Static buoyancy

Figure 11. Volume change vs. strain for a polyurethane propellant (105). Volume change was determined by static buoyancy...

The most common sources of TG disturbances and errors in the sequence of the frequency of their occurrence [682] are unstable buoyancy forces, convection forces, electrostatic forces, and condensation of volatile products on sample suspension, thermal expansion of the balance beam (as a severe problem in horizontal TG s) or turbulent drag forces from gas flow. Note that the first item is the static buoyancy (related to the gas density), and the last one is the influence of dynamic forces of flow and of thermal convection. It is a common... 

The lapse rate in the PBL is imstable and vertical motion leads to the transport of significant amounts of energy upward, due to the buoyancy of air that has been in contact with the surface. A mixed layer forms up to a height where static stability of the air forms a barrier to thermally induced upward motion. This extreme occurs practically daily over the arid areas of the world and the barrier to upward mixing is often the tropopause itself. On the average in mid-latitudes, the imstable or mixed PBL is typically 1-2 km deep. 

Bubble size at departure. At departure from a heated surface, the bubble size may theoretically be obtained from a dynamic force balance on the bubble. This should include allowance for surface forces, buoyancy, liquid inertia due to bubble growth, viscous forces, and forces due to the liquid convection around the bubble. For a horizontally heated surface, the maximum static bubble size can be determined analytically as a function of contact angle, surface tension, and... 

Krishnamurthy et ah (K13) have confirmed the above conclusion and have developed an expression for evaluating the bubble volume under conditions of flow when the corresponding volume for static conditions is given. These authors used capillaries of different diameters ground at the tip as nozzles. The capillaries were arranged horizontally, and the fluid travelled vertically so as to add to the buoyancy. The liquid viscosity was varied from 1 to 30 cp and the surface tension from 62 to 70 dyn/cm. 

Mark 5 Flares, Types 1 and 2 consisted of a cylindrical buoyancy chamber which contd two candles. To these were attached a fuze, a static cord and a pull igniter. The static cord functioned either the arming device of the fuze or the pull igniter. When the device was released (from a container) over the water it went under the surface and then came up. It floated with the head of the flare just clear of the water. When the 1st candle was about % burned out, a piece of safety fuse running to the 2nd candle was ignited and, after a short delay, die 2nd candle started to bum. Each candle burned for about 2% min... 

The forces acting on a droplet attached to the pore opening can be conveniently subdivided into perpendicular and parallel direction with respect to the membrane surface. Considering the former case, the Young-Laplace FYl [39] (named also static pressure force), the dynamic lift FDL and buoyancy FBg forces [26] are generally taken into account. They are defined as ... 

When a parcel of air in the atmosphere is moved rapidly from an equilibrium condition and its tendency to come back to its undisturbed position is noted, then we term the atmosphere as statically stable. The movement of the packet is considered as impulsive, to preclude any heat transfer from the parcel to the ambience. This tendency of static stability- when exists, is due to the buoyancy force caused by the density differential due to temperature variation with height and such body force acts upon the displaced air-parcel. 

In nonagitated (static) extractors, drops are formed by flow through small holes in sieve plates or inlet distributor pipes. The maximum size of drops issuing from the holes is determined not by the hole size but primarily by the balance between buoyancy and interfacial tension forces acting on the stream or jet emerging from the hole. Neglecting any viscosity effects (i.e., assuming low dispersed-phase viscosity), the maximum drop size is proportional to the square root of interfacial tension a divided by density difference Ap ... 

As for many immobilised enz3nnes, the hydraulic behaviour Is not adequately described by classical fluid mechanics. It was, therefore, necessary to develop a detailed mathematical model of the column hydraulics which together with a laboratory test procedure, would provide data on the basic mechanical properties of the enzyme pellet. The model Is based on a force balance across a differential element of the enzyme bed. The primary forces involved are fluid friction, wall friction, solids cohesion, static weight and buoyancy. The force balance Is integrated to provide generating functions for fluid pressure drop and solid stress pressure down the length of the column under given conditions. 

There are two main disadvantages to this technique. First, the sample is placed in a static sample bucket i.e. there is no possibility of stirring. Thus, equilibrium is reached solely by diffusion of the gas into the IL sample. For the more viscous samples this can require equilibration times of as much as several hours. Second, the weight gain must be corrected for the buoyancy of the sample in order to determine the actual gas solubility. While the mass is measured accurately, the density of the sample must also be known accurately for the buoyancy correction. 

As demonstrated in Fig. 7, which shows the results for a pipette checked using the gravimetric method, the coefficient of variation (CV, defined as the ratio of the standard deviation to the mean) for a run consisting of ten measurements of a given volume increases considerably when the dispensed liquid volumes are less than 5 pi. Evaporation, buoyancy, vibration, and the effects of static electricity are the primary causes of these results. A possible alternative method to determine microscale droplet volumes is one based on measurement of the concentration of a liquid solution. Two practices, also recommended by the ISO standard [5], are commoifly used, namely, the photometric and titrimetric methods. 

The term static instabihty refers to those motions that tend to amplify when the basic state is at rest (static). Because buoyancy is a key restoring force in a stratified fluid, static instabihty typically arises in situations in which the buoyancy force drives a parcel vertically away from its initial position. A simple analog is the heating of water on a stove the heat somce warms the bottom of the water pot, which then warms the water molecules very close to the bottom. Because warm water is generally less dense than cold water, any small random pertmbation wiU cause the warm water to be displaced upward by conservation of mass, the cold water above is displaced downward. This turbulent convective process transfers the heat through the entire fluid much more rapidly than molecular diffusion, and therefore is the dominant heat-transfer mechanism. 

Since it can be argued that the geometry is not affected by flow rate, equation (16) is valid when flow occurs, and thus C = CT ". Assuming that equation (13) is valid for the three air-oil-solid systems, a tabulation of the resulting flow-induced maximum mobilization force data is shown in Table 8. The differences between the buoyancy-induced mobilization results and the flow-induced data are only between 0.6% and 4.3%. Both experiments therefore result in equivalent conclusions about mobilization pressure drop in a static system. It can be seen, as in the buoyancy experiment, that the maximum pressure drop necessary to initiate mobilization increases with increases in contact angle. 

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