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  • Author or Editor: D. H. Lenschow x
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D. H. Lenschow

Abstract

Measurements of air velocity and temperature from an airplane in the planetary boundary layer with strong surface heating are used to calculate vertical heat, momentum and energy fluxes, as well as spectral densities and probability distributions of velocity and temperature. Airplane traverses parallel to the wind are compared to crosswind traverses and a definite elongation of the heat transporting eddies, or thermals, parallel to the wind is observed. The terms in the turbulent kinetic energy balance equation (with the exception of the pressure fluctuation term) and the temperature variance balance equation are estimated. The turbulent kinetic energy dissipation is almost constant with height between the lowest flight level of 100 m above the surface, and the highest flight level of 1000 m, which is just below the top of the boundary layer, while the generation term due to the buoyancy force decreases and the divergence of the vertical transport of kinetic energy increases with height to maintain an approximate balance. The temperature variance dissipation decreases rapidly with height and the generation of temperature variance and the divergence of the vertical transport of temperature variance become small above 100 m.

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D. H. Lenschow

Abstract

Two types of vanes that were used to measure the angle of the airstream with respect to an aircraft are described, analyzed and compared. One type is a rotating vane that is free to align itself with the airstream and the angle is sensed by an angle transducer. The other type is constrained from rotating and the angle is obtained by measuring the force exerted on the vane by the airstream and dividing by the pitot-static pressure. The free vane measures the angle directly and is not sensitive to acceleration, while the constrained vane has a faster response time and has no bearing friction. At an aircraft speed of 70 m sec−1, both vanes are able to resolve changes in angle of less than 0.02°, which corresponds to a gust velocity of about 2 cm sec−1, and respond to within 5% of a step-function change in angle in a distance of less than 5 m. An inflight comparison between the two vanes indicates that they both measure the same angle with a correlation coefficient of 0.97.

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E. N. Brown, C. A. Friehe, and D. H. Lenschow

Abstract

An air-motion sensing technique is described for measurement of attack and sideslip angles and dynamicpressure. The sensing probe consists of an array of five pressure holes in the standard radome of a twin-jetresearch aircraft. Comparisons are made with air motion measurements (angle of attack and dynamic pressure) obtained from a conventional differential pressure flow angle sensor at the tip of a nose boom 1.5fuselage diameters ahead of the aircraft body. The results indicate that the radome system works well downto scale sizes slightly larger than the fuselage diameter. (Finer scale measurements were limited by pressuretransducer response.) An insitu calibration technique is described for the determination of the empiricalradome angle-pressure difference sensitivity factor k, as a function of aircraft Mach number. The value ofk, so determined at low Mach numbers, is in approximate agreement with that calculated for potential flowfor a spherical radome. The in-situ technique applied to the conventional nose boom sensor indicates thatthe value of k based on wind tunnel calibrations may not apply for the present installation.

The time response of the conventional pressure system on the NCAR Sabreliner twin-jet aircraft is estimated on the basis of an in-flight comparison between the conventional pressure probe and a fast-responsegust probe flown together on a nose boom. Comparison of the power spectra of the conventional and radomeangles of attack for a traverse in boundary-layer turbulence indicates that the response of the radome systemis superior to the conventional system due to the shorter pressure lines that can be used.

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R. L. Schwiesow, S. D. Mayor, V. M. Glover, and D. H. Lenschow

Abstract

The NCAR Airborne Infrared Lidar System (NAILS) observed the edge of an extended, sloping aerosol layer that intersected a stratocumulus cloud deck over the Pacific Ocean during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment, 260 km WNW of San Diego. In situ measurements support the interpretation of the lidar observations as arising from a particle-laden layer with relatively clean air above, below, and to the SW. Intersection of these sloping layers with cloud top leads to substantial horizontal variability of boundary-layer structure in the intersection region. The intersection of the aerosol layer with cloud top also corresponded closely to a quasi-linear trough in the cloud top that showed enhanced brightness and an enhanced number of small particles.

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