High-Rayleigh number turbulent thermal convection: A study of structures and statistics of the temperature and velocity fields

This thesis is an experimental investigation of both the temperature and the velocity fields in the Rayleigh-Bénard (RB) system. Our goal is to understand the nature of the turbulent motion in this system. With water as the working fluid, temperature field was measured in a cylindrical cell of height 19.6 cm and of diameter 19.0 cm with Ra from 4 × 10 8 to 2 × 1010, whereas velocity field was measured in a rectangular cell with length 81 cm, width 20 cm and height 81 cm at Ra ∼5 × 1011.; For the temperature field, we find that the peak frequency of the temperature dissipation spectra may be identified as the “Bolgiano frequency” with respect to which the temperature power spectra are universal functions; the scaling exponent of the buoyancy subrange assumes the value of 2/5 predicted for the Bolgiano-Obukhov scaling only in the central region of the cell; and in the mixing zone the exponent for the high frequency scaling exponent has a value of 2/3. We show that the skewness of the plus and minus temperature increments can be used to quantitatively characterize the mixing zone in the convective flow and the result reveals how the mixing zone evolves with the Rayleigh number. Thermal plumes are identified from temperature time series and the results indicate that the plumes have a log-normal distribution and the width of thermal cap is an intrinsic scale in the system. It is also found that the transition in the thermal dissipation power appears to be related to a similar transition in plume dynamics.; From the velocity measurement, we obtain some fundamental quantities of turbulence in convective flow. The instantaneous Reynolds shear stress shows strong temporal and spatial intermittency. The flow field is found to be inhomogeneous and anisotropic in the bulk of the cell and is also different from those found in the cylindrical and cubical cells. The maximum integral scale is found to be consistent with the system size and the maximum Taylor scale is found to be the order of the viscous boundary layer thickness. For spectrum of turbulent energy fluctuations, one scaling range is found in the central core region of the cell, outside which two scaling ranges are resolved with position-dependent scaling laws.