Wednesday, 5 June 2002: 9:30 AM

Effects of flow accelerations on collisions of small drops in a turbulent flow

The effects of Largangian accelerations in turbulent flow on collision efficiency and collision kernels of small cloud droplets is investigated using results of recent laboratory experiments by La Porta et al, (2001) conducted under high flow having pronounced intermittency.
Effect of turbulence on drop collisions was found to be significant: for drop pairs containing drop collector exceeding 10 micron, collision efficiency and collision kernels increase by up to 25% and 40%, respectively, at dissipation rates typical of weak cumulus clouds, and by factor of 2.5 and 5, at dissipation rates typical of well developed deep cumulus clouds. The effect of turbulence follows mainly from increase in the collision efficiency, which is very sensitive even to weak variations of interdrop relative velocity. The increase in the swept volume is responsible for only small fraction of the increase in the collision kernel.
Effects of turbulent flow intermittency, manifested itself in high values of probability density function (PDF) flatness, on drop collisions are found to be not significant, and seem are not responsible for the acceleration of collision rate. High values of flatness of turbulent velocity distribution in a high turbulent flow (under the same variation of the acceleration), leads to a decrease in collision efficiency and collision kernels. In spite of the increase in the probability of extremely high accelerations under large flatness conditions, it remains too small. The fact that a)the magnitude of collision efficiency is limited from above, and that 2)a great number of collisions is necessary to obtain significant drop growth makes the effect of elongated tail of acceleration on collision drop growth negligibly small. On the other hand, under the assumption of the Gaussian PDF distributions (no intermittency), the PDF of acceleration of the same variation provide significantly higher probability of significant, but not so extreme values of Lagrangian accelerations and drop velocity deviations. As a result, mean value of collision efficiency and collision kernels in a turbulent flow turns out to be higher under assumption of normal distribution of acceleration.
It is concluded that the problem of drop collisions remains incomplete in case only accelerations is taken into account. In other words, in addition to radial velocities one has to take into account tangential component of drop relative velocities caused by turbulent shears, affecting the geometry of drop approaching.

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