The center of buoyancy of an arbitrary shaped body is defined in analogy to the center of gravity. The definitions of the buoyant force and center of buoyancy in terms of integrals over the area of the body are converted to volume integrals and shown to have simple intuitive interpretations
Buoyancy organic Rankine cycle
Schoenmaker, J.; Rey, J.F.Q. [Centro de Engenharia, Modelagem e Ciencias Sociais Aplicadas, Universidade Federal do ABC (CECS-UFABC), Rua Santa Adelia 166, Bairro Bangu, 09210-170 Santo Andre, SP (Brazil); Pirota, K.R. [Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas (UNICAMP), C.P. 6165, Campinas, SP (Brazil)
2011-03-15
In the scope of renewable energy, we draw attention to a little known technique to harness solar and geothermal energy. The design here proposed and analyzed is a conceptual hybrid of several patents. By means of a modified organic Rankine cycle, energy is obtained utilizing buoyancy force of a working fluid. Based on thermodynamic properties we propose and compare the performance of Pentane and Dichloromethane as working fluids. Theoretical efficiencies up to 0.26 are estimated for a 51 m (Pentane) and 71.5 m (Dichloromethane) high column of water in a regime below 100 C operation temperature. These findings are especially relevant in the scope of distributed energy systems, combined cycle plants, and low-temperature Rankine cycles. (author)
Buoyancy effects on smoldering combustion
Dosanjh, S.; Peterson, J.; Fernandez-Pello, A. C.; Pagni, P. J.
1985-01-01
The effect of buoyancy on the rate of spread of a concurrent smolder reaction through a porous combustible material is investigated theoretically and experimentally. In the experiments, buoyant forces are controlled by varying the density difference, and the smolder rate spread through porous alpha cellulose (0.83 void fraction) is measured as a function of the ambient air pressure. The smolder velocity is found to increase with the ambient pressure; extinction occurs when the buoyancy forces cannot overcome the drag forces, indicating that diffusion by itself cannot support the spread of a smolder reaction. Theoretical predictions are found to be in good qualitative agreement with the experimental results.
Wave Dragon Buoyancy Regulation Study
Jakobsen, Jens; Kofoed, Jens Peter
Wave Dragon is a wave energy converter, which was deployed offshore at Nissum Bredning in Denmark in 2003. The experience gained from operating Wave Dragon during 2003 and 2004 has shown that the buoyancy regulation system can be improved in a number of ways. This study describes the current...... situation, and proposes a number of activities in order to improve the buoyancy regulation system. This work was performed under EU ENERGIE contract no. ENK5-CT-2002-00603, and is a contribution to WP 2.3/2.4 and D40/D41....
Buoyancy Waves in Earth's Magnetosphere
Wolf, Richard Alan; Moore Schutza, Aaron; Rocco Toffoletto, Frank
2015-04-01
Thin-filament simulations raised the possibility that underpopulated flux tubes moving earthward through the plasma sheet from the distant plasma sheet might oscillate a few times before coming to rest near the inner edge. Such oscillations, called braking or interchange oscillations, have been observed, and their periods agree fairly well with the predictions of the thin-filament model. However, the thin-filament model assumes a highly idealized geometry and so does not provide a fully adequate theory of the oscillations. This paper addresses two questions: (1) How do the thin-filament oscillations relate to linear eigenmodes of the magnetosphere? (2) What do the corresponding eigenfunctions look like? We investigate those questions by focusing on a simple wedge-shaped plasma configuration with circular field lines that resembles the Earth’s magnetosphere in that it exhibits interchange oscillations in the thin filament approximation. However, the wedge configuration is also simple enough that linear eigenfunctions can easily be calculated. If we consider wavelengths smaller than the scale length for spatial variations in the wedge and frequencies far below the fast-mode speed, the resulting wave equation has exactly the form of an equation for buoyancy oscillation of the neutral atmosphere. The frequency of the thin-filament oscillation appears in the wave equation in exactly the way that the buoyancy frequency ωb (also known as the Brunt-Väisälä frequency) appears in the neutral-atmosphere equation. As in the neutral-atmosphere case, the magnetospheric buoyancy wave of frequency ω propagates through the region where the buoyancy frequency exceeds ω, but is evanescent in the region where the buoyancy frequency is less than ω.
Buoyancy instability of homologous implosions
Johnson, Bryan M
2015-01-01
I consider the hydrodynamic stability of imploding gases as a model for inertial confinement fusion capsules, sonoluminescent bubbles and the gravitational collapse of astrophysical gases. For oblate modes under a homologous flow, a monatomic gas is governed by the Schwarzschild criterion for buoyant stability. Under buoyantly unstable conditions, fluctuations experience power-law growth in time, with a growth rate that depends upon mean flow gradients and is independent of mode number. If the flow accelerates throughout the implosion, oblate modes amplify by a factor (2C)^(|N0| ti)$, where C is the convergence ratio of the implosion, N0 is the initial buoyancy frequency and ti is the implosion time scale. If, instead, the implosion consists of a coasting phase followed by stagnation, oblate modes amplify by a factor exp(pi |N0| ts), where N0 is the buoyancy frequency at stagnation and ts is the stagnation time scale. Even under stable conditions, vorticity fluctuations grow due to the conservation of angular...
14 CFR 29.755 - Hull buoyancy.
2010-01-01
... 14 Aeronautics and Space 1 2010-01-01 2010-01-01 false Hull buoyancy. 29.755 Section 29.755 Aeronautics and Space FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION AIRCRAFT AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY ROTORCRAFT Design and Construction Floats and Hulls § 29.755 Hull buoyancy. Water-based and amphibian rotorcraft....
Short wavelength magnetic buoyancy instability
Mizerski, K A; Hughes, D W
2013-01-01
Magnetic buoyancy instability plays an important role in the evolution of astrophysical magnetic fields. Here we revisit the problem introduced by \\citet{Gilman_1970} of the short wavelength linear stability of a plane layer of compressible isothermal fluid permeated by a horizontal magnetic field of strength decreasing with height. Dissipation of momentum and magnetic field is neglected. By the use of a Rayleigh-Schr\\"odinger perturbation analysis, we explain in detail the limit in which the transverse horizontal wavenumber of the perturbation, denoted by $k$, is large (i.e.\\ short horizontal wavelength) and show that the fastest growing perturbations become localized in the vertical direction as $k$ is increased. The growth rates are determined by a function of the vertical coordinate $z$ since, in the large $k$ limit, the eigenmodes are strongly localized in the vertical direction. We consider in detail the case of two-dimensional perturbations varying in the directions perpendicular to the magnetic field,...
Buoyancy effects of a growing, isolated dendrite
Canright, D.; Davis, S. H.
1991-01-01
The buoyancy effect of a growing isolated dendrite on the solidification process in the undercooling liquid material was investigated by developing an analytic solution to the growth/convection problem in powers of a buoyancy parameter G. The solution depends on the Prandtl number P and the Stefan number S (undercooling) for the local velocity and thermal fields and also the buoyant alteration of the interface shape. Results suggest that buoyancy effect for metals (low P) may be qualitatively different from that for organics (high P).
Surfactants for Bubble Removal against Buoyancy
Md. Qaisar Raza; Nirbhay Kumar; Rishi Raj
2016-01-01
The common phenomenon of buoyancy-induced vapor bubble lift-off from a heated surface is of importance to many areas of science and technology. In the absence of buoyancy in zero gravity of space, non-departing bubbles coalesce to form a big dry patch on the heated surface and heat transfer deteriorates despite the high latent heat of vaporization of water. The situation is worse on an inverted heater in earth gravity where both buoyancy and surface tension act upwards to oppose bubble remova...
Buoyancy instability of homologous implosions
Johnson, Bryan
2015-11-01
Hot spot turbulence is a potential contributor to yield degradation in inertial confinement fusion (ICF) capsules, although its origin, if present, remains unclear. In this work, a perturbation analysis is performed of an analytical homologous solution that mimics the hot spot and surrounding cold fuel during the late stages of an ICF implosion. It is shown that the flow is governed by the Schwarzschild criterion for buoyant stability, and that during stagnation, short wavelength entropy and vorticity fluctuations amplify by a factor exp (π |N0 | ts) , where N0 is the buoyancy frequency at stagnation and ts is the stagnation time scale. This amplification factor is exponentially sensitive to mean flow gradients and varies from 103-107 for realistic gradients. Comparisons are made with a Lagrangian hydrodynamics code, and it is found that a numerical resolution of ~ 30 zones per wavelength is required to capture the evolution of vorticity accurately. This translates to an angular resolution of ~(12 / l) ∘ , or ~ 0 .1° to resolve the fastest growing modes (Legendre mode l > 100).
The island wind–buoyancy connection
De Boer, Agatha M.; Nof, Doron
2005-01-01
A variety of recent studies have suggested that the meridional overturning circulation (MOC) is at least partially controlled by the Southern Ocean (SO) winds. The paradoxical implication is that a link exists between the global surface buoyancy flux to the ocean (which is needed for the density transformation between surface and deep water) and the SO winds. Although the dependency of buoyancy forcing on local wind is obvious, the global forcings are usually viewed independently with regard ...
Energy Spectrum of Buoyancy-driven Turbulence
Verma, Mahendra K; Chatterjee, Anando G
2014-01-01
Using direct numerical simulation we demonstrate that stably stratified flows with large Richardson number follow Bolgiano-Obukhov scaling, i.e, the kinetic energy spectrum $E_u(k) \\sim k^{-11/5}$, the entropy spectrum $E_\\theta(k) \\sim k^{-7/5}$, and kinetic energy flux $\\Pi_u(k) \\sim k^{-4/5}$. This is due to the conversion of kinetic energy to potential energy because of buoyancy. We also demonstrate that $E_u(k) \\sim k^{-5/3}$ for stratified flow with weaker buoyancy or smaller Richardson number. We argue that due to the positive energy supply by buoyancy and non-decreasing $\\Pi_u(k)$, Rayleigh B\\'{e}nard convection should follow Kolmogorov-Obukhov scaling ($E_u(k) \\sim k^{-5/3}$).
Energy spectrum of Buoyancy-driven Flows
Kumar, Abhishek; Verma, Mahendra K
2014-01-01
Using high-resolution direct numerical simulation and arguments based on the kinetic energy flux $\\Pi_u$, we demonstrate that for stably stratified flows, the kinetic energy spectrum $E_u(k) \\sim k^{-11/5}$, the entropy spectrum $E_\\theta(k) \\sim k^{-7/5}$, and $\\Pi_u(k) \\sim k^{-4/5}$ (Bolgiano-Obukhov scaling). This scaling is due to the depletion of kinetic energy because of buoyancy. For weaker buoyancy in stratified flows, $E_u(k)$ follows Kolmgorov's spectrum with a constant energy flux. We also argue that for Rayleigh B\\'{e}nard convection, the Bolgiano-Obukhov scaling will not hold for the bulk flow due to the positive energy supply by buoyancy and non-decreasing $\\Pi_u(k)$.
A buoyancy profile for the Earth's core
Davies, C. J.; Gubbins, D.
2011-11-01
We investigate the thermal and chemical buoyancy forces that drive convection in the Earth's liquid outer core and derive a radial buoyancy profile that can be used in geodynamo models. We assume the core is well mixed, adiabatic and cools as a result of mantle convection. The buoyancy profile is developed for a Boussinesq fluid and incorporates secular cooling, latent heat release at the inner core boundary, radiogenic heating, the effect of the adiabat, and compositional buoyancy due to inner core freezing. Surprisingly, these complex effects can be modelled accurately by a simple combination of bottom heating and near-uniform heat sinks, which is implemented using a cotemperature formulation that converts compositional effects into effective thermal effects. The relative importance of internal and bottom heating is then defined by just two parameters, the cooling rate at the core-mantle boundary (CMB) and the uniform rate of internal radiogenic heat production, both of which can be obtained from core evolution calculations. We vary these parameters in geodynamo models and compare basic features of the generated fields with the geomagnetic field; in this manner we link core evolution models, geodynamo simulations and geomagnetic observations. We consider three end-member scenarios for core evolution: (1) rapid cooling and a young inner core; (2) moderate cooling and neutral stability at the CMB; (3) slow cooling and enough radiogenic heating to allow the inner core to be 3.5 Gyr old. We find that compositional buoyancy dominates thermal buoyancy everywhere except near the CMB, even with large amounts of radiogenic heating, and buoyancy forces are far larger at depth than higher up. Reducing the cooling rate and increasing radiogenic heating reduces the drop in the superadiabatic gradient between the inner and outer boundaries: for rapid cooling the drop is by a factor 50; for slow cooling it is a factor of 5. We demonstrate the effects of these different buoyancy
Energy Spectrum of Buoyancy-Driven Turbulence
Kumar, Abhishek; Chatterjee, Anando G.; Verma, Mahendra K.
2014-01-01
Using high-resolution direct numerical simulation and arguments based on the kinetic energy flux $\\Pi_u$, we demonstrate that for stably stratified flows, the kinetic energy spectrum $E_u(k) \\sim k^{-11/5}$, the entropy spectrum $E_\\theta(k) \\sim k^{-7/5}$, and $\\Pi_u(k) \\sim k^{-4/5}$, consistent with the Bolgiano-Obukhov scaling. This scaling arises due to the conversion of kinetic energy to the potential energy by buoyancy. For weaker buoyancy, this conversion is weak, hence $E_u(k)$ follo...
Exploring Titan with Autonomous, Buoyancy Driven Gliders
Morrow, M. T.; Woolsey, C. A.; Hagerman, G. M.
Buoyancy driven underwater gliders are highly efficient winged underwater vehicles which locomote by modifying their internal shape. The concept, which is already well-proven in Earth's oceans, is also an appealing technology for remote terrain exploration and environmental sampling on worlds with dense atmospheres. Because of their high efficiency and their gentle, vertical take-off and landing capability, buoyancy driven gliders might perform long duration, global mapping tasks as well as light-duty, local sampling tasks. Moreover, a sufficiently strong gradient in the planetary boundary layer may enable the vehicles to perform dynamic soaring, achieving even greater locomotive efficiency. Shape Change Actuated, Low Altitude Robotic Soarers (SCALARS) are an appealing alternative to more conventional vehicle technology for exploring planets with dense atmospheres. SCALARS are buoyancy driven atmospheric gliders with a twin-hulled, inboard wing configuration. The inboard wing generates lift, which propels the vehicle forward. Symmetric changes in mass distribution induce gravitational pitch moments that provide longitudinal control. Asymmetric changes in mass distribution induce twist in the inboard wing that provides directional control. The vehicle is actuated solely by internal shape change; there are no external seals and no exposed moving parts, save for the inflatable buoyancy ballonets. Preliminary sizing analysis and dynamic modeling indicate the viability of using SCALARS to map the surface of Titan and to investigate features of interest.
Buoyancy Driven Natural Ventilation through Horizontal Openings
Heiselberg, Per; Li, Zhigang
2009-01-01
An experimental study of the phenomenon of buoyancy driven natural ventilation through single-sided horizontal openings was performed in a full-scale laboratory test rig. The measurements were made for opening ratios L/D ranging from 0.027 to 4.455, where L and D are the length of the opening and...
Surfactants for Bubble Removal against Buoyancy.
Raza, Md Qaisar; Kumar, Nirbhay; Raj, Rishi
2016-01-01
The common phenomenon of buoyancy-induced vapor bubble lift-off from a heated surface is of importance to many areas of science and technology. In the absence of buoyancy in zero gravity of space, non-departing bubbles coalesce to form a big dry patch on the heated surface and heat transfer deteriorates despite the high latent heat of vaporization of water. The situation is worse on an inverted heater in earth gravity where both buoyancy and surface tension act upwards to oppose bubble removal. Here we report a robust passive technique which uses surfactants found in common soaps and detergents to avoid coalescence and remove bubbles downwards, away from an inverted heater. A force balance model is developed to demonstrate that the force of repulsion resulting from the interaction of surfactants adsorbed at the neighboring liquid-vapor interfaces of the thin liquid film contained between bubbles is strong enough to overcome buoyancy and surface tension. Bubble removal frequencies in excess of ten Hz resulted in more than twofold enhancement in heat transfer in comparison to pure water. We believe that this novel bubble removal mechanism opens up opportunities for designing boiling-based systems for space applications. PMID:26743179
Surfactants for Bubble Removal against Buoyancy
Raza, Md. Qaisar; Kumar, Nirbhay; Raj, Rishi
2016-01-01
The common phenomenon of buoyancy-induced vapor bubble lift-off from a heated surface is of importance to many areas of science and technology. In the absence of buoyancy in zero gravity of space, non-departing bubbles coalesce to form a big dry patch on the heated surface and heat transfer deteriorates despite the high latent heat of vaporization of water. The situation is worse on an inverted heater in earth gravity where both buoyancy and surface tension act upwards to oppose bubble removal. Here we report a robust passive technique which uses surfactants found in common soaps and detergents to avoid coalescence and remove bubbles downwards, away from an inverted heater. A force balance model is developed to demonstrate that the force of repulsion resulting from the interaction of surfactants adsorbed at the neighboring liquid-vapor interfaces of the thin liquid film contained between bubbles is strong enough to overcome buoyancy and surface tension. Bubble removal frequencies in excess of ten Hz resulted in more than twofold enhancement in heat transfer in comparison to pure water. We believe that this novel bubble removal mechanism opens up opportunities for designing boiling-based systems for space applications.
Twisted magnetic structures emerging from buoyancy instabilities
Jouve, L; Proctor, M R E
2012-01-01
We here report calculations of magnetic buoyancy instabilities of a sheared magnetic layer where two separate zones are unstable. The idea is to study the possible generation of large-scale helical structures which could then rise through a stellar convection zone and emerge at the surface to create active regions. The calculations shown here are a follow-up of the work of Favier et al. (2012) where the instability developed in a weakly magnetized atmosphere, consisting of a uniform field oriented in a different direction from the unstable layer below. Here, the top layer representing the atmosphere is itself unstable to buoyancy instabilities and thus quickly creates a more complex magnetic configuration with which the layer below will interact. We also find in this case that the accumulation of magnetic tension between the two unstable layers favors the creation of large-scale helical structures.
Twisted magnetic structures emerging from buoyancy instabilities
Jouve, L.; Silvers, L. J.; Proctor, M. R. E.
2012-12-01
We here report calculations of magnetic buoyancy instabilities of a sheared magnetic layer where two separate zones are unstable. The idea is to study the possible generation of large-scale helical structures which could then rise through a stellar convection zone and emerge at the surface to create active regions. The calculations shown here are a follow-up of the work of Favier et al. (2012) where the instability developed in a weakly magnetized atmosphere, consisting of a uniform field oriented in a different direction from the unstable layer below. Here, the top layer representing the atmosphere is itself unstable to buoyancy instabilities and thus quickly creates a more complex magnetic configuration with which the layer below will interact. We also find in this case that the accumulation of magnetic tension between the two unstable layers favors the creation of large-scale helical structures.
Buoyancy Instabilities in Degenerate, Collisional, Magnetized Plasmas
Chang, P
2009-01-01
In low-collisionality plasmas, anisotropic heat conduction due to a magnetic field leads to buoyancy instabilities for any nonzero temperature gradient. We study analogous instabilities in degenerate {\\it collisional} plasmas, i.e., when the electron collision frequency is large compared to the electron cyclotron frequency. Although heat conduction is nearly isotropic in this limit, the small residual anisotropy ensures that collisional degenerate plasmas are also convectively unstable independent of the sign of the temperature gradient. We show that the range of wavelengths that are unstable is independent of the magnetic field strength, while the growth time increases with decreasing magnetic field strength. We discuss the application of these collisional buoyancy instabilities to white dwarfs and neutron stars. Magnetic tension and the low specific heat of a degenerate plasma significantly limit their effectiveness; the most promising venues for growth are in the liquid oceans of young, weakly magnetized n...
Topology Optimization including Inequality Buoyancy Constraints
Picelli, R.; Dijk, R.; Vicente, W.M.; Pavanello, R.; Langelaar, M.; Van Keuen, A.
2014-01-01
This paper presents an evolutionary topology optimization method for applications in design of completely submerged buoyant devices with design-dependent fluid pressure loading. This type of structures aid rig installations and pipeline transportation in all water depths in offshore structural engineering. The proposed optimization method seeks the buoy design that presents higher stiffness, less material and a prescribed buoyancy effect. A hydrostatic fluid is used to simulate the underwater...
The effect of buoyancy on opposed smoldering
Bar-Ilan, Amnon; Rein, Guillermo; Walther, David C; Fernandez-Pello, A. C; Torero, Jose L; Urban, David L.
2004-01-01
An experimental investigation on the effects of buoyancy on opposed-flow smolder is presented. Tests were conducted on cylindrical samples of open-cell, unretarded polyurethane foams at a range of ambient pressures using the Microgravity Smoldering Combustion (MSC) experimental apparatus. The samples were tested in the opposed configuration, in which the flow of oxidizer is induced in the opposite direction of the propagation of the Smolder front. These data were compared with opposed-forced-...
陈振诚
2012-01-01
In nature, there exists the hydrostatic buoyancy and the hydrodynamic buoyancy practically. A body in static state under or on the water surface bears the hydrostatic buoyancy. Archimedes discovered that the quantitative value of the hydrostatic buoyancy equals the weight of the same volume water displaced by the said body. A moving body keeping an attack angle against the horizontal plane under or on the water surface would arouse the hydrodynamic buoyancy. Inferring from physical qualitative analysis, the hydrodynamic buoyancy is closely related with the physical factors of velocity, size, draft depth, attack angle of moving body, water density, and gravity acceleration embodying the action of gravity field to the fluid field. If the quantitative value of the hydrodynamic buoyancy is expressed by mathematical analytical expression, then it is the function of these physical factors. People hope to know the hydrodynamic buoyancy. Here we present a research into this problem, applying a new integral transform to solve the problem of the hydrodynamic buoyancy, and an analytical expression of the quantitative value of the hydrodynamic buoyancy has been acquired. The said expression generalizes the related physical factors of the moving body that contribute to the hydrodynamic buoyancy and the mutual-restricting relationship among these factors, which agrees with the physical qualitative analysis. Using a boat we designed by the result of this paper, the experiments in navigation show that the result of the theoretical calculation is in good agreement with the data acquired from practical measurements in the experiments. This proves that the researching result of this paper agrees with practice and has general significance. The said method may solve many problems in the design and production of hydrodynamic engineering. Finally in this paper, compared with our researching result, the forefather's approximate calculation formulae of the hydrodynamic buoyancy have been
DNS of MHD turbulent flow with buoyancy
Liquid-metals as coolant material in fusion reactor have a significant role in the design of advanced reactors. The investigation of thermal behavior in the actual facility environment, such as in the case of low Pr number fluid flow, is needed with the buoyancy effect under a magnetic field. In the present study, a direct numerical simulation (DNS) for the low Pr number fluid flow of turbulent heat transfer with buoyancy effect has been carried out under a magnetic field. The values of Hartmann number Ha were 0, 8, 16, 32, 64 and 128, and Prandtl number was 0.06. Grashof number was 4.8 x 106. The turbulent quantities such as the mean temperature, turbulent heat flux, and temperature variant were obtained by DNS. The Reynolds number for channel flow based on friction velocity, viscosity, and channel half width was set to be constant as Reτ=150. A uniform magnetic field was applied in a direction perpendicular to the wall of the channel. On the large Hartmann number, large-scale thermal convection by the effect of buoyancy can not be found at the whole region of the channel even if Grashof number is large, that is, the thermal transport is restricted by the effect of the magnetic field. (author)
Energy spectrum of buoyancy-driven turbulence
Kumar, Abhishek
2014-08-25
Using high-resolution direct numerical simulation and arguments based on the kinetic energy flux Πu, we demonstrate that, for stably stratified flows, the kinetic energy spectrum Eu(k)∼k-11/5, the potential energy spectrum Eθ(k)∼k-7/5, and Πu(k)∼k-4/5 are consistent with the Bolgiano-Obukhov scaling. This scaling arises due to the conversion of kinetic energy to the potential energy by buoyancy. For weaker buoyancy, this conversion is weak, hence Eu(k) follows Kolmogorov\\'s spectrum with a constant energy flux. For Rayleigh-Bénard convection, we show that the energy supply rate by buoyancy is positive, which leads to an increasing Πu(k) with k, thus ruling out Bolgiano-Obukhov scaling for the convective turbulence. Our numerical results show that convective turbulence for unit Prandt number exhibits a constant Πu(k) and Eu(k)∼k-5/3 for a narrow band of wave numbers. © 2014 American Physical Society.
Spontaneous chiral parity breaking by hydromagnetic buoyancy
Chatterjee, Piyali; Brandenburg, Axel; Rheinhardt, Matthias
2010-01-01
Evidence for a parity-breaking nature of the magnetic buoyancy instability in a stably stratified gas is reported. In the absence of rotation, no helicity is produced, but the non-helical state is found to be unstable to small helical perturbations during the development of the instability. The parity-breaking nature of an instability in magnetohydrodynamics appears to be the first of its kind and is similar to chiral symmetry breaking in biochemistry. Applications to the production of mean fields in galaxy clusters are being discussed.
Spontaneous chiral symmetry breaking by hydromagnetic buoyancy
Chatterjee, Piyali; Mitra, Dhrubaditya; Brandenburg, Axel; Rheinhardt, Matthias
2011-08-01
Evidence for the parity-breaking nature of the magnetic buoyancy instability in a stably stratified gas is reported. In the absence of rotation, no helicity is produced, but the nonhelical state is found to be unstable to small helical perturbations during the development of the instability. The parity-breaking nature of this magnetohydrodynamic instability appears to be the first of its kind and has properties similar to those in chiral symmetry breaking in biochemistry. Applications to the production of mean fields in galaxy clusters are discussed.
Effects of Buoyancy on Langmuir Circulation
SONG Jun; SONG Jin-Bao
2008-01-01
Based on the Navier-Stokes equation,an equation describing the Langmuir circulation is derived by a perturbation method when the influences of Coriolis force and buoyancy force are both considered.The approach used in the analysis is similar to the works carried out by Craik and Leibovich[J.Fluid Mech.73 (1976)401],Leibovich [J.Fluid Mech.79 (1977) 715]and Huang[J.Fluid Mech.91 (1979) 191].Potential applications of the equation proposed are discussed in the area of Antarctic circumpolar current.
The stability of protostellar disks with Hall effect and buoyancy
Urpin, V.; Rüdiger, G.
2003-01-01
The stability properties of inviscid protostellar disks are examined taking into account the Hall effect and buoyancy. Depending on the parameters, different types of instabilities can exist in different regions of disks. In a very low ionized region, the instability associated with baroclinic effects of buoyancy is likely most efficient. The Hall-driven shear instability can lead to destabilization of regions with a higher ionization. The magnetorotational instability modified by buoyancy ca...
Skylab Astronauts' Neutral Buoyancy Simulator Training
1970-01-01
After the end of the Apollo missions, NASA's next adventure into space was the marned spaceflight of Skylab. Using an S-IVB stage of the Saturn V launch vehicle, Skylab was a two-story orbiting laboratory, one floor being living quarters and the other a work room. The objectives of Skylab were to enrich our scientific knowledge of the Earth, the Sun, the stars, and cosmic space; to study the effects of weightlessness on living organisms, including man; to study the effects of the processing and manufacturing of materials utilizing the absence of gravity; and to conduct Earth resource observations. At the Marshall Space Flight Center (MSFC), astronauts and engineers spent hundreds of hours in an MSFC Neutral Buoyancy Simulator (NBS) rehearsing procedures to be used during the Skylab mission, developing techniques, and detecting and correcting potential problems. The NBS was a 40-foot deep water tank that simulated the weightlessness environment of space. This photograph shows astronaut Ed Gibbon (a prime crew member of the Skylab-4 mission) during the neutral buoyancy Skylab extravehicular activity training at the Apollo Telescope Mount (ATM) mockup. One of Skylab's major components, the ATM was the most powerful astronomical observatory ever put into orbit to date.
On the buoyancy force and the metacentre
Mégel, Jacques
2009-01-01
We address the point of application A of the buoyancy force (also known as the Archimedes force) by using two different definitions of the point of application of a force, derived one from the work-energy relation and another one from the equation of motion. We present a quantitative approach to this issue based on the concept of the hydrostatic energy, considered for a general shape of the immersed cross-section of the floating body. We show that the location of A depends on the type of motion experienced by the body. In particular, in vertical translation, from the work-energy viewpoint, this point is fixed with respect to the centre of gravity G of the body. In contrast, in rolling/pitching motion there is duality in the location of A ; indeed, the work-energy relation implies A to be fixed with respect to the centre of buoyancy C, while from considerations involving the rotational moment it follows that A is located at the metacentre M. We obtain analytical expressions of the location of M for a general s...
Buoyancy instabilities in degenerate, collisional, magnetized plasmas
Chang, Philip; Quataert, Eliot
2010-03-01
In low-collisionality plasmas, anisotropic heat conduction due to a magnetic field leads to buoyancy instabilities for any non-zero temperature gradient. We study analogous instabilities in degenerate collisional plasmas, i.e. when the electron collision frequency is large compared to the electron cyclotron frequency. Although heat conduction is nearly isotropic in this limit, the small residual anisotropy ensures that collisional degenerate plasmas are also convectively unstable independent of the sign of the temperature gradient. We show that the range of wavelengths that are unstable is independent of the magnetic field strength, while the growth time increases with decreasing magnetic field strength. We discuss the application of these collisional buoyancy instabilities to white dwarfs and neutron stars. Magnetic tension and the low specific heat of a degenerate plasma significantly limit their effectiveness; the most promising venues for growth are in the liquid oceans of young, weakly magnetized neutron stars (B <~ 109 G) and in the cores of young, high magnetic field white dwarfs (B ~ 109 G).
Kimberlite ascent by assimilation-fuelled buoyancy.
Russell, James K; Porritt, Lucy A; Lavallée, Yan; Dingwell, Donald B
2012-01-19
Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons. During ascent, they travel through about 150 kilometres of cratonic mantle lithosphere and entrain seemingly prohibitive loads (more than 25 per cent by volume) of mantle-derived xenoliths and xenocrysts (including diamond). Kimberlite magmas also reputedly have higher ascent rates than other xenolith-bearing magmas. Exsolution of dissolved volatiles (carbon dioxide and water) is thought to be essential to provide sufficient buoyancy for the rapid ascent of these dense, crystal-rich magmas. The cause and nature of such exsolution, however, remains elusive and is rarely specified. Here we use a series of high-temperature experiments to demonstrate a mechanism for the spontaneous, efficient and continuous production of this volatile phase. This mechanism requires parental melts of kimberlite to originate as carbonatite-like melts. In transit through the mantle lithosphere, these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions, and causing a marked drop in carbon dioxide solubility. The solubility drop manifests itself immediately in a continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of the increasingly kimberlitic magma. Our model provides an explanation for continuous ascent of magmas laden with high volumes of dense mantle cargo, an explanation for the chemical diversity of kimberlite, and a connection between kimberlites and cratons. PMID:22258614
Pitching effects of buoyancy during four competitive swimming strokes.
Cohen, Raymond C Z; Cleary, Paul W; Harrison, Simon M; Mason, Bruce R; Pease, David L
2014-10-01
The purpose of this study was to determine the pitching effects of buoyancy during all competitive swimming strokes--freestyle, backstroke, butterfly, and breaststroke. Laser body scans of national-level athletes and synchronized multiangle swimming footage were used in a novel markerless motion capture process to produce three-dimensional biomechanical models of the swimming athletes. The deforming surface meshes were then used to calculate swimmer center-of-mass (CoM) positions, center-of-buoyancy (CoB) positions, pitch buoyancy torques, and sagittal plane moments of inertia (MoI) throughout each stroke cycle. In all cases the mean buoyancy torque tended to raise the legs and lower the head; however, during part of the butterfly stroke the instantaneous buoyancy torque had the opposite effect. The swimming strokes that use opposing arm and leg strokes (freestyle and backstroke) had smaller variations in CoM positions, CoB positions, and buoyancy torques. Strokes with synchronized left-right arm and leg movement (butterfly and breaststroke) had larger variations in buoyancy torques, which impacts the swimmer's ability to maintain a horizontal body pitch for these strokes. The methodology outlined in this paper enables the rotational effects of buoyancy to be better understood by swimmers, allowing better control of streamlined horizontal body positioning during swimming to improve performance. PMID:24979812
14 CFR 27.751 - Main float buoyancy.
2010-01-01
... 14 Aeronautics and Space 1 2010-01-01 2010-01-01 false Main float buoyancy. 27.751 Section 27.751 Aeronautics and Space FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION AIRCRAFT AIRWORTHINESS STANDARDS: NORMAL CATEGORY ROTORCRAFT Design and Construction Floats and Hulls § 27.751 Main float buoyancy. (a) For main floats, the...
An Antarctic Circumpolar Current driven by surface buoyancy forcing
Hogg, Andrew McC.
2010-12-01
Simulations of an idealised, but eddy-resolving, channel model of the Antarctic Circumpolar Current (ACC) are used to investigate the sensitivity of ACC transport to wind and surface buoyancy forcing. The results are consistent with theoretical predictions of the eddy-saturated limit, where transport is independent of wind stress. In this parameter regime, buoyancy forcing provides the primary control over ACC transport.
14 CFR 29.751 - Main float buoyancy.
2010-01-01
... 14 Aeronautics and Space 1 2010-01-01 2010-01-01 false Main float buoyancy. 29.751 Section 29.751 Aeronautics and Space FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION AIRCRAFT AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY ROTORCRAFT Design and Construction Floats and Hulls § 29.751 Main float buoyancy. (a) For main floats,...
46 CFR 197.342 - Buoyancy-changing devices.
2010-10-01
... 46 Shipping 7 2010-10-01 2010-10-01 false Buoyancy-changing devices. 197.342 Section 197.342 Shipping COAST GUARD, DEPARTMENT OF HOMELAND SECURITY (CONTINUED) MARINE OCCUPATIONAL SAFETY AND HEALTH STANDARDS GENERAL PROVISIONS Commercial Diving Operations Equipment § 197.342 Buoyancy-changing devices. (a) A dry suit or other...
14 CFR 25.751 - Main float buoyancy.
2010-01-01
... 14 Aeronautics and Space 1 2010-01-01 2010-01-01 false Main float buoyancy. 25.751 Section 25.751 Aeronautics and Space FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION AIRCRAFT AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES Design and Construction Floats and Hulls § 25.751 Main float buoyancy. Each main float must have— (a)...
14 CFR 23.751 - Main float buoyancy.
2010-01-01
... 14 Aeronautics and Space 1 2010-01-01 2010-01-01 false Main float buoyancy. 23.751 Section 23.751 Aeronautics and Space FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION AIRCRAFT AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES Design and Construction Floats and Hulls § 23.751 Main float buoyancy....
Buoyancy driven turbulence and distributed chaos
Bershadskii, A
2016-01-01
It is shown, using results of recent direct numerical simulations, laboratory experiments and atmospheric measurements, that buoyancy driven turbulence exhibits a broad diversity of the types of distributed chaos with its stretched exponential spectrum $\\exp(-k/k_{\\beta})^{\\beta}$. The distributed chaos with $\\beta = 1/3$ (determined by the helicity correlation integral) is the most common feature of the stably stratified turbulence (due to the strong helical waves presence). These waves mostly dominate spectral properties of the vertical component of velocity field, while the horizontal component is dominated by the diffusive processes both for the weak and strong stable stratification ($\\beta =2/3$). For the last case influence of the low boundary can overcome the wave effects and result in $\\beta =1/2$ for the vertical component of the velocity field (the spontaneous breaking of the space translational symmetry - homogeneity). For the unstably stratified turbulence in the Rayleigh-Taylor mixing zone the di...
Microgravity Flow Regime Data: Buoyancy and Mixing Apparatus Effects
Shephard, Adam; Best, Frederick
2010-01-01
Zero-g two-phase flow data set qualification and flight experiment design have not been standardized and as a result, agreement among researchers has not been reached regarding what experimental conditions adequately approximate those of microgravity. The effects of buoyancy forces and mixing apparatus on the flow regime transitions are presented in this study. The gravity conditions onboard zero-g aircraft are at best 10-3 g which is used to approximate the 10-5 g conditions of microgravity, thus the buoyancy forces present on zero-g aircraft can become significantly large and unrepresentative of microgravity. When buoyancy forces approach those of surface tension forces, buoyancy induced coalescence occurs. When discussing flow regime transitions, these large buoyancy forces lead to flow regime transitions which otherwise would not occur. The buoyancy attributes of the two-phase flow data sets available in the literature are evaluated to determine which data sets exhibit buoyancy induced transitions. Upon comparison of the representative data sets, the affects of different mixing apparatus can be seen in the superficial velocity flow regime maps.
Neutral Buoyancy Test - NB23 - Space Telescope
1979-01-01
Once the United States' space program had progressed from Earth's orbit into outerspace, the prospect of building and maintaining a permanent presence in space was realized. To accomplish this feat, NASA launched a temporary workstation, Skylab, to discover the effects of low gravity and weightlessness on the human body, and also to develop tools and equipment that would be needed in the future to build and maintain a more permanent space station. The structures, techniques, and work schedules had to be carefully designed to fit this unique construction site. The components had to be lightweight for transport into orbit, yet durable. The station also had to be made with removable parts for easy servicing and repairs by astronauts. All of the tools necessary for service and repairs had to be designed for easy manipulation by a suited astronaut. And construction methods had to be efficient due to limited time the astronauts could remain outside their controlled environment. In lieu of all the specific needs for this project, an environment on Earth had to be developed that could simulate a low gravity atmosphere. A Neutral Buoyancy Simulator (NBS) was constructed by NASA Marshall Space Flight Center (MSFC) in 1968. Since then, NASA scientists have used this facility to understand how humans work best in low gravity and also provide information about the different kinds of structures that can be built. Included in the plans for the space station was a space telescope. This telescope would be attached to the space station and directed towards outerspace. Astronomers hoped that the space telescope would provide a look at space that is impossible to see from Earth because of Earth's atmosphere and other man made influences. In an effort to make replacement and repairs easier on astronauts the space telescope was designed to be modular. Practice makes perfect as demonstrated in this photo: an astronaut practices moving modular pieces of the space telescope in the Neutral
Is VHF Fresnel reflectivity due to low frequency buoyancy waves?
Vanzandt, T. E.; Vincent, R. A.
1983-01-01
VHF radar echoes are greatly enhanced near the zenith relative to other directions. This enhancement must be due to reflection from horizontally stratified laminate of refractive index. The refractivity laminate are due to the displacements of low frequency buoyancy (internal gravity) waves acting on the background vertical gradient of refractivity. VANZANDT (1982) has shown that the observed spectra of mesoscale wind fluctuations in the troposphere and lower stratosphere are modeled by a universal spectrum of buoyancy (internal gravity) waves. Since the observed frequency spectrum is red, the buoyancy wave model of the vertical displacement spectrum is strongly enhanced near the zenith. In other terms, the resulting refractivity irregularities are strongly stratified.
Short-wavelength Magnetic Buoyancy Instability
Mizerski, K. A.; Davies, C. R.; Hughes, D. W.
2013-04-01
Magnetic buoyancy instability plays an important role in the evolution of astrophysical magnetic fields. Here we revisit the problem introduced by Gilman of the short-wavelength linear stability of a plane layer of compressible isothermal fluid permeated by a horizontal magnetic field of strength decreasing with height. Dissipation of momentum and magnetic field is neglected. By the use of a Rayleigh-Schrödinger perturbation analysis, we explain in detail the limit in which the transverse horizontal wavenumber of the perturbation, denoted by k, is large (i.e., short horizontal wavelength) and show that the fastest growing perturbations become localized in the vertical direction as k is increased. The growth rates are determined by a function of the vertical coordinate z since, in the large k limit, the eigenmodes are strongly localized in the vertical direction. We consider in detail the case of two-dimensional perturbations varying in the directions perpendicular to the magnetic field, which, for sufficiently strong field gradients, are the most unstable. The results of our analysis are backed up by comparison with a series of initial value problems. Finally, we extend the analysis to three-dimensional perturbations.
SHORT-WAVELENGTH MAGNETIC BUOYANCY INSTABILITY
Mizerski, K. A.; Davies, C. R.; Hughes, D. W., E-mail: kamiz@igf.edu.pl, E-mail: tina@maths.leeds.ac.uk, E-mail: d.w.hughes@leeds.ac.uk [Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT (United Kingdom)
2013-04-01
Magnetic buoyancy instability plays an important role in the evolution of astrophysical magnetic fields. Here we revisit the problem introduced by Gilman of the short-wavelength linear stability of a plane layer of compressible isothermal fluid permeated by a horizontal magnetic field of strength decreasing with height. Dissipation of momentum and magnetic field is neglected. By the use of a Rayleigh-Schroedinger perturbation analysis, we explain in detail the limit in which the transverse horizontal wavenumber of the perturbation, denoted by k, is large (i.e., short horizontal wavelength) and show that the fastest growing perturbations become localized in the vertical direction as k is increased. The growth rates are determined by a function of the vertical coordinate z since, in the large k limit, the eigenmodes are strongly localized in the vertical direction. We consider in detail the case of two-dimensional perturbations varying in the directions perpendicular to the magnetic field, which, for sufficiently strong field gradients, are the most unstable. The results of our analysis are backed up by comparison with a series of initial value problems. Finally, we extend the analysis to three-dimensional perturbations.
TAO/TRITON, RAMA, and PIRATA Buoys, Quarterly, Buoyancy Flux
National Oceanic and Atmospheric Administration, Department of Commerce — This dataset has quarterly Buoyancy Flux data from the TAO/TRITON (Pacific Ocean, http://www.pmel.noaa.gov/tao/), RAMA (Indian Ocean,...
Vertigo and positional alcohol nystagmus. The buoyancy mechanism
Franco-Gutiérrez V, Pérez-Vázquez P.
2013-02-01
Full Text Available Ethanol can cause nistagmus and dizziness by a buoyancy mechanism. Its differential diagnosis includes atypical or central positional vertigo. We report the case of a woman whose positional symptoms were caused by ethanol contained in some mixtures.
The treatment of magnetic buoyancy in flux transport dynamo models
Choudhuri, Arnab Rai; Hazra, Gopal
2015-01-01
One important ingredient of flux transport dynamo models is the rise of the toroidal magnetic field through the convection zone due to magnetic buoyancy to produce bipolar sunspots and then the generation of the poloidal magnetic field from these bipolar sunspots due to the Babcock-Leighton mechanism. Over the years, two methods of treating magnetic buoyancy, a local method and a non-local method have been used widely by different groups in constructing 2D kinematic models of the flux transpo...
Transient buoyancy-driven ventilation: Part 2. Modelling heat transfer
Steven D. Sandbach and Gregory F. Lane-Serff
2011-01-01
A new mathematical model for buoyancy-driven ventilation [Sandbach SD, Lane-Serif GF. Transient buoyancy-driven ventilation: Part 1. Modelling advection. Building and Environment, 2011] is modified to include heat transfer at the boundaries. Heat transfers at the ceiling and floor are included, using Newton's law of cooling to model convective heat transfer between the air and the solid boundaries, Fourier's law to model conductive heat transfer through the floor and ceiling, and a linear ver...
Field Effects of Buoyancy on Lean Premixed Turbulent Flames
Cheng, R. K.; Johnson, M. R.; Greenberg, P. S.; Wernet, M. P.
2003-01-01
The study of field effects of buoyancy on premixed turbulent flames is directed towards the advancement of turbulent combustion theory and the development of cleaner combustion technologies. Turbulent combustion is considered the most important unsolved problem in combustion science and laboratory studies of turbulence flame processes are vital to theoretical development. Although buoyancy is dominant in laboratory flames, most combustion models are not yet capable to consider buoyancy effects. This inconsistency has impeded the validation of theories and numerical simulations with experiments. Conversely, the understanding of buoyancy effects is far too limited to help develop buoyant flame models. Our research is also relevant to combustion technology because lean premixed combustion is a proven method to reduce the formation of oxides of nitrogen (NOx). In industrial lean premixed combustion systems, their operating conditions make them susceptible to buoyancy thus affecting heat distribution, emissions, stability, flashback and blowoff. But little knowledge is available to guide combustion engineers as to how to avoid or overcome these problems. Our hypothesis is that through its influence on the mean pressure field, buoyancy has direct and indirect effects on local flame/turbulence interactions. Although buoyancy acts on the hot products in the farfield the effect is also felt in the nearfield region upstream of the flame. These changes also influence the generation and dissipation of turbulent kinetic energy inside the flame brush and throughout the flowfield. Moreover, the plume of an open flame is unstable and the periodic fluctuations make additional contributions to flame front dynamics in the farfield. Therefore, processes such as flame wrinkling, flow acceleration due to heat release and flame- generated vorticity are all affected. Other global flame properties (e.g. flame stabilization limits and flame speed) may all be coupled to buoyancy. This
Modelling and Linear Control of a Buoyancy-Driven Airship
Wu, Xiaotao,; Moog, Claude; Hu, Yueming
2009-01-01
We describe the modelling and control of a newkind airship which is propelled by buoyancy. Based on the Newton-Euler equations and Kirchhoff equations, and referred to the models of underwater gliders and aircraft, a 6DOF nonlinear mathematical model of a buoyancy-driven airship is derived, with features distributed internal mass, and no thrust, elevators and rudders. The attitudes are controlled by the motion of internal mass. The performances of the airship are studied in the vertical plane...
Rigorous buoyancy driven bubble mixing for centrifugal microfluidics.
Burger, S; Schulz, M; von Stetten, F; Zengerle, R; Paust, N
2016-01-21
We present batch-mode mixing for centrifugal microfluidics operated at fixed rotational frequency. Gas is generated by the disk integrated decomposition of hydrogen peroxide (H2O2) to liquid water (H2O) and gaseous oxygen (O2) and inserted into a mixing chamber. There, bubbles are formed that ascent through the liquid in the artificial gravity field and lead to drag flow. Additionaly, strong buoyancy causes deformation and rupture of the gas bubbles and induces strong mixing flows in the liquids. Buoyancy driven bubble mixing is quantitatively compared to shake mode mixing, mixing by reciprocation and vortex mixing. To determine mixing efficiencies in a meaningful way, the different mixers are employed for mixing of a lysis reagent and human whole blood. Subsequently, DNA is extracted from the lysate and the amount of DNA recovered is taken as a measure for mixing efficiency. Relative to standard vortex mixing, DNA extraction based on buoyancy driven bubble mixing resulted in yields of 92 ± 8% (100 s mixing time) and 100 ± 8% (600 s) at 130g centrifugal acceleration. Shake mode mixing yields 96 ± 11% and is thus equal to buoyancy driven bubble mixing. An advantage of buoyancy driven bubble mixing is that it can be operated at fixed rotational frequency, however. The additional costs of implementing buoyancy driven bubble mixing are low since both the activation liquid and the catalyst are very low cost and no external means are required in the processing device. Furthermore, buoyancy driven bubble mixing can easily be integrated in a monolithic manner and is compatible to scalable manufacturing technologies such as injection moulding or thermoforming. We consider buoyancy driven bubble mixing an excellent alternative to shake mode mixing, in particular if the processing device is not capable of providing fast changes of rotational frequency or if the low average rotational frequency is challenging for the other integrated fluidic operations. PMID:26607320
Failure Prediction in Multiphase Deep-Water Buoyancy Systems
Hoel, Eirik
2013-01-01
The objective of this thesis is to explore how a new type of buoyancy material foroshore applications will perform under operating conditions. This includes buoy-ancy loads and extreme hydrostatic pressure. The new material system, Compbuoy,consists of porous, low cost pellets in a polymer matrix. Conventional buoyancyelements today are lled with syntactic foam, a much more expensive material. Asthe promising material Compbuoy has been developed, critical failure mechanismsmust be investigate...
40 CFR 1065.690 - Buoyancy correction for PM sample media.
2010-07-01
... insignificant effect on buoyancy correction, air density is primarily a function of atmospheric pressure... 40 Protection of Environment 32 2010-07-01 2010-07-01 false Buoyancy correction for PM sample... Buoyancy correction for PM sample media. (a) General. Correct PM sample media for their buoyancy in air...
Physics of Buoyancy, pressure potential and Buoyancy reversal for CO2 and hydrocarbon migration
Udo Weyer, K. [WDA Consultants Inc., Calgary, Alberta (Canada)], email: weyer@wda-consultants.com
2011-07-01
Buoyancy reversal has been proven to exist by the three means of theoretical derivation, field evidence, and mathematical modelling. It is becoming urgent to use Hubbert's force potential and buoyancy reversal for studying carbon sequestration and the accumulation and production of hydrocarbons, in order to better understand the physical processes involved. As well, this study is important for optimization of both the methods of carbon sequestration and the recovery rate of hydrocarbons from reservoirs and unconventional gas plays, such as CBM and shale gas. The application of correct physics, through the existing models of regional groundwater flow, to the long-term migration of CO2, helps identify the eventual discharge points of injected CO2 and gives an estimation of the time required. With proper selection of the injection sites, it will take more than thousands or tens of thousands of years before the CO2 will re-enter the atmosphere, and the amount of CO2 discharged at that time will be significantly reduced by geochemical processes.
Buoyancy-Driven Polymerase Chain Reaction (PCR) Devices
Ness, K D; Wheeler, E K; Benett, W; Stratton, P; Christian, A; Chen, A; Ortega, J; Weisgraber, T H; Goodson, K E
2004-09-28
Polymerase chain reaction (PCR) facilitates DNA detection by significantly increasing the concentration of specific DNA segments. A new class of PCR instruments uses a buoyancy-driven re-circulating flow to thermally cycle the DNA sample and benefits from reduced cycle times, low sample volumes, a miniaturized format, and low power consumption. This paper analyzes a specific buoyancy PCR device in a micro-channel ''race-track'' geometry to determine key parameters about PCR cycle times and other figures of merit as functions of device dimensions. The 1-D model balances the buoyancy driving force with frictional losses. A hydrostatic pressure imbalance concept is used between the left and right sides of the fluid loop to calculate the buoyancy driving force. Velocity and temperature distributions within the channels are determined from two-dimensional analysis of the channel section, with developing region effects included empirically through scaled values of the local Nusselt number. Good agreement between four independent verification steps validate the 1-D simulation approach: (1) analytical expressions for the thermal entrance length are compared against, (2) comparison with a full 3-D finite element simulation, (3) comparison with an experimental flow field characterization, and (4) calculation of the minimum PCR runtime required to get a positive PCR signal from the buoyancy-driven PCR device. The 1-D approach closely models an actual buoyancy-driven PCR device and can further be used as a rapid design tool to simulate buoyancy PCR flows and perform detailed design optimizations studies.
The treatment of magnetic buoyancy in flux transport dynamo models
Choudhuri, Arnab Rai
2015-01-01
One important ingredient of flux transport dynamo models is the rise of the toroidal magnetic field through the convection zone due to magnetic buoyancy to produce bipolar sunspots and then the generation of the poloidal magnetic field from these bipolar sunspots due to the Babcock-Leighton mechanism. Over the years, two methods of treating magnetic buoyancy, a local method and a non-local method have been used widely by different groups in constructing 2D kinematic models of the flux transport dynamo. We review both these methods and conclude that neither of them is fully satisfactory, presumably because magnetic buoyancy is an inherently 3D process. We also point out so far we do not have proper understanding of why sunspot emergence is restricted to rather low latitudes.
Numerical Simulations of Buoyancy Effects in low Density Gas Jets
Satti, R. P.; Pasumarthi, K. S.; Agrawal, A. K.
2004-01-01
This paper deals with the computational analysis of buoyancy effects in the near field of an isothermal helium jet injected into quiescent ambient air environment. The transport equations of helium mass fraction coupled with the conservation equations of mixture mass and momentum were solved using a staggered grid finite volume method. Laminar, axisymmetric, unsteady flow conditions were considered for the analysis. An orthogonal system with non-uniform grids was used to capture the instability phenomena. Computations were performed for Earth gravity and during transition from Earth to different gravitational levels. The flow physics was described by simultaneous visualizations of velocity and concentration fields at Earth and microgravity conditions. Computed results were validated by comparing with experimental data substantiating that buoyancy induced global flow oscillations present in Earth gravity are absent in microgravity. The dependence of oscillation frequency and amplitude on gravitational forcing was presented to further quantify the buoyancy effects.
Buoyancy effects on flames spreading down thermally thin fuels
Altenkirch, R. A.; Eichhorn, R.; Shang, P. C.
1980-01-01
Experiments show that buoyancy influences the downward spread rate of flames consuming thermally thin fuel beds. For index cards (0.0098 cm half-thickness) and adding-machine tape (0.0043 cm half-thickness), an increase in the buoyancy level causes the spread rate to drop until no flame propagation is possible. A dimensionless spread rate is found to correlate with a Damkoehler number. As the Damkoehler number increases with decreasing buoyancy level brought about by an increase in pressure or a decrease in gravity, the dimensionless spread rate approaches unity. It is also found that a small change in orientation with respect to the vertical is equivalent to a change in the magnitude of gravity in the direction of spread, and power-law relations between the dimensional spread rate and pressure are only valid over a small pressure range.
Axisymmetric large eddy simulation of a circular source of buoyancy
Nihous, Gerard C., E-mail: nihous@hawaii.ed [University of Hawaii, Hawaii Natural Energy Institute, 1680 East-West Road, POST 109, Honolulu, HI 96822 (United States)
2009-12-15
Axisymmetric large-eddy simulations (LES) of published experiments on vertical mixing above a circular source of buoyancy are performed. Numerical results confirm the existence of a mixing boundary layer just above the buoyancy source (freshwater injected into saline water). As experiments suggested, the calculated normalized shape of the boundary layer seems independent of source size, freshwater injection rate and background solute concentration. This validates a restrictive but numerically efficient assumption of axisymmetric turbulence for the LES calculations. It also lends additional credibility to the theoretical boundary-layer analysis of Epstein and Burelbach [Epstein, M., Burelbach, J.P., 2001. Vertical mixing above a steady circular source of buoyancy. Int. J. Heat Mass Trans. 44, 525-536], even at marginally high Froude numbers.
Axisymmetric large eddy simulation of a circular source of buoyancy
Axisymmetric large-eddy simulations (LES) of published experiments on vertical mixing above a circular source of buoyancy are performed. Numerical results confirm the existence of a mixing boundary layer just above the buoyancy source (freshwater injected into saline water). As experiments suggested, the calculated normalized shape of the boundary layer seems independent of source size, freshwater injection rate and background solute concentration. This validates a restrictive but numerically efficient assumption of axisymmetric turbulence for the LES calculations. It also lends additional credibility to the theoretical boundary-layer analysis of Epstein and Burelbach [Epstein, M., Burelbach, J.P., 2001. Vertical mixing above a steady circular source of buoyancy. Int. J. Heat Mass Trans. 44, 525-536], even at marginally high Froude numbers.
How did Archimedes discover the law of buoyancy by experiment?
Kuroki, Hidetaka
2016-03-01
After Archimedes and Vitruvius era, for more than 2000 years, it has been believed that the displaced water measurement of golden crown is impossible, and at his Eureka moment, Archimedes discovered the law of buoyancy (Proposition 7 of his principles) and proved the theft of a goldsmith by weighing the golden crown in water. A previous study showed that a small amount of displaced water was able to be measured with enough accuracy by the introduced method. Archimedes measured the weight of displaced water. He did not find the law of buoyancy but rather specific gravity of things at the moment. After which, Archimedes continued to measure the specific gravity of various solids and fluids. Through these measurements, he reached the discovery of the law of buoyancy directly by experiment. In this paper, the process to the discovery of Archimedes' principle (Proposition 5) is presented.
Trapping of swimming microorganisms at lower surfaces by increasing buoyancy.
Jung, Ilyong; Guevorkian, Karine; Valles, James M
2014-11-21
Models suggest that mechanical interactions alone can trap swimming microorganisms at surfaces. Testing them requires a method for varying the mechanical interactions. We tuned contact forces between Paramecia and surfaces in situ by varying their buoyancy with nonuniform magnetic fields. Remarkably, increasing their buoyancy can lead to ∼100% trapping at lower surfaces. A model of Paramecia in surface contact passively responding to external torques quantitatively accounts for the data implying that interactions with a planar surface do not engage their mechanosensing network and illuminating how their trapping differs from other smaller microorganisms. PMID:25479523
EVA Development and Verification Testing at NASA's Neutral Buoyancy Laboratory
Jairala, Juniper C.; Durkin, Robert; Marak, Ralph J.; Sipila, Stepahnie A.; Ney, Zane A.; Parazynski, Scott E.; Thomason, Arthur H.
2012-01-01
As an early step in the preparation for future Extravehicular Activities (EVAs), astronauts perform neutral buoyancy testing to develop and verify EVA hardware and operations. Neutral buoyancy demonstrations at NASA Johnson Space Center's Sonny Carter Training Facility to date have primarily evaluated assembly and maintenance tasks associated with several elements of the International Space Station (ISS). With the retirement of the Shuttle, completion of ISS assembly, and introduction of commercial players for human transportation to space, evaluations at the Neutral Buoyancy Laboratory (NBL) will take on a new focus. Test objectives are selected for their criticality, lack of previous testing, or design changes that justify retesting. Assembly tasks investigated are performed using procedures developed by the flight hardware providers and the Mission Operations Directorate (MOD). Orbital Replacement Unit (ORU) maintenance tasks are performed using a more systematic set of procedures, EVA Concept of Operations for the International Space Station (JSC-33408), also developed by the MOD. This paper describes the requirements and process for performing a neutral buoyancy test, including typical hardware and support equipment requirements, personnel and administrative resource requirements, examples of ISS systems and operations that are evaluated, and typical operational objectives that are evaluated.
An Analytic Model for Buoyancy Resonances in Protoplanetary Disks
Lubow, Stephen H
2014-01-01
Zhu, Stone, and Rafikov (2012) found in 3D shearing box simulations a new form of planet-disk interaction that they attributed to a vertical buoyancy resonance in the disk. We describe an analytic linear model for this interaction. We adopt a simplified model involving azimuthal forcing that produces the resonance and permits an analytic description of its structure. We derive an analytic expression for the buoyancy torque and show that the vertical torque distribution agrees well with results of Athena simulations and a Fourier method for linear numerical calculations carried out with the same forcing. The buoyancy resonance differs from the classic Lindblad and corotation resonances in that the resonance lies along tilted planes. Its width depends on damping effects and is independent of the gas sound speed. The resonance does not excite propagating waves. At a given large azimuthal wavenumber k_y > 1/h (for disk thickness h), the buoyancy resonance exerts a torque over a region that lies radially closer to...
An analytic model for buoyancy resonances in protoplanetary disks
Lubow, Stephen H. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Zhu, Zhaohuan, E-mail: lubow@stsci.edu, E-mail: zhzhu@astro.princeton.edu [Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)
2014-04-10
Zhu et al. found in three-dimensional shearing box simulations a new form of planet-disk interaction that they attributed to a vertical buoyancy resonance in the disk. We describe an analytic linear model for this interaction. We adopt a simplified model involving azimuthal forcing that produces the resonance and permits an analytic description of its structure. We derive an analytic expression for the buoyancy torque and show that the vertical torque distribution agrees well with the results of the Athena simulations and a Fourier method for linear numerical calculations carried out with the same forcing. The buoyancy resonance differs from the classic Lindblad and corotation resonances in that the resonance lies along tilted planes. Its width depends on damping effects and is independent of the gas sound speed. The resonance does not excite propagating waves. At a given large azimuthal wavenumber k{sub y} > h {sup –1} (for disk thickness h), the buoyancy resonance exerts a torque over a region that lies radially closer to the corotation radius than the Lindblad resonance. Because the torque is localized to the region of excitation, it is potentially subject to the effects of nonlinear saturation. In addition, the torque can be reduced by the effects of radiative heat transfer between the resonant region and its surroundings. For each azimuthal wavenumber, the resonance establishes a large scale density wave pattern in a plane within the disk.
The effect of mechanical stirring on buoyancy-driven circulations
Tailleux, Remi
2009-01-01
The theoretical analysis of the energetics of mechanically-stirred horizontal convection for a Boussinesq fluid yields the formula: G(APE) = \\gamma_{mixing} G(KE) + (1+\\gamma_{mixing}) W_{r,laminar} where G(APE) and G(KE) are the work rate done by the buoyancy and mechanical forcing respectively, \\gamma_{mixing} is the mixing efficiency, and W_{r,laminar} is the background rate of increase in gravitational potential energy due to molecular diffusion. The formula shows that mechanical stirring can easily induce a very strong buoyancy-driven overturning cell (meaning a large G(APE)) even for a relatively low mixing efficiency, whereas this is only possible in absence of mechanical stirring if \\gamma_{mixing} >> 1. Moreover, the buoyancy-driven overturning becomes mechanically controlled when $\\gamma_{mixing} G(KE) >> (1+\\gamma_{mixing}) W_{r,laminar}$. This result explains why the buoyancy-driven overturning cell in the laboratory experiments by \\cite{Whitehead2008} is amplified by the lateral motions of a stir...
An Analytic Model for Buoyancy Resonances in Protoplanetary Disks
Lubow, Stephen H.; Zhu, Zhaohuan
2014-04-01
Zhu et al. found in three-dimensional shearing box simulations a new form of planet-disk interaction that they attributed to a vertical buoyancy resonance in the disk. We describe an analytic linear model for this interaction. We adopt a simplified model involving azimuthal forcing that produces the resonance and permits an analytic description of its structure. We derive an analytic expression for the buoyancy torque and show that the vertical torque distribution agrees well with the results of the Athena simulations and a Fourier method for linear numerical calculations carried out with the same forcing. The buoyancy resonance differs from the classic Lindblad and corotation resonances in that the resonance lies along tilted planes. Its width depends on damping effects and is independent of the gas sound speed. The resonance does not excite propagating waves. At a given large azimuthal wavenumber ky > h -1 (for disk thickness h), the buoyancy resonance exerts a torque over a region that lies radially closer to the corotation radius than the Lindblad resonance. Because the torque is localized to the region of excitation, it is potentially subject to the effects of nonlinear saturation. In addition, the torque can be reduced by the effects of radiative heat transfer between the resonant region and its surroundings. For each azimuthal wavenumber, the resonance establishes a large scale density wave pattern in a plane within the disk.
Semi-Empirical Models for Buoyancy-Driven Ventilation
Terpager Andersen, Karl
2015-01-01
A literature study is presented on the theories and models dealing with buoyancy-driven ventilation in rooms. The models are categorised into four types according to how the physical process is conceived: column model, fan model, neutral plane model and pressure model. These models are analysed a...
Using Surface Integrals for Checking Archimedes' Law of Buoyancy
Lima, F. M. S.
2012-01-01
A mathematical derivation of the force exerted by an "inhomogeneous" (i.e. compressible) fluid on the surface of an "arbitrarily shaped" body immersed in it is not found in the literature, which may be attributed to our trust in Archimedes' law of buoyancy. However, this law, also known as Archimedes' principle (AP), does not yield the force…
Longitudinal Modelling of Academic Buoyancy and Motivation: Do the 5Cs Hold Up over Time?
Martin, Andrew J.; Colmar, Susan H.; Davey, Louise A.; Marsh, Herbert W.
2010-01-01
Background: Academic buoyancy is students' ability to successfully deal with setbacks and challenges that are typical of academic life. The present study extends previous preliminary cross-sectional work that tentatively identified five motivational predictors of academic buoyancy--referred to as the "5Cs" of academic buoyancy: confidence…
Buoyancy statistics in moist turbulent Rayleigh-Benard convection
Schumacher, Joerg
2009-01-01
We study shallow moist Rayleigh-Benard convection in the Boussinesq approximation in three-dimensional direct numerical simulations. The thermodynamics of phase changes is approximated by a piecewise linear equation of state close to the phase boundary. The impact of phase changes on the turbulent fluctuations and the transfer of buoyancy through the layer is discussed as a function of the Rayleigh number and the ability to form liquid water. The enhanced buoyancy flux due to phase changes is compared with dry convection reference cases and related to the cloud cover in the convection layer. This study indicates that the moist Rayleigh-Benard problem offers a practical framework for the development and evaluation of parametrizations for atmospheric convection.
Buoyancy Effect on MHD Flow Past a Permeable Bed
S. Venkataramana
1986-10-01
Full Text Available In this paper, the effect of buoyancy force on the parallel flows bounded above by a rigid permeable plate which may be moving or stationary and below, by a permeable bed has been investigated. To discuss the solution, the flow region is divided into two zones. In Zone 1, the flow is laminar and is governed by the Navier-Stokes equations from the impermeable upper rigid plate to the permeable bed. In Zone 2, the flow is governed by the Darcy law in the permeable bed below the nominal surface. The expressions for velocity and temparature distributions, Slip velocity, slip temperature, mass flow rate and the rates of heat transfer coefficients are obtained. The effects of magnetic, porous, slip and buoyancy parameters and Biot number on the above physical quantities are investigated. The thickness of the boundary layer in Zone 2 has been evaluated.
Surface buoyancy flux in Bay of Bengal and Arabian Sea
G. Anitha
2008-03-01
Full Text Available The seasonal variation of thermal, haline, net surface buoyancy flux, the Monin-Obukhov length (M-O length, L and stability parameter, i.e. the ratio of M-O length to mixed layer depth (h were studied in the Bay of Bengal (BoB and the Arabian Sea (AS for the years 2003 and 2004 using Argo temperature and salinity profiles. The relative quantitative influence of winds to surface buoyancy and the applicability of scaling mixed layer using M-O length in BoB and AS was brought out. Rotation and light penetration modify the mixed layer depth from M-O length during shoaling in spring giving L/h<1.
The global Meridional Overturning Circulation's response to variable buoyancy forcing
Butler, Edward D.
2015-01-01
The meridional overturning circulation (MOC) is a large-scale circulation throughout the global ocean and plays a significant role in the complex global climate system. However, our traditional understanding of the processes driving the MOC has been questioned in recent years. In particular, it has been suggested that surface buoyancy forcing plays little energetic role in driving the MOC. Furthermore, doubt has also been cast over the relationship between meridional overturning and meridiona...
Floating rings in vertical soap films : capillary driven bidimensional buoyancy
Adami, N
2013-01-01
The present study aims to investigate the motion of buoyant rings in vertical soap films. Thickness differences and related bi-dimensional densities are considered as the motor leading to bi-dimensional buoyancy. We show how this effect can be re-interpreted thanks to surface tension profiles in soap films. We propose a model involving surface tension profiles in order to describe the motion of buoyant particles in vertical soap films, and compare it to experimental data.
SuperLig Ion Exchange Resin Swelling and Buoyancy Study
The objective of this study was to achieve a fundamental understanding of SuperLig resin swelling and shrinking characteristics, which lead to channeling and early breakthrough during loading cycles. The density of salt solution that causes resin floating was also determined to establish a limit for operation. Specific tests performed include (a) pH dependence, (b) ionic strength dependence and (c) buoyancy effect vs. simulant composition
Investigation on the Performance of the Universal Buoyancy System
Staalesen, Odd Eivind Solø
2014-01-01
Future increase in the market demand for liquefied natural gas (LNG) will pose new challenges to the existing infrastructure for LNG distribution. The Universal Buoyancy System (UBS) seeks to contribute to the solution of these challenges by offering a new, cost-efficient, and safe solution for small scale LNG transfer, as an alternative to land based distribution. A critical component in the UBS is the floating cryogenic pipeline, connecting an onshore LNG terminal to an inshore loading buoy...
Design and Analysis of Typical Buoyancy Tank Riser Tensioner Systems
Zhuang Kang; Lusheng Jia; Liping Sun; Wenzhou Liang
2012-01-01
The method for design and analysis of a buoyancy tank riser tensioner system (BTRTS) was put forward in this paper,taking the free standing hybrid riser's top buoyancy tank as an example.The design procedure was discussed and was also illustrated in a flowchart,after a short description of the global arrangement,structure configuration,and the function of different types of buoyancy tanks (BT).The objective of this paper is to describe a way of developing a BT with minimal hydro force,maximal net lift,and no redundancy of comparunents.The method of determining the main dimensions of the BT,namely the length and the outer diameter,was outlined.A series of investigations was conducted for a West Africa FSHR BT design,and the effect of the ratio of the length to the outer diameter (L/D) on the hydrodynamics and the weight of the BT was discussed.The methodology of designing the internal structure of the BT was presented.The effects of the number of compartments and the dimension of the inner stem on the BT weight and strength were compared.The relationship between inner structure and the number one index of the BT as well as the riser's top tension factor (TTF) were illustrated for normal operating conditions and conditions with one or more compartments (or inner stem) damaged.A design instance was given in this paper,when L/D is 4-6,the BT weight and the drag force are compromised.When the BT is divided into 10 compartments,the riser TTF will reach the maximum value,and the ratio of the stem OD to shell OD is about 0.3.A global strength analysis method of the BT and the main load case matrix was also included in the paper,together with the local strength analysis of the buoyancy tank's pad-eye assembly.
Oscillatory convection in binary mixtures: thermodiffusion, solutal buoyancy, and advection
Jung, D.; Matura, P.; Luecke, M.
2005-01-01
The role of thermodiffusive generation of concentration fluctuations via the Soret effect, their contribution to the buoyancy forces that drive convection, the advective mixing effect of the latter, and the diffusive homogenisation are compared and elucidated for oscillatory convection. Numerically obtained solutions of the field equations in the form of spatially extended relaxed traveling waves, of standing waves, and of the transient growth of standing waves and their transition to traveli...
Neutral Buoyancy Portable Life Support System performance study
Chang, Chi-Min; Conger, Bruce C.; Iovine, John V.
1991-01-01
The Neutral Buoyancy Portable Life Support System (NBPSS) has been designed to support astronaut underwater training activities associated with EVA operations. The performance of competing NBPSS configurations has been analyzed on the basis of a modified 'Metabolic Man' program. NBPSS success is dependent on the development of novel cryogen supply tank and liquid-cooling garment vaporizer. Attention is given to mass and thermal balances and the evaluation results for the vent-loop ejector and heat-exchanger designs.
Buoyancy waves in Pluto's high atmosphere: Implications for stellar occultations
Hubbard, W B; Kulesa, C A; Benecchi, S D; Person, M J; Elliot, J L; Gulbis, A A S
2009-01-01
We apply scintillation theory to stellar signal fluctuations in the high-resolution, high signal/noise, dual-wavelength data from the MMT observation of the 2007 March 18 occultation of P445.3 by Pluto. A well-defined high wavenumber cutoff in the fluctuations is consistent with viscous-thermal dissipation of buoyancy waves (internal gravity waves) in Pluto's high atmosphere, and provides strong evidence that the underlying density fluctuations are governed by the gravity-wave dispersion relation.
Compositional vs. thermal buoyancy and the evolution of subducted lithosphere
Gaherty, James B.; Hager, Bradford H.
1994-01-01
We formulate 2-D Cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle. Subducted lithosphere is represented as a cold, stiff, layered composite of denser eclogite underlain by more buoyant harzburgite. Slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle. Approximately a factor of two thickening occurs via pure shear just above the discontinuity, with additional enhancement due to folding by over a factor of two. No separation of the individual slab components occurs at the discontinuity, and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy. These results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle.
Buoyancy Effects in Strongly-Pulsed, Turbulent Diffusion Flames
Hermanson, James; Johari, Hamid; Stocker, Dennis; Hegde, Uday
2004-11-01
Buoyancy effects in pulsed, turbulent flames are studied in microgravity in a 2.2 s drop-tower. The fuel is pure ethylene or a 50/50 mixture with nitrogen; the oxidizer co-flow is either air or 30% oxygen in nitrogen. A fast solenoid valve fully modulates (shuts off) the fuel flow between pulses. The jet Reynolds number is 5000 with a nozzle i.d. of 2 mm. For short injection times and small duty cycle (jet-on fraction), compact, puff-like flames occur. The invariance in flame length of these puffs with buoyancy is due to offsetting changes in puff celerity and burnout time. Buoyancy does impact interacting flame puffs, with the flame length generally increasing with injection duty cycle. The mean centerline temperatures for all flames are generally higher in microgravity than in normal gravity. The transition in temperatures with increasing injection time is more gradual in micro-g than in 1-g. These observations can be explained in terms of the local duty cycle in the flame and differences in entrainment in normal- vs. microgravity.
Buoyancy Effects in Turbulent Jet Flames in Crossflow
Boxx, Isaac; Idicheria, Cherian; Clemens, Noel
2003-11-01
The aim of this study is to investigate the effects of buoyancy on the structure of turbulent, non-premixed hydrocarbon jet-flames in crossflow (JFICF). This was accomplished using a small jet-in-crossflow facility which can be oriented at a variety of angles with respect to the gravity vector. This facility enables us to alter the relative influence of buoyancy on the JFICF without altering the jet-exit Reynolds number, momentum flux ratio or the geometry of the system. Results are compared to similar, but non-buoyant, JFICF studied in microgravity. Departures of jet-centerline trajectory from the well-known power-law scaling of turbulent JFICF were used to explore the transition from a buoyancy-influenced regime to a momentum dominated one. The primary diagnostic was CCD imaging of soot-luminosity. We present results on ethylene jet flames with jet-exit Reynolds numbers of 1770 to 8000 and momentum flux ratios of 5 to 13.
Computational study of buoyancy effects in a laminar starting jet
Vortical structures formed in evolving jets are important in applications such as fuel injection in diesel engines and fuel leaks. When the jet fluid is different from the ambient fluid, the buoyancy can play an important role in determining the jet flow structure, and hence, the entrainment and fluid mixing processes. In the present study, a jet of helium injected in air is investigated, with emphasis placed on delineating the buoyancy effects on vector-scalar fields during the starting phase. We utilize a computational model, previously validated to predict the flow field of low-density gas jets. The model incorporates finite volume approach to solve the transport equation of helium mass fraction coupled with conservation equations of mixture mass and momentum. Computations were performed for a laminar jet to characterize the advancing jet front, and to capture the formation and propagation of vortex rings and the related pinch-off process. Results show significant effects of buoyancy on jet advancement, as well as on vorticity and helium concentration in the core of the vortex rings
Effect of the radial buoyancy on a circular Couette flow
Meyer, Antoine; Yoshikawa, Harunori N.; Mutabazi, Innocent
2015-11-01
The effect of a radial temperature gradient on the stability of a circular Couette flow is investigated when the gravitational acceleration is neglected. The induced radial stratification of the fluid density coupled with the centrifugal acceleration generates radial buoyancy which is centrifugal for inward heating and centripetal for outward heating. This radial buoyancy modifies the Rayleigh discriminant and induces the asymmetry between inward heating and outward heating in flow behavior. The critical modes are axisymmetric and stationary for inward heating while for outward heating, they can be oscillatory axisymmetric or nonaxisymmetric depending on fluid diffusion properties, i.e., on the Prandtl number Pr. The dependence of the critical modes on Pr is explored for different values of the radius ratio of the annulus. The power input of the radial buoyancy is compared with other power terms. The critical frequency of the oscillatory axisymmetric modes is linked to the Brunt-Väisälä frequency due to the density stratification in the radial gravity field induced by the rotation. These modes are associated with inertial waves. The dispersion relation of the oscillatory axisymmetric modes is derived in the vicinity of the critical conditions. A weakly nonlinear amplitude equation with a forcing term is proposed to explain the domination of these axisymmetric oscillatory modes over the stationary centrifugal mode.
Buoyancy, transport, and head loss of fibrous reactor insulation
In the event of a loss of coolant accident (LOCA) in a nuclear power plant, it is possible that insulation for pipes or other items inside the containment building could be dislodged by the high energy break jet. This insulation debris could affect the recirculation of water from the sump of the emergency core cooling system (ECCS) by collecting on the screen surrounding this sump. To help in assessing the possible effect of detached insulation on the ECCS, buoyancy, transport, and head loss characteristics of the insulation were studied experimentally. Three types of insulation pillows with mineral wool and fiberglass cores were tested in undamaged state, with their covers opened and with the insulation core in broken-up and shredded conditions. Small samples of reflective metallic and closed cell insulations were also tested for transport and buoyancy. The buoyancy studies revealed that the insulation core material sank more rapidly in hot water than in cold, that the tested mineral wool insulation did not readily sink, but that the fiberglass insulation did and that undamaged pillows could remain afloat for several days due to trapped air pockets forming inside the pillow covers. The transport studies furnished values of the flow velocity needed (i) to initiate the movement of sunken insulation towards a screen, (ii) to bring all the insulation on the screen, and (iii) to flip the insulation vertically on the screen. The head loss studies resulted in graphs of head loss versus approach velocity for undisturbed insulation and for beds of accumulated insulation fragments
Effects of Buoyancy in Hydrogen Jet Diffusion Flames
Agrawal, A. K.; Al-Ammar, K.; Gollahalli, S. R.; Griffin, D. W.
1999-01-01
This project was carried out to understand the effects of heat release and buoyancy on the flame structure of diffusion flames. Experiments were conducted at atmospheric pressure in both normal gravity and microgravity conditions in the NASA LeRC 2.2 s drop tower. Experiments were also conducted in a variable pressure combustion facility in normal gravity to scale buoyancy and thus, to supplement the drop tower experiments. Pure H2 or H2 mixed with He was used as the jet fluid to avoid the complexities associated with soot formation. Fuel jet burning in quiescent air was visualized and quantified by the Rainbow Schlieren Deflectometry (RSD) to obtain scalar profiles (temperature, oxygen concentration) within the flame. Burner tube diameter (d) was varied from 0.3 to 1.19 mm producing jet exit Reynolds numbers ranging from 40 to 1900, and generating flames encompassing laminar and transitional (laminar to turbulent) flow structure. Some experiments were also complemented with the CFD analysis. In a previous paper, we have presented details of the RSD technique, comparison of computed and measured scalar distributions, and effects of buoyancy on laminar and transitional H2 gas-jet diffusion flames. Results obtained from the RSD technique, variable pressure combustion chamber, and theoretical models have been published. Subsequently, we have developed a new drop rig with improved optical and image acquisition. In this set up, the schlieren images are acquired in real time and stored digitally in RAM of an onboard computer. This paper deals with laminar diffusion flames of pure H2 in normal and microgravity.
A continuous and prognostic convection scheme based on buoyancy, PCMT
Guérémy, Jean-François; Piriou, Jean-Marcel
2016-04-01
A new and consistent convection scheme (PCMT: Prognostic Condensates Microphysics and Transport), providing a continuous and prognostic treatment of this atmospheric process, is described. The main concept ensuring the consistency of the whole system is the buoyancy, key element of any vertical motion. The buoyancy constitutes the forcing term of the convective vertical velocity, which is then used to define the triggering condition, the mass flux, and the rates of entrainment-detrainment. The buoyancy is also used in its vertically integrated form (CAPE) to determine the closure condition. The continuous treatment of convection, from dry thermals to deep precipitating convection, is achieved with the help of a continuous formulation of the entrainment-detrainment rates (depending on the convective vertical velocity) and of the CAPE relaxation time (depending on the convective over-turning time). The convective tendencies are directly expressed in terms of condensation and transport. Finally, the convective vertical velocity and condensates are fully prognostic, the latter being treated using the same microphysics scheme as for the resolved condensates but considering the convective environment. A Single Column Model (SCM) validation of this scheme is shown, allowing detailed comparisons with observed and explicitly simulated data. Four cases covering the convective spectrum are considered: over ocean, sensitivity to environmental moisture (S. Derbyshire) non precipitating shallow convection to deep precipitating convection, trade wind shallow convection (BOMEX) and strato-cumulus (FIRE), together with an entire continental diurnal cycle of convection (ARM). The emphasis is put on the characteristics of the scheme which enable a continuous treatment of convection. Then, a 3D LAM validation is presented considering an AMMA case with both observations and a CRM simulation using the same initial and lateral conditions as for the parameterized one. Finally, global
Buoyancy waves in Pluto's high atmosphere: Implications for stellar occultations
Hubbard, W. B.; McCarthy, D. W.; Kulesa, C. A.; Benecchi, S. D.; Person, M. J.; Elliot, J. L.; Gulbis, A.A.S.
2009-01-01
We apply scintillation theory to stellar signal fluctuations in the high-resolution, high signal/noise, dual-wavelength data from the MMT observation of the 2007 March 18 occultation of P445.3 by Pluto. A well-defined high wavenumber cutoff in the fluctuations is consistent with viscous-thermal dissipation of buoyancy waves (internal gravity waves) in Pluto’s high atmosphere, and provides strong evidence that the underlying density fluctuations are governed by the gravity-wave dispersion rela...
The effect of mechanical stirring on buoyancy-driven circulations
Tailleux, Remi; Rouleau, Lucie
2009-01-01
The theoretical analysis of the energetics of mechanically-stirred horizontal convection for a Boussinesq fluid yields the formula: G(APE) = \\gamma_{mixing} G(KE) + (1+\\gamma_{mixing}) W_{r,laminar} where G(APE) and G(KE) are the work rate done by the buoyancy and mechanical forcing respectively, \\gamma_{mixing} is the mixing efficiency, and W_{r,laminar} is the background rate of increase in gravitational potential energy due to molecular diffusion. The formula shows that mechanical stirring...
Buoyancy Effect on MHD Flow Past a Permeable Bed
S. Venkataramana; D. Bathaiah
1986-01-01
In this paper, the effect of buoyancy force on the parallel flows bounded above by a rigid permeable plate which may be moving or stationary and below, by a permeable bed has been investigated. To discuss the solution, the flow region is divided into two zones. In Zone 1, the flow is laminar and is governed by the Navier-Stokes equations from the impermeable upper rigid plate to the permeable bed. In Zone 2, the flow is governed by the Darcy law in the permeable bed below the nominal surface....
Alpha effect due to buoyancy instability of a magnetic layer
Chatterjee, Piyali; Mitra, Dhrubaditya; Rheinhardt, Matthias; Brandenburg, Axel
2010-01-01
A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. We calculate the alpha effect due to helical motions caused by a disintegrating magnetic layer in a rotating density-stratified system with angular velocity Omega making an angle theta with the vertical. We also study the dependence of the alpha effect on theta and the strength of the ini...
The Evolution of a Double Diffusive Magnetic Buoyancy Instability
Silvers, Lara J; Brummell, Nicholas H; Proctor, Michael R E
2011-01-01
Recently, Silvers, Vasil, Brummell, & Proctor (2009), using numerical simulations, confirmed the existence of a double diffusive magnetic buoyancy instability of a layer of horizontal magnetic field produced by the interaction of a shear velocity field with a weak vertical field. Here, we demonstrate the longer term nonlinear evolution of such an instability in the simulations. We find that a quasi two-dimensional interchange instability rides (or "surfs") on the growing shear-induced background downstream field gradients. The region of activity expands since three-dimensional perturbations remain unstable in the wake of this upward-moving activity front, and so the three-dimensional nature becomes more noticeable with time.
The Evolution of a Double Diffusive Magnetic Buoyancy Instability
Silvers, Lara J.; Vasil, Geoffrey M.; Brummell, Nicholas H.; Proctor, Michael R. E.
2011-08-01
Recently, Silvers et al. (2009b), using numerical simulations, confirmed the existence of a double diffusive magnetic buoyancy instability of a layer of horizontal magnetic field produced by the interaction of a shear velocity field with a weak vertical field. Here, we demonstrate the longer term nonlinear evolution of such an instability in the simulations. We find that a quasi two-dimensional interchange instability rides (or ``surfs'') on the growing shear-induced background downstream field gradients. The region of activity expands since three-dimensional perturbations remain unstable in the wake of this upward-moving activity front, and so the three-dimensional nature becomes more noticeable with time.
Non-Gaussian buoyancy statistics in fingering convection
von Hardenberg, Jost
2010-01-01
We examine the statistics of active scalar fluctuations in high-Rayleigh number fingering convection with high-resolution three-dimensional numerical experiments. The one-point distribution of buoyancy fluctuations is found to present significantly non-Gaussian tails. A modified theory based on an original approach by Yakhot (1989) is used to model the active scalar distributions as a function of the conditional expectation values of scalar dissipation and fluxes in the flow. Simple models for these two quantities highlight the role of blob-like coherent structures for scalar statistics in fingering convection.
An approach to modelling of intermittent buoyancy induced flow
Spencer, J. [Dept. of Engineering Physics, McMaster Univ., Hamilton, Ontario (Canada)], E-mail: spencejh@mcmaster.ca
2009-07-01
In the event of a loss of forced circulation while at low power, heat sinks may be maintained by natural circulation phenomena including Intermittent Buoyancy Induced Flow (IBIF). The sensitivity of IBIF to a variety of parameters is an important consideration in determining its effectiveness as a cooling mechanism and in planning heat sinks. Although there has been some work done on IBIF in industry, there are no publicly available models of IBIF and very limited material available in open literature. This paper presents an outline of a proposed best-estimate IBIF model, to be used for uncertainty analysis. (author)
Non-Gaussian buoyancy statistics in fingering convection
Hardenberg, Jost von, E-mail: j.vonhardenberg@isac.cnr.i [Institute of Atmospheric Sciences and Climate, CNR, Torino (Italy); Paparella, Francesco, E-mail: francesco.paparella@unisalento.i [Dip. di Matematica, Universita del Salento and CMCC, Lecce (Italy)
2010-06-07
We examine the statistics of active scalar fluctuations in high-Rayleigh number fingering convection with high-resolution three-dimensional numerical experiments. Marked non-Gaussian tails are found in the one-point distribution of buoyancy fluctuations. A modified theory based on an original approach by Yakhot (1989) is used to model the active scalar distributions as a function of the conditional expectation values of scalar dissipation and fluxes in the flow. Simple models for these two quantities highlight the role of blob-like coherent structures for scalar statistics in fingering convection.
An approach to modelling of intermittent buoyancy induced flow
In the event of a loss of forced circulation while at low power, heat sinks may be maintained by natural circulation phenomena including Intermittent Buoyancy Induced Flow (IBIF). The sensitivity of IBIF to a variety of parameters is an important consideration in determining its effectiveness as a cooling mechanism and in planning heat sinks. Although there has been some work done on IBIF in industry, there are no publicly available models of IBIF and very limited material available in open literature. This paper presents an outline of a proposed best-estimate IBIF model, to be used for uncertainty analysis. (author)
Modeling the Buoyancy System of a Wave Energy Power Plant
Pedersen, Tom S.; Nielsen, Kirsten M.
2009-01-01
A nonlinear dynamic model of the buoyancy system in a wave energy power plant is presented. The plant ("Wave Dragon") is a floating device using the potential energy in overtopping waves to produce power. A water reservoir is placed on top of the WD, and hydro turbines lead the water to the sea...... producing electrical power. Through air chambers it is possible to control the level of the WD. It is important to control the level in order to maximize the power production in proportion to the wave height, here the amount of overtopping water and the amount of potential energy is conflicting. The...
Using surface integrals for checking the Archimedes' law of buoyancy
Lima, F. M. S.
2011-01-01
A mathematical derivation of the force exerted by an \\emph{inhomogeneous} (i.e., compressible) fluid on the surface of an \\emph{arbitrarily-shaped} body immersed in it is not found in literature, which may be attributed to our trust on Archimedes' law of buoyancy. However, this law, also known as Archimedes' principle (AP), does not yield the force observed when the body is in contact to the container walls, as is more evident in the case of a block immersed in a liquid and in contact to the ...
Buoyancy Effects on Flow Transition in Low-Density Inertial Gas Jets
Pasumarthi, Kasyap S.; Agrawal, Ajay K.
2005-01-01
Effects of buoyancy on transition from laminar to turbulent flow are presented for momentum-dominated helium jet injected into ambient air. The buoyancy was varied in a 2.2-sec drop tower facility without affecting the remaining operating parameters. The jet flow in Earth gravity and microgravity was visualized using the rainbow schlieren deflectometry apparatus. Results show significant changes in the flow structure and transition behavior in the absence of buoyancy.
Unexpected Positive Buoyancy in Deep Sea Sharks, Hexanchus griseus, and a Echinorhinus cookei
Itsumi Nakamura; Meyer, Carl G.; Katsufumi Sato
2015-01-01
We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Sharks, for example, rely on oil-filled livers instead of gas-filled swim bladders to increase their buoyancy, but are nonetheless ubiquitously regarded as either negatively or neutrally buoyant. Deep-sea sharks have particularly large, oil-filled livers, and are believed to be neutrally buoyant in their natural habitat, but this has never been confirmed. To empirically determine the buoyancy status of two specie...
Buoyancy Effects upon Vapor Flame and Explosion Processes
Edelman, R. B.; Harsha, P. T.
1985-01-01
The objective of this microgravity project is to develop an experimental and theoretical analyses critical to the understanding of the coupling of buoyancy and turbulence generation and its effect on fuel-air mixing, flame intensity and flame propagation in jet diffusion flames. The experiment is designed to examine certain effects of buoyancy acting on a diffusion flame in which the flame is directed either upward or downward. This change from negative to positive g is observed to significantly alter the flame shape although all other operating conditions are the same for both configurations. However, to perform this experiment a significant coaxial secondary air flow is needed in order to prevent flow reversal when the flame is inverted. The theoretical analysis that has been developed handles the secondary air flow and the extreme change in gravity vector direction. Thus the data will provide a measure of credibility of the analysis which will then be used to assist in the design of the actual zero-g experiment.
Response of mantle transition zone thickness to plume buoyancy flux
Das Sharma, S.; Ramesh, D. S.; Li, X.; Yuan, X.; Sreenivas, B.; Kind, R.
2010-01-01
The debate concerning thermal plumes in the Earth's mantle, their geophysical detection and depth characterization remains contentious. Available geophysical, petrological and geochemical evidence is at variance regarding the very existence of mantle plumes. Utilizing P-to-S converted seismic waves (P receiver functions) from the 410 and 660 km discontinuities, we investigate disposition of these boundaries beneath a number of prominent hotspot regions. The thickness of the mantle transition zone (MTZ), measured as P660s-P410s differential times (tMTZ), is determined. Our analyses suggest that the MTZ thickness beneath some hotspots correlates with the plume strength. The relationship between tMTZ, in response to the thermal perturbation, and the strength of plumes, as buoyancy flux B, follows a power law. This B-tMTZ behavior provides unprecedented insights into the relation of buoyancy flux and excess temperature at 410-660 km depth below hotspots. We find that the strongest hotspots, which are located in the Pacific, are indeed plumes originating at the MTZ or deeper. According to the detected power law, even the strongest plumes may not shrink the transition zone by significantly more than ~40 km (corresponding to a maximum of 300-400° excess temperature).
Equilibrium models of coronal loops that involve curvature and buoyancy
Hindman, Bradley W. [JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0440 (United States); Jain, Rekha, E-mail: hindman@solarz.colorado.edu [School of Mathematics and Statistics, University of Sheffield, Sheffield S3 7RH (United Kingdom)
2013-12-01
We construct magnetostatic models of coronal loops in which the thermodynamics of the loop is fully consistent with the shape and geometry of the loop. This is achieved by treating the loop as a thin, compact, magnetic fibril that is a small departure from a force-free state. The density along the loop is related to the loop's curvature by requiring that the Lorentz force arising from this deviation is balanced by buoyancy. This equilibrium, coupled with hydrostatic balance and the ideal gas law, then connects the temperature of the loop with the curvature of the loop without resorting to a detailed treatment of heating and cooling. We present two example solutions: one with a spatially invariant magnetic Bond number (the dimensionless ratio of buoyancy to Lorentz forces) and the other with a constant radius of the curvature of the loop's axis. We find that the density and temperature profiles are quite sensitive to curvature variations along the loop, even for loops with similar aspect ratios.
On the general concept of buoyancy in sedimentation and ultracentrifugation.
Piazza, Roberto; Buzzaccaro, Stefano; Secchi, Eleonora; Parola, Alberto
2013-08-01
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. PMID:23913160
Experimental studies and CFD calculations for buoyancy driven mixing phenomena
Silva, Marco Jose da, E-mail: M.dasilva@fzd.d [Institute of Safety Research, Forschungszentrum Dresden-Rossendorf e.V., Dresden (Germany); Thiele, Sebastian; Hoehne, Thomas; Vaibar, Roman; Hampel, Uwe [Institute of Safety Research, Forschungszentrum Dresden-Rossendorf e.V., Dresden (Germany)
2010-09-15
In nuclear reactor safety the mixing of borated and deborated water is a critical issue that needs investigation, assessment and prediction. Such mixing is buoyancy driven and numerical codes must correctly model momentum transfer between fluids of different density. To assess and develop CFD models for buoyancy driven mixing we set up a simple vertical mixing test facility (VeMix) and equipped it with a newly developed planar electrical imaging sensor. This imaging sensor acquires conductivity images of the liquid at the rear channel wall with a speed of 2,500 frames/s. By adding NaCl tracer to the denser fluid we were able to visualize the mixing process in high spatial and temporal detail. Furthermore, an image processing algorithm based on the optical flow concept was implemented and tested which allows the measurement of flow pattern velocities. Selected experiments at different Richardson numbers were run with two components of different density (pure water and glucose-water mixture) simulating borated and deborated water in a light water reactor scenario. These experiments were compared to CFD calculations using standard turbulence models. Good agreement between experimental data and CFD simulations was found.
Equilibrium Models of Coronal Loops That Involve Curvature and Buoyancy
Hindman, Bradley W.; Jain, Rekha
2013-12-01
We construct magnetostatic models of coronal loops in which the thermodynamics of the loop is fully consistent with the shape and geometry of the loop. This is achieved by treating the loop as a thin, compact, magnetic fibril that is a small departure from a force-free state. The density along the loop is related to the loop's curvature by requiring that the Lorentz force arising from this deviation is balanced by buoyancy. This equilibrium, coupled with hydrostatic balance and the ideal gas law, then connects the temperature of the loop with the curvature of the loop without resorting to a detailed treatment of heating and cooling. We present two example solutions: one with a spatially invariant magnetic Bond number (the dimensionless ratio of buoyancy to Lorentz forces) and the other with a constant radius of the curvature of the loop's axis. We find that the density and temperature profiles are quite sensitive to curvature variations along the loop, even for loops with similar aspect ratios.
Equilibrium models of coronal loops that involve curvature and buoyancy
Hindman, Bradley W
2013-01-01
We construct magnetostatic models of coronal loops in which the thermodynamics of the loop is fully consistent with the shape and geometry of the loop. This is achieved by treating the loop as a thin, compact, magnetic fibril that is a small departure from a force-free state. The density along the loop is related to the loop's curvature by requiring that the Lorentz force arising from this deviation is balanced by buoyancy. This equilibrium, coupled with hydrostatic balance and the ideal gas law, then connects the temperature of the loop with the curvature of the loop without resorting to a detailed treatment of heating and cooling. We present two example solutions: one with a spatially invariant magnetic Bond number (the dimensionless ratio of buoyancy to Lorentz forces) and the other with a constant radius of curvature of the loop's axis. We find that the density and temperature profiles are quite sensitive to curvature variations along the loop, even for loops with similar aspect ratios.
Buoyancy Wave Interaction with Critical Levels in the Atmosphere
Rõõm, Rein; Zirk, Marko
2013-04-01
General non-linear internal buoyancy wave equation (BWE) is developed, which has the second (differential) order in space but the third order in time. The wave operator splitting method is then applied to the stationary BWE to get the orographically generated buoyancy waves in the thermally stratified atmosphere for altitude-variable wind conditions. The splitting method is further generalized to a critical level (CL) containing atmosphere. The CLs, which appear in the atmosphere (including the planetary boundary layer) if the wind weakens and changes direction or rotates with the altitude, will provide a break-up of the atmosphere to the regular (for BWE) layers separated by singular critical levels at which the differential order of the BWE is lowered. This lowering will cause either partial or full reflection of waves, though in the special fast wind altering case, the CL can prove transparent, too. The wave modelling examples for different CLs are provided. Classification of the wind situations with respect to the various reflection-transparency types of CL is vital for wave stress (vertical flux of mean horizontal momentum) and upper-level wave breaking study. ------------------------------------------------------------------ This research is supported by Estonian Science Foundation Grant 9134 and Estonian Research Council TF Project SF0180038s08.
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. (paper)
Mechanical power input from buoyancy and wind to the circulation in an ocean model
Saenz, J. A.; Hogg, A. M.; Hughes, G. O.; Griffiths, R. W.
2012-07-01
We make a systematic quantitative comparison of the effects that surface buoyancy forcing and wind stress have on the energy balance of an idealized, rotating, pole-to-pole ocean model with a zonally re-entrant channel in the south, forced by realistic heat (buoyancy) fluxes and wind stresses representative of global climatology. Surface buoyancy fluxes and wind stress forcing are varied independently; both have significant effects on the reservoirs of various forms of energy and the rates of transfer between them. Importantly, we show for the first time that in the ocean, each power input has a positive feedback on the other. Changes in the rate of generation of available potential energy by buoyancy fluxes at the surface lead to similar changes in the rate of conversion of potential energy to kinetic energy by buoyancy forces (sinking) in the interior, and to changes in the rate of generation of kinetic energy by wind stress. Conversely, changes in the rate of generation of kinetic energy by wind stress lead to changes in the rate of generation of available potential energy by buoyancy forcing. We discuss how this feedback is mediated by the circumpolar current, and processes involving buoyancy, mixing and geostrophic balances. Our results support the notion that surface buoyancy forcing, along with wind and tidal forcing, plays an active role in the energy balance of the oceans. The overturning circulation in the oceans is not the result of a single driving force. Rather, it is a manifestation of a complex and subtle balance.
Field evidence for buoyancy-driven water flow in a Sphagnum dominated peat bog
Adema, E.B.; Baaijens, G. J.; van Belle, J.; Rappoldt, C.; Grootjans, A. P.; Smolders, A. J. P.
2006-01-01
Nocturnal buoyancy-driven water flow in bogs is proposed as a mechanism to replenish the nutrient availability in the top of the acrotelm. In an earlier paper, we provided evidence for buoyancy-driven water flow on theoretical and experimental grounds. In this paper, field evidence is given for the
Effects of Buoyancy on the Flowfields of Lean Premixed Turbulent V-Flames
Cheng, R. K.; Bedat, B.; Yegian, D. T.; Greenberg, P.
1999-01-01
Open laboratory turbulent flames used for investigating fundamental flame turbulence interactions are greatly affected by buoyancy. Though much of our current knowledge is based on observations made in open flames, buoyancy effects are usually not considered in data interpretation, numerical analysis or theories. This inconsistency remains an obstacle to merging experimental observations and theoretical predictions. To better understanding the effects of buoyancy, our research focuses on steady lean premixed flames propagating in fully developed turbulence. We hypothesize that the most significant role of buoyancy forces on these flames is to influence their flowfields through a coupling with the mean and the fluctuating pressure fields. This coupling relates to the elliptical problem that emphasizes the importance of the upstream, wall and downstream boundary conditions in determining all aspects of flame propagation. Therefore, buoyancy has the same significance as other parameters such as flow configuration, and flame geometry.
Statistical Change Detection for Diagnosis of Buoyancy Element Defects on Moored Floating Vessels
Blanke, Mogens; Fang, Shaoji; Galeazzi, Roberto; Leira, Bernt J.
Floating platforms with mooring systems are used extensively in off-shore operations. Part of the mooring systems are underwater buoyancy elements that are attached to the mooring lines. Loss or damage of a buoyancy element is invisible but changes the characteristics of the mooring system and...... alters its ability to provide the necessary responses to withstand loads from weather. Damage of a buoyancy element increases the operation risk and could even cause abortion during an oil-offloading. The objective of this paper is to diagnose the loss of a buoyancy element using diagnostic methods....... After residual generation, statistical change detection scheme is derived from mathematical models supported by experimental data. To experimentally verify loss of an underwater buoyancy element, an underwater line breaker is designed to create realistic replication of abrupt faults. The paper analyses...
Bivariate conditional sampling of buoyancy flux during an intense cold-air outbreak
Chou, Shu-Hsien; Zimmerman, Jeffrey
1989-01-01
The joint frequency distribution technique was used to analyze buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime noted during the Genesis of Atlantic Lows Experiment. It is found that for the lower half of the MABL, the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top. If the buoyancy flux is primarily driven by the temperature flux, these buoyancy-flux generating processes should be the same for the lower boundary layers over land and ocean. The results of the scale analysis of the buoyancy flux agree well with those obtained for mesoscale cellular convection during the Air-Mass Transformation Experiment.
Magma zonation - Effects of chemical buoyancy and diffusion
Spera, Frank J.; Oldenburg, Curtis M.; Yuen, David A.
1989-01-01
Numerical simulations and scale analysis are used to assess the viability of the marginal box-filling mechanism for producing compositional zonation in magma bodies. Scale analysis and two-dimensional numerical experiments both show that box-filling occurs provided a critical ratio of compositional-to-thermal buoyancy is exceeded. This critical ratio depends on the ratio of thermal-to-chemical diffusivity; application of this result to magma bodies suggests that box-filling may occur for components with relatively high-chemical diffusivities such as water. However, box-filling will not produce significant zonation for components with small chemical diffusivities, such as silica, unless diffusive coupling increases silica diffusivity.
The mean electromotive force resulting from magnetic buoyancy instability
Davies, C R
2010-01-01
Motivated both by considerations of the generation of large-scale astrophysical magnetic fields and by potential problems with mean magnetic field generation by turbulent convection, we investigate the mean electromotive force (emf) resulting from the magnetic buoyancy instability of a rotating layer of stratified magnetic field, considering both unidirectional and sheared fields. We discuss why the traditional decomposition into $\\alpha$ and $\\beta$ effects is inappropriate in this case, and that it is only consideration of the entire mean emf that is meaningful. By considering a weighted average of the unstable linear eigenmodes, and averaging over the horizontal plane, we obtain depth-dependent emfs. For the simplified case of isothermal, ideal MHD we are able to obtain an analytic expression for the emf; more generally the emf has to be determined numerically. We calculate how the emf depends on the various parameters of the problem, particularly the rotation rate and the latitude of the magnetic layer.
Buoyancy Instabilities in a Weakly Collisional Intracluster Medium
Kunz, Matthew W; Reynolds, Christopher S; Stone, James M
2012-01-01
The intracluster medium of galaxy clusters is a weakly collisional, high-beta plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign, the magnetothermal instability (MTI) in the outskirts of non-isothermal clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We employ the Athena MHD code to investigate the nonlinear evolution of these instabilities, self-consistently including the effects of anisotropic viscosity (i.e. Braginskii pressure anisotropy), anisotropic conduction, and radiative cooling. We highlight the importance of the microscale instabilities that inevitably accompany and regulate the pressure anisotropies generated by the HBI and MTI. We find that, in all but the innermost regions of cool-core clusters, anisotropic viscosity significantly impairs the ability of the HBI to reorient magnetic-field lines orthogonal...
The Mean Electromotive Force Resulting from Magnetic Buoyancy Instability
Davies, C. R.; Hughes, D. W.
2011-02-01
Motivated both by considerations of the generation of large-scale astrophysical magnetic fields and by potential problems with mean magnetic field generation by turbulent convection, we investigate the mean electromotive force (emf) resulting from the magnetic buoyancy instability of a rotating layer of stratified magnetic field, considering both unidirectional and sheared fields. We discuss why the traditional decomposition into α and β effects is inappropriate in this case, and that it is only consideration of the entire mean emf that is meaningful. By considering a weighted average of the unstable linear eigenmodes, and averaging over the horizontal plane, we obtain depth-dependent emfs. For the simplified case of isothermal, ideal MHD, we are able to obtain an analytic expression for the emf; more generally, the emf has to be determined numerically. We calculate how the emf depends on the various parameters of the problem, particularly the rotation rate and the latitude of the magnetic layer.
Prediction of turbulent cavity flow driven by buoyancy and shear
A finite-difference numerical procedure, incorporating a buoyant of the k-turbulence model, is employed to compute turbulent, mixed convection in a square cavity for Reynolds number Re=104 to 2x105, and Archimedes number Ar=0 to 0.4. For Ar>0.04 stable advancement of the solution procedure is obtained only through the use of an 'inertial relaxation' method. The calculated flow, temperature, and turbulence fields are presented and compared with available experimental data. At high Re and low Ar the predicted heat transfer agrees with the correlation Nu=1.16 Resup(0.5). When buoyancy is pronounced the computed Nu is some 15 per cent higher than the data; this is suspected to be due to the turbulence model, and possible improvements to the model are suggested. (U.K.)
Buoyancy-driven mixing of fluids in a confined geometry
The present work based on Direct Numerical Simulations is devoted to the study of mixing between two miscible fluids of different densities. The movement of these fluids is induced by buoyancy. Three geometries are considered: a cylindrical tube, a square channel and a plane two-dimensional flow. For cylindrical tubes, the results of numerical simulations fully confirm previous experimental findings by Seon et al., especially regarding the existence of three different flow regimes, depending on the tilt angle. The comparison of the various geometries shows that tridimensional flows in tubes or channels are similar, whereas the two-dimensional model fails to give reliable information about real 3D flows, either from a quantitative point of view or for a phenomenological understanding. A peculiar attention is put on a joint analysis of the concentration and vorticity fields and allows us to explain several subtle aspects of the mixing dynamics. (author)
Buoyancy and Pressure Effects on Bulk Metal-Oxygen Reactions
Abbud-Madrid, A.; McKnight, C.; Branch, M. C.; Daily, J. W.; Friedman, R. (Technical Monitor)
1998-01-01
The combustion behavior of metal-oxygen reactions if a weakly buoyant environment is studied to understand the rate-controlling mechanisms in the homogeneous and heterogeneous combustion of bulk metals. Cylindrical titanium and magnesium specimens are ignited in pure-oxygen at pressures ranging from 0.1 to 4.0 atm. Reduced gravity is obtained from an aircraft flying parabolic trajectories. A weakly buoyant environment is generated at low pressures under normal gravity and also at 1 atm under reduced gravity (0.01g). The similarity between these two experimental conditions comes from the p(exp 2)g buoyancy scale extracted from the Grashof number. Lower propagation rates of the molten interface on titanium samples are found at progressively lower pressures at 1 g. These rates are compared to theoretical results from heat conduction analyses with a diffusion/convection controlled reaction. The close agreement found between experimental and theoretical values indicate the importance values indicate the importance of natural convection enhanced oxygen transport on combustion rates. For magnesium, progressively longer burning times are experienced at lower pressures and 1 g. Under reduced gravity conditions at 1 atm, a burning time twice as long as in 1 g is exhibited. However, in this case, the validity of the p(exp 2)g buoyancy scale remains untested due to the inability to obtain steady gas-phase burning of the magnesium sample at 0.1 atm. Nevertheless, longer burning times and larger flame standoff distance at low pressures and at low gravity points to a diffusion/convection controlled reaction.
Buoyancy driven flow within an inclined elliptic enclosure
Mahfouz, F.M. [Mechanical Engineering Department, UET, Taxila (Pakistan)
2011-10-15
Buoyancy driven flow and associated heat convection in an elliptical enclosure has been investigated. The enclosure which is the space between two horizontal concentric confocal elliptic tubes is heated through its inner tube surface which is maintained at either uniform temperature or uniform heat flux. The induced buoyancy driven flow and the associated heat convection are predicted at different enclosure orientations. The full governing equations in terms of vorticity, stream function and temperature are solved numerically using Fourier Spectral Method. Beside Rayleigh and Prandtl numbers the heat convection process in the enclosure depends on the geometry of the enclosure and the angle of inclination with respect to gravity vector. The geometry of the enclosure is represented in terms of major axes ratio and axis ratio of inner tube. The study considered a moderate range of Rayleigh numbers between 5 x 10{sup 3} and 1 x 10{sup 5} while Prandtl number is fixed at 0.7. The inner tube axis ratio is considered between 0 and 1 while the ratio between the two major axes is considered up to 3. The angle of inclination of the minor axes with respect to gravity vector is varied from 0 to 90 deg.. The results for local and average Nusselt numbers as well as temperature distribution are obtained and discussed together with the details of both flow and thermal fields. For isothermal heating conditions, the study has shown an optimum value for major axes ratio that minimizes the rate of heat transfer in the enclosure. While in case of heating at uniform heat flux the study revealed existence of major axes ratio at which the mean temperature of the inner wall is maximum. Another aspect of this paper is the prediction of global flow circulation around the inner tube in case of asymmetrical orientation of the enclosure with respect to the gravity vector. (authors)
Buoyancy, bending, and seismic visibility in deep slab stagnation
Bina, Craig R.; Kawakatsu, Hitoshi; Suetsugu, D.; Bina, C.; Inoue, T.; Wiens, D.; Jellinek, M.
2010-11-01
The petrological consequences of deep subhorizontal deflection ("stagnation") of subducting slabs should affect both apparent seismic velocity structures and slab morphology. We construct kinematic thermal models of stagnant slabs and perform thermodynamic modeling of the consequent perturbation of high-pressure phase transitions in mantle minerals, focusing upon Japan as our study area. We calculate associated thermo-petrological buoyancy forces and bending moments which (along with other factors such as viscosity variations and rollback dynamics) may contribute to slab deformation. We consider effects of variations in depth of stagnation, post-stagnation dip angle, phase transition sharpness, transition triplication due to multiple intersection of geotherms with phase boundaries, and potential persistence of metastable phases due to kinetic hindrance. We also estimate seismic velocity anomalies, as might be imaged by seismic tomography, and corresponding seismic velocity gradients, as might be imaged by receiver-function analysis. We find that buoyant bending moment gradients of petrological origin at the base of the transition zone may contribute to slab stagnation. Such buoyancy forces vary with the depth at which stagnation occurs, so that slabs may seek an equilibrium slab stagnation depth. Metastable phase bending moment gradients further enhance slab stagnation, but they thermally decay after ∱/4600•700 km of horizontal travel, potentially allowing stagnant slabs to descend into the lower mantle. Stagnant slabs superimpose zones of negative velocity gradient onto a depressed 660-km seismic discontinuity, affecting the seismological visibility of such features. Seismologically resolvable details should depend upon both stagnation depth and the nature of the imaging technique (travel-time tomography vs. boundary-interaction phases). While seismic tomography appears to yield images of stagnant slabs, discontinuity topography beneath Japan resolved by
Effect of buoyancy on power deposition in microwave cavity hydrogen plasma source
Prasanna, S.; Rond, C.; Michau, A.; Hassouni, K.; Gicquel, A.
2016-08-01
A self-consistent model describing the coupling of resonant microwave radiation and plasma has been constructed. This model improves upon the models developed by Hassouni et al and Hagelaar et al, in 1999 and 2004, respectively with inclusion of hydrodynamic effects. The model has been used to study the effect of buoyancy on power deposition in microwave assisted hydrogen plasmas at different operating pressures over the range 25–300 mbar and power over the range 400 and 4000 W. Three cases viz. normal reactor (g = ‑9.81 m s‑2, negative buoyancy), pure diffusion (g = 0 m s‑2) and the inverted case (g = 9.81 m s‑2, positive buoyancy) were considered. Buoyancy effects in the cavity become important at high power / pressure operating conditions. The formation of a secondary plasma zone is strongly increased in the presence of negative buoyancy, while positive buoyancy and diffusion cases are more stable. Also the density of atomic hydrogen close to the substrate is larger with a wider radial spread for the positive buoyancy case over normal operating conditions which augurs well for achieving good deposition of diamond.
Gregg, Patricia M.; Grosfils, Eric B.; de Silva, Shanaka L.
2015-10-01
Recent analytical investigations have suggested that magma buoyancy is critical for triggering catastrophic caldera forming eruptions. Through detailed assessment of these approaches, we illustrate how analytical models have been misapplied for investigating buoyancy and are, therefore, incorrect and inconclusive. Nevertheless, the hypothesis that buoyancy is the critical trigger for larger eruptions warrants further investigation. As such, we utilize viscoelastic finite element models that incorporate buoyancy to test overpressure evolution and mechanical failure in the roof due to the coalescence of large buoyant magma bodies for two model cases. In the first case, we mimic empirical approaches and include buoyancy as an explicit boundary condition. In the second set of models, buoyancy is calculated implicitly due to the density contrast between the magma in the reservoir and the host rock. Results from these numerical experiments indicate that buoyancy promotes only minimal overpressurization of large silicic magma reservoirs (implementations and the results from the numerical experiments, we conclude that buoyancy does not provide an eruption triggering mechanism for large silicic systems. Therefore, correlations of buoyancy with magma residence times, the eruption frequency-volume relationship, and the dimensions of calderas are re-assessed. We find a causal relationship with magma reservoir volume that implicates the mechanical conditions of the host rock as a primary control on eruption frequency. As magma reservoirs grow in size (> 100 km3) they surpass a rheological threshold where their subsequent evolution is controlled by host rock mechanics. Consequently, this results in a thermomechanical division between small systems that are triggered "internally" by magmatic processes and large systems that are triggered "externally" by faulting related to roof uplift or tectonism. Finally, critical assessment of recent analytical approaches illustrates that care
Kinematic features of whole-body reaching movements underwater: Neutral buoyancy effects.
Macaluso, T; Bourdin, C; Buloup, F; Mille, M-L; Sainton, P; Sarlegna, F R; Taillebot, V; Vercher, J-L; Weiss, P; Bringoux, L
2016-07-01
Astronauts' training is conventionally performed in a pool to reproduce weightlessness by exploiting buoyancy which is supposed to reduce the impact of gravity on the body. However, this training method has not been scientifically validated yet, and requires first to study the effects of underwater exposure on motor behavior. We examined the influence of neutral buoyancy on kinematic features of whole-body reaching underwater and compared them with those produced on land. Eight professional divers were asked to perform arm reaching movements toward visual targets while standing. Targets were presented either close or far from the subjects (requiring in the latter case an additional whole-body displacement). Reaching movements were performed on land or underwater in two different contexts of buoyancy. The divers either wore a diving suit only with neutral buoyancy applied to their center of mass or were additionally equipped with a submersible simulated space suit with neutral buoyancy applied to their body limbs. Results showed that underwater exposure impacted basic movement features, especially movement speed which was reduced. However, movement kinematics also differed according to the way buoyancy was exerted on the whole-body. When neutral buoyancy was applied to the center of mass only, some focal and postural components of whole-body reaching remained close to land observations, notably when considering the relative deceleration duration of arm elevation and concomitant forward trunk bending when reaching the far target. On the contrary, when neutral buoyancy was exerted on body segments, movement kinematics were close to those reported in weightlessness, as reflected by the arm deceleration phase and the whole-body forward displacement when reaching the far target. These results suggest that astronauts could benefit from the application of neutral buoyancy across the whole-body segments to optimize underwater training and acquire specific motor skills which
Basal buoyancy and fast-moving glaciers: in defense of analytic force balance
van der Veen, C. J.
2016-06-01
The geometric approach to force balance advocated by T. Hughes in a series of publications has challenged the analytic approach by implying that the latter does not adequately account for basal buoyancy on ice streams, thereby neglecting the contribution to the gravitational driving force associated with this basal buoyancy. Application of the geometric approach to Byrd Glacier, Antarctica, yields physically unrealistic results, and it is argued that this is because of a key limiting assumption in the geometric approach. A more traditional analytic treatment of force balance shows that basal buoyancy does not affect the balance of forces on ice streams, except locally perhaps, through bridging effects.
Nearshore bar migration and sediment-induced buoyancy effects
Falchetti, Silvia; Conley, Daniel C.; Brocchini, Maurizio; Elgar, Steve
2010-02-01
A 1-D General Ocean Turbulence Model that includes the effects of sediment-induced stratification is shown to simulate the observed onshore and offshore migration of a nearshore sandbar. The only two free parameters of the model, the bed reference concentration and the sediment diffusivity, are taken from the literature, rather than tuned to the data used here. The model results suggest that predictions of onshore bar migration, in which wave-induced sediment transport confined to within a few centimeters of the bottom dominates, are not greatly affected by accounting for buoyancy effects. The model results also suggest that both mean flows and waves transport sediment during offshore bar migration, with different components of transport dominating at different cross-shore locations across the bar-trough bathymetry. Neglecting the effects of sediment-induced stratification results in higher model skill during the largest waves, likely because the excess turbulence production simulated by the non-stratified model is counterbalanced by neglected breaking-wave-generated turbulence. Considering both onshore and offshore migration, the model that includes sediment-induced stratification has higher skill than the model without stratification.
Buoyancy Effects in Fully-Modulated, Turbulent Diffusion Flames
Hermanson, J. C.; Johari, H.; Ghaem-Maghami, E.; Stocker, D. P.; Hegde, U. G.; Page, K. L.
2003-01-01
Pulsed combustion appears to have the potential to provide for rapid fuel/air mixing, compact and economical combustors, and reduced exhaust emissions. The objective of this experiment (PuFF, for Pulsed-Fully Flames) is to increase the fundamental understanding of the fuel/air mixing and combustion behavior of pulsed, turbulent diffusion flames by conducting experiments in microgravity. In this research the fuel jet is fully-modulated (i.e., completely shut off between pulses) by an externally controlled valve system. This gives rise to drastic modification of the combustion and flow characteristics of flames, leading to enhanced fuel/air mixing compared to acoustically excited or partially-modulated jets. Normal-gravity experiments suggest that the fully-modulated technique also has the potential for producing turbulent jet flames significantly more compact than steady flames with no increase in exhaust emissions. The technique also simplifies the combustion process by avoiding the acoustic forcing generally present in pulsed combustors. Fundamental issues addressed in this experiment include the impact of buoyancy on the structure and flame length, temperatures, radiation, and emissions of fully-modulated flames.
Effects of buoyancy on gas jet diffusion flames
Bahadori, M. Yousef; Edelman, Raymond B.
1993-01-01
The objective of this effort was to gain a better understanding of the fundamental phenomena involved in laminar gas jet diffusion flames in the absence of buoyancy by studying the transient phenomena of ignition and flame development, (quasi-) steady-state flame characteristics, soot effects, radiation, and, if any, extinction phenomena. This involved measurements of flame size and development, as well as temperature and radiation. Additionally, flame behavior, color, and luminosity were observed and recorded. The tests quantified the effects of Reynolds number, nozzle size, fuel reactivity and type, oxygen concentration, and pressure on flame characteristics. Analytical and numerical modeling efforts were also performed. Methane and propane flames were studied in the 2.2 Second Drop Tower and the 5.18-Second Zero-Gravity Facility of NASA LeRC. In addition, a preliminary series of tests were conducted in the KC-135 research aircraft. Both micro-gravity and normal-gravity flames were studied in this program. The results have provided unique and new information on the behavior and characteristics of gas jet diffusion flames in micro-gravity environments.
Alpha effect due to buoyancy instability of a magnetic layer
Chatterjee, Piyali; Rheinhardt, Matthias; Brandenburg, Axel
2010-01-01
A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. We calculate the alpha effect due to helical motions caused by a disintegrating magnetic layer in a rotating density-stratified system with angular velocity Omega making an angle theta with the vertical. We also study the dependence of the alpha effect on theta and the strength of the initial magnetic field. We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent EMF due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer. We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting alpha effect is inho...
Where is the level of neutral buoyancy for deep convection?
Takahashi, Hanii; Luo, Zhengzhao
2012-08-01
This study revisits an old concept in meteorology - level of neutral buoyancy (LNB). The classic definition of LNB is derived from the parcel theory and can be estimated from the ambient sounding (LNB_sounding) without having to observe any actual convective cloud development. In reality, however, convection interacts with the environment in complicated ways; it will eventually manage to find its own effective LNB and manifests it through detraining masses and developing anvils (LNB_observation). This study conducts a near-global survey of LNB_observation for tropical deep convection using CloudSat data and makes comparison with the corresponding LNB_sounding. The principal findings are as follows: First, although LNB_sounding provides a reasonable upper bound for convective development, correlation between LNB_sounding and LNB_observation is low suggesting that ambient sounding contains limited information for accurately predicting the actual LNB. Second, maximum mass outflow is located more than 3 km lower than LNB_sounding. Hence, from convective transport perspective, LNB_sounding is a significant overestimate of the “destination” height level of the detrained mass. Third, LNB_observation is consistently higher over land than over ocean, although LNB_sounding is similar between land and ocean. This difference is likely related to the contrasts in convective strength and environment between land and ocean. Finally, we estimate the bulk entrainment rates associated with the observed deep convection, which can serve as an observational basis for adjusting GCM cumulus parameterization.
Using surface integrals for checking the Archimedes' law of buoyancy
Lima, F M S
2011-01-01
A mathematical derivation of the force exerted by an \\emph{inhomogeneous} (i.e., compressible) fluid on the surface of an \\emph{arbitrarily-shaped} body immersed in it is not found in literature, which may be attributed to our trust on Archimedes' law of buoyancy. However, this law, also known as Archimedes' principle (AP), does not yield the force observed when the body is in contact to the container walls, as is more evident in the case of a block immersed in a liquid and in contact to the bottom, in which a \\emph{downward} force that \\emph{increases with depth} is observed. In this work, by taking into account the surface integral of the pressure force exerted by a fluid over the surface of a body, the general validity of AP is checked. For a body fully surrounded by a fluid, homogeneous or not, a gradient version of the divergence theorem applies, yielding a volume integral that simplifies to an upward force which agrees to the force predicted by AP, as long as the fluid density is a \\emph{continuous func...
Control of a Buoyancy-Based Pilot Underwater Lifting Body
Finn Haugen
2010-04-01
Full Text Available This paper is about position control of a specific small-scale pilot underwater lifting body where the lifting force stems from buoyancy adjusted with an air pocket in the lifting body. A mathematical model is developed to get a basis for a simulator which is used for testing and for designing the control system, including tuning controller parameters. A number of different position controller solutions were tried both on a simulator and on the physical system. Successful control on both the simulator and the physical system was obtained with cascade control based on feedback from measured position and height of the air pocket in the lifting body. The primary and the secondary controllers of the cascade control system were tuned using Skogestad's model-based PID tuning rules. Feedforward from estimated load force was implemented in combination with the cascade control system, giving a substantial improvement of the position control system, both with varying position reference and varying disturbance (load mass.
Alpha effect due to buoyancy instability of a magnetic layer
Chatterjee, P.; Mitra, D.; Rheinhardt, M.; Brandenburg, A.
2011-10-01
Context. A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. Aims: We calculate the α effect due to helical motions caused by an unstable magnetic layer in a rotating density-stratified system with angular velocity Ω making an angle θ with the vertical. We also study the dependence of the α effect on θ and the strength of the initial magnetic field. Methods: We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent electromotive force (EMF) due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer. Results: We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting α effect is non-uniform and increases with the strength of the initial magnetic field. It is thus an example of an "anti-quenched" α effect. The α effect is also nonlocal, i.e. scale dependent, requiring around 8-16 Fourier modes to reconstruct the actual mean EMF based on the actual mean field.
Buoyancy Instabilities in Weakly Magnetized Low Collisionality Plasmas
Quataert, Eliot
2007-01-01
I calculate the linear stability of a stratified low collisionality plasma in the presence of a weak magnetic field. Heat is assumed to flow only along magnetic field lines. In the absence of a heat flux in the background plasma, Balbus (2000) demonstrated that plasmas in which the temperature increases in the direction of gravity are buoyantly unstable to convective-like motions (the ``magnetothermal instability''). I show that in the presence of a background heat flux, an analogous instability is present when the temperature decreases in the direction of gravity. The instability is driven by the background heat flux and the fastest growing mode has a growth time of order the local dynamical time. Thus, independent of the sign of the temperature gradient, weakly magnetized low collisionality plasmas are unstable on a dynamical time to magnetically-mediated buoyancy instabilities. The instability described in this paper is predicted to be present in clusters of galaxies at radii from ~ 0.1-100 kpc, where the ...
Buoyancy and Penrose Process Produce Jets from Rotating Black Holes
Semenov, V S; Heyn, M F
2014-01-01
The exact mechanism by which astrophysical jets are formed is still unknown. It is believed that necessary elements are a rotating (Kerr) black hole and a magnetised accreting plasma. We model the accreting plasma as a collection of magnetic flux tubes/strings. If such a tube falls into a Kerr black hole, then the leading portion loses angular momentum and energy as the string brakes, and to compensate for this loss, momentum and energy is redistributed to the trailing portion of the tube.} {We found that buoyancy creates a pronounced helical magnetic field structure aligned with the spin axis. Along the field lines, the plasma is centrifugally accelerated close to the speed of light. This process leads to unlimited stretching of the flux tube since one part of the tube continues to fall into the black hole and simultaneously the other part of the string is pushed outward. Eventually, reconnection cuts the tube, the inner part is filled with new material and the outer part forms a collimated bubble-structured...
Using surface integrals for checking Archimedes' law of buoyancy
A mathematical derivation of the force exerted by an inhomogeneous (i.e. compressible) fluid on the surface of an arbitrarily shaped body immersed in it is not found in the literature, which may be attributed to our trust in Archimedes' law of buoyancy. However, this law, also known as Archimedes' principle (AP), does not yield the force observed when the body is in contact with the container walls, as is more evident in the case of a block immersed in a liquid and in contact with the bottom, in which a downward force that increases with depth is observed. In this work, by taking into account the surface integral of the pressure force exerted by a fluid over the surface of a body, the general validity of AP is checked. For a body fully surrounded by a fluid, homogeneous or not, a gradient version of the divergence theorem applies, yielding a volume integral that simplifies to an upward force which agrees with the force predicted by AP, as long as the fluid density is a continuous function of depth. For the bottom case, this approach yields a downward force that increases with depth, which contrasts to AP but is in agreement with experiments. It also yields a formula for this force which shows that it increases with the area of contact.
Annual and seasonal mean buoyancy fluxes for the tropical Indian Ocean
Prasad, T.G.
INSAT-derived monthly mean precipitation, combined with estimates of evaporation from COADS, are used to prepare the annual mean and seasonal distributions of evaporation-precipitation (E-P)) and buoyancy fluxes for the tropical Indian Ocean...
Guilet, Jerome
2015-01-01
The magneto-rotational instability (MRI) is considered to be a promising mechanism to amplify the magnetic field in fast rotating protoneutron stars. In contrast to accretion disks, radial buoyancy driven by entropy and lepton fraction gradients is expected to have a dynamical role as important as rotation and shear. We investigate the poorly known impact of buoyancy on the non-linear phase of the MRI, by means of three dimensional numerical simulations of a local model in the equatorial plane of a protoneutron star. The use of the Boussinesq approximation allows us to utilise a shearing box model with clean shearing periodic boundary conditions, while taking into account the buoyancy driven by radial entropy and composition gradients. We find significantly stronger turbulence and magnetic fields in buoyantly unstable flows. On the other hand, buoyancy has only a limited impact on the strength of turbulence and magnetic field amplification for buoyantly stable flows in the presence of a realistic thermal diff...
Sensitivity of the Southern Ocean overturning circulation to surface buoyancy forcing
Morrison, Adele K.; Hogg, Andrew M.; Ward, Marshall L.
2011-07-01
The sensitivity of the Southern Ocean overturning to altered surface buoyancy forcing is investigated in a series of eddy-permitting, idealised simulations. The modelled response indicates that heat and freshwater fluxes in the Southern Hemisphere mid-latitudes may play a significant role in setting the strength of the overturning circulation. Enhanced buoyancy fluxes act to increase the meridional overturning up to a limit approaching the wind-driven Ekman transport. The sensitivity of the overturning to surface buoyancy forcing is strongly dependent on the relative locations of the wind stress profile, buoyancy forcing and upwelling region. The numerical simulations provide support for the hypothesis that changes in upwelling during deglaciations may have been driven by changes in heat and freshwater fluxes, instead of, or in addition to, changes in wind stress.
Buoyancy package for self-contained acoustic doppler current profiler mooring
Venkatesan, R.; Krishnakumar, V.
A buoyancy package for self-contained Acoustic Doppler Current Profiler(SC-ADCP 1200 RD instruments USA) was designed and fabricated indigenously, for subsurface mooring in coastal waters. The system design is discussed. The design to keep SC...
The Role of Magnetic Buoyancy in a Babcock-Leighton Type Solar Dynamo
Dibyendu Nandy; Arnab Rai Choudhuri
2000-09-01
We study the effects of incorporating magnetic buoyancy in a model of the solar dynamo—which draws inspiration from the Babcock-Leighton idea of surface processes generating the poloidal field. We present our main results here.
Buoyancy-driven flow reversal phenomena in radially rotating serpentine ducts
Hwang, J.J.; Wang, W.J.; Chen, C.K.
2000-02-01
Convective characteristics are analyzed numerically in a rotating multipass square duct connecting with 180-deg sharp returns. Isoflux is applied to each duct wall and periodic conditions are used between the entrance and exit of a typical two-pass module. Emphasis is placed on the phenomenon of buoyancy-driven reversed flow in the serpentine duct. Predictions reveal that the radial distance from the rotational axis to the location of flow separation in the radial-outward duct decreases with increasing the Richardson number. In addition, the local buoyancy that is required to yield the radial flow reversal increases with increasing the rotation number. This buoyancy-driven reversed flow in the radial-outward duct always results in local hot spots in the cooling channels. The critical buoyancy for the initiation of flow reversal is therefore concluded for the design purpose.
Sexton, J. D.
1992-06-01
The transfer orbit stage (TOS) will propel the advanced communications technology satellite (ACTS) from the Space Shuttle to an Earth geosynchronous transfer orbit. Two neutral buoyancy test series were conducted at MSFC to validate the extravehicular activities (EVA) contingency operations for the ACTS/TOS/mission. The results of the neutral buoyancy tests are delineated and a brief history of the TOS EVA program is given.
Estimating the Federal Direct Tax Buoyancy for Pakistan in Post-1973 Era
Shaikh, Salman
2012-01-01
This study used the simple co-integration technique to estimate the direct tax buoyancy for Pakistan economy for the 36 year period starting from FY-1974 to FY-2009. The buoyancy estimated was more than unity which represents slight improvement over previous estimates in past studies. The study attributes the improvement to factors such as expansion of tax base, diversification and deepening of manufacturing sector and structural change in the economy with the size of agriculture sector outpu...
Buoyancy-driven convection may switch between reactive states in three-dimensional chemical waves
Šebestíková, L. (Lenka); Hauser, M J B
2012-01-01
Traveling waves in an extended reactor, whose width cannot be neglected, represent a three-dimensional (3D) reaction-diffusion-convection system. We investigate the effects of buoyancy-driven convection in such a setting. The 3D waves traveled through horizontal layers of the iodate–arsenous acid (IAA) reaction solution containing excess of arsenous acid. The depth of the reaction solution was the examined parameter. An increase in the intensity of buoyancy-driven flow caused an increase of t...
Brito López, Ricardo; Soto, R.
2009-01-01
It has been recently reported that a granular mixture in which grains differ in their restitution coefficients presents segregation: the more inelastic particles sink to the bottom. When other segregation mechanisms as buoyancy and the Brazil nut effect are present, the inelasticity induced segregation can compete with them. First, a detailed analysis, based on numerical simulations of two dimensional systems, of the competition between buoyancy and the inelasticity induced segregation is pre...
Effects of Buoyancy on Lean Premixed V-Flames Part I: Laminar and Turblent Flame Structure
Cheng, Robert K.; Bedat, Benoit; Kostiuk, Larry W.
1998-01-01
Laser schlieren and planar laser-induced fluorescence techniques have been used to investigate laminar and turbulent v-flames in +g, -g, and micro g under flow conditions that span the regimes of momentum domination (Ri buoyancy domination (Ri > 0.1). Overall flame features shown by schlieren indicate that buoyancy dominates the entire flow field for conditions close to Ri = 1. With decreasing Ri, buoyancy effects are observed only in the far-field regions. Analyses of the mean flame angles demonstrate that laminar and turbulent flames do not have similar responses to buoyancy. Difference in the laminar +g and -g flame angles decrease with Ri (i.e., increasing Re) and converge to the microgravity flame angle at the momentum limit (Ri - 0). This is consistent with the notion that the effects of buoyancy diminish with increasing flow momentum. The +g and -g turbulent flame angles, however, do not converge at Ri = 0. As shown by OH-PLIF images, the inconsistency in +g and -g turbulent flame angles is associated with the differences in flame wrinkles. Turbulent flame wrinkles evolve more slowly in +g than in -g. The difference in flame wrinkle structures, however, cannot be explained in terms of buoyancy effects on flame instability mechanisms. It seems to be associated with the field effects of buoyancy that stretches the turbulent flame brushes in +g and compresses the flame brush in -g. Flame wrinkling offers a mechanism through which the flame responds to the field effects of buoyancy despite increasing flow momentum. These observations point to the need to include both upstream and downstream contributions in theoretical analysis of flame turbulence interactions.
Errors caused by incompatible wind and buoyancy forcing in the ocean general circulation models.
Kuo, Yu-Heng
1992-01-01
Approved for public release; distribution is unlimited The Geophysical Fluid Dynamics Laboratory Modular Ocean Model (GFDL MOM) is used to investigate the model difference between compatible and incompatible surface wind and buoyancy forcing. The atmosphere is a physical system in which surface wind and temperature fields are related, however in most ocean numerical models, the wind stress and buoyancy forcing are usually specified separately, i.e., no constraint between the...
THE INFLUENCE OF BUOYANCY ON FLOW AND POLLUTANT DISPERSION IN STREET CANYONS
Buccolieri, Riccardo; Pulvirenti, Beatrice; Di Sabatino, Silvana; Britter, Rex
2008-01-01
Abstract: In this paper, the effect of buoyancy on flow and pollutant dispersion within street canyons is studied by means of computational fluid dynamics simulations. We consider a neutral boundary layer approaching a 3D street canyon assuming a wind direction perpendicular to the street canyon. The Boussinesq hypothesis for incompressible fluids is chosen for modelling buoyancy. We distinguish three cases: leeward, ground and windward wall heating. Thermal effects on both the flow ...
Drag, but not buoyancy, affects swim speed in captive Steller sea lions
Ippei Suzuki; Katsufumi Sato; Andreas Fahlman; Yasuhiko Naito; Nobuyuki Miyazaki; Andrew W Trites
2014-01-01
ABSTRACT Swimming at an optimal speed is critical for breath-hold divers seeking to maximize the time they can spend foraging underwater. Theoretical studies have predicted that the optimal swim speed for an animal while transiting to and from depth is independent of buoyancy, but is dependent on drag and metabolic rate. However, this prediction has never been experimentally tested. Our study assessed the effects of buoyancy and drag on the swim speed of three captive Steller sea lions (Eumet...
Effects of body condition on buoyancy in endangered North Atlantic right whales.
Nousek-McGregor, Anna E; Miller, Carolyn A; Moore, Michael J; Nowacek, Douglas P
2014-01-01
Buoyancy is an important consideration for diving marine animals, resulting in specific ecologically relevant adaptations. Marine mammals use blubber as an energy reserve, but because this tissue is also positively buoyant, nutritional demands have the potential to cause considerable variation in buoyancy. North Atlantic right whales Eubalaena glacialis are known to be positively buoyant as a result of their blubber, and the thickness of this layer varies considerably, but the effect of this variation on buoyancy has not been explored. This study compared the duration and rate of ascending and descending glides, recorded with an archival tag, with blubber thickness, measured with an ultrasound device, in free-swimming right whales. Ascending whales with thicker blubber had shorter portions of active propulsion and longer passive glides than whales with thinner blubber, suggesting that blubber thickness influences buoyancy because the buoyant force is acting in the same direction as the animal's movement during this phase. Whales with thinner layers also used similar body angles and velocities when traveling to and from depth, while those with thicker layers used shallower ascent angles but achieved higher ascent velocities. Such alterations in body angle may help to reduce the cost of transport when swimming against the force of buoyancy in a state of augmented positive buoyancy, which represents a dynamic response to reduce the energetic consequences of physiological changes. These results have considerable implications for any diving marine animal during periods of nutritional stress, such as during seasonal migrations and annual variations in prey availability. PMID:24457930
Field Effects of Buoyancy on a Premixed Turbulent Flame Studied by Particle Image Velocimetry
Cheng, Robert K.
2003-01-01
Typical laboratory flames for the scientific investigation of flame/turbulence interactions are prone to buoyancy effects. Buoyancy acts on these open flame systems and provides upstream feedbacks that control the global flame properties as well as local turbulence/flame interactions. Consequently the flame structures, stabilization limits, and turbulent reaction rates are directly or indirectly coupled with buoyancy. The objective of this study is to characterize the differences between premixed turbulent flames pointing upwards (1g), pointing downwards (-1g), and in microgravity (mg). The configuration is an inverted conical flame stabilized by a small cone-shaped bluff body that we call CLEAN Flames (Cone-Stabilized Lean Flames). We use two laser diagnostics to capture the velocity and scalar fields. Particle image velocimetry (PIV) measures the mean and root mean square velocities and planar imaging by the flame fronts method outlines the flame wrinkle topology. The results were obtained under typical conditions of small domestic heating systems such as water heaters, ovens, and furnaces. Significant differences between the 1g and -1g flames point to the need for including buoyancy contributions in theoretical and numerical calculations. In Earth gravity, there is a complex coupling of buoyancy with the turbulent flow and heat release in the flame. An investigation of buoyancy-free flames in microgravity will provide the key to discern gravity contributions. Data obtained in microgravity flames will provide the benchmark for interpreting and analyzing 1g and -1g flame results.
BUOYANCY INSTABILITIES IN A WEAKLY COLLISIONAL INTRACLUSTER MEDIUM
Kunz, Matthew W.; Stone, James M. [Department of Astrophysical Sciences, Princeton University, Peyton Hall, 4 Ivy Lane, Princeton, NJ 08544 (United States); Bogdanovic, Tamara; Reynolds, Christopher S., E-mail: kunz@astro.princeton.edu, E-mail: jstone@astro.princeton.edu, E-mail: tamarab@astro.umd.edu, E-mail: chris@astro.umd.edu [Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
2012-08-01
The intracluster medium (ICM) of galaxy clusters is a weakly collisional plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign: the magnetothermal instability (MTI) in the outskirts of clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We employ the Athena magnetohydrodynamic code to investigate the nonlinear evolution of these instabilities, self-consistently including the effects of anisotropic viscosity (i.e., Braginskii pressure anisotropy), anisotropic conduction, and radiative cooling. We find that, in all but the innermost regions of cool-core clusters, anisotropic viscosity significantly impairs the ability of the HBI to reorient magnetic-field lines orthogonal to the temperature gradient. Thus, while radio-mode feedback appears necessary in the central few Multiplication-Sign 10 kpc, heat conduction may be capable of offsetting radiative losses throughout most of a cool core over a significant fraction of the Hubble time. Magnetically aligned cold filaments are then able to form by local thermal instability. Viscous dissipation during cold filament formation produces accompanying hot filaments, which can be searched for in deep Chandra observations of cool-core clusters. In the case of MTI, anisotropic viscosity leads to a nonlinear state with a folded magnetic field structure in which field-line curvature and field strength are anti-correlated. These results demonstrate that, if the HBI and MTI are relevant for shaping the properties of the ICM, one must self-consistently include anisotropic viscosity in order to obtain even qualitatively correct results.
Buoyancy Instabilities in a Weakly Collisional Intracluster Medium
Kunz, Matthew W.; Bogdanović, Tamara; Reynolds, Christopher S.; Stone, James M.
2012-08-01
The intracluster medium (ICM) of galaxy clusters is a weakly collisional plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign: the magnetothermal instability (MTI) in the outskirts of clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We employ the Athena magnetohydrodynamic code to investigate the nonlinear evolution of these instabilities, self-consistently including the effects of anisotropic viscosity (i.e., Braginskii pressure anisotropy), anisotropic conduction, and radiative cooling. We find that, in all but the innermost regions of cool-core clusters, anisotropic viscosity significantly impairs the ability of the HBI to reorient magnetic-field lines orthogonal to the temperature gradient. Thus, while radio-mode feedback appears necessary in the central few× 10 kpc, heat conduction may be capable of offsetting radiative losses throughout most of a cool core over a significant fraction of the Hubble time. Magnetically aligned cold filaments are then able to form by local thermal instability. Viscous dissipation during cold filament formation produces accompanying hot filaments, which can be searched for in deep Chandra observations of cool-core clusters. In the case of MTI, anisotropic viscosity leads to a nonlinear state with a folded magnetic field structure in which field-line curvature and field strength are anti-correlated. These results demonstrate that, if the HBI and MTI are relevant for shaping the properties of the ICM, one must self-consistently include anisotropic viscosity in order to obtain even qualitatively correct results.
Buoyancy-driven CO2/brine flow at reservoir conditions
Oh, J.; Kim, K.; Han, W.; Kim, T.; Kim, J.; Park, E.
2013-12-01
Suitable geological formations should guarantee a long-term safe and reliable storage of the injected supercritical CO2. In this study we targeted the cases of gravity-driven CO2 plume migration in a storage formation and the resulting CO2 leakage to overlying formation through a possible fractures or abandoned wells. A laboratory experiment and numerical model for two-phase core-flooding tests were designed to understand the buoyancy effect on supercritical CO2 migration under reservoir conditions. A series of core flooding tests were performed with Berea sandstone cores which have 20 % porosity and 1.7×10-13 m2 permeability. Unlike the normal core-flooding tests, the core was set up in a vertical direction and the CO2 was released at the bottom of the core to investigate the gravity effect on CO2 migration. During the test, the downstream pressure was maintained at 10 MPa, and the confining pressure was kept at 20 MPa. The temperature was set to be 40 °C to reflect the 1 km subsurface environment. The CO2-flooding (drainage) tests with brine-saturated core were performed with various CO2-release periods. The CO2 saturation was measured with a linear X-ray scanner. In addition to laboratory experiments, numerical simulations were performed to provide further insight into the CO2 migration behavior. TOUGH2 with ECO2N module was used to simulate CO2/brine core-flooding tests. Dimensionless numbers (Capillary number and Bond number) were calculated with the simulation results at various time points covering both the release and monitoring period.
Gravitaxis of Euglena gracilis depends only partially on passive buoyancy
Richter, Peter R.; Schuster, Martin; Lebert, Michael; Streb, Christine; Häder, Donat-Peter
In darkness, the unicellular freshwater flagellate Euglena gracilis shows a pronounced negative gravitactic behavior, and the cells swim actively upward in the water column. Up to now it was unclear whether this behavior is based on a passive (physical) alignment mechanism (e.g., buoyancy due to a fore-aft asymmetry of the cell body) or on an active physiological mechanism. A sounding rocket experiment was performed in which the effect of sub-1g-accelerations (0.05, 0.08, 0.12, and 0.2g) on untreated living cells and immobilized (fixation with liquid nitrogen) cells was observed. By means of computerized image analysis the angles of the cells long axis with respect to the acceleration vector were analyzed in order to calculate and compare the reorientation kinetics of the immobilized cells versus that of the controls. In both groups, the reorientation kinetics depended on the dose, but the reorientation of the living cells was about five times faster than that of the immobilized cells. This indicates that in young cells gravitaxis can be explained by a physical mechanism only to a small extend. In older cultures, in which the cells often have a drop shaped cell body, the physical reorientation is considerably faster, and a more pronounced influence of passive alignment caused by fore/aft asymmetry (drag-gravity model) can not be excluded. In addition to these results, Euglena gracilis cells seem to respond very sensitively to small accelerations when they are applied after a longer microgravity period. The data indicate that gravitactic orientation occurred at an acceleration as low as 0.05g.
Martin, Andrew J.
2014-01-01
Background: Academic buoyancy is students' capacity to successfully overcome setback and challenge that is typical of the ordinary course of everyday academic life. It may represent an important factor on the psycho-educational landscape assisting students who experience difficulties in school and schoolwork. Aims: This study investigated the…
Drag, but not buoyancy, affects swim speed in captive Steller sea lions
Ippei Suzuki
2014-04-01
Full Text Available Swimming at an optimal speed is critical for breath-hold divers seeking to maximize the time they can spend foraging underwater. Theoretical studies have predicted that the optimal swim speed for an animal while transiting to and from depth is independent of buoyancy, but is dependent on drag and metabolic rate. However, this prediction has never been experimentally tested. Our study assessed the effects of buoyancy and drag on the swim speed of three captive Steller sea lions (Eumetopias jubatus that made 186 dives. Our study animals were trained to dive to feed at fixed depths (10–50 m under artificially controlled buoyancy and drag conditions. Buoyancy and drag were manipulated using a pair of polyvinyl chloride (PVC tubes attached to harnesses worn by the sea lions, and buoyancy conditions were designed to fall within the natural range of wild animals (∼12–26% subcutaneous fat. Drag conditions were changed with and without the PVC tubes, and swim speeds were recorded and compared during descent and ascent phases using an accelerometer attached to the harnesses. Generalized linear mixed-effect models with the animal as the random variable and five explanatory variables (body mass, buoyancy, dive depth, dive phase, and drag showed that swim speed was best predicted by two variables, drag and dive phase (AIC = −139. Consistent with a previous theoretical prediction, the results of our study suggest that the optimal swim speed of Steller sea lions is a function of drag, and is independent of dive depth and buoyancy.
Drag, but not buoyancy, affects swim speed in captive Steller sea lions.
Suzuki, Ippei; Sato, Katsufumi; Fahlman, Andreas; Naito, Yasuhiko; Miyazaki, Nobuyuki; Trites, Andrew W
2014-01-01
Swimming at an optimal speed is critical for breath-hold divers seeking to maximize the time they can spend foraging underwater. Theoretical studies have predicted that the optimal swim speed for an animal while transiting to and from depth is independent of buoyancy, but is dependent on drag and metabolic rate. However, this prediction has never been experimentally tested. Our study assessed the effects of buoyancy and drag on the swim speed of three captive Steller sea lions (Eumetopias jubatus) that made 186 dives. Our study animals were trained to dive to feed at fixed depths (10-50 m) under artificially controlled buoyancy and drag conditions. Buoyancy and drag were manipulated using a pair of polyvinyl chloride (PVC) tubes attached to harnesses worn by the sea lions, and buoyancy conditions were designed to fall within the natural range of wild animals (∼12-26% subcutaneous fat). Drag conditions were changed with and without the PVC tubes, and swim speeds were recorded and compared during descent and ascent phases using an accelerometer attached to the harnesses. Generalized linear mixed-effect models with the animal as the random variable and five explanatory variables (body mass, buoyancy, dive depth, dive phase, and drag) showed that swim speed was best predicted by two variables, drag and dive phase (AIC = -139). Consistent with a previous theoretical prediction, the results of our study suggest that the optimal swim speed of Steller sea lions is a function of drag, and is independent of dive depth and buoyancy. PMID:24771620
Buoyancy-activated cell sorting using targeted biotinylated albumin microbubbles.
Yu-Ren Liou
Full Text Available Cell analysis often requires the isolation of certain cell types. Various isolation methods have been applied to cell sorting, including fluorescence-activated cell sorting and magnetic-activated cell sorting. However, these conventional approaches involve exerting mechanical forces on the cells, thus risking cell damage. In this study we applied a novel isolation method called buoyancy-activated cell sorting, which involves using biotinylated albumin microbubbles (biotin-MBs conjugated with antibodies (i.e., targeted biotin-MBs. Albumin MBs are widely used as contrast agents in ultrasound imaging due to their good biocompatibility and stability. For conjugating antibodies, biotin is conjugated onto the albumin MB shell via covalent bonds and the biotinylated antibodies are conjugated using an avidin-biotin system. The albumin microbubbles had a mean diameter of 2 μm with a polydispersity index of 0.16. For cell separation, the MDA-MB-231 cells are incubated with the targeted biotin-MBs conjugated with anti-CD44 for 10 min, centrifuged at 10 g for 1 min, and then allowed 1 hour at 4 °C for separation. The results indicate that targeted biotin-MBs conjugated with anti-CD44 antibodies can be used to separate MDA-MB-231 breast cancer cells; more than 90% of the cells were collected in the MB layer when the ratio of the MBs to cells was higher than 70:1. Furthermore, we found that the separating efficiency was higher for targeted biotin-MBs than for targeted avidin-incorporated albumin MBs (avidin-MBs, which is the most common way to make targeted albumin MBs. We also demonstrated that the recovery rate of targeted biotin-MBs was up to 88% and the sorting purity was higher than 84% for a a heterogenous cell population containing MDA-MB-231 cells (CD44(+ and MDA-MB-453 cells (CD44-, which are classified as basal-like breast cancer cells and luminal breast cancer cells, respectively. Knowing that the CD44(+ is a commonly used cancer
Buoyancy-activated cell sorting using targeted biotinylated albumin microbubbles.
Liou, Yu-Ren; Wang, Yu-Hsin; Lee, Chia-Ying; Li, Pai-Chi
2015-01-01
Cell analysis often requires the isolation of certain cell types. Various isolation methods have been applied to cell sorting, including fluorescence-activated cell sorting and magnetic-activated cell sorting. However, these conventional approaches involve exerting mechanical forces on the cells, thus risking cell damage. In this study we applied a novel isolation method called buoyancy-activated cell sorting, which involves using biotinylated albumin microbubbles (biotin-MBs) conjugated with antibodies (i.e., targeted biotin-MBs). Albumin MBs are widely used as contrast agents in ultrasound imaging due to their good biocompatibility and stability. For conjugating antibodies, biotin is conjugated onto the albumin MB shell via covalent bonds and the biotinylated antibodies are conjugated using an avidin-biotin system. The albumin microbubbles had a mean diameter of 2 μm with a polydispersity index of 0.16. For cell separation, the MDA-MB-231 cells are incubated with the targeted biotin-MBs conjugated with anti-CD44 for 10 min, centrifuged at 10 g for 1 min, and then allowed 1 hour at 4 °C for separation. The results indicate that targeted biotin-MBs conjugated with anti-CD44 antibodies can be used to separate MDA-MB-231 breast cancer cells; more than 90% of the cells were collected in the MB layer when the ratio of the MBs to cells was higher than 70:1. Furthermore, we found that the separating efficiency was higher for targeted biotin-MBs than for targeted avidin-incorporated albumin MBs (avidin-MBs), which is the most common way to make targeted albumin MBs. We also demonstrated that the recovery rate of targeted biotin-MBs was up to 88% and the sorting purity was higher than 84% for a a heterogenous cell population containing MDA-MB-231 cells (CD44(+)) and MDA-MB-453 cells (CD44-), which are classified as basal-like breast cancer cells and luminal breast cancer cells, respectively. Knowing that the CD44(+) is a commonly used cancer-stem-cell biomarker, our
Effects of Buoyancy on Laminar and Turbulent Premixed V-Flame
Cheng, Robert K.; Bedat, Benoit
1997-01-01
Turbulent combustion occurs naturally in almost all combustion systems and involves complex dynamic coupling of chemical and fluid mechanical processes. It is considered as one of the most challenging combustion research problems today. Though buoyancy has little effect on power generating systems operating under high pressures (e.g., IC engines and turbines), flames in atmospheric burners and the operation of small to medium furnaces and boilers are profoundly affected by buoyancy. Changes in burner orientation impacts on their blow-off, flash-back and extinction limits, and their range of operation, burning rate, heat transfer, and emissions. Theoretically, buoyancy is often neglected in turbulent combustion models. Yet the modeling results are routinely compared with experiments of open laboratory flames that are obviously affected by buoyancy. This inconsistency is an obstacle to reconciling experiments and theories. Consequently, a fundamental understanding of the coupling between turbulent flames and buoyancy is significant to both turbulent combustion science and applications. The overall effect of buoyancy relates to the dynamic interaction between the flame and its surrounding, i.e., the so-called elliptical problem. The overall flame shape, its flowfield, stability, and mean and local burning rates are dictated by both upstream and downstream boundary conditions. In steady propagating premixed flames, buoyancy affects the products region downstream of the flame zone. These effects are manifested upstream through the mean and fluctuating pressure fields to influence flame stretch and flame wrinkling. Intuitively, the effects buoyancy should diminish with increasing flow momentum. This is the justification for excluding buoyancy in turbulent combustion models that treats high Reynolds number flows. The objectives of our experimental research program is to elucidate flame-buoyancy coupling processes in laminar and turbulent premixed flames, and to
Double-diffusive magnetic buoyancy instability in a quasi-two-dimensional Cartesian geometry
Skinner, D. M.; Silvers, L. J.
2013-11-01
Magnetic buoyancy, believed to occur in the solar tachocline, is both an important part of large-scale solar dynamo models and the picture of how sunspots are formed. Given that in the tachocline region the ratio of magnetic diffusivity to thermal diffusivity is small it is important, for both the dynamo and sunspot formation pictures, to understand magnetic buoyancy in this regime. Furthermore, the tachocline is a region of strong shear and such investigations must involve structures that become buoyant in the double-diffusive regime which are generated entirely from a shear flow. In a previous study, we have illustrated that shear-generated double-diffusive magnetic buoyancy instability is possible in the tachocline. However, this study was severely limited due to the computational requirements of running three-dimensional magnetohydrodynamic simulations over diffusive time-scales. A more comprehensive investigation is required to fully understand the double-diffusive magnetic buoyancy instability and its dependency on a number of key parameters; such an investigation requires the consideration of a reduced model. Here we consider a quasi-two-dimensional model where all gradients in the x direction are set to zero. We show how the instability is sensitive to changes in the thermal diffusivity and also show how different initial configurations of the forced shear flow affect the behaviour of the instability. Finally, we conclude that if the tachocline is thinner than currently stated then the double-diffusive magnetic buoyancy instability can more easily occur.
Guilet, Jérôme; Müller, Ewald
2015-06-01
The magnetorotational instability (MRI) is considered to be a promising mechanism to amplify the magnetic field in fast-rotating protoneutron stars. In contrast to accretion discs, radial buoyancy driven by entropy and lepton fraction gradients is expected to have a dynamical role as important as rotation and shear. We investigate the poorly known impact of buoyancy on the non-linear phase of the MRI, by means of three-dimensional numerical simulations of a local model in the equatorial plane of a protoneutron star. The use of the Boussinesq approximation allows us to utilize a shearing box model with clean shearing periodic boundary conditions, while taking into account the buoyancy driven by radial entropy and composition gradients. We find significantly stronger turbulence and magnetic fields in buoyantly unstable flows. On the other hand, buoyancy has only a limited impact on the strength of turbulence and magnetic field amplification for buoyantly stable flows in the presence of a realistic thermal diffusion. The properties of the turbulence are, however, significantly affected in the latter case. In particular, the toroidal components of the magnetic field and of the velocity become even more dominant with respect to the poloidal ones. Furthermore, we observed in the regime of stable buoyancy the formation of long-lived coherent structures such as channel flows and zonal flows. Overall, our results support the ability of the MRI to amplify the magnetic field significantly even in stably stratified regions of protoneutron stars.
Use of A-Train data to estimate convective buoyancy and entrainment rate
Luo, Zhengzhao Johnny; Liu, G. Y.; Stephens, Graeme L.
2010-05-01
This study describes a satellite-based method to estimate simultaneously convective buoyancy (B) and entrainment rate (λ). The measurement requirements are cloud-top height (CTH), cloud-top temperature (CTT), cloud profiling information (from radar and lidar), and ambient sounding. Initial results of the new method applied to A-Train data are presented. It is observed that tropical oceanic convection above the boundary layer fall into two groups: deep convection (DC) and cumulus congestus (Cg). DC tend to have negative buoyancy near cloud top and λ < 10%/km. Cg are further divided into two groups due to the snapshot view of the A-Train: one has positive buoyancy and λ ≤ 10%/km and the other has negative buoyancy and λ reaching up to 50%/km. Uncertainty analysis is conducted showing that CTT and CTH are the primary source of errors, but they do not affect our conclusions qualitatively. Brief comparisons with previous studies indicate the results of this study are broadly consistent with these earlier studies. Although most of the initial results are expected, this study represents the first time, to our knowledge, that satellite data are used to estimate convective buoyancy and entrainment rate. This new, space-borne method presents an opportunity for a number of follow-up investigations. For example, it serves as a bridge to connect A-Train observations (and follow-up missions) to GCM cumulus parameterizations.
Design and testing of a shape memory alloy buoyancy engine for unmanned underwater vehicles
Angilella, Alex J.; Gandhi, Farhan S.; Miller, Timothy F.
2015-11-01
The US Navy’s 2004 Unmanned Underwater Vehicle (UUV) Master Plan outlines the Navy’s aim to expand the role of UUVs, and one of the key areas of interest is the increase in UUV range and endurance. A class of UUVs known as underwater gliders achieves this objective by cyclically modifying its buoyancy and covering horizontal distance with a climb/dive pattern. The present study proposes the use of shape memory alloys (SMAs) in a buoyancy heat engine where the oceanic thermocline would be exploited to produce martensite-austenite phase transformations that in turn change the buoyancy of a piston-cylinder prototype. The working principle of the device involves transitioning between the following two states. At low temperature (at depth) the SMA wires are tensioned into a detwinned martensitic state by a parallel compressed spring. This moves the piston within the cylinder to increase the chamber dry volume and device buoyancy. At higher temperatures (near the surface) the SMA wires undergo a martensite-to-austenite phase transformation, recover part of the applied strain, and reduce the volume and buoyancy of the piston-cylinder. This paper presents the analysis, design, fabrication, and testing of a prototype device. The prototype was immersed in a water bath, and it was demonstrated that its volume would change, as expected, with change in temperature of the water bath. Simulation results showed good correlation with test data.
Hybrid ventilation in two interconnected rooms with a buoyancy source
Tovar, R. [Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Apdo. Postal 34, Temixco Mor. 62580 (Mexico); Linden, P.F. [Department of Mechanical and Aerospace Engineering, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 920903-0411 (United States); Thomas, L.P. [Instituto de Fisica Arroyo Seco, Universidad Nacional del Centro, Pinto 399, 7000 Tandil (Argentina)
2007-05-15
The design of energy efficient buildings and the potential for using solar energy for heating and cooling is contingent upon optimizing the building ventilation systems. In this paper, we study the ventilation of two interconnected spaces, such as adjacent offices or areas in an open plan office. The goal is to locate return vents to increase the efficiency of night ventilation and to reduce energy consumption. The flow in two interconnected rooms of similar sizes is studied experimentally using a tank divided by an interior vertical wall. A forced buoyancy source with a finite volume flux is located in the ceiling of one-room and an unforced vent is opened in the ceiling of the other room. The goal of the study is to understand the transient cooling/heating that occurs in this two-room system when a forced cold-air vent is located in the ceiling of the first room and a return ventilation exit is located in the second. In particular, we investigate the effects of varying the number of openings and their vertical positions in the interconnecting wall. First, a single opening at the bottom, middle or top of the shared wall is examined. Second, the case of two openings in the wall is considered, with the openings located at the top-bottom, top-middle, bottom-middle, and finally at two mid locations in the wall. The results are compared with the one-room case, which represents the reference case. It was found that, irrespective of the number and locations of the openings, the flow evolves into a quasi-stationary stably stratified two-layer system, with the depths of the layers being different in each room. The average temperature inside each room initially decreases linearly with time and approaches the supply-air temperature at large times. This initial linear decrease holds until cold-air leaves the unforced room through the top-vent at time t{sub e}. Subsequently, temperature decreases as an exponential function of time with a characteristic filling time {tau} V
The effects of buoyancy on the critical heat flux in forced convection
Brusstar, Matthew J.; Merte, Herman, Jr.
1993-01-01
The critical heat flux (CHF) in forced convection over a flat surface at relatively low flow velocities has been found, not unexpectedly, to depend upon the orientation of the buoyancy. The CHF for R-113 was measured at various heating surface orientations for test section Reynolds numbers ranging between 3000 and 6500. In this flow range, the buoyancy force acting on the vapor generally dominates over the flow inertia, yet the inertia would still be substantial were gravity to be reduced. In the experiments of this study, the CHF is determined for heating surface orientations ranging from 0 deg to 360 deg, for flow velocities between 4 cm/s and 35 cm/s, and for subcoolings between 2.8 C and 22.2 C. The results presented here demonstrate the strong influence of buoyancy at low flow velocities, which diminishes as the flow velocity and subcooling are increased.
Debris size and buoyancy influence the dispersal distance of stranded litter.
Fazey, Francesca M C; Ryan, Peter G
2016-09-15
Recent at sea surveys of floating macro-debris in the southeast Atlantic Ocean found that debris increases in size with distance from shore, suggesting that many smaller items, which dominate litter close to urban source areas, sink before dispersing far into the ocean. We test whether this pattern is evident in beach litter in the same region. Freshly stranded beach litter was collected at increasing distances (0km, 100km, 200km and 2800km) from Cape Town, a major urban litter source. Mean size and buoyancy of litter items increased significantly with distance from Cape Town. Size-specific sedimentation due to the ballasting effect of biofouling is a plausible explanation for the disappearance of smaller, less buoyant items. Our results provide further evidence that many low buoyancy items sink and support the hypothesis that size and buoyancy are strong predictors of dispersal distance for floating debris. PMID:27389460
Analysis of buoyancy effect on fully developed laminar heat transfer in a rotating tube
Siegel, R.
1985-01-01
Laminar heat transfer is analyzed in a tube rotating about an axis perpendicular to the tube axis. The solution applies for flow that is either radially outward from the axis of rotation, or radially inward toward the axis of rotation. The conditions are fully developed, and there is uniform heat addition at the tube wall. The analysis is performed by expanding velocities and temperature in power series using the Taylor number as a perturbation parameter. Coriolis and buoyancy forces caused by tube rotation are included, and the solution is calculated through second-order terms. The secondary flow induced by the Coriolis terms always tends to increase the heat transfer coefficient; this effect can dominate for small wall heating. For radial inflow, buoyancy also tends to improve heat transfer. For radial outflow, however, buoyancy tends to reduce heat transfer; for large wall heating this effect can dominate, and there is a net reduction in heat transfer coefficient.
An improving method for micro-G simulation with magnetism-buoyancy hybrid system
Zhu, Zhanxia; Yuan, Jianping; Song, Jiangzhou; Cui, Rongxin
2016-06-01
This paper presents a novel solution for the micro-G experiment with magnetism-buoyancy hybrid system. The improvement includes two parts, (i) proposing an innovative system called general balance test bed (GBTB), and (ii) designing a resistance effect compensation system. The GBTB, a special platform, can be used to realize the effect of neutral buoyancy, by using controllable electromagnetic force instead of conventional weight or foam module to eliminate the difference between gravity and liquid buoyancy. In this paper, principles, components, and functions of the GBTB are developed. Then, in order to improve test fidelity, a compensation system is designed to counteract the water resistance effect during maneuver, and a novel prediction law is proposed to make water resistance force prediction more coincident with the real value by introducing control errors and error rates. Finally, the feasibility and effectiveness of the proposed solution are demonstrated through micro-G experiments and tests.
Buoyancy effects on the vapor condensation rate on a horizontal liquid surface
Hasan, Mohammad M.; Lin, Chin-Shun
1989-01-01
The results are presented of a numerical study of the effects of buoyancy on the direct condensation of saturated or nearly saturated vapor on a horizontal liquid surface in a cylindrical tank. The liquid motion beneath the liquid-vapor interface is induced by an axisymmetric laminar jet of subcooled liquid. Analysis and numerical results show that the dominant parameter which determines the influence of buoyancy on the condensation rate is the Richardson number. However, the effect of buoyancy on the condensation rate cannot be quantified in terms of the Richardson number alone. The critical value of the Richardson number below which the condensation rate is not significantly reduced depends on the Reynolds number as well as the Prandtl number.
Rasera, Yann
2008-01-01
In clusters of galaxies, the specific entropy of intracluster plasma increases outwards. Nevertheless, a number of recent studies have shown that the intracluster medium is subject to buoyancy instabilities due to the effects of cosmic rays and anisotropic thermal conduction. In this paper, we present a new numerical algorithm for simulating such instabilities. This numerical method treats the cosmic rays as a fluid, accounts for the diffusion of heat and cosmic rays along magnetic field lines, and enforces the condition that the temperature and cosmic-ray pressure remain positive. We carry out several tests to ensure the accuracy of the code, including the detailed matching of analytic results for the eigenfunctions and growth rates of linear buoyancy instabilities. This numerical scheme will be useful for simulating convection driven by cosmic-ray buoyancy in galaxy cluster plasmas and may also be useful for other applications, including fusion plasmas, the interstellar medium, and supernovae remnants.
Yuan, Jianping; Zhu, Zhanxia; Ming, Zhenfeng; Luo, Qiuyue
2015-07-01
This paper proposes an innovative method for simulating space microgravity effects. The new approach combines the neutral buoyancy and the electromagnetic force on the tested-body to balance the gravity and simulate the microgravity effects. In the paper, we present in some detail the magnetism-buoyancy hybrid microgravity simulation system, its components, functions and verification. We describe some key techniques such as ground-space similarity, the homogenization of electromagnetic field, the precise control of microgravity effects in dynamic environment, measurement in the hybrid suspension system. With this innovative microgravity simulation system, we prove through experiments and tests that our innovative method is feasible and effective and that the simulation fidelity is even higher than the neutral buoyancy system.
Buoyancy of a thin plate pressing a floating oil film on water.
Ji, Xiang-Ying; Feng, Xi-Qiao
2013-06-01
Because of the superhydrophobicity of their legs, water striders and many other aquatic creatures can stand and walk effortlessly on water. Because of pollution, an oil film may exist on water in some practical situations. To date, however, it remains unclear how the presence of an oil film would affect the wetting behavior of an object floating on water. In this work, we investigated, both theoretically and experimentally, the buoyancy of a thin plate pressing the surface of a bilayered liquid system. In particular, the effect of the oil layer on the buoyancy force was examined. The critical depth and the corresponding buoyancy at the penetration of the plate into the liquids were obtained analytically. For a plate vertically pressing the liquid surface, the force-displacement loop during a complete advancing-receding cycle was analyzed. Experiments were also performed to verify the theoretical results. PMID:23659380
Sharma, P; Quataert, E; Parrish, I J
2009-01-01
Using a linear stability analysis and two and three-dimensional nonlinear simulations, we study the physics of buoyancy instabilities in a combined thermal and relativistic (cosmic ray) plasma, motivated by the application to clusters of galaxies. We argue that cosmic ray diffusion is likely to be slow compared to the buoyancy time on large length scales, so that cosmic rays are effectively adiabatic. If the cosmic ray pressure $p_{cr}$ is $\\gtrsim 25 %$ of the thermal pressure, and the cosmic ray entropy ($p_{\\rm cr}/\\rho^{4/3}$; $\\rho$ is the thermal plasma density) decreases outwards, cosmic rays drive an adiabatic convective instability analogous to Schwarzschild convection in stars. Global simulations of galaxy cluster cores show that this instability saturates by reducing the cosmic ray entropy gradient and driving efficient convection and turbulent mixing. At larger radii in cluster cores, the thermal plasma is unstable to the heat flux-driven buoyancy instability (HBI), a convective instability genera...
Buoyancy-driven interannual sea level changes in the southeast tropical Pacific
Piecuch, Christopher G.; Ponte, Rui M.
2012-03-01
It is commonly held that interannual-to-decadal sea level variability patterns mainly represent the ocean's response to wind forcing. This view is based in part on modeling studies of wind-driven sea level changes along the tropical Pacific. However, because buoyancy forcing (and other generating mechanisms) are usually ignored, this paradigm may overemphasize the role of winds. Focusing on the southeast tropical Pacific, we use a data-constrained ocean state estimate to demonstrate that distinct mechanisms—including the ocean's response to buoyancy forcing as well as nonlinear processes—can also contribute to interannual sea level variability. Contrary to the notion that buoyancy-driven sea level changes are dynamically passive, such changes exhibit a strongly nonlocal, dynamically active character, made manifest in westward propagating waves. As similar findings apply elsewhere, accurate modeling of interannual-to-decadal regional sea level changes requires consideration of a variety of forcing mechanisms, including, but not limited to, the winds.
Karatay, Elif; Mani, Ali
2016-01-01
Recent investigations have revealed that ion transport from aqueous electrolytes to ion-selective surfaces is subject to electroconvective instability that stems from coupling of hydrodynamics with electrostatic forces. Electroconvection is shown to enhance ion mixing and the net rate of transport. However, systems subject to electroconvection inherently involve fluid density variation set by salinity gradient in the bulk fluid. In this study we thoroughly examine the interplay of gravitational convection and chaotic electroconvection. Our results reveal that buoyant forces can significantly influence the transport rates, otherwise set by electroconvection, when the Rayleigh number $Ra$ of the system exceeds a value $Ra \\sim 1000$. We show that buoyancy forces can significantly alter the flow patterns in these systems. When the buoyancy acts in the stabilizing direction, it limits the extent of penetration of electroconvection, but without eliminating it. When the buoyancy destabilizes the flow, it alters the...
High accuracy measurement of ThO2 kernel density by buoyancy method
Accuracy was compared between pycnometer and buoyancy methods in measuring densities of ThO2 kernels. It was found that, in order to measure with accuracy less than 0.3%, 25 g of samples were required by pycnometer method but only 1 g was by the buoyancy method. Densities of ThO2 kernels prepared by a sol-gel process were measured by the buoyancy method; sample weight dependence of the accuracy was found to agree with one calculated on the basis of limits estimated for the apparatus and attachments used. Sintered ThO2 kernel density was found to depend considerably on the conditions of each step of the sol-gel process. (author)
Swirl and buoyancy effects on mixing performance of baffle-plate-type miniature confined multijet
Tatsumi, Kazuya, E-mail: tatsumi@mbox.kudpc.kyoto-u.ac.j [Department of Mechanical Engineering and Science, Kyoto University, Kyoto 606-8501 (Japan); Tanaka, Miyako [Department of Mechanical Engineering, Osaka Prefecture University, Osaka 599-8531 (Japan); Woodfield, Peter L. [Research Center for Hydrogen Industrial Use and Storage, National Institute of Advanced Industrial Science and Technology, Fukuoka 819-0395 (Japan); Nakabe, Kazuyoshi [Department of Mechanical Engineering and Science, Kyoto University, Kyoto 606-8501 (Japan)
2010-02-15
A three-dimensional numerical simulation was carried out to examine the effects of swirl and buoyancy-driven flows on the mixing performance of a baffle-plate-type miniature confined multijet. Swirl flow was produced by inclining the jet nozzles surrounding the central jet in the circumferential direction. The results obtained were compared with those of the non-swirl case. The swirl flow interrupted the radial secondary flow generated in the region adjacent to the baffle plate. This interruption decreased the size of the reverse flow region, resulting in a deterioration of the mixing performance. This behavior was more noticeable in the case of a large swirl number. During the study of the buoyancy effect, the operating direction of the gravitational force was changed with respect to the central axis of the mixing chamber, i.e. the attack angles were 0 deg., 90 deg. and 180 deg. The buoyancy effect was negligible when the operating direction was parallel to the chamber axis. However, when the attack angle was 90 deg., asymmetric distributions of mixture fraction and flow velocity were obtained, and the mixing performance was found to be poor. In the swirl-accompanied cases, the effect of buoyancy force was apparent even when the operating direction was parallel to the central axis. The swirl flow, which reduced the secondary flow effects of the reverse and radial flows, tended to increase the effect of the buoyancy force on the multijet flow. On the other hand, the swirl flow prevented the mixture fraction distribution to be asymmetric in the case of buoyancy with 90 deg. attack angle. These tendencies became more conspicuous for flows with stronger swirl.
Overview of thermal-buoyancy-induced phenomena in reactor-plant components
Studies related to delineating the influence of thermal-buoyancy forces on the thermal-hydraulics of Liquid Metal Fast Breeder Reactor plant components under low-flow thermal transient and steady state conditions have generated unique information which will aid design of these components. Various buoyancy force induced phenomena such as thermal stratification, flow recirculation, stagnation, and channeling are described and the importance to component performance are discussed. The water based studies have been conducted in the Mixing Components Test Facility, a large multi program facility capable of performing generic studies of fluid flow and heat transfer in reactor components under programmed transient and steady state conditions
Study of Buoyancy Effects in Diffusion Flames Using Rainbow Schlieren Deflectometry
Agrawal, Ajay K.; Gollahalli, Subramanyam R.; Griffin, DeVon
1997-01-01
Diffusion flames are extensively encountered in many domestic and industrial processes. Even after many decades of research, a complete understanding of the diffusion flame structure is not available. The structure and properties of the flames are governed by the mixing (laminar or turbulent), chemical kinetics, radiation and soot processes. Another important phenomenon that affects flame structure in normal gravity is buoyancy. The presence of buoyancy has long hindered the rational understanding of many combustion processes. In gas jet diffusion flames, buoyancy affects the structure of the shear layer, the development of fluid instabilities, and formation of the coherent structures in the near nozzle region of the gas jets. The buoyancy driven instabilities generate vorticial structures outside the flame resulting in flame flicker. The vortices also strongly interact with the small-scale structures in the jet shear layer. This affects the transitional and turbulence characteristics of the flame. For a fundamental understanding of diffusion flames it is essential to isolate the effects of buoyancy. This is the primary goal of the experiments conducted in microgravity. Previous investigations, have shown dramatic differences between the jet flames in microgravity and normal gravity. It has been observed that flames in microgravity are taller and more sooty than in normal gravity. The fuels used in these experiments were primarily hydrocarbons. In the absence of buoyancy the soot resides near the flame region, which adversely affects the entrainment of reactants. It is very important to eliminate the interference of soot on flame characteristics in microgravity. The present work, therefore, focuses on the changes in the flame structure due to buoyancy without the added complexities of heterogeneous reactions. Clean burning hydrogen is used as the fuel to avoid soot formation and minimize radiative losses. Because of the low luminosity of hydrogen flames, we use
The role of negative buoyancy in convective Cepheid models. Double-mode pulsations revisited
Smolec, R
2008-01-01
The longstanding problem of modeling double-mode behaviour of classical pulsators was solved with the incorporation of turbulent convection into pulsation hydrocodes. However, the reasons for the computed double-mode behaviour were never clearly identified. In our recent papers (Smolec & Moskalik 2008a,b) we showed that the double-mode behaviour results from the neglect of negative buoyancy effects in some of the hydrocodes. If these effects are taken into account, no stable non-resonant double-mode behaviour can be found. In these proceedings we focus our attention on the role of negative buoyancy effects in classical Cepheid models.
Investigation of small-scale unintended releases of hydrogen: Buoyancy effects
Schefer, R.W.; Houf, W.G.; Williams, T.C. [Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551 (United States)
2008-09-15
Measurements were performed in small-scale hydrogen leaks to characterize the dimensional properties and flow characteristics of the resulting ignitable hydrogen cloud. The data are intended to provide a technological basis for determining hazardous length scales associated with the formation of ignitable mixtures due to unintended releases. In contrast to a previous study where momentum-dominated releases were considered, the present study focuses on smaller-scale releases at lower flow rates where buoyancy becomes important. A turbulent jet flow is selected as representative of releases in which the leak geometry is circular. Laser-based Rayleigh scattering is used to characterize the hydrogen concentration field downstream of the leak. Particle Image Velocimetry (PIV) is also used to characterize the flow velocity. Time-averaged mean and fluctuating hydrogen concentration statistics are presented and compared with results in momentum-dominated flows to elucidate the effects of buoyancy on the H{sub 2} dispersion process. Over the range of Froude numbers investigated (Fr=268, 152 and 99), increasing effects of buoyancy are seen as the Fr is reduced and at downstream locations where the influence of buoyancy increases relative to jet momentum. The primary effect of buoyancy is to increase the centerline decay rate of the time-averaged H{sub 2} mass fraction relative to momentum-dominated flows. Acceleration due to buoyancy also results in a slower decay of the time-averaged axial velocity component along the centerline. Radial profiles of the time-averaged H{sub 2} mass fraction also collapse onto the same curves as results in momentum-dominated flows when plotted against the same similarity/scaling variables. While buoyancy is found to have a negligible effect on centerline velocity fluctuations, the maximum H{sub 2} mass fraction fluctuation intensity increases by 70 percent in the buoyant regime and the peak value shifts from the mixing region to the jet
Release of radon contaminants from Yucca Mountain: The role of buoyancy driven flow
The potential for the repository heat source to promote buoyancy driven flow and thereby cause release of radon gas out of Yucca Mountain has been examined through a critical review of the theoretical and experimental studies of this process. The review indicates that steady-state buoyancy enhanced release of natural radon and other contaminant gases should not be a major concern at Yucca Mountain. Barometric pumping and wind pumping are identified as two processes that will have a potentially greater effect on surface releases of gases
Core-annular flow through a horizontal pipe: Hydrodynamic counterbalancing of buoyancy force on core
Ooms, G.; Vuik, C.; Poesio, P.
2007-01-01
A theoretical investigation has been made of core-annular flow: the flow of a high-viscosity liquid core surrounded by a low-viscosity liquid annular layer through a horizontal pipe. Special attention is paid to the question of how the buoyancy force on the core, caused by a density difference betwe
Coupling between Buoyancy Forces and Electroconvective Instability near Ion-Selective Surfaces
Karatay, Elif; Andersen, Mathias Bækbo; Wessling, Matthias; Mani, Ali
2016-05-01
Recent investigations have revealed that ion transport from aqueous electrolytes to ion-selective surfaces is subject to electroconvective instability that stems from coupling of hydrodynamics with electrostatic forces. These systems inherently involve fluid density variation set by salinity gradients. However, the coupling between the buoyancy effects and electroconvective instability has not yet been investigated although a wide range of electrochemical systems are naturally prone to these interplaying effects. In this study we thoroughly examine the interplay of gravitational convection and chaotic electroconvection. Our results reveal that buoyant forces can significantly influence the transport rates, otherwise set by electroconvection, when the Rayleigh number Ra of the system exceeds a value Ra ˜1000 . We show that buoyancy forces can significantly alter the flow patterns in these systems. When the buoyancy acts in the stabilizing direction, it limits the extent of penetration of electroconvection, but without eliminating it. When the buoyancy destabilizes the flow, it alters the electroconvective patterns by introducing upward and downward fingers of respectively light and heavy fluids.
Buoyancy frequency profiles and internal semidiurnal tide turning depths in the oceans
King, B.; Stone, M.; Zhang, H.P.; Gerkema, T.; Marder, M.; Scott, R.B.; Swinney, H.L.
2012-01-01
We examine the possible existence of internal gravity wave "turning depths," depths below which the local buoyancy frequency N(z) becomes smaller than the wave frequency. At a turning depth, incident gravity waves reflect rather than reaching the ocean bottom as is generally assumed. Here we conside
Kofoed, Jens Peter; Tedd, James; Friis-Madsen, E.;
2007-01-01
This paper reports on the real sea performance of the buoyancy control system of Wave Dragon, a floating wave energy converter using the overtopping principle. The device operates with the full independent control system which has been tested during three years of operation. The impact of the...
Coupling between Buoyancy Forces and Electroconvective Instability near Ion-Selective Surfaces.
Karatay, Elif; Andersen, Mathias Bækbo; Wessling, Matthias; Mani, Ali
2016-05-13
Recent investigations have revealed that ion transport from aqueous electrolytes to ion-selective surfaces is subject to electroconvective instability that stems from coupling of hydrodynamics with electrostatic forces. These systems inherently involve fluid density variation set by salinity gradients. However, the coupling between the buoyancy effects and electroconvective instability has not yet been investigated although a wide range of electrochemical systems are naturally prone to these interplaying effects. In this study we thoroughly examine the interplay of gravitational convection and chaotic electroconvection. Our results reveal that buoyant forces can significantly influence the transport rates, otherwise set by electroconvection, when the Rayleigh number Ra of the system exceeds a value Ra∼1000. We show that buoyancy forces can significantly alter the flow patterns in these systems. When the buoyancy acts in the stabilizing direction, it limits the extent of penetration of electroconvection, but without eliminating it. When the buoyancy destabilizes the flow, it alters the electroconvective patterns by introducing upward and downward fingers of respectively light and heavy fluids. PMID:27232024
Dissipation of buoyancy waves and turbulence in the atmosphere of venus
Izakov, M. N.
2010-12-01
The turbulent energy dissipation rate and the coefficients of turbulent diffusion and viscosity caused by breaking buoyancy waves (BWs) have been calculated. From the comparison of these values with other data, the contribution of BWs to the generation of turbulence has been determined. The comparison confirms the validity of the turbulence characteristics of the Venusian troposphere previously calculated from experimental data.
46 CFR 160.010-5 - Buoyant apparatus with plastic foam buoyancy.
2010-10-01
... 46 Shipping 6 2010-10-01 2010-10-01 false Buoyant apparatus with plastic foam buoyancy. 160.010-5 Section 160.010-5 Shipping COAST GUARD, DEPARTMENT OF HOMELAND SECURITY (CONTINUED) EQUIPMENT, CONSTRUCTION, AND MATERIALS: SPECIFICATIONS AND APPROVAL LIFESAVING EQUIPMENT Buoyant Apparatus for Merchant Vessels § 160.010-5 Buoyant apparatus...
Non-uniqueness of the point of application of the buoyancy force
Kliava, Janis; Megel, Jacques, E-mail: janis.kliava@u-bordeaux1.f [CPMOH UMR 5798 Universite de Bordeaux1-CNRS, 351 cours de la Liberation, 33405 Talence cedex (France)
2010-07-15
Even though the buoyancy force (also known as the Archimedes force) has always been an important topic of academic studies in physics, its point of application has not been explicitly identified yet. We present a quantitative approach to this problem based on the concept of the hydrostatic energy, considered here for a general shape of the cross-section of a floating body and for an arbitrary angle of heel. We show that the location of the point of application of the buoyancy force essentially depends (i) on the type of motion experienced by the floating body and (ii) on the definition of this point. In a rolling/pitching motion, considerations involving the rotational moment lead to a particular dynamical point of application of the buoyancy force, and for some simple shapes of the floating body this point coincides with the well-known metacentre. On the other hand, from the work-energy relation it follows that in the rolling/pitching motion the energetical point of application of this force is rigidly connected to the centre of buoyancy; in contrast, in a vertical translation this point is rigidly connected to the centre of gravity of the body. Finally, we consider the location of the characteristic points of the floating bodies for some particular shapes of immersed cross-sections. The paper is intended for higher education level physics teachers and students.
Buoyancy driven mixing of miscible fluids by volumetric energy deposition of microwaves.
Wachtor, Adam J; Mocko, Veronika; Williams, Darrick J; Goertz, Matthew P; Jebrail, Farzaneh F
2013-01-01
An experiment that seeks to investigate buoyancy driven mixing of miscible fluids by microwave volumetric energy deposition is presented. The experiment involves the use of a light, non-polar fluid that initially rests on top of a heavier fluid which is more polar. Microwaves preferentially heat the polar fluid, and its density decreases due to thermal expansion. As the microwave heating continues, the density of the lower fluid eventually becomes less than that of the upper, and buoyancy driven Rayleigh-Taylor mixing ensues. The choice of fluids is crucial to the success of the experiment, and a description is given of numerous fluid combinations considered and characterized. After careful consideration, the miscible pair of toluene/tetrahydrofuran (THF) was determined as having the best potential for successful volumetric energy deposition buoyancy driven mixing. Various single fluid calibration experiments were performed to facilitate the development of a heating theory. Thereafter, results from two-fluid mixing experiments are presented that demonstrate the capability of this novel Rayleigh-Taylor driven experiment. Particular interest is paid to the onset of buoyancy driven mixing and unusual aspects of the experiment in the context of typical Rayleigh-Taylor driven mixing. PMID:24779141
Sukoriansky, Semion; Galperin, Boris
2013-01-13
The buoyancy subrange of stably stratified turbulence is defined as an intermediate range of scales larger than those in the inertial subrange. This subrange encompasses the crossover from internal gravity waves (IGWs) to small-scale turbulence. The energy exchange between the waves and small-scale turbulence is communicated across this subrange. At the same time, it features progressive anisotropization of flow characteristics on increasing spatial scales. Despite many observational and computational studies of the buoyancy subrange, its theoretical understanding has been lagging. This article presents an investigation of the buoyancy subrange using the quasi-normal scale elimination (QNSE) theory of turbulence. This spectral theory uses a recursive procedure of small-scale modes elimination based upon a quasi-normal mapping of the velocity and temperature fields using the Langevin equations. In the limit of weak stable stratification, the theory becomes completely analytical and yields simple expressions for horizontal and vertical eddy viscosities and eddy diffusivities. In addition, the theory provides expressions for various one-dimensional spectra that quantify turbulence anisotropization. The theory reveals how the dispersion relation for IGWs is modified by turbulence, thus alleviating many unique waves' features. Predictions of the QNSE theory for the buoyancy subrange are shown to agree well with various data. PMID:23185059
Non-uniqueness of the point of application of the buoyancy force
Even though the buoyancy force (also known as the Archimedes force) has always been an important topic of academic studies in physics, its point of application has not been explicitly identified yet. We present a quantitative approach to this problem based on the concept of the hydrostatic energy, considered here for a general shape of the cross-section of a floating body and for an arbitrary angle of heel. We show that the location of the point of application of the buoyancy force essentially depends (i) on the type of motion experienced by the floating body and (ii) on the definition of this point. In a rolling/pitching motion, considerations involving the rotational moment lead to a particular dynamical point of application of the buoyancy force, and for some simple shapes of the floating body this point coincides with the well-known metacentre. On the other hand, from the work-energy relation it follows that in the rolling/pitching motion the energetical point of application of this force is rigidly connected to the centre of buoyancy; in contrast, in a vertical translation this point is rigidly connected to the centre of gravity of the body. Finally, we consider the location of the characteristic points of the floating bodies for some particular shapes of immersed cross-sections. The paper is intended for higher education level physics teachers and students.
Investigating Students' Ideas About Buoyancy and the Influence of Haptic Feedback
Minogue, James; Borland, David
2016-04-01
While haptics (simulated touch) represents a potential breakthrough technology for science teaching and learning, there is relatively little research into its differential impact in the context of teaching and learning. This paper describes the testing of a haptically enhanced simulation (HES) for learning about buoyancy. Despite a lifetime of everyday experiences, a scientifically sound explanation of buoyancy remains difficult to construct for many. It requires the integration of domain-specific knowledge regarding density, fluid, force, gravity, mass, weight, and buoyancy. Prior studies suggest that novices often focus on only one dimension of the sinking and floating phenomenon. Our HES was designed to promote the integration of the subconcepts of density and buoyant forces and stresses the relationship between the object itself and the surrounding fluid. The study employed a randomized pretest-posttest control group research design and a suite of measures including an open-ended prompt and objective content questions to provide insights into the influence of haptic feedback on undergraduate students' thinking about buoyancy. A convenience sample (n = 40) was drawn from a university's population of undergraduate elementary education majors. Two groups were formed from haptic feedback (n = 22) and no haptic feedback (n = 18). Through content analysis, discernible differences were seen in the posttest explanations sinking and floating across treatment groups. Learners that experienced the haptic feedback made more frequent use of "haptically grounded" terms (e.g., mass, gravity, buoyant force, pushing), leading us to begin to build a local theory of language-mediated haptic cognition.
Buoyancy-driven flow in a peat moss layer as a mechanism for solute transport
Rappoldt, C; Pieters, GJJM; Adema, EB; Baaijens, GJ; Grootjans, AP; van Duijn, CJ; Pieters, Gert-Jan J.M.; Adema, Erwin B.; Baaijens, Gerrit J.; Grootjans, Ab P.; Duijn, Cornelis J. van; Jury, William A.
2003-01-01
Transport of nutrients, CO2, methane, and oxygen plays an important ecological role at the surface of wetland ecosystems. A possibly important transport mechanism in a water-saturated peat moss layer (usually Sphagnum cuspidatum) is nocturnal buoyancy flow, the downward flow of relatively cold surfa
Lifetime of oil drops pressed by buoyancy against a planar interface: Large drops
Rojas, Clara; García-Sucre, Máximo; Urbina-Villalba, Germán
2010-01-01
In a previous report [10] it was shown that emulsion stability simulations are able to reproduce the lifetime of micrometer-size drops of hexadecane pressed by buoyancy against a planar water-hexadecane interface. It was confirmed that small drops (ri
Martin, Andrew J.
2013-01-01
Academic buoyancy has been defined as a capacity to overcome setbacks, challenges, and difficulties that are part of everyday academic life. Academic resilience has been defined as a capacity to overcome acute and/or chronic adversity that is seen as a major threat to a student's educational development. This study is the first to examine the…
Buoyancy driven flow in a hot water tank due to standby heat loss
Fan, Jianhua; Furbo, Simon
2012-01-01
show that the CFD model predicts satisfactorily water temperatures at different levels of the tank during cooling by standby heat loss. It is elucidated how the downward buoyancy driven flow along the tank wall is established by the heat loss from the tank sides and how the natural convection flow is...
Buoyancy-driven convection may switch between reactive states in three-dimensional chemical waves
Šebestíková, Lenka; Hauser, M. J. B.
2012-01-01
Roč. 85, č. 3 (2012), 036303. ISSN 1539-3755 R&D Projects: GA ČR GAP105/10/0919 Institutional research plan: CEZ:AV0Z20600510 Keywords : buoyancy-driven convection * chemical waves * iodate-arsenous acid reaction Subject RIV: BK - Fluid Dynamics Impact factor: 2.313, year: 2012
Unexpected Positive Buoyancy in Deep Sea Sharks, Hexanchus griseus, and a Echinorhinus cookei.
Nakamura, Itsumi; Meyer, Carl G; Sato, Katsufumi
2015-01-01
We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Sharks, for example, rely on oil-filled livers instead of gas-filled swim bladders to increase their buoyancy, but are nonetheless ubiquitously regarded as either negatively or neutrally buoyant. Deep-sea sharks have particularly large, oil-filled livers, and are believed to be neutrally buoyant in their natural habitat, but this has never been confirmed. To empirically determine the buoyancy status of two species of deep-sea sharks (bluntnose sixgill sharks, Hexanchus griseus, and a prickly shark, Echinorhinus cookei) in their natural habitat, we used accelerometer-magnetometer data loggers to measure their swimming performance. Both species of deep-sea sharks showed similar diel vertical migrations: they swam at depths of 200-300 m at night and deeper than 500 m during the day. Ambient water temperature was around 15°C at 200-300 m but below 7°C at depths greater than 500 m. During vertical movements, all deep-sea sharks showed higher swimming efforts during descent than ascent to maintain a given swimming speed, and were able to glide uphill for extended periods (several minutes), indicating that these deep-sea sharks are in fact positively buoyant in their natural habitats. This positive buoyancy may adaptive for stealthy hunting (i.e. upward gliding to surprise prey from underneath) or may facilitate evening upward migrations when muscle temperatures are coolest, and swimming most sluggish, after spending the day in deep, cold water. Positive buoyancy could potentially be widespread in fish conducting daily vertical migration in deep-sea habitats. PMID:26061525
Unexpected Positive Buoyancy in Deep Sea Sharks, Hexanchus griseus, and a Echinorhinus cookei.
Itsumi Nakamura
Full Text Available We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Sharks, for example, rely on oil-filled livers instead of gas-filled swim bladders to increase their buoyancy, but are nonetheless ubiquitously regarded as either negatively or neutrally buoyant. Deep-sea sharks have particularly large, oil-filled livers, and are believed to be neutrally buoyant in their natural habitat, but this has never been confirmed. To empirically determine the buoyancy status of two species of deep-sea sharks (bluntnose sixgill sharks, Hexanchus griseus, and a prickly shark, Echinorhinus cookei in their natural habitat, we used accelerometer-magnetometer data loggers to measure their swimming performance. Both species of deep-sea sharks showed similar diel vertical migrations: they swam at depths of 200-300 m at night and deeper than 500 m during the day. Ambient water temperature was around 15°C at 200-300 m but below 7°C at depths greater than 500 m. During vertical movements, all deep-sea sharks showed higher swimming efforts during descent than ascent to maintain a given swimming speed, and were able to glide uphill for extended periods (several minutes, indicating that these deep-sea sharks are in fact positively buoyant in their natural habitats. This positive buoyancy may adaptive for stealthy hunting (i.e. upward gliding to surprise prey from underneath or may facilitate evening upward migrations when muscle temperatures are coolest, and swimming most sluggish, after spending the day in deep, cold water. Positive buoyancy could potentially be widespread in fish conducting daily vertical migration in deep-sea habitats.
An underground nuclear waste repository produces heat that will induce a buoyancy flow of groundwater in fractures and other permeable regions in the surrounding rock. The radioactive material may then, in case of penetrated canisters, possibly reach the biosphere. Measurements of ground water in crystalline rock show an increasing salt content with depth. The resulting increase of water density counteracts the thermal buoyancy, and it may create a natural barrier for the groundwater flow between the repository and the biosphere. The aim of the study is to analyse this barrier effect and to assess the maximum upward displacement of water starting from the vicinity of the repository. The coupled flow process for groundwater, salt and heat with buoyancy due to temperature and salt concentration differences is studied. The equations have been analysed in great detail, and a numerical model has been developed for the case of groundwater flow in a fracture plane. The largest upward displacement from the repository has been determined with the model for any heat release. Approximate formulas, which are shown to be sufficiently accurate for assessments, have been derived. The main formula concerns the case, when the canisters are stacked on the top of each other in a very deep borehole. There are no restrictions in the position of the fracture plane. The borehole may even lie directly in the fracture plane. We find a strong barrier effect. In a reference case with a salt concentration increase of 2% per km downwards and with 300 canisters placed over a length of 2000 m in the borehole (the total amount of released heat is 0.32 TWh), the largest upward displacement from the top of canisters becomes, according to the formula, 60 m
Yonetsu, Daigo; Tanaka, Kazufumi; Hara, Takehisa
In recent years, induction-heating (IH) cookers that can be used to heat nonmagnetic metals such as aluminum have been produced. Occasionally, a light pan moves on a glass plate due to buoyancy when heated by an IH cooker. In some IH cookers, an aluminum plate is mounted between the glass plate and the coil in order to reduce the buoyancy effect. The objective of this research is to evaluate the buoyancy-reduction effect and the heating effect of buoyancy-reduction plates. Eddy current analysis is carried out by 3D finite element method, and the electromagnetic force and the heat distribution on the heating plate are calculated. After this calculation is performed, the temperature distribution of the heating plate is calculated by heat transfer analysis. It is found that the shape, area, and the position of the buoyancy reduction plate strongly affect the buoyancy and the heat distribution. The impact of the shape, area, and position of the buoyancy reduction plate was quantified. The phenomena in the heating were elucidated qualitatively.
Two separate effects experiments concerned with buoyancy-influenced convective heat transfer in vertical passages which have relevance to the problem of nuclear reactor containment cooling by means of buoyancy-driven airflow are described. A feature of each is that local values of heat transfer coefficient are determined on surfaces maintained at uniform temperature. Experimental results are presented which highlight the need for buoyancy-induced impairment of turbulent convective heat transfer to be accounted for in the design of such passive cooling systems. A strategy is presented for predicting the heat removal by combined convective and radiative heat transfer from a full scale nuclear reactor containment shell using such experimental results
Kareem, Semiu O.; Adesanya, Samuel O.; Vincent, Uchechukwu E.
2016-08-01
This paper examines the combined effects of the buoyancy force and of the magnetic field on the entropy generation rate in the flow of a couple stress fluid through a porous vertical channel. The flow's dynamical equations were non-dimensionalised and solved via the application of the Adomian decomposition method (ADM). Variations of some thermo-physical parameters were conducted and discussed, with regard to the physics of the fluid. Our result shows that the entropy generation rate increases as the buoyancy increases in the fluid. In addition, the irreversibility in the flow system results mainly from the fluid's viscosity, ohmic heating, and the buoyancy.
A study of the effects of macrosegregation and buoyancy-driven flow in binary mixture solidification
Sinha, S. K.; Sundararajan, T.; Garg, V. K.
1993-01-01
A generalized anisotropic porous medium approach is developed for modelling the flow, heat and mass transport processes during binary mixture solidification. Transient predictions are obtained using FEM, coupled with an implicit time-marching scheme, for solidification inside a two-dimensional rectangular enclosure. A parametric study focusing attention on the effects of solutal buoyancy and thermal buoyancy is presented. It is observed that three parameters, namely the thermal Rayleigh number, the solutal Rayleigh number, and the relative density change parameter, significantly alter the flow fields in the liquid and the mushy regions. Depending upon the nature of these flow fields, the solute enrichment caused by macrosegregation may occur in the top or the bottom region of the enclosure.
Mathematical modelling of nucleation and growth of crystals with buoyancy effects
Alexandrov, D. V.
2016-04-01
A complete analytical solution of the integro-differential model describing the nucleation of crystals and their subsequent growth in a binary system with allowance for buoyancy forces is constructed. An exact analytical solution of the Fokker-Planck-type equation for the three-parameter density distribution function is found for arbitrary nucleation kinetics. Two important cases of the Weber-Volmer-Frenkel-Zel'dovich and Meirs kinetics are considered in some detail. It is shown that the solute concentration decreases and the distribution function increases with increasing the melt supercooling (with increasing the depth of a metastable system). It is demonstrated that the distribution function attains its minimum at a certain size of crystals owing to buoyancy forces.
Buoyancy Effects on Flow Transition in Hydrogen Gas Jet Diffusion Flames
Albers, Burt W.; Agrawal, Ajay K.; Griffin, DeVon (Technical Monitor)
2000-01-01
Experiments were performed in earth-gravity to determine how buoyancy affected transition from laminar to turbulent flow in hydrogen gas jet diffusion flames. The jet exit Froude number characterizing buoyancy in the flame was varied from 1.65 x 10(exp 5) to 1.14 x 10(exp 8) by varying the operating pressure and/or burner inside diameter. Laminar fuel jet was discharged vertically into ambient air flowing through a combustion chamber. Flame characteristics were observed using rainbow schlieren deflectometry, a line-of-site optical diagnostic technique. Results show that the breakpoint length for a given jet exit Reynolds number increased with increasing Froude number. Data suggest that buoyant transitional flames might become laminar in the absence of gravity. The schlieren technique was shown as effective in quantifying the flame characteristics.
The effects of buoyancy on turbulent nonpremixed jet flames in crossflow
Boxx, Isaac G.
An experimental research study was conducted to investigate what effect buoyancy had on the mean and instantaneous flow-field characteristics of turbulent jet-flames in crossflow (JFICF). The study used an experimental technique wherein a series of normal-gravity, hydrogen-diluted propane JFICF were compared with otherwise identical ones in low-gravity. Experiments were conducted at the University of Texas Drop Tower Facility, a new microgravity science laboratory built for this study at the University of Texas at Austin. Two different diagnostic techniques were employed, high frame-rate digital cinematographic imaging and planar laser Mie scattering (PLMS). The flame-luminosity imaging revealed significant elongation and distortion of the large-scale luminous structure of the JFICF. This was seen to affect the flametip oscillation and burnout characteristics. Mean and root-mean-square (RMS) images of flame-luminosity were computed from the flame-luminosity image sequences. These were used to compare visible flame-shapes, flame chord-lengths and jet centerline-trajectories of the normal- and low-gravity flames. In all cases the jet-centerline penetration and mean luminous flame-width were seen to increase with decreasing buoyancy. The jet-centerline trajectories for the normal-gravity flames were seen to behave differently to those of the low-gravity flames. This difference led to the conclusion that the jet transitions from a momentum-dominated forced convection limit to a buoyancy-influenced regime when it reaches xiC ≈ 3, where xiC is the Becker and Yamazaki (1978) buoyancy parameter based on local flame chord-length. The mean luminous flame-lengths showed little sensitivity to buoyancy or momentum flux ratio. Consistent with the flame-luminosity imaging experiments, comparison of the instantaneous PLMS flow-visualization images revealed substantial buoyancy-induced elongation and distortion of the large-scale shear-layer vortices in the flow. This effect
Nekrasov, Anatoly K
2010-01-01
We develop a theory of buoyancy instabilities of the electron-ion plasma with the heat flux based on not the MHD equations, but using the multicomponent plasma approach. We investigate a geometry in which the background magnetic field, gravity, and stratification are directed along one axis. No simplifications usual for the MHD-approach in studying these instabilities are used. The background electron thermal flux and collisions between electrons and ions are included. We derive the simple dispersion relation, which shows that the thermal flux perturbation generally stabilizes an instability. There is a narrow region of the temperature gradient, where an instability is possible. This result contradicts to a conclusion obtained in the MHD-approach. We show that the reason of this contradiction is the simplified assumptions used in the MHD analysis of buoyancy instabilities and the role of the longitudinal electric field perturbation, which is not captured by the MHD equations. Our dispersion relation also show...
Nekrasov, Anatoly
2011-01-01
We investigate electromagnetic buoyancy instabilities of the electron-ion plasma with the heat flux based on not the magnetohydrodynamic (MHD) equations, but using the multicomponent plasma approach when the momentum equations are solved for each species. We consider a geometry in which the background magnetic field, gravity, and stratification are directed along one axis. The nonzero background electron thermal flux is taken into account. Collisions between electrons and ions are included in the momentum equations. No simplifications usual for the one-fluid MHD-approach in studying these instabilities are used. We derive a simple dispersion relation, which shows that the thermal flux perturbation generally stabilizes an instability for the geometry under consideration. This result contradicts to conclusion obtained in the MHD-approach. We show that the reason of this contradiction is the simplified assumptions used in the MHD analysis of buoyancy instabilities and the role of the longitudinal electric field ...
Low-buoyancy thermochemical plumes resolve controversy of classical mantle plume concept.
Dannberg, Juliane; Sobolev, Stephan V
2015-01-01
The Earth's biggest magmatic events are believed to originate from massive melting when hot mantle plumes rising from the lowermost mantle reach the base of the lithosphere. Classical models predict large plume heads that cause kilometre-scale surface uplift, and narrow (100 km radius) plume tails that remain in the mantle after the plume head spreads below the lithosphere. However, in many cases, such uplifts and narrow plume tails are not observed. Here using numerical models, we show that the issue can be resolved if major mantle plumes contain up to 15-20% of recycled oceanic crust in a form of dense eclogite, which drastically decreases their buoyancy and makes it depth dependent. We demonstrate that, despite their low buoyancy, large enough thermochemical plumes can rise through the whole mantle causing only negligible surface uplift. Their tails are bulky (>200 km radius) and remain in the upper mantle for 100 millions of years. PMID:25907970
Tropical cloud buoyancy is the same in a world with or without ice
Seeley, Jacob T.; Romps, David M.
2016-04-01
When convective clouds grow above the melting line, where temperatures fall below 0°C, condensed water begins to freeze and water vapor is deposited. These processes release the latent heat of fusion, which warms cloud air, and many previous studies have suggested that this heating from fusion increases cloud buoyancy in the upper troposphere. Here we use numerical simulations of radiative-convective equilibrium with and without ice processes to argue that tropical cloud buoyancy is not systematically higher in a world with fusion than in a world without it. This insensitivity results from the fact that the environmental temperature profile encountered by developing tropical clouds is itself determined by convection. We also offer a simple explanation for the large reservoir of convective available potential energy in the tropical upper troposphere that does not invoke ice.
Numerical and Experimental Study on Negative Buoyance Induced Vortices in N-Butane Jet Flames
Xiong, Yuan
2015-07-26
Near nozzle flow field in flickering n-butane diffusion jet flames was investigated with a special focus on transient flow patterns of negative buoyance induced vortices. The flow structures were obtained through Mie scattering imaging with seed particles in a fuel stream using continuous-wave (CW) Argon-ion laser. Velocity fields were also quantified with particle mage velocimetry (PIV) system having kHz repetition rate. The results showed that the dynamic motion of negative buoyance induced vortices near the nozzle exit was coupled strongly with a flame flickering instability. Typically during the flame flickering, the negative buoyant vortices oscillated at the flickering frequency. The vortices were distorted by the flickering motion and exhibited complicated transient vortical patterns, such as tilting and stretching. Numerical simulations were also implemented based on an open source C++ package, LaminarSMOKE, for further validations.
Low-buoyancy thermochemical plumes resolve controversy of classical mantle plume concept
Dannberg, Juliane; Sobolev, Stephan V.
2015-01-01
The Earth's biggest magmatic events are believed to originate from massive melting when hot mantle plumes rising from the lowermost mantle reach the base of the lithosphere. Classical models predict large plume heads that cause kilometre-scale surface uplift, and narrow (100 km radius) plume tails that remain in the mantle after the plume head spreads below the lithosphere. However, in many cases, such uplifts and narrow plume tails are not observed. Here using numerical models, we show that the issue can be resolved if major mantle plumes contain up to 15–20% of recycled oceanic crust in a form of dense eclogite, which drastically decreases their buoyancy and makes it depth dependent. We demonstrate that, despite their low buoyancy, large enough thermochemical plumes can rise through the whole mantle causing only negligible surface uplift. Their tails are bulky (>200 km radius) and remain in the upper mantle for 100 millions of years. PMID:25907970
BUOYANCY INSTABILITY IN THE NATURAL CONVECTION BOUNDARY LAYER AROUND A VERTICAL HEATED FLAT PLATE
颜大椿; 张汉勋
2002-01-01
A systematic research on the buoyancy instability in the natural convection boundary layer was conducted, including the basic characteristics such as its spectral components, wave length and velocity, the location of its critical layer,and amplitude distributions of the triple independent eigenmodes with the linear instability theory, the growth rates of its temperature and velocity fluctuations and the corresponding neutral curves for the buoyancy eigenmode were also obtained.Results indicated that the neutral curve of the velocity fluctuation had a nose shape consistent with that obtained in the numerical calculation, but for the temperature fluctuation, a ring-like region could be measured at a lower Grashof number before the nose-shaped main portion of the neutral curve.
The effect of surface buoyancy gradients on oceanic Rossby wave propagation
Xiao, Xiao(Institute for Strings, Cosmology and Astroparticle Physics (ISCAP) and Physics Department, Columbia University, 538 West 120th Street, New York, NY, 10027 U.S.A.); Smith, K. Shafer; Keating, Shane R.
2014-01-01
Motivated by the discrepancy between satellite observations of coherent westward propagating surface features and Rossby wave theory, this paper revisits the planetary wave propagation problem, taking into account the effects of lateral buoyancy gradients at the ocean's surface. The standard theory for long baroclinic Rossby waves is based on an expansion of the quasigeostrophic stretching operator in normal modes, $\\phi_n(z)$, satisfying a Neumann boundary condition at the surface, $\\phi_n'(...
Asymptotic conditions for the use of linear ventilation models in the presence of buoyancy forces
Cao, Shijie; Meyers, Johan
2014-01-01
Low-dimensional discrete linear ventilation models have been studied by Cao and Meyers (2012). In the present study, we investigate the validity and applicability of linear ventilation models for heavy-gas dispersion by employing Reynolds-averaged Navier-Stokes (RANS) simulations. A simple benchmark ventilation case is considered under isothermal condition. Considering large density differences from pollutant gas and fresh air, the effect of buoyancy force has been taken into account in turbu...
Folkins, I.
2005-01-01
International audience In regions of the tropics undergoing active deep convection, the variation of lower tropospheric lapse rates (2.0 km to 5.2 km) with height is inconsistent with both reversible moist adiabatic and pseudoadiabatic assumptions. It is argued that this anomalous behavior arises from the tendency for the divergence of a convective buoyancy anomaly to be primarily offset by the collective divergence of other updrafts and downdrafts within one Rossby radius of deformation. ...
Kiliyanpilakkil, V P; Ruiz-Columbié, A; Araya, G; Castillo, L; Hirth, B; Burgett, W
2015-01-01
We have analyzed long-term wind speed time-series from five field sites up to a height of 300 m from the ground. Structure function-based scaling analysis has revealed that the scaling exponents in the mesoscale regime systematically depend on height. This anomalous behavior is shown to be caused by the buoyancy effects. In the framework of the extended self-similarity, the relative scaling exponents portray quasi-universal behavior.
Jeetendra Singh Negi; Abhinav Trivedi; Praveen Khanduri; Vandana Negi; Nikhil Kasliwal
2011-01-01
The purpose of this study was to investigate effect of bioadhesion on the initial in vitro buoyancy behaviour of effervescent matrix tablets of ciprofloxacin HCl (CIPRO). Tablets were prepared by direct compression using HPMC K4M and Carbopol 971P as hydrophilic-controlled release polymers, sodium bicarbonate (NaHCO 3 ) as gas-generating agent, polyplasdone XL, Explotab and Ac-Di-Sol as swelling agents. Tablets were evaluated for normal and modified initial in vitro floating behavior, floatin...
The buoyancy convection during directional solidification of AlZn eutectic
PrazÁk, M.; Procio, M.; Holecek, S.
1993-01-01
A study has been made of the effect of buoyancy convection during the directional solidification of AlZn eutectic alloy. Experiments have been conducted using a Bridgman-Stockbarger arrangement with the furnace moving along the specimen. The apparatus rotated around the horizontal axis, which made it possible to carry out measurements at different angles β contained by the gravity and temperature gradient vectors in the specimen. The anisotropy of both the linear thermal expansion coefficient...
Experimental parameterisation of principal physics in buoyancy variations of marine teleost eggs
Kyung-Mi Jung; Arild Folkvord; Olav Sigurd Kjesbu; Svein Sundby
2014-01-01
It is generally accepted that the high buoyancy of pelagic marine eggs is due to substantial influx of water across the cell membrane just before ovulation. Here we further develop the theoretical basis by applying laboratory observations of the various components of the fertilized egg in first-principle equations for egg specific gravity (ρ(egg)) followed by statistical validation. We selected Atlantic cod as a model animal due to the affluent amount of literature on this species, but also u...
Craton stability and longevity: The roles of composition-dependent rheology and buoyancy
Wang, Hongliang; van Hunen, Jeroen; Pearson, D. Graham; Allen, Mark B.
2014-01-01
Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. High strength of the cratonic root is generally considered to be the most important factor, but the role of lithospheric mantle depletion and dehydration in this strengthening is still debated. Geodynamical models often argue for a significant strength or buoyancy contrast between cratonic and non-cratonic mantle lithosphere, induced by mantle de...
Mangor-Jensen, Anders; Jelmert, Anders
1986-01-01
The effect of ambient salinity on buoyancy and the formation of perivitelline fluid in eggs from the Atlantic halibut Hippoglossus hippoglossus have been investigated. The results clearly demonstrate that the water balance of the eggs are independant of the ambient salinity the first days after fertilization. The water loss from eggs fertilized in 17 ppt saline sea water was not less than from eggs fertilized in 34 ppt sea water in spite of a reduced osmotic gradient. Nei...
Turbulence-induced secondary motion in a buoyancy-driven flow in a circular pipe
Hallez, Yannick; Magnaudet, Jacques
2009-01-01
We analyze the results of a direct numerical simulation of the turbulent buoyancy-driven flow that sets in after two miscible fluids of slightly different densities have been initially superimposed in an unstable configuration in an inclined circular pipe closed at both ends. In the central region located midway between the end walls, where the flow is fully developed, the resulting mean flow is found to exhibit nonzero secondary velocity components in the tube cross section. We present a det...
Idowu Amos Sesan; Joseph K. Moses; Onwubuoya Cletus; Joseph W D
2013-01-01
The viscous dissipation and buoyancy effects on laminar convection with transpiration are investigated. Uniform and asymmetric temperatures are prescribed at the channel walls. The velocity field is considered as parallel. A perturbation method is employed to solve the momentum balance equation and the energy balance equation. A comparison with the velocity and temperature profiles in the case of laminar forced convection with various dissipation is performed in order to point out the effect ...
Combined Effect of Buoyancy Force and Navier Slip on Entropy Generation in a Vertical Porous Channel
Oluwole Daniel Makinde; Adetayo Samuel Eegunjobi
2012-01-01
In this paper, we investigate the combined effects of buoyancy force and Navier slip on the entropy generation rate in a vertical porous channel with wall suction/injection. The nonlinear model problem is tackled numerically using Runge–Kutta–Fehlberg method with shooting technique. Both the velocity and temperature profiles are obtained and utilized to compute the entropy generation number. The effects of slip parameter, Brinkmann number, the Peclet number and suction/injection Reynolds numb...
Knutsen, David Mikal
2012-01-01
A new loading and discharge concept for ships, referred to as the Universal Buoyancy Concept (UBC), is proposed as an effective alternative to land based infrastructure for distribution of primarily LNG. The UBC system consists of a slack moored stepped spar buoy equipped with pads for shipside vacuum attachment. The buoy is connected to shore based tank facilities with a flexible cryogenic riser, thus providing opportunity for offshore loading and discharge of cryogenic liquids. In the follo...
The ALTEC- Neutral Buoyancy Test Facility: A Powerful Tool for Physiology and Neuroscience Research
Benassai, Mario
2013-02-01
Recently introduced by the European Space Agency in the list of Ground Based Facilities (GBF), the ALTEC Neutral Buoyancy Test Facility (NBTF) is a unique item: this is the greatest pool completely out of the ground in Europe. Its four huge windows - one facing on a dedicated control room connected to subjects via audio and video links - allow a complete monitoring of the subject / experiment scenario
Buoyancy waves in Pluto’s high atmosphere: Implications for stellar occultations
Hubbard, W. B.; McCarthy, D. W.; Kulesa, C. A.; Benecchi, S. D.; Person, M. J.; Elliot, J. L.; Gulbis, A. A. S.
2009-11-01
We apply scintillation theory to stellar signal fluctuations in the high-resolution, high signal/noise, dual-wavelength data from the MMT observation of the 2007 March 18 occultation of P445.3 by Pluto. A well-defined high wavenumber cutoff in the fluctuations is consistent with viscous-thermal dissipation of buoyancy waves (internal gravity waves) in Pluto's high atmosphere, and provides strong evidence that the underlying density fluctuations are governed by the gravity-wave dispersion relation.
Experimental Study of Wind-Opposed Buoyancy-Driven Natural Ventilation
Andersen, A.; Bjerre, M.; Chen, Z. D.;
Natural ventilation driven by natural forces, i.e. wind and thermal buoyancy, is an environmentally friendly system for buildings and has been increasingly used around the world in recent years to mitigate the impact on the global environment due to the significant energy consumption by heating......, ventilation and air-conditioning (HV AC). There is a need for the understanding and development of theories and tools related to the design, operation and control of natural ventilation systems....
Buoyancy efects on three-dimensional waves of iodate-arsenous acid reactions
Šebestíková, Lenka
Gothenburg: University of Gothenburg, 2012 - (Mehlig, B.; Ghavami, O.; Östlund, S.; Hanstorp, D.). s. 136 ISBN 978-91-637-1501-3. [Dynamics Days Europe /32./. 02.09.2012-07.09.2012, Gothenburg] R&D Projects: GA ČR GAP105/10/0919 Institutional research plan: CEZ:AV0Z20600510 Keywords : buoyancy * chemical waves * iodate-arsenous acid reaction Subject RIV: BK - Fluid Dynamics
Piazza, Roberto; Buzzaccaro, Stefano; Secchi, Eleonora; Parola, Alberto
2012-01-01
Particle settling is a pervasive process in nature, and centrifugation is a much versatile separation technique. Yet, the results of settling and ultracentrifugation experiments often appear to contradict the very law on which they are based: Archimedes Principle - arguably, the oldest Physical Law. The purpose of this paper is delving at the very roots of the concept of buoyancy by means of a combined experimental-theoretical study on sedimentation profiles in colloidal mixtures. Our analysi...
Buoyant balaenids: the ups and downs of buoyancy in right whales.
Nowacek, D. P.; Johnson, M P; Tyack, P.L.; Shorter, K. A.; McLellan, W. A.; Pabst, D.A.
2001-01-01
A variety of marine mammal species have been shown to conserve energy by using negative buoyancy to power prolonged descent glides during dives. A new non-invasive tag attached to North Atlantic right whales recorded swim stroke from changes in pitch angle derived from a three-axis accelerometer. These results show that right whales are positively buoyant near the surface, a finding that has significant implications for both energetics and management. Some of the most powerful fluke strokes o...
Predictions of buoyancy-induced flow in asymmetrical heated rotating cavity system
This paper presents the finite difference solutions for buoyancy-induced flow in the asymmetrical heated rotating cavity system for the range of rotational Reynolds numbers Re =6.13x10/sup 5/< Re/sub theta/=4.4*10/sup 6/ and the mass flow rates C/sub w/ <28000< C/sub w/ <3000. All the simulations have been carried out through the CFD (computational Fluid Dynamics) commercial code, ANSYS Fluent 12.0, by adopting axisymmetric, steady-state and elliptic technique. Two well know models namely k-epsilon and the Reynolds stress models have been employed. The simulated results illustrate the important aspects of the heated rotating cavity flow system. The noteworthy influence of buoyancy-induced flow have been observed on the predicted stream line, static temperature contours and the local Nusselt numbers for the rotating cavity flow system. The noteworthy influence of buoyancy-induced flow have been observed on the predicted stream lines, static temperature contours and the local Nusselt numbers for the rotating cavity space. A comparison of the predicted local nusselt numbers for the hot and cold discs showed a good level of agreement with the measurement. (author)
Schlieren Measurements of Buoyancy Effects on Flow Transition in Low-Density Gas Jets
Pasumarthi, Kasyap S.; Agrawal, Ajay K.
2005-01-01
The transition from laminar to turbulent flow in helium jets discharged into air was studied using Rainbow Schlieren Deflectometry technique. In particular, the effects of buoyancy on jet oscillations and flow transition length were considered. Experiments to simulate microgravity were conducted in the 2.2s drop tower at NASA Glenn Research Center. The jet Reynolds numbers varied from 800 to1200 and the jet Richardson numbers ranged between 0.01 and 0.004. Schlieren images revealed substantial variations in the flow structure during the drop. Fast Fourier Transform (FFT) analysis of the data obtained in Earth gravity experiments revealed the existence of a discrete oscillating frequency in the transition region, which matched the frequency in the upstream laminar regime. In microgravity, the transition occurred farther downstream indicating laminarization of the jet in the absence of buoyancy. The amplitude of jet oscillations was reduced by up to an order of magnitude in microgravity. Results suggest that jet oscillations were buoyancy induced and that the brief microgravity period may not be sufficient for the oscillations to completely subside.