Transport and Boundary Layers in Rayleigh-Benard Convection.

In this thesis, we study the transport of passive impurities and of heat in Rayleigh-Benard convection. These processes are often accompanied by the formation of thin boundary layers in the relevant field (concentration or temperature). A major focus of these studies is the relationship between the structure of the boundary layers and the transport rates. The research is composed of two experimental studies. In the first experiment, we investigate passive transport in laminar, cellular, (nearly) two-dimensional convection. Dispersion in this system is limited by slow transport of the impurity across boundary layers at separatrices between adjacent rolls. Two velocity regimes are examined. If the flow is time-independent, transport across the boundary layers is predominantly diffusive. Enhanced diffusion coefficients D^* measured in this regime agree well with theories based on analysis of the boundary layer thickness. If the flow is time-periodic, transport across the boundary layers is advective, resulting in substantial additional enhancements of D^*. Numerical integration of a simplified model of this velocity regime shows that particle motion near the separatrices is chaotic. The simulations capture many of the essential features of the experimental transport. Numerical estimates of D ^* agree reasonably well with experimental measurements in the time-periodic regime. The second experiment tests certain theories of turbulent convection in which winds in the vicinity of the thermal boundary layers are proposed to affect their stability and change the overall heat flux. Artificial winds in and near the lower boundary layer are created with a flowing layer of mercury that shears the bottom of the convecting fluid. Changes in horizontal winds affect the frequency and spatial organization of eruptions, but have almost no effect on the heat flux. Enhancements in vertical boundary layer winds, however, result in increases of up to 70% in the heat flux. The scaling between the heat flux and the Rayleigh number with enhanced vertical winds is discussed quantitatively. The effects of shear -induced changes in the boundary layer eruptions on temperature statistics in the interior are also discussed in the context of theories about "soft" and "hard" turbulence.