensuremath{\sim} { B}_{s}^{1/ 6} {d}^{1/ 3} {t}^{1/ 2} $ and that the horizontally averaged reduced gravity $ \overline{{g}^{\ensuremath{\prime} } } = g( \overline{\rho } \ensuremath{-} {\rho }_{0} )/ {\rho }_{0} $ at the top of tank increases as $ \overline{{g}^{\ensuremath{\prime} } } (0)\ensuremath{\sim} { B}_{s}^{5/ 6} {d}^{\ensuremath{-} 7/ 3} {t}^{1/ 2} $, where $d$ is the width of the tank. Once the mixed region reaches the bottom of the tank, the turbulent mixing continues in an intermediate mixing phase, and we demonstrate that the reduced gravity at each height increases approximately linearly with time. This suggests that the buoyancy flux is uniformly distributed over the full height of the tank. The overall density gradient between the top and bottom of the mixed region is hence time-independent for both the mixing phases before and after the mixed region has reached the bottom of the tank. Our results are consistent with previous models developed for the mixing of an unstable density gradient in a confined geometry, based on Prandtl’s mixing length theory, which suggest that the turbulent diffusion coefficient and the magnitude of the local turbulent flux are given by the nonlinear relations ${ \kappa }_{T}^{\mathit{nl}} = {\lambda }^{2} {d}^{2} \mathop{ (\partial \overline{{g}^{\ensuremath{\prime} } } / \partial z)}\nolimits ^{1/ 2} $ and ${J}^{\mathit{nl}} = {\lambda }^{2} {d}^{2} \mathop{ (\partial \overline{{g}^{\ensuremath{\prime} } } / \partial z)}\nolimits ^{3/ 2} $, respectively. The $O(1)$ constant $\lambda $ relates the width of the tank to the characteristic mixing length of the turbulent eddies. Since the mixed region is characterized by a time-independent overall density gradient, we also tested the predictions based on a linear model in which the turbulent diffusion coefficient is approximated by a constant ${ \kappa }_{T}^{l} $. We solve the corresponding nonlinear and linear turbulent diffusion equations for both mixing phases, and show a good agreement with experimental profiles measured by a dye attenuation technique, in particular for the solutions based on the nonlinear model.

Transient pollutant flushing of buoyancy-driven natural ventilation

The transient flushing of neutrally-buoyant pollutants from a naturally ventilated enclosure is investigated. A simplified transient model for buoyancy-driven natural ventilation produced by a point source of heat is presented to describe the ventilation development from the plume generation to its steady state. The instantaneous thermal stratification interface height and ventilation flow rate and the time taken for the flow to reach the steady state are then examined by the transient model. The results indicate that the decrease of the thermal stratification interface height with dimensionless time, the steady-state interface height and the dimensionless time taken for the flow to reach the steady state are only determined by the dimensionless effective area of the vents. The ventilation flow rate can be increased by decreasing the enclosure floor area or increasing the effective vent area, enclosure height or source buoyancy flux. Accordingly, for rooms with smaller floor area, larger effective vent area or larger source buoyancy flux, ventilation airflow provides more effective flushing of neutrally-buoyant pollutant. Nevertheless, increasing the enclosure height is only beneficial to flush the pollutant from the lower layer rapidly and is disadvantageous to reduce the pollutant concentration of the upper layer.

The influence of thermo-compositional boundary conditions on convection and dynamos in a rotating spherical shell

Today’s geodynamo is driven by a combination of secular cooling and of latent heat and light core constituents emanating from a growing inner core. The early dynamos of Earth and Mars, however, functioned without an inner core and were thus exclusively driven by secular cooling. Dynamo simulations model secular cooling by internal buoyancy sources and the inner core-related driving by bottom sources. Adopting a codensity approach, we explore how the different combination of thermo-compositional boundary conditions and source distributions affects nonmagnetic convection and dynamo simulations. The impact of the outer boundary condition, fixed codensity or fixed codensity flux (temperature or heat flux when no compositional contribution is present), is only large when the convection is mainly driven by internal sources. When bottom sources dominate, the lower boundary condition becomes more important. In both cases, a fixed flux condition promotes larger convective scales than a fixed codensity condition. A magnetic field can further increase the flow scale and is important to obtain large-scale structures at high Rayleigh numbers. The thermo-compositional outer boundary condition thus plays an important role for the early dynamos in Earth and Mars. Using the more realistic fixed flux condition promotes dipole dominated fields here. For today’s geodynamo, however, the lower boundary condition may be more influential.

3-D numerical simulations of eruption column collapse: Effects of vent size on pressure-balanced jet/plumes

Buoyant columns or pyroclastic flows form during explosive volcanic eruptions. In the transition-state, these two eruption styles can develop simultaneously. We investigated the critical condition that separates the two eruption styles (referred to as “the column collapse condition”) by performing a series of three-dimensional numerical simulations. In the simulation results, we identify two types of flow regime: a turbulent jet that efficiently entrains ambient air (jet-type) and a fountain with a high-concentration of the ejected material (fountain-type). Hence, there are two types of column collapse (jet-type and fountain-type). Which type of collapse occurs at the column collapse condition depends on whether the critical mass discharge rate for column collapse (MDRCC) is larger or smaller than that for the generation of a fountain (MDRJF) for a given exit velocity. Temperature controls the relative magnitude of MDRCC relative to MDRJF, and hence the type of collapse. For given magma properties (e.g., temperature and water content), the column collapse condition is expressed by a critical value of the Richardson number (RiCC≡g′0L0/w02, where g′0 is the source buoyancy, L0 is the vent radius, and w0 is the exit velocity). When the jet-type collapse occurs at the column collapse condition, RiCC is independent of the exit velocity. When the fountain type collapse occurs at the column collapse condition, on the other hand, RiCC decreases as the exit velocity increases. As the exit velocity exceeds the sound velocity, a robust flow structure with a series of standing shock waves develops in the fountain, which suppresses entrainment of ambient air and enhances column collapse.

Numerical study on double-diffusive convection in the Earth’s core

Our numerical study focuses on convection in a rotating spherical shell with the objective to model combined thermal and compositional convection as proposed for the Earth’s core. Since the core is cooling, a thermal gradient is established, which can drive thermal convection. Simultaneously, due to the solidification of the inner core latent heat is released at the freezing front and the concentration of the light constituents of the liquid phase increases thus providing a source for compositional buoyancy. Typically, the molecular diffusivities of both driving components differ by some orders of magnitude. To account for this difference it is indicated to adopt a double-diffusive convection model in treating Earth’s core dynamics. As opposed to purely thermal or purely compositional convection the double-diffusive system is controlled by two Rayleigh numbers associated with the respective buoyancy sources. Using the Rayleigh numbers as control parameters neutral curves of the linear onset of convection in the rotating shell are determined for different Ekman numbers and diffusivity ratios. It is found that the neutral curves depend significantly on the system parameters. By comparison with the analytical solutions of the rotating cylindrical annulus it is shown that the neutral curves represent a superposition of curves associated with solutions for different azimuthal wave numbers. Furthermore, fully non-linear simulations are presented in order to elucidate the effect of isochemical and fixed chemical flux boundary conditions on the convection. We consider three driving scenarios with varying thermo-chemical forcing ratios. Both the forcing ratio and the chemical boundary condition have distinct effects on the system that are discussed separately.

Convection-Driven Melting near the Grounding Lines of Ice Shelves and Tidewater Glaciers

Abstract Subglacial meltwater draining along the bed of fast-flowing, marine-terminating glaciers emerges at the grounding line, where the ice either goes afloat to form an ice shelf or terminates in a calving face. The input of freshwater to the ocean provides a source of buoyancy and drives convective motion alongside the ice–ocean interface. This process is modeled using the theory of buoyant plumes that has previously been applied to the study of the larger-scale circulation beneath ice shelves. The plume grows through entrainment of ocean waters, and the heat brought into the plume as a result drives melting at the ice–ocean interface. The equations are nondimensionalized by using scales appropriate for the region where the subglacial drainage, rather than the subsequent addition of meltwater, supplies the majority of the buoyancy forcing. It is found that the melt rate within this region can be approximated reasonably well by a function that is linear in ocean temperature, has a cube root dependence on the flux of subglacial meltwater, and has a complex dependency on the slope of the ice–ocean interface. The model is used to investigate variability in melting induced by changes in both ocean temperature and subglacial discharge for a number of realistic examples of ice shelves and tidewater glaciers. The results show how warming ocean waters and increasing subglacial drainage both generate increases in melting near the grounding line.

Gravity Currents from Instantaneous Sources Down a Slope

Gravity currents from instantaneous sources down a slope were modeled with classic thermal theory, which has formed the basis for many subsequent studies. Considering entrainment of ambient fluid and conservation of total buoyancy, thermal theory predicted the height, length, and velocity of the gravity current head. In this study, the problem with direct numerical simulations was re-investigated, and the results compared with thermal theory. The predictions based on thermal theory are shown to be appropriate only for the acceleration phase, not for the entire gravity current motion. In particular, for the current head forms on a 10° slope produced from an instantaneous buoyancy source, the contained buoyancy in the head is approximately 58% of the total buoyancy at most and is not conserved during the motion as assumed in thermal theory. In the deceleration phase, the height and aspect ratio of the head and the buoyancy contained within it may all decrease with downslope distance. Thermal theory relies o...

An Integrated TKE-Based Eddy Diffusivity/Mass Flux Boundary Layer Closure for the Dry Convective Boundary Layer

Abstract This study presents a new approach to the eddy diffusivity/mass flux (EDMF) framework for the modeling of convective boundary layers. At the root of EDMF lies a decomposition of turbulent transport mechanisms into strong ascending updrafts and smaller-scale turbulent motions. The turbulent fluxes can be therefore described using two conventional approaches: mass flux (MF) for the organized thermals and eddy diffusivity (ED) for the remaining turbulent field. Since the intensities of both MF and ED transports depend on the kinetic energy of the turbulent motions, it seems reasonable to formulate an EDMF framework based on turbulent kinetic energy (TKE). Such an approach allows for more physical and less arbitrary formulations of parameters in the model. In this study the EDMF–TKE coupling is achieved through the use of (i) a new parameterization for the lateral entrainment coefficient ɛ and (ii) the MF contribution to the buoyancy source of TKE. Some other important features of the EDMF parameterization presented here include a revised mixing length formulation and Monin–Obukhov stability scaling for the surface layer. The scheme is implemented in a one-dimensional (1D) model. Several cases of dry convective boundary layers (CBL) with different surface sensible heat fluxes in the free-convection limit are investigated. Results are compared to large-eddy simulation (LES). Good agreement between LES and the 1D model is achieved with respect to mean profiles, boundary layer evolution, and updraft characteristics. Some disagreements between the models are found to most likely relate to deficiencies in the TKE simulation in the 1D model. Comparison with other previously established ɛ parameterizations shows that the new TKE-based formulation leads to equally accurate, and in many respects better, simulation of the CBL. The encouraging results obtained with the proposed EDMF framework indicate that full integration of EDMF with higher-order closures is possible and can further improve boundary layer simulations.

Imaging crustal and upper mantle structure beneath the Colorado Plateau using finite frequency Rayleigh wave tomography

A new 3-D shear velocity model of the crust and upper mantle beneath the Colorado Plateau and surrounding regions of the southwestern United States was made with finite frequency Rayleigh wave tomography using EarthScope/USArray data. The goal of our study is to examine the Colorado Plateau lithospheric modification that has resulted from Cenozoic tectonism and magmatism. We have inverted for the isotropic Vs model from a grid of Rayleigh wave dispersion curves obtained by a modified two-plane wave method for periods from 20 to 167 s. We map the lithosphere-asthenosphere boundary under the Colorado Plateau by identifying the middle of the shallowest upper mantle negative Vs gradient. The depths of the lithosphere-asthenosphere boundary inferred here agree well with receiver function estimates made independently. The strong lateral heterogeneity of shear velocity can be mainly attributed to 200–400 K variations in temperature together with ∼1% partial melt fraction in the shallow upper mantle. The resulting Vs structures clearly image the upper mantle low-velocity zones under the Colorado Plateau margins that are associated with magmatic encroachment. These upper mantle low-velocity zones resulted from the convective removal of the Colorado Plateau lithosphere that had been rehydrated by subduction-released water, refertilizing and destabilizing it. This convective erosion by the asthenosphere at the low-viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended Basin and Range since the Mid-Cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast, south, and northwest. Moreover, the Rayleigh wave tomography model images parts of a high-velocity drip in the western Colorado Plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography. The widespread edge convective erosion, which the regional delamination-style downwelling processes are a 3-D manifestation of, could provide additional buoyancy sources to support the excess uplift at the margins of the plateau.

Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell

We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded on broadband seismic stations deployed during the 1997–1999 Southern African Seismic Experiment. The velocity in the lithosphere beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the Kalahari craton, and hence the southern African portion of the African Superswell.

Laboratory experiments on two coalescing axisymmetric turbulent plumes in a rotating fluid

We investigate the early-time coalescence of two co-flowing axisymmetric turbulent plumes and the later-time flow of the induced vortices in a rotating, homogeneous fluid using laboratory experiments. The experiments demonstrate the critical importance of the rotation period Tf=2π/f, where f is the Coriolis parameter of the background rotation. We find that if the plumes’ sources are sufficiently “close” for the plumes to merge initially at an “early time” tm≲tr=3Tf/4, the experimentally observed merging height zme agrees well with the non-rotating theoretical relationship of zmt≈(0.44/α)x0<zr=5.5F01/4f-3/4, where α is the entrainment “constant” of the turbulent plumes, x0 is the separation distance between the two plume sources, F0 is the source buoyancy flux of each plume, and zr is the distance that the plume rises in the time tr before rotational effects become significant. Therefore, rotation does not affect the initial time to merger or the initial merger height of such “close” plumes. For “late” times t>tr, however, the flow dynamics are substantially more complicated, as the flow becomes significantly affected by rotation. The propagation and entrainment of the plumes becomes strongly affected by the vortices induced by the entrainment flow in a rotating environment. Also, the plume fluid itself starts to interact with these vortices. If the plumes have already initially merged by the time t=tr, a single vortex (initially located at the midpoint of the line connecting the two plume sources) develops, which both advects and modifies the geometry of the merging plumes. Coupled with the various suppressing effects of rotation on the radial plume entrainment, the “apparent” observed height of merger can vary substantially from its initial value. Conversely, for more widely separated “distant” plumes, where x0>xc=(25α/2)F01/4f-3/4, the plumes do not merge before the critical time tr when rotation becomes significant in the flow dynamics and two vortices are observed, each located over a plume source. The combined effect of these vortices with the associated suppression of entrainment by rotation thus significantly further delays the merger of the two plumes, which apparently becomes possible only through the merger of the induced vortices.

COMPOSITIONALLY DRIVEN CONVECTION IN THE OCEANS OF ACCRETING NEUTRON STARS

We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and argue that the retention of light elements in the liquid acts as a source of buoyancy that drives a slow but continual mixing of the ocean, enriching it substantially in light elements, and leading to a relatively uniform composition with depth. We first consider the timescales associated with different processes that can redistribute elements in the ocean, including convection, sedimentation, crystallization, and diffusion. We then calculate the steady state structure of the ocean of a neutron star for an illustrative model in which the accreted hydrogen and helium burns to produce a mixture of O and Se. Even though the H/He burning produces only 2% oxygen by mass, the steady state ocean has an oxygen abundance more than ten times larger, almost 40% by mass. Furthermore, we show that the convective motions transport heat inwards, with a flux of ~ 0.2 MeV per nucleon for an O-Se ocean, heating the ocean and steepening the outwards temperature gradient. The enrichment of light elements and heating of the ocean due to compositionally-driven convection likely have important implications for carbon ignition models of superbursts.

Time-dependent ventilation flows driven by opposing wind and buoyancy

We consider transient flow in a box containing an isolated buoyancy source, ventilated by a windward high-level opening and a leeward low-level opening, so that prevailing wind acts to oppose buoyancy-driven flow. Hunt & Linden (J. Fluid Mech., vol. 527, 2005, p. 27) demonstrated that two stable steady states can exist above a critical wind strength: buoyancy-driven displacement ventilation with a two-layer stratification and wind-driven mixing ventilation with the whole interior contaminated by buoyant fluid. We present two time-dependent models for this system: a nonlinear ordinary differential equation (ODE) model following Kaye & Hunt (J. Fluid Mech., vol. 520, 2004, p. 135), assuming ‘perfect’ vertical mixing of fluid within each layer, and a partial differential equation model assuming zero vertical mixing, following Germeles (J. Fluid Mech., vol. 71, 1975, p. 601).We apply these models to an initial-value problem – the filling box with constant opposing wind. The interface between the upper hot plume fluid and the lower cool ambient air can dramatically overshoot its final level before relaxing to equilibrium; in some cases, a fully contaminated transient can occur before the buoyancy-driven two-layer steady state is reached. However, we find that for an initially completely uncontaminated box, the system converges to a stable wind-driven steady state whenever it exists. By analysing phase diagrams of the ODE model for the flow, we establish a general method of determining which final state is attained and also explain the hysteresis observed by Hunt & Linden (2005). We confirm these transient behaviours by conducting salt bath experiments in a recirculating flume tank and establish quantitative agreement between theory and experiment. Our ‘zero mixing’ model is more accurate than our ‘perfect mixing’ model for our experiments, as the upper layer remains stratified for a substantial time.

Large eddy simulation of pollutant gas dispersion with buoyancy ejected from building into an urban street canyon

The dispersion of buoyancy driven smoke soot and carbon monoxide (CO) gas, which was ejected out from side building into an urban street canyon with aspect ratio of 1 was investigated by large eddy simulation (LES) under a perpendicular wind flow. Strong buoyancy effect, which has not been revealed before, on such pollution dispersion in the street canyon was studied. The buoyancy release rate was 5 MW. The wind speed concerned ranged from 1 to 7.5 m/s. The characteristics of flow pattern, distribution of smoke soot and temperature, CO concentration were revealed by the LES simulation. Dimensionless Froude number (Fr) was firstly introduced here to characterize the pollutant dispersion with buoyancy effect counteracting the wind. It was found that the flow pattern can be well categorized into three regimes. A regular characteristic large vortex was shown for the CO concentration contour when the wind velocity was higher than the critical re-entrainment value. A new formula was theoretically developed to show quantitatively that the critical re-entrainment wind velocities, u c, for buoyancy source at different floors, were proportional to −1/3 power of the characteristic height. LES simulation results agreed well with theoretical analysis. The critical Froude number was found to be constant of 0.7.

Particle laden flows through an inverted chimney with applications to ocean carbon sequestration

Plumes of negatively buoyant hydrate particles, formed by reacting liquid CO2 with seawater at ocean depths of 1000–1500 m, have been suggested as a way to help sequester CO2. The vertical flux of CO2 can be increased by constructing a shroud around the hydrate particle source to shelter the plume from effects of ambient stratification and current. The shroud also serves as an inverted chimney, inducing a down draft that will transport the dissolving particles to a depth of lower ambient disturbance. Laboratory PIV measurements are compared to an analysis of an idealized shroud that is long, frictionless and driven by a single phase source of buoyancy distributed uniformly over the shroud base. Results indicate that induced draft, and hence dilution of dissolved CO2, increases with plume buoyancy, and shroud length and diameter, but efficiency decreases with increasing ratio of particle slip velocity divided by the characteristic induced draft velocity. While larger particles show reduced plume-like behavior and hence are less efficient in inducing draft, they still generated about half of the theoretically predicted flow.

Stack-Driven Ventilation in Two Interconnected Rooms Sharing a Single Opening and Connected to the Exterior by a Lower Vent

This paper describes the transient ventilation of two interconnected rooms. One of them has a negative buoyancy source located at ceiling level, similar to an overhead split type air-conditioning system with ductless distribution, whereas the other has a vent at floor level. The flow evolution from short- and long-term analytical models was determined and confirmed with scale-model salt bath experiments. The short-term model predicts the depth of the dense-layer ‘first front’ until it reaches the ceiling of the first room, and the long-term model predicts the density evolution of the fluid until it reaches a steady value. The advance of the first front depends on the location of the interior opening but is independent of the size of the exterior vent. The long-term density evolution depends on both parameters. As the interior opening is lowered, the volume implicated in the dynamics decreases and the dense fluid density increases at a higher rate. Furthermore, increasing the size of the exterior vent causes a turbulent exchange flow to develop through that vent, which inhibits the increase of the average density by introducing ambient fluid to the rooms.