Three-Dimensional Trajectory Analysis of Two Drop Sizing instruments: PMS* OAP and PMS* FSSP

Abstract

Flow-induced distortions of water drop flux and speed are predicted by three-dimensional calculations. The instruments are studied in isolation and mounted under the wing of a DeHavilland Twin Otter airplane. Several free stream air speeds and angles of attack of 0° and 4° are studied for drop diameters ranging from 2 to 1000 μm. For the PMS Optical Array Probe (OAP) in isolation and under the Twin Otter wing with the airplane at 0° angle of attack, distortions of practical consequence are not found. At 4° airplane angle of attack, 17% under-measurement of both flux and speed is predicted for cloud-size droplets. The PMS Forward Scattering Spectrometer Probe (FSSP) presents greater flow obstruction than the OAP, and requires in addition that air and drops traverse the measurement tube. As expected, larger flow-induced effects are predicted under all circumstances than for the OAP. For the FSSP in isolation and mounted on the Twin Otter at 0° angle of attack, both speed and flux are predicted to be undermeasured by about 10% for cloud-size droplets. At 4° airplane angle of attack, 24% undermeasurement of both flux and speed is predicted for cloud-size droplets. For the wing-mounted instruments we find that a large part of the flow-induced effects (approximately half) is caused by the instruments themselves. This shows that it is not always justified to assume that instrument-induced flow effects are insignificant compared with aircraft-induced effects.