Aerodynamic interaction

J ct Spra.y, The mechanism that controls the breakup of a Hquid jet has been analy2ed by many researchers (22,23). These studies indicate that Hquid jet atomisation can be attributed to various effects such as Hquid—gas aerodynamic interaction, gas- and Hquid-phase turbulence, capillary pinching, gas pressure fluctuation, and disturbances initiated inside the atomiser. In spite of different theories and experimental observations, there is agreement that capillary pinching is the dominant mechanism for low velocity jets. As jet velocity increases, there is some uncertainty as to which effect is most important in causing breakup. 

It has been postulated that jet breakup is the result of aerodynamic interaction between the Hquid and the ambient gas. Such theory considers a column of Hquid emerging from a circular orifice into a surrounding gas. The instabiHty on the Hquid surface is examined by using first-order linear theory. A small perturbation is imposed on the initially steady Hquid motion to simulate the growth of waves. The displacement of the surface waves can be obtained by the real component of a Fourier expression ... 

Atomization Unknown, but Plausibly Aerodynamic Interaction, Turbulence, Cavitation, Bursting Effect We >40.3 or Oh lOORei 092 [220] ... 

Formulations for SMD of secondary droplets have also been derived by other researchers, for example, O Rourke and Amsden)3101 and Reitz.[316] O Rourke and Amsden[310] used the % -square distri-bution[317] for determining size distribution of the secondary droplets. They speculated that a breakup process may result in a distribution of droplet sizes because many modes are excited by aerodynamic interactions with the surrounding gas. Each mode may produce droplets of different sizes. In addition, during the breakup process, there might be collisions and coalescences of the secondary droplets, giving rise to collisional broadening of the size distribution. 

Separation in these devices known as winnowing machines [3], is achieved due to the difference between trajectories of coarse and fine particles in the separation zone (Fig. lb). Their operation and efficiency are strongly affected by the stochastic factors of the process, in particular by uncertainties in feeding and particles aerodynamic interactions. In most cases coarse particles prevent proper classification of fines. Separation efficiency of these devices is usually low. They are normally used for separation of solid particles according to densities (e.g. grain from peel), rather than by size. Sometimes crossflow separation in horizontal streams is used in combination with other separation principles. 

Lee, K. C. (1979). Aerodynamic Interactions Between Two Spheres at Reynolds Number Around 104. Aerosp. Q., 30, 371. 

An aerodynamic interaction between the gas and the solids, mainly controlled by the dynamic viscosity of the gas and the elasticity of the packed solids. 

Flames in crossflow are largely influenced by complex fluid mechanics and aerodynamic interactions between... 

Spangler et al. [93] considered nonlinear instability of a straight liquid jet xmder the influence of both capillary forces and aerodynamic interactions with an external gas. They showed that the gas phase interaction is important even at relatively low jet velocities. The presence of the gas leads to a swelling in the trough region of the wave. Aerodynamic interactions had very httle effect on predicted droplet sizes... 

The effect of the Weber number on the breakup time (and length) is shown in Fig. 3.12 for several values of initial disturbance amplitude and two values of the density ratio e. It can be seen that breakup time decreases as the Weber number is increased. This is because the bigger the Weber number, the larger the aerodynamic interactions between the liquid sheet and the surrounding gas, and the latter is what enables the growth of the surface waves and the eventual disintegration of the sheet. In addition, the breakup time is reduced by a larger value of the initial disturbance... 

The amplitude of the fluctuations summarized in Fig. 27.13 are significant (relative to linear theory) the sharper inlets show pulsations greater than 1% for aU conditions assessed. These large scale pulsations can be further amplified by either boundary layer instabilities or aerodynamic interactions outside the nozzle. This unsteadiness will lead to finite-amplitude waves on the free-surface immediately downstream of the orifice exit, i.e., as a small-amplitude Klystron effect. Depending on the capillary length scale, these waves could be amplified to the point... 

Due to lower evaporation rates and instability growth of the liquid in combination with lower aerodynamic interactions, the characteristic droplet sizes get larger for higher Capillary Numbers (see Fig. 16.33). 

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