Numerical Studies of Sampling Efficiencies of the ASCME and PMS Aspirator Hydrometeor Measurement Instruments

Abstract

A three-dimensional first-order panel code is used to calculate airflow around and into the Airborne Snow Concentration Measuring Equipment (ASCME), and an axisymmetric second-order panel code is used to calculate flow to and through the Particle Measuring Systems (PMS) Aspirator horn. Air intake by the ASCME inlet tube is taken to be either zero or at the free stream flow rate. Calculations are done for three aspirator horn axis angles relative to horizontal: 27°, 60° and 90°, with a uniform exterior horizontal wind of 2 m s−1 along with a fan-induced horn throat airspeed of 12.1 m s−1. The flow codes are combined with three dimensional trajectory codes to calculate hydrometeor fluxes through the sampling volumes of the instruments. Excellent sampling efficiencies for the ASCME are indicated for all flow conditions studied, and for comprehensive size ranges of water drops and hexagonal plate ice crystals. Convergent flow to the PMS Aspirator measurement volume causes large flux distortions for small waterdrops. Accessibility of larger drops to the measurement volume is dependent on the horn axis angle relative to the horizontal. When they can reach the sampling volume, drops over a wide range of diameters (approximately 40-–400 μm) show extreme flux distortions owing to combined effects of flow convergence and the variation of trajectory curvature required for drops to reach all parts of the measurement volume. Finally, large drops are found to pass through the measurement volume at speeds substantially less than the airspeed.