title: Persistent skewness of a strongly active scalar.
abstract: We present a skewness measurement of the density fluctuations deltarho[over ]_{l} on scales l in a vertical turbulent soap film driven by a temperature gradient. Unlike the temperature difference deltaT_{l} in Rayleigh-Bénard convection, the density difference in the film is strongly active because deltarho[over ]_{l}/rho[over ]>>deltaT_{l}T[over ] , where the over-bar indicates the mean. Our results show persistent nonzero skewness S_{y}(l) identical with <deltarho[over ]_{l};{3}>/<deltarho[over ]_{l};{2};{3/2}> on different scales along the gravity direction y[over ]; S_{y}(l) is positive for small l and negative for large l. The sign switch occurs approximately at the Bolgiano length scale l_{B}. The simultaneous presence of positive and negative skewnesses is interpreted here as a result of the structures of rising and falling plumes in a strongly stratified medium.
authors.affiliation: Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhang
authors.forename: X L
authors.initials: XL
authors.lastname: Wu