ON LOCAL ISOTROPY OF SMALL SCALE TEMPERATURE FLUCTUATIONS.

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 organized 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. 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.