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  • Author or Editor: Holger Siebert x
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Holger Siebert and Andreas Muschinski

Abstract

The performance of a new type of sonic anemometer–thermometer (called a sonic), the Solent-Research HS, manufactured by Gill Instruments, Lymington, United Kingdom, was investigated. Measurements of the three wind-velocity components u, υ, w, and temperature T were taken in the laboratory under quiet conditions and in the field at wind speeds of about 10 m s−1. The power spectra of u, υ, w, and T measured in the laboratory follow a −5/3 power law at moderate frequencies. At frequencies higher than u/l (here u is the mean wind speed along a given path of length l), there is a roll-off in all spectra, an expected effect caused by the spatial averaging along the finite pathlength. Over the bandwidth of f s/2 = 50 Hz, the standard deviations due to uncorrelated noise amount to 0.02 m s−1 for u, υ, and w and to 0.02 K for T. In the field, the spectra of u, υ, and w show a clean −5/3 power law, except for a flattening at frequencies larger than 30 Hz. The ratio of the spectra of the transverse and longitudinal velocity components was close to 4/3, the ratio predicted by classical theory for isotropic turbulence. The T spectra measured in the field were severely contaminated at frequencies larger than about 5 Hz. Closer inspection of the T time series revealed amplitude-modulated artifacts. These artifacts were presumed to be the result of oscillations of the sonic's pathlengths induced by oscillations of the tower, which was exposed to a turbulently changing wind. The artifacts were reproduced in the laboratory by controlled blows on the sonic's attachment. The mechanical oscillations, which the authors refer to as the tuning-fork effect, were measured with a strain gauge attached to the sonic. The tuning-fork effect was observed simultaneously and independently in the strain-gauge measurements and as artifacts in the temperature time series.

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Holger Siebert, Katrin Lehmann, and Raymond A. Shaw

Abstract

The use of a hot-wire anemometer for high-resolution turbulence measurements in a two-phase flow (e.g., atmospheric clouds) is discussed. Experiments in a small wind tunnel (diameter of 0.2 and 2 m in length) with a mean flow velocity in the range between 5 and 16 m s−1 are performed. In the wind tunnel a spray with a liquid water content of 0.5 and 2.5 g m−3 is generated. After applying a simple despiking algorithm, power spectral analysis shows the same results as spectra observed without spray under similar flow conditions. The flattening of the spectrum at higher frequencies due to impacting droplets could be reduced significantly. The time of the signal response of the hot wire to impacting droplets is theoretically estimated and compared with observations. Estimating the fraction of time during which the velocity signal is influenced by droplet spikes, it turns out that the product of liquid water content and mean flow velocity should be minimized. This implies that for turbulence measurements in atmospheric clouds, a slowly flying platform such as a balloon or helicopter is the appropriate instrumental carrier. Examples of hot-wire anemometer measurements with the helicopter-borne Airborne Cloud Turbulence Observation System (ACTOS) are presented.

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