PROBABILITY DENSITY-FUNCTIONS, SKEWNESS, AND FLATNESS IN LARGE REYNOLDS-NUMBER TURBULENCE

Probability density functions (PDF) of longitudinal velocity increment s are measured in an experiment using low temperature helium gas, in w hich a large domain of microscale Reynolds numbers R(lambda) (from 150 to 5040) is explored. The technique of measurement, which is essentia lly hot wire anemometry, but operating in nonstandard conditions, is d escribed. In the inertial range of scales, and for a given scale of se paration, the PDF are found to be independent of the Reynolds number a t large R(lambda). Measurements of the skewness and the flatness of th e velocity derivatives are performed in a range of R(lambda) lying bet ween 150 and 3200. The surprising result that we find is that these qu antities first increase with the Reynolds number up to R(lambda) appro ximate to 700 and then cease to increase further, indicating a transit ional behavior.