Conditions for Cloud Settling and Rayleigh-Taylor Instability

Most aerosol motion can be analyzed by individual particle motion or by the motion of the suspending gas. There are, however, two related situations in which an aerosol can exhibit bulk motion: cloud settling and Rayleigh-Taylor instability. In both cases, the aerosol particles move faster as a cloud than they do as individual particles. In the case of cloud settling, the aerosol is usually a spheroidal cloud surrounded by clean air. Rayleigh-Taylor instability occurs when a dense aerosol layer overlies a layer of clean air. This instability is characterized by abrupt breakthrough of the aerosol layer into the clean air layer at multiple points. High-concentration, submicrometer test aerosols were generated in two experimental systems that permitted observation of the transition from particle-dominated motion to cloud, or bulk, dominated motion and measurement of cloud settling velocities and characteristics. In both systems aerosol concentration could be controlled over two orders of magnitude. One system used commercial ventilation smoke tubes to release a dense stream of aerosol into a low velocity wind tunnel. The other used diluted mainstream cigarette smoke from a smoking machine in an aerosol centrifuge. Based on these experiments, theoretical equations for cloud settling predict cloud settling velocity within an order of magnitude. The transition from individual particle motion to observable bulk motion occurs when predicted cloud settling velocity is from 0.01 to 0.05 m/s. Cloud settling appears to be initiated from an aerosol stream or layer by Rayleigh-Taylor instability. The ratio of cloud settling velocity to particle settling velocity does not appear to be a reliable predictor of the transition from particle to bulk motion.