Concept Of Buoyancy Research Articles

The unbearable heaviness of colloids: facts, surprises, and puzzles in sedimentation

Sedimentation has played a key role in the development of colloid science. In fact, it is because of the celebrated experiments by Perrin, yielding a concrete demonstration of molecular reality and giving strong support to Einstein’s theory of Brownian motion, that colloids enter the realm of basic physics. Subsequent investigations have shown that a lot more can be learnt both from sedimentation equilibrium and from particle settling dynamics. These advances, together with new experimental approaches, will be reviewed in this paper. Yet, we shall also show that inquiring about gravity settling is far from being a closed matter: for instance, the concept of buoyancy for a settling colloidal mixture is far from being obvious. Moreover, sedimentation holds novel surprises, such as colloidal inflations and settling disasters, showing that a simple external field like gravity may induce mind-boggling, and theoretically challenging effects.

On the modeling of aiding mixed convection in vertical channels

This paper aims at showing that to prescribe a flow rate at the inlet section of a vertical channel with heated walls leads to surprising and counterintuitive physical solutions, especially when the problem is modeled as elliptical. Such an approach can give rise to the onset of recirculation cells in the entry region while the heat transfer is slightly increased under the influence of the buoyancy force. We suggest an alternative model based on more realistic boundary conditions based on a prescribed total pressure at the inlet and a fixed pressure at the outlet sections. In this case, the pressure and buoyancy forces act effectively in the same direction and, the concept of buoyancy aiding convection makes sense. The numerical results emphasize the large differences between solutions based on prescribed inlet velocity and those obtained with the present pressure-based boundary conditions.

How Heavy Is a Balloon? Using the Ideal Gas Law

This activity makes use of the Ideal Gas Law to determine the mass of a helium-filled Mylar balloon, if it were empty. The concept of buoyancy is explored as weights are attached to the balloon to bring it to neutral buoyancy, at which point the mass of the air displaced by the balloon is equal to the mass of the empty balloon, the helium inside, and the attached weights. Out of this relationship, the mass of the empty balloon is calculated and compared to its actual empty mass.

Buoyancy in tropical cyclones and other rapidly rotating atmospheric vortices

Motivated primarily by its application to understanding tropical-cyclone intensification and maintenance, we re-examine the concept of buoyancy in rapidly rotating vortices, distinguishing between the buoyancy of the symmetric balanced vortex or system buoyancy, and the local buoyancy associated with cloud dynamics. The conventional definition of buoyancy is contrasted with a generalized form applicable to a vortex, which has a radial as well as a vertical component. If, for the special case of axisymmetric motions, the balanced density and pressure distribution of a rapidly rotating vortex are used as the reference state, the buoyancy field then characterizes the unbalanced density perturbations, i.e. the local buoyancy. We show how to determine such a reference state without approximation. The generation of the toroidal circulation of a vortex, which is necessary for vortex amplification, is characterized in the vorticity equation by the baroclinicity vector. This vector depends, inter-alia, on the horizontal (or radial) gradient of buoyancy evaluated along isobaric surfaces. We show that for a tropical-cyclone-scale vortex, the buoyancy so calculated is significantly different from that calculated at constant height or on surfaces of constant σ ( σ = ( p − p *)/( p s − p *), where p is the actual pressure, p * some reference pressure and p s is the surface pressure). Since many tropical-cyclone models are formulated using σ-coordinates, we examine the calculation of buoyancy on σ-surfaces and derive an expression for the baroclinicity vector in σ-coordinates. The baroclinic forcing term in the azimuthal vorticity equation for an axisymmetric vortex is shown to be approximately equal to the azimuthal component of the curl of the generalized buoyancy. A scale analysis indicates that the vertical gradient of the radial component of generalized buoyancy makes a comparatively small contribution to the generation of toroidal vorticity in a tropical cyclone, but may be important in tornadoes and possibly also in dust devils. We derive also a form of the Sawyer–Eliassen equation from which the toroidal (or secondary) circulation of a balanced vortex may be determined. The equation is shown to be the time derivative of the toroidal vorticity equation in which the time rate-of-change of the material derivative of potential toroidal vorticity is set to zero. In analogy with the general case, the diabatic forcing term in the Sawyer–Eliassen equation is shown to be approximately equal to the time rate-of-change of the azimuthal component of the curl of generalized buoyancy. Finally, we discuss the generation of buoyancy in tropical cyclones and contrast the definitions of buoyancy that have been used in recent studies of tropical cyclones. We emphasize the non-uniqueness of the buoyancy force, which depends on the choice of a reference density and pressure, and note that different, but equivalent interpretations of the flow dynamics may be expected to arise if different reference quantities are chosen.

Conceptual Acquisition and Change through Social Interaction

The present article begins with an examination of where collective knowledge construction is likely to occur. After a brief discussion of the conditions necessary for inventing rather than simply reproducing knowledge through social interaction (horizontal information flow, prior domain knowledge, and availability of intellectual tools), a prototype of such interaction is offered – a class taught using the Hypothesis-Experiment-Instruction method. Experimental data are then presented on the effects of discussion on college students’ ability to learn the concept of buoyancy. Finally, it is proposed that conceptual acquisition or change is induced through social interaction, such as a science activity, although the acquisition or change occurs in the individual’s mind. The role that meta-cognitive beliefs play in scientific knowledge is emphasized.

Errors from Using Conservation of Buoyancy Concept in Plume Computations

The conservation of buoyancy concept as currently applied to buoyant jets and plumes in water is reviewed with regard to its origin, validity, and accuracy. Although the original buoyant plume model was based on the assumption of a constant thermal coefficient of volumetric expansion, and thus was limited to temperature differences of a few degrees, the original model has been widely applied to cases involving temperature differences as high as 17°C (30.6°F). Such application results in buoyancy forces which are significantly larger than the more accurate values based on models using conservation of energy instead of buoyancy. In the case of the computed trajectories the errors are most noticeable for lower ambient temperatures and lower jet exit velocities. The basic point is that conservation of buoyancy is an approximation and not a fundamental conservation principle. Because of the current use of numerical solutions to the governing equations there is no real justification for its use in place of the more rigorous conservation of energy equation.