An Axial-Flow Cyclone for Aircraft-Based Cloud Water Sampling

A new aircraft-based cloud water collection system has been developed to provide samples of cloud water for chemical analysis. The collection system makes use of centrifugal separation in an axial-flow cyclone to remove cloud drops from the airstream. An automated sample storage system allows up to seven independent samples to be obtained during a single research flight. The entire collection system is housed in a Particle Measurement Systems (PMS) canister to permit the collector to be used on a range of research aircraft without extensive modification to the collector or the aircraft structure. Computational fluid dynamics (CFD) analysis was used extensively throughout the development of the new collector for component design and to predict internal flow dynamics. CFD-based cloud drop trajectory simulations provided an estimate of collection efficiency as a function of drop size. Based on the numerical modeling, the 50% cut diameter was predicted to be 8 μm. Through a quantitative laboratory calibration using fluorescein-tagged monodisperse drops, CFD predictions of drop deposition patterns in the interior of the axial-flow cyclone were verified. The numerical and experimental evaluations were performed to ensure that the population of collected cloud drops is well characterized. Initial flight testing of the system occurred during the Dynamics and Chemistry of Marine Stratocumulus, Phase II (DYCOMS-II) field project in July 2001. Although the major components of the prototype collection system operated as expected during flight testing, sample collection rates were lower than expected because of the inefficient removal and storage of cloud water collected in the axial-flow cyclone. Actual sample collection rates ranged between 0.1 and 1.2 mL min−1.