Physical and geometrical properties of thermal plumes in turbulent Rayleigh-Benard convection

We present a systematic experimental study of geometric and physical properties of thermal plumes in turbulent Rayleigh-Benard convection using the thermochromic-liquid-crystal (TLC) technique. The experiments were performed in three water-filled cylindrical convection cells with aspect ratios 2, 1 and 0.5 and over the Rayleigh number range 5 x 10(7) <= Ra <= 10(11). TLC thermal images of horizontal plane cuts at various depths below the top plate were acquired. Three-dimensional (3D) images of thermal plumes were then reconstructed from the 2D slices of the temperature field. The results show that the often-called sheetlike plumes are really 1D structures and may be called rodlike plumes. We find that the number densities for both sheetlike/rodlike and mushroomlike plumes have power-law dependence on Ra with scaling exponents of similar to 0.3, which is close to that between the Nusselt number Nu and Ra. This result suggests that it is the plume number that primarily determines the scaling exponent of the Nu-Ra scaling relation. The evolution of the aspect ratio of sheetlike/rodlike plumes reveals that, as Ra increases, the plume geometry changes from more-elongated to less-elongated. Our study of the plume area fraction (fraction of coverage over the surface of the plate) further reveals that the increase in plume numbers with Ra mainly comes from an increase in plume emission, rather than fragmentation of plumes. In addition, the area, perimeter and shape complexity of the 2D horizontal cuts of sheetlike/rodlike plumes were studied and all are found to obey log-normal distributions.