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Investigation of high wave number temperature and velocity spectra in air Boston, Noel Edward James

Abstract

Turbulent velocity and temperature fluctuations in air were measured at a height of 4 meters over a tidal mud flat. Particular attention was focused on the high wave number, small scale region of the spectra of these fluctuations. The measurements of the velocity fluctuations were made with a constant temperature hot wire anemometer; the hot wire consisted of a platinum wire 5 µm in diameter and approximately 1 mm in length. Temperature fluctuations were measured with a platinum resistance thermometer which consisted of a platinum wire 0.25 µm in diameter and about 0.30 mm in length. The velocity spectra results agree well with the classical results of Grant, Stewart and Moilliet (1962) and Pond, Stewart and Burling (1963). In addition, they extend the velocity spectrum in air to slightly higher wave numbers. Pour "clean" spectra agree slightly better with the universal curve suggested by Nasmyth (1970) than with the classical results. This suggests that the one-dimensional Kolmogorov constant. K’ previously estimated may be too low. The mean value obtained with these data was 0.50. The temperature spectra clearly show the shape of the one-dimensional temperature spectrum in air beyond the -5/3 region. These spectra show that in air there is no -1 region and that temperature and velocity spectra are very similar. The temperature spectrum falls off from the -5/3 slope at slightly higher wave numbers than the velocity spectrum. The value of the scalar constant K’[sub θ], which appears in the scalar -5/3 law, computed from these data was 0.81. Direct measurement was made of all parameters that enter into the calculation of it. The coefficient of excess and skewness of derivatives of velocity and temperature signals were computed for sixteen cases. Large absolute values were obtained indicating the non-Gaussian character of high wave number, high Reynolds number turbulence. A wide range of values of both parameters was obtained. In general the coefficient of excess of the temperature derivative exceeds that of the velocity derivative. The effect of velocity sensitivity on the skewness of the temperature derivative signal was investigated and found to be non-negligible.

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