Abstract:

Statistical properties of cliff structures of active and passive scalar fields in a waterfilled turbulent RayleighBénard convection cell are investigated experimentally at Ra=1.2×10¹⁰ and Pr=5.3. An arithmetic with a parameter β used to describe “intensity” of cliffs is applied to extract cliff structures from time series of the local concentration (passive scalar) of fluorescence dye and the local temperature (active scalar). Both scalars are measured near the sidewall of a rectangular cell. Histograms of the cliffs’ width, amplitude and size are studied quantitatively. It is found that, for both scalars, shapes of the histograms for cliffs’ width are invariant with respective to the β value, while the cliffs’ most probable amplitude shifts to higher values with increasing β. The obtained results show that the sizes of cliff structures of both scalars are log-normal distributed and the passive scalar mixing in turbulent thermal convection is determined by thermal plumes. 

Key words: Rayleigh-Bénard; turbulent thermal convection; scalar turbulence; active scalar; passive scalar; Ramp-Cliff structure