Persistent skewness of a strongly active scalar

We present a skewness measurement of the density fluctuations δρ̃_l on scales l in a vertical turbulent soap film driven by a temperature gradient. Unlike the temperature difference δT_l in Rayleigh-Bénard convection, the density difference in the film is strongly active because δρ̃_l∕ρ̅ ⪢δT_l∕T̅ , where the over-bar indicates the mean. Our results show persistent nonzero skewness S_y(l)≡⟨δρ̃_l³⟩∕⟨δρ̃_l²⟩^3∕2 on different scales along the gravity direction ŷ; 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.