CBI Colloquium with Prof. Dr. Schumacher

Title: Boundary layer analysis in high-Rayleigh number turbulent convection

Turbulent convection processes are ubiquituous in nature and technology. We study the structure of the thermal and viscous boundary layers in three-dimensional Rayleigh-Bénard convection, a paradigm of these flows, for a range of Rayleigh numbers Ra=1e+5 to 1e+11 at Prandtl numbes of Pr=0.7 in direct numerical simulations. The convection flow evolves in a plane layer of aspect ratio 4H:4H:H with H being the layer height. We report the following findings: For the highest Rayleigh numbers, a hierarchy of thermal plumes is observed. Finer plumes close to the wall merge to bigger ones further away from the wall. The plume spacing is found to be self-similar, which suggests local boundary layer instability mechanisms at work. The velocity field close to the wall can be decomposed into shear-dominated (with different directions) and shear-free regions. There is no coherent shear flow covering the whole boundary plate, as assumed in several theories of turbulent heat transfer. We discuss possible connections of these findings to the scaling of the turbulent heat transfer with Rayleigh number which is close to a 1/3-power law.