Boundary layer structure in turbulent Rayleigh-Bénard convection in air

Abstract

We present Particle Image Velocimetry measurements in the boundary layer in turbulent Rayleigh-Bénard (RB) convection for the Rayleigh number Ra=1.4×10E10 and the Prandtl number Pr=0.7. The measurements have been undertaken in a large-scale RB experiment 7.15 m in diameter and 6.30 m in height which is called the “Barrel of Ilmenau”. They give detailed insight into the near-wall flow field in turbulent RB convection and provide experimental data to evaluate various competing theories on the heat transport which essentially based on the boundary layer. We found that the convective boundary layer becomes turbulent locally and temporarily although its shear Reynolds number Re_s=(U_\infty \delta)⁄\nu ≈ 265 (U_\infty - outer velocity, \delta - boundary layer thickness, \nu - kinematic viscosity) is considerably smaller than the value 420 underlying existing phenomenological theories.