REFINED SIMILARITY HYPOTHESES FOR PASSIVE SCALARS MIXED BY TURBULENCE

In analogy with Kolmogorov's refined similarity hypotheses for the vel ocity field, two hypotheses are stated for passive scalar fields mixed by high-Reynolds-number turbulence. A 'refined' Yaglom equation is de rived under the new assumption of local isotropy in pure ensembles, wh ich is stronger than the usual assumption of local isotropy but weaker than the isotropy of the large scale. The new theoretical result is s hown to be consistent with the hypotheses of refined similarity for pa ssive scalars. These hypotheses are approximately verified by experime ntal data on temperature fluctuations obtained (in air) at moderate Re ynolds numbers in the wake of a heated cylinder. The fact that the ref ined similarity hypotheses are stated for high Reynolds (and Peclet) n umbers, but verified at moderate Reynolds (and Peclet) numbers suggest s that these hypotheses are not sufficiently sensitive tests of univer sality. It is conjectured that possible departures from universality a re hidden by the process of taking conditional expectations.