Abstract
We present measurements of the near-wall velocity field in highly turbulent Rayleigh-Bénard(RB) convection with rough horizontal surfaces. The measurements have been undertaken in a large-scale RB experiment which is called "Barrel of Ilmenau". We used the technique of stereoscopic Particle Image Velocimetry to measure the three components of the velocity field in horizontal planes parallel to the heated bottom plate. The measurements reported here comprise two Rayleigh numbers Ra=8.6×10^{10} and Ra=6.3×10^{11}. The Prandtl number of the working fluid was Pr=0.7. We used a chess board-like pattern of cuboids with a base area of 30 mm by 30 mm and a height of h=12 mm. Our measurements confirm the observation from previous experiments that the relation Nu=f(Ra) remains unchanged with respect to RB convection with smooth surfaces as long as the thickness of the thermal boundary layer \delta_{th}=H/(2Nu) exceeds the height of the roughness elements h. If \delta_{th} falls below h, Nu becomes larger and grows faster with increasing Ra then it does in RB convection with smooth plates. We also could identify the valleys in between the cuboids as the main contribution for the increase of Nu
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