Buoyancy phenomenon

In quiescent liquids and in bubble columns, buoyancy-driven coalescence is more important. Large fluid particles with a freely moving surface will also have a low-pressure region at the edge of the particle where the velocity is maximum. This low-pressure region will not only allow the bubble to stretch out and form a spherical cap but also allow other bubbles to move into that area and coalesce. Figure 15.14 shows an example of this phenomenon. 

Because of nonlinear Interactions between buoyancy, viscous and Inertia terms multiple stable flow fields may exist for the same parameter values as also predicted by Kusumoto et al (M.). The bifurcations underlying this phenomenon may be computed by the techniques described In the numerical analysis section. The solution structure Is Illustrated In Figure 7 In terms of the Nusselt number (Nu, a measure of the growth rate) for varying Inlet flow rate and susceptor temperature. Here the Nusselt number Is defined as ... 

The molecnlar weight of water vapor (MW= 18) is less than that of air (MW= 29). As snch, the diffnsion of water vapor into the surrounding atmosphere, which consists of a mixtnre of water and air, leads to a buoyant force with upward macroscopic movement. The natural evaporation phenomenon is not only the effect of heat transfer but also a buoyancy-induced motion. The system is at steady state when the vapor pressure of the water at the surface is less than that in the air above, and the resulting condensation is governed by the slow process of molecular diffusion and lamilar flow. 

The phenomenon of formation of a new NBF when a very small bubble is pressed into the solution/gas surface by buoyancy force, can be used for determining of positive line tension values only. The nascency and expansion of the new contact (NBF) between the bubble and the bulk gas phase are hindered by a force barrier due to the positive linear energy. The buoyancy force (necessary to overcome this force barrier) must be larger than a critical value which depends on the value of k. This principle has been realised in the critical bubble method developed by Platikanov et. al. [474]. The results obtained by this method for 0.05% aqueous solutions of NaDoS are presented in Fig. 3.100 [475]. 

In Grahame (1957), the presence at a liquid surface of a quasi-ice structure was hypothesised. Moreover, study of droplets on inclined planes, and of bubbles clinging to vertical surfaces, reveals behaviour (contact angle hysteresis) which cannot be accounted for without such an ice film. On a vertical wall, a bubble experiences an upward buoyancy force, but in the absence of an ice film, a zero restraining force. The restraining force is made evident by the phenomenon of contact angle hysteresis, followed by film rupture as bubble growth occurs, and leads to bubble detachment. 

Heat is transferred from or to a region by the motion of fluids and the phenomenon of convection. In natural convection, the movement is caused by buoyancy forces induced by variations in the density of the fluid these variations are caused by differences in temperature. In forced convection, movement is created by an external agency such as a pump. 

A period of particularly rapid accumulation of carbon from CH4 was observed during the initial 5 min at 873 K. This effect (Fig. 2d) is favored by higher CH4 pressure. This phenomenon is not observed at 973 K or 1073 K. Buoyancy is observed as a weight loss at the moment when CH4 enters the reactor and is thus excluded as a potential explanation for the rapid weight increase. Calculations show that the Ni/C ratio is very close to 1 at the end of this initial period (at 4.5 bar) indicating that a rapid saturation of the nickel phase could possibly be of importance. 

Schlieren production and motions. This is a fascinating phenomenon that can easily be observed during ice-cube melting in brines or sugar water solutions. Thin-layer schlieren of dense solution represent cold, dense boundary layers that are shed from ice cubes due to their sinking buoyancy. Such observations are small-scale counterparts of thermohaline circulation in the oceans and the fluid counterpart of subduction of the cold, dense boundary layer of the Earth (the lithosphere) into hot, less-dense mantle below. 

These quantitative relations are very approximate. The force exerted by the cone involves momentum as well as weight the cone falls into the sample. The reaction force of the material involves friction exerted by the material and depends on the volume of material that does yield, which may not depend in a simple manner on Lp also the buoyancy force of the material on the submersed part of the cone may not be negligible finally, the phenomenon of stress overshoot (see Figure 17.6c) may upset the relations. Careful comparison for a number of margarine samples with a well-defined test for the yield stress led to the relation [Pg.713]

Kinematic models of this phenomenon supposed the rise due to buoyancy of the less dense clarified liquid to be balanced by the fall of the more dense suspension. With most of the clarified liquid assumed to accumulate above the suspension, it is a simple matter to estimate the rate of production of clarified fluid. This rate is obtained by setting the interface fall rate multiplied by the horizontal projection of the channel cross-sectional area equal to the vertical settling velocity of the particles multiplied by the sum of the horizontal projection of the area of the channel end plus the horizontal projection of the area of the lower channel wall. The result is... 

Figures 3a and 3b show the calculated flow fields for the 260 mm immersion case. The predominant flow direction in the slag was upwards from the interface to the top and back down in a loop. This flow was driven by both natural convection and the buoyancy generated by the carbon monoxide gas produced by the electrode-slag reactions. Hence, it was strongest around the electrodes, oftheordra-of 0.1 m/s, and decreased to die ordra-ofO.OI m/s away from the electrodes. In addition to the principal recirculatory flow loop around the electrodes, a second weaker recirculatory loop formed below the electrodes due to the temperature difference between the electrode and the interface. By contrast, the bullion was relatively quiescent, except for a weak flow loop near the side wall, apparently in a counterintuitive direction (Figure 3b). More work is required to establish whether this loop is a real physical phenomenon or an artefact of the mesh geometry.

When the bubbles migrate toward the center, three phenomena occur. First, the bubbles become spherical because there is no buoyancy. The second is that fewer small bubbles are formed because the bubbles are essentially at rest, and most of the evolved gasses will feed into pre-existing bubbles. The third phenomenon is that as the bubbles grow larger, they begin to pinch off. (A drop can only be stretched to 7C times its diameter before it breaks. (28) ) This offers a validation of the results obtained on the Conquest I sounding rocket. (21)... 

If sizeable fluid density changes occur as a function of temperature, the preceding approaches must be altered. Basically in such situations, fluid motion near a heated or cooled surface occurs because of buoyancy effects. This phenomenon in which velocity and temperature distributions are intimately connected is called natural or free convection. The mode of convection where some external force accounts for the motion is termed forced convection. 

It optimizes analysis. For each system we studied - the pendulum, walking, and spheres moving through fluids - a complex phenomenon described by several variables was reduced to a function of a few dimensionless groupings of the variables. In the analysis of walking the parameters h, /, g, and m reduced to s/1 = f v /gl). In the analysis of the terminal velocity of spheres, the six parameters v, D, g, buoyancy, Pfluid, and fx reduced to a relation between three FI groups,... 

The equilibrium between these two effects causes mixed convection and stratified flow. The stratification interface oscillates and may induce severe damage to the neighbouring structures. Furthermore, the support structures at the bottom of the hot pool have to be protected from hot sodium. The behaviour of such a region has been studied, namely for SPXl [8.27] and EFR [8.19]. Studies were mainly conducted through scale model tests, because computations are not yet able to predict the fluctuation characteristics for such complex situations. As the main physical phenomenon of interest is the interaction between buoyancy forces (natural convection) and inertia forces (forced convection from the main pool... 

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