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Hillyer G. Norment

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.

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Hillyer G. Norment

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.

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Hillyer G. Norment

Abstract

Methods that use potential-flow calculations to correct turbulence measurements for instrument-induced flow distortions are discussed in general terms for two- and three-dimensional geometries. A simple calculation method is described for the Wyngaard distortion coefficients, from which the corrections are determined, that previously had not been fully exploited. Panel methods, which are especially well suited to the simple method and allow calculations for arbitrary instrument geometries, are also discussed. Applications of all methods are illustrated by use of examples.

Distortions caused by wakes that result from flow separation from the instruments themselves cannot be corrected for by use of potential-flow calculations. Neglect of these distortions may compromise accuracy of the corrections. Severity of this problem for atmospheric turbulence measurements is illustrated by results calculated for some simple geometries.

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Andrew Detwiler and Hillyer G. Norment

Abstract

The M-meter is designed to measure total mass concentration of hydrometeors from aircraft aloft. Its essence is a free-rotating, vaned disk with face normal to the freestream; rotation is driven by airflow through the vanes. Hydrometeors collected on the disk surface are expelled by being flung tangentially from its periphery. This expulsion causes a reduction of equilibrium rotation rate that is proportional to freestream hydrometeor mass concentration.

A prototype was carried under the wing of a research airplane as it probed precipitating clouds at levels from just above to just below the melting layer. M-meter measurements of hydrometeor mass concentrations are demonstrated to be in qualitative agreement with independent measurements.

This unique, simple, rugged instrument with high sampling volume probably can be refined to provide accurate in situ measurement of cloud plus precipitation water mass—a capability sorely needed. It warrants additional research and development, which is not planned by those involved at present. This note is for the purpose of stimulating further interest in this promising concept.

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