ABSTRACT
The structure of steady, non-penetrative turbulent thermal convection over various two-dimensional, periodic roughness elements has been investigated experimentally by light-sheet visualization of the flow and temperature fields and by instantaneous measurements of the three-dimensional temperature fields to determine the interactions between the roughness elements and the turbulent convective sheets and plumes in the vicinity of the heated plate. Convective structures over smooth surfaces are similar to the 'spoke' pattern discovered by Clever and Busse (1989), which consists of sheets of buoyant plumes arranged in polygonal cell walls with thermals at the intersection of these polygonal cells. Roughness improves heat transfer, as shown by measurements of Nusselt number increasing as the wavelength and amplitude of the roughness elements increase. Increased surface roughness distorts the polygonal cells and affects the distribution of thermals in the vicinity of the bottom surface. It also produces thermals that penetrate significantly further into the core of the convective fluid layer. Over rough surfaces, the majority of the thermal plumes are generated in the depressions of the surface at the intersection of the polygonal cells. Three-dimensional quantitative visualization of the temperature, obtained by scanning thermochroic particle image thermometry, shows the structure and penetration into the core region of these thermals.
