Effective Buoyancy Research Articles

Effective buoyancy ratio: a new parameter for characterizing thermo-chemical mixing in the Earth's mantle

Abstract. Numerical modeling has been carried out in a 2-D cylindrical shell domain to quantify the evolution of a primordial dense layer around the core–mantle boundary. Effective buoyancy ratio, Beff was introduced to characterize the evolution of the two-layer thermo-chemical convection in the Earth's mantle. Beff decreases with time due to (1) warming of the compositionally dense layer, (2) cooling of the overlying mantle, (3) eroding of the dense layer through thermal convection in the overlying mantle and (4) diluting of the dense layer through inner convection. When Beff reaches the instability point, Beff = 1, effective thermo-chemical convection starts, and the mantle will be mixed (Beff = 0) over a short time period. A parabolic relationship was revealed between the initial density difference of the layers and the mixing time. Morphology of large low-shear-velocity provinces and results from seismic tomography and normal mode data suggest a value of Beff ≥ 1 for the mantle.

Local Buckling Analysis of Free Span for Submarine Pipeline

Free span resulting from unevenness and scour of seabed often occurs and endanger the safety of submarine pipeline. Local buckling which is an important failure mode for free span should be considered for the safety assessment of pipeline. In this paper, considering the real service status of submarine pipeline, the structural response of free span under loads induced by vortex shedding, effective axial force, gravity and buoyancy was analyzed with numerical simulation and theoretical analysis method. Local buckling analysis for free span with different length was investigated based on the ULS criterion under load controlled condition given by DNV-OS-F101 standard.

On the general concept of buoyancy in sedimentation and ultracentrifugation

Gravity or ultracentrifuge settling of colloidal particles and macromolecules usually involves several disperse species, either because natural and industrial colloids display a large size polydispersity, or because additives are put in on purpose to allow for density-based fractionation of the suspension. Such ‘macromolecular crowding’, however, may have surprising effects on sedimentation, for it strongly affects the buoyant force felt by a settling particle. Here we show that, as a matter of fact, the standard Archimedes' principle is just a limiting law, valid only for mesoscopic particles settling in a molecular fluid, and we obtain a fully general expression for the actual buoyancy force providing a microscopic basis to the general thermodynamic analysis of sedimentation in multi-component mixtures. The effective buoyancy also depends on the particle shape, being much more pronounced for thin rods and discs. Our model is successfully tested on simple colloidal mixtures, and used to predict rather unexpected effects, such as denser particles floating on top of a lighter fluid, which we actually observe in targeted experiments. This ‘generalized Archimedes principle’ may provide a tool to devise novel separation methods sensitive to particle size and shape.

Simulation of wind-driven dispersion of fire pollutants in a street canyon using FDS

Air quality in urban areas attracts great attention due to increasing pollutant emissions and their negative effects on human health and environment. Numerous studies, such as those by Mouilleau and Champassith (J Loss Prevent Proc 22(3): 316-323, 2009), Xie et al. (J Hydrodyn 21(1): 108-117, 2009), and Yassin (Environ Sci Pollut Res 20(6): 3975-3988, 2013) focus on the air pollutant dispersion with no buoyancy effect or weak buoyancy effect. A few studies, such as those by Hu et al. (J Hazard Mater 166(1): 394-406, 2009; J Hazard Mater 192(3): 940-948, 2011; J Civ Eng Manag (2013)) focus on the fire-induced dispersion of pollutants with heat buoyancy release rate in the range from 0.5 to 20MW. However, the air pollution source might very often be concentrated and intensive, as a consequence of the hazardous materials fire. Namely, transportation of fuel through urban areas occurs regularly, because it is often impossible to find alternative supply routes. It is accompanied with the risk of fire accident occurrences. Accident prevention strategies require analysis of the worst scenarios in which fire products jeopardize the exposed population and environment. The aim of this article is to analyze the impact of wind flow on air pollution and human vulnerability to fire products in a street canyon. For simulation of the gasoline tanker truck fire as a result of a multivehicle accident, computational fluid dynamics large eddy simulation method has been used. Numerical results show that the fire products flow vertically upward, without touching the walls of the buildings in the absence of wind. However, when the wind velocity reaches the critical value, the products touch the walls of the buildings on both sides of the street canyon. The concentrations of carbon monoxide and soot decrease, whereas carbon dioxide concentration increases with the rise of height above the street canyon ground level. The longitudinal concentration of the pollutants inside the street increases with the rise of the wind velocity at the roof level of the street canyon.

Dynamics of cyanobacterial bloom formation during short-term hydrodynamic fluctuation in a large shallow, eutrophic, and wind-exposed Lake Taihu, China

Short-term hydrodynamic fluctuations caused by extreme weather events are expected to increase worldwide because of global climate change, and such fluctuations can strongly influence cyanobacterial blooms. In this study, the cyanobacterial bloom disappearance and reappearance in Lake Taihu, China, in response to short-term hydrodynamic fluctuations, was investigated by field sampling, long-term ecological records, high-frequency sensors and MODIS satellite images. The horizontal drift caused by the dominant easterly wind during the phytoplankton growth season was mainly responsible for cyanobacterial biomass accumulation in the western and northern regions of the lake and subsequent bloom formation over relatively long time scales. The cyanobacterial bloom changed slowly under calm or gentle wind conditions. In contrast, the short-term bloom events within a day were mainly caused by entrainment and disentrainment of cyanobacterial colonies by wind-induced hydrodynamics. Observation of a westerly event in Lake Taihu revealed that when the 30 min mean wind speed (flow speed) exceeded the threshold value of 6 m/s (5.7 cm/s), cyanobacteria in colonies were entrained by the wind-induced hydrodynamics. Subsequently, the vertical migration of cyanobacterial colonies was controlled by hydrodynamics, resulting in thorough mixing of algal biomass throughout the water depth and the eventual disappearance of surface blooms. Moreover, the intense mixing can also increase the chance for forming larger and more cyanobacterial colonies, namely, aggregation. Subsequently, when the hydrodynamics became weak, the cyanobacterial colonies continuously float upward without effective buoyancy regulation, and cause cyanobacterial bloom explosive expansion after the westerly. Furthermore, the results of this study indicate that the strong wind happening frequently during April and October can be an important cause of the formation and expansion of cyanobacterial blooms in Lake Taihu.

Fluid-solid interaction of resistance loss of flexible hose in deep ocean mining

The resistance loss of transportation was studied and the influences of buoyancy layout, mineral content and elastic modulus of flexible hose were investigated based on three-dimensional finite element model of fluid-solid interaction by MSC.MARC/MENTAT software. The numerical results show that the resistance losses increase with the increase of mineral content C v and velocity of internal fluid v and decrease with the increase of elastic modulus E of flexible hose. The buoyancy layout and the velocity of internal fluid have greater impacts on the resistance losses than the elastic modulus of flexible hose. In order to reduce the resistance losses and improve the efficiency of the deep-ocean mining, C v and v must be restricted in a suitable range (e.g. 10%–25% and 2.5–4 m/s). Effective buoyancy layout (such as Scheme C and D) should be adopted and the suitable material of moderate E should be used for the flexible hose in deep-ocean mining.

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.

Onset of Rayleigh–Bénard–Marangoni convection with time-dependent nonlinear concentration profiles

Taking into account the effects of the time-dependent nonlinear base concentration profile on the concentration and velocity disturbances in the Rayleigh–Bénard, Bénard–Marangoni, and Rayleigh–Bénard–Marangoni problems, a spatial base-profile influenced frozen-time marginal state analysis has been developed to predict the onset of cellular convection caused by the Rayleigh effect (buoyancy) and Marangoni effect (surface tension gradient). The relations between the base velocity and concentration profiles and the velocity and concentration fluctuations have been established, analogous to the theory of statistical physics which describes the relation between the fluctuating forces and velocities for particles in non-equilibrium systems. Compared with the quasi-static analysis and the initial-value technique, the numerical calculations of this spatial base-profile influenced frozen-time marginal state analysis are straightforward and involve no subjective decisions on either the initial conditions of disturbances or the criterion for the onset time of convection. Using the spatial base-profile influenced frozen-time marginal state analysis, the calculated critical parameters for the onset of convection are in good agreement with the published experimental results.

Boundary currents in a meridional channel subject to seasonally varying buoyancy forcing: application to the Tsushima Current

The theoretical problem of formation of boundary currents in an idealized basin subject to seasonally varying buoyancy forcing is considered in an attempt to apply it to the seasonally varying Tsushima Current. Until now, all the theories intended to explain the branching of the Tsushima Current have been for the annual mean Tsushima Current—the seasonally varying Tsushima Current has never been properly explained. A simple numerical experiment shows that eastern and western boundary currents change in time, concurrently, when local buoyancy forcing is sufficient, as it is in the Tsushima Current. However, this is not true for ineffective local buoyancy forcing. The importance of the role played by local buoyancy forcing is further supported by simple theoretical considerations. Overall, this study suggests that effective local buoyancy forcing is probably essential to the formation of seasonally varying eastern and western boundary currents of the Tsushima Current.

Dyke propagation and sill formation in a compressive tectonic environment

Sills could potentially form as a result of dykes modifying their trajectory in response to remote tectonic compression. Here, we use analogue experiments to investigate how a buoyant vertical dyke adjusts its trajectory to a compressive remote stress to form a sill, and over which vertical distance this sill formation does occur. Our investigation is restricted to an intrusion propagating through a homogeneous solid, which enables us to identify the characteristic length‐scale over which a dyke responds to remote stress compression, independently of the presence of crustal layers. The experiments involve the injection of air in a gelatine solid that experiences lateral deviatoric compression. The response of the buoyant air crack to the compressive stress in not instantaneous but operates over some distance. An important observation is that some cracks reach the surface despite the compressive environment. Dyke‐to‐sill rotation occurs only for large compressive stress or small effective buoyancy. Dimensional analysis shows that the length‐scale over which this rotation takes place increases exponentially with the ratio of crack effective buoyancy to horizontal compressive stress. Up‐scaled to geological conditions, our analysis indicates that a dyke‐to‐sill transition in response to tectonic compression in homogeneous rocks cannot occur over less than two hundred meters and would need several kilometers in most cases. This is typically greater than the average thickness of lithological units, which supports the idea that crustal heterogeneities play an important role in determining the fate of dykes and in controlling where sills could form.

Effects of multi-component diffusion and heat release on laminar diffusion flame liftoff

Numerical simulations were conducted of the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C 3H 8 and CH 4 fuels. Both non-reacting and reacting jets were investigated, including multi-component diffusivities and heat release effects (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously arrived at in similarity solutions. The cold-flow simulation for He-diluted CH 4 fuel does not predict flame liftoff; however, adding heat release reaction lead to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C 3H 8 flames, with the flame base location differing from that in the similarity solution – the intersection of the stoichiometric and iso-velocity (equal to 1-D flame speed) is not necessary for flame stabilization (and thus liftoff). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena.

Formation dry‐out from CO2 injection into saline aquifers: 1. Effects of solids precipitation and their mitigation

Injection of CO2 into saline aquifers may cause formation dry‐out and precipitation of salt near the injection well, which may reduce formation porosity, permeability, and injectivity. This paper uses numerical simulation to explore the role of different processes and parameters in the salt precipitation process and to examine injection strategies that could mitigate the effects. The main physical mechanisms affecting the dry‐out and salt precipitation process include (1) displacement of brine away from the injection well by injected CO2, (2) dissolution (evaporation) of brine into the flowing CO2 stream, (3) upflow of CO2 due to gravity effects (buoyancy), (4) backflow of brine toward the injection point due to capillary pressure gradients that oppose the pressure gradient in the CO2‐rich (“gas”) phase, and (5) molecular diffusion of dissolved salt. The different mechanisms operate on a range of spatial scales. CO2 injection at constant rate into a homogeneous reservoir with uniform initial conditions is simulated in 1‐D radial geometry, to resolve multiscale processes by taking advantage of the similarity property, i.e., the evolution of system conditions as a function of radial distance R and time t depends only on the similarity variable R2/t. Simulations in 2‐D vertical cross sections are used to examine the role of gravity effects. We find that counterflow of CO2 and brine can greatly increase aqueous phase salinity and can promote substantial salt precipitation even in formations with low dissolved solids. Salt precipitation can accentuate effects of gravity override. We find that injecting a slug of fresh water prior to commencement of CO2 injection can reduce salt precipitation and permeability loss near the injection well.

Prediction of single particle settling velocities through liquid fluidized beds

Single particle settling velocities through water fluidized beds of mono-sized glass spheres ( d p = 0.645, 1.20, 1.94, 2.98 and 5 mm in diameter) were studied experimentally using a column, 40 mm in diameter. The settling spherical particles ( D p = 10 and 19.5 mm) had different densities (1237 to 8320 kg/m 3), while the settling particles ( D p = 5 and 2.98 mm) were glass spheres. The pseudo-fluid model, which considers a liquid fluidized bed as a homogenous pseudo-fluid, predicts single particle settling velocities quite well if the ratio D p/ d p is larger than about 10. With decreasing ratio D p/ d p, the overall friction between the settling particle and the fluidized media increases. A method for predicting single particle settling velocities through a liquid fluidized bed is proposed and discussed. Following the approach of Van der Wielen et al. [L.A.M. Van der Wielen, M.H.H Van Dam, K.C.A.M. Van Luyben, On the relative motion of a particle in a swarm of different particles, Chem. Eng. Sci. 51 (2006) 995–1008], the overall friction is decomposed into a particle–fluid and a particle–particle component. The effective buoyancy force is calculated using the transition function proposed by Ruzicka [M.C. Ruzicka, On buoyancy in dispersion, Chem. Eng. Sci. 61 (2006) 2437–2446]. A simple model for predicting the collision force is proposed, as well as a correlation for the collision coefficient. The mean absolute deviation between the experimental and calculated slip velocities was 5.08%.

Effect of Scaling Parameters on Waterflood Performance with Horizontal and Vertical Wells

This paper presents numerical study on the effect of scaling parameters on the waterflood performance with different well configurations. The oil recoveries obtained from experiments on a laboratory scale model have been compared with those obtained from a model, which is scaled up using scaling relationships which are in the form of dimensionless numbers. The effects of dimensionless scaling groups like effective aspect ratio, mobility ratio, buoyancy number, and capillary number on breakthrough oil recovery (BOR) with four different well configurations, viz., vertical injection-vertical production (VI-VP), vertical injection-horizontal production at top (VI-HPT), vertical injection-horizontal production at bottom (VI-HPB), and horizontal injection at bottom-horizontal production at top (HIB-HPT), are reported. It is found that the HIB-HPT well configuration gives higher oil recovery and that VI-HPB gives lower recovery than other well configurations under most of the conditions considered. For higher bu...

On the Growth of Layers of Nonprecipitating Cumulus Convection

A prototype problem of a nonprecipitating convective layer growing into a layer of uniform stratification and exponentially decreasing humidity is introduced to study the mechanism by which the cumulus-topped boundary layer grows. The problem naturally admits the surface buoyancy flux, outer layer stratification, and moisture scale as governing parameters. Large-eddy simulations show that many of the well-known properties of the cumulus-topped boundary layer (including a well-mixed subcloud layer, a cloud-base transition layer, a conditionally unstable cloud layer, and an inversion layer) emerge naturally in the simulations. The simulations also quantify the differences between nonprecipitating moist convection and its dry counterpart. Whereas dry penetrative convective layers grow proportionally to the square root of time (diffusively) the cumulus layers grow proportionally to time (ballistically). The associated downward transport of warm, dry air results in a significant decrease in the surface Bowen ratio. The linear-in-time growth of the cloud layer is shown to result from the transport and subsequent evaporation of liquid water into the inversion layer. This process acts as a sink of buoyancy, which acts to imbue the free troposphere with the properties of the cloud layer. A simple model, based on this mechanism, and formulated in terms of an effective dry buoyancy flux (which is constrained by the subcloud layer’s similarity to a dry convective layer), is shown to provide good predictions of the growth of the layer across a wide range of governing parameters.

The effect of a depth-dependent bubble distribution on the normal modes of internal waves: quasistatic approximation

We consider internal and surface waves propagating horizontally in the fluid waveguide, formed by an underlying density stratification. As well as the usual basic density stratification, we are concerned with the effect of a nearly stationary depth-dependent distribution of bubbles. While our previous work was restricted to the case of a locally monodisperse mixture, in this paper we show that by using a quasistatic approximation (where attention is confined to those modes whose typical frequencies are much less than the natural frequency for bubble oscillations), we can extend that work to the case of more general discrete and continuous bubble distributions. The equations of motion are formulated in terms of the usual fluid variables and the void fraction of bubbles. Then, to leading order in the Boussinesq approximation, we obtain the usual equation for internal wave modes, but the value of the buoyancy frequency in the fluid is replaced by an effective buoyancy frequency which takes account of the bubble distribution. Indeed, in some circumstances the bubble distribution could be entirely responsible for the density stratification, and hence can give rise to its own internal wave field. Two physical factors are shown to affect the buoyancy frequency in the mixture: the effective stratification due to the bubbles adds to the effect of stratification in the liquid, while the compressibility of the mixture due to the bubbles reduces the buoyancy frequency. Having in mind possible applications of our model to oceanic situations and in accordance with existing observational evidence that the void fraction profile in the ocean decays exponentially with depth, we obtain an explicit description of the normal modes for several simple models of the basic density profile and show that bubble distributions, when present, may considerably change the properties of the internal waveguide.

Motion of a magnetic flow follower in two‐phase flow — application to the study of airlift reactor hydrodynamics

AbstractA low‐cost and simple magnetic particle tracer method was adapted to characterize the hydrodynamic behavior of an internal‐ and an external‐loop airlift reactor (ALR). The residence time distribution of three magnetic particles differing in diameter (5.5, 11.0 and 21.2 mm) and with a density very close to that of water was measured in individual reactor sections. The measured data were analyzed and used to determine the velocity of the liquid phase. Validation of the experimental results for liquid velocity was done by means of the data obtained by an independent reference method. Furthermore, analysis of the differences found in the settling velocity of the particle in single‐liquid and gas‐liquid phases was carried out, using a simplified 3D momentum transfer model. The model considering particle‐bubble interaction forces resulting from changes in the liquid velocity field due to bubble motion was able to predict satisfactorily the increase in the particle settling velocity in the homogeneous bubbly regime. The effective drag coefficient in two‐phase flow was found to be directly dependent on particle Reynolds number to the power of − 2 but independent of gas flow‐rate for all particle diameters studied. Based on the experimental and theoretical investigations, the valid exact formulation of the effective buoyancy force necessary for the calculation of the correct particle settling velocity in two‐phase flow was done. In addition, recommendations concerning the use of flow‐following particles in internal‐loop ALRs for liquid velocity measurements are presented. Copyright © 2006 Society of Chemical Industry

Swimming Paramecium in magnetically simulated enhanced, reduced, and inverted gravity environments.

Earth's gravity exerts relatively weak forces in the range of 10-100 pN directly on cells in biological systems. Nevertheless, it biases the orientation of swimming unicellular organisms, alters bone cell differentiation, and modifies gene expression in renal cells. A number of methods of simulating different strength gravity environments, such as centrifugation, have been applied for researching the underlying mechanisms. Here, we demonstrate a magnetic force-based technique that is unique in its capability to enhance, reduce, and even invert the effective buoyancy of cells and thus simulate hypergravity, hypogravity, and inverted gravity environments. We apply it to Paramecium caudatum, a single-cell protozoan that varies its swimming propulsion depending on its orientation with respect to gravity, g. In these simulated gravities, denoted by f(gm), Paramecium exhibits a linear response up to f(gm) = 5 g, modifying its swimming as it would in the hypergravity of a centrifuge. Moreover, experiments from f(gm) = 0 to -5 g show that the response is symmetric, implying that the regulation of the swimming speed is primarily related to the buoyancy of the cell. The response becomes nonlinear for f(gm) >5 g. At f(gm) = 10 g, many paramecia "stall" (i.e., swim in place against the force), exerting a maximum propulsion force estimated to be 0.7 nN. These findings establish a general technique for applying continuously variable forces to cells or cell populations suitable for exploring their force transduction mechanisms.

Varying the effective buoyancy of cells using magnetic force

We introduce a magnetic force buoyancy variation (MFBV) technique that employs intense inhomogeneous magnetic fields to vary the effective buoyancy of cells and other diamagnetic systems in solution. Nonswimming Paramecia have been suspended, forced to sediment and driven to rise in solution using MFBV. Details of their response to MFBV have been used to determine the magnetic susceptibility of a single Paramecium. The use of MFBV as a means by which to suspend cell cultures indefinitely is also described.

Suppression of buoyancy-driven vortex flow resulting from a low speed jet impinging onto a heated disk in a vertical cylinder by cylinder top tilting

Buoyancy-driven vortex flow resulting from a low speed round gas jet impinging vertically downwards onto a heated horizontal circular disk confined in an adiabatic vertical cylindrical chamber can be strong and even unstable as the buoyancy-to-inertia ratio exceeds certain critical level. An experiment combining flow visualization and temperature measurement is conducted in the present study to explore the suppression of the above buoyancy-driven vortex gas flow by inclining the top of the cylindrical chamber. The chamber top is chosen to incline linearly in the radial direction so that the mean flow in the wall-jet region is accelerated and meanwhile the effective buoyancy signified by the local Rayleigh number reduces in that direction. Specifically, in the present experiment the disk-to-chamber top separation distance decreases from 20.0 mm at the jet axis to 10.0 mm at the chamber side. Tests are conducted for the jet flow rate varied from 1.0 to 5.0 slpm (standard liter per minute) and the jet-to-disk temperature difference varied from 0 to 35.0 °C for two injection pipes with diameter 10.0 and 22.1 mm for the chambers with horizontal and inclined tops. The results from the flow visualization indicate that the chamber top inclination can effectively suppress the unstable buoyancy-induced vortex roll and the temporal flow oscillation at high buoyancy-to-inertia ratios. However, the effects of the chamber top inclination on the inertia-driven rolls are much milder. The non-monotonic air temperature distribution in the radial direction is found to result from the unique vortex flow structure in the chamber. A universal criterion based on the local flow and thermal conditions in the wall-jet region for the onset of the buoyancy-driven roll is proposed. To quantify the characteristics of the vortex flow in the chamber with the inclined top, empirical correlations have been proposed for the size and location of the vortex rolls.