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ISSN Online: 2377-424X

ISBN Print: 0-89116-130-9

International Heat Transfer Conference 6
August, 7-11, 1978, Toronto, Canada

ON LOCAL ISOTROPY OF SMALL SCALE TEMPERATURE FLUCTUATIONS

Get access (open in a dialog) DOI: 10.1615/IHTC6.1510
pages 359-364

Abstract

The measured skewness of the streamwise temperature derivative in both laboratory and atmospheric turbulent flows has been found to be non-zero (and of either sign) and essentially independent of Reynolds number, for a sufficiently large value of the Reynolds number. The apparent contradiction of this result with the hypothesis of local isotropy is examined here in the context of accumulating evidence for the presence of a highly organised large scale motion in turbulent shear flows. In the temperature trace, the signature of the large scale motion appears as a 'ramp-like' structure on which small scale fluctuations are superimposed. With the assumption that small scale and large scale fluctuations are essentially independent, the decomposition between these two scales has been attempted for temperature signals obtained in a slightly heated turbulent jet. The large scale component of the signal is obtained by ensemble averaging over a number of ramps (of the order of 100) of given duration (in practice, within a narrow band about this duration). The ensemble averaged large scale structure strongly supports an 'exponential' shape for ramps whose length scale is comparable to the scale of mean motion. Small scale fluctuations are determined by subtracting the ensemble averaged temperature from the instant-aneous temperature. Statistical properties of these fluctuations are found to be consistent with the concept of local isotropy. In particular, the measured non-zero skewness of the temperature derivative does not contradict local isotropy, as it appears to be almost entirely due to the non-isotropic large scale structure.