Mean Velocity Profile in Confined Turbulent Convection

In this Letter, highly resolved measurements of the horizontal velocity inside the boundary layer of turbulent Rayleigh-Bénard convection are reported. They were performed in a cylindrical box with an aspect ratio $\Gamma =1.13$ which was filled with air with a Prandtl number $\mathrm{Pr}=0.7$. The horizontal velocity was measured along the central axis close to the cooling plate in a range of Rayleigh numbers between $\mathrm{Ra}={10}^{11}$ and $\mathrm{Ra}={10}^{12}$ using a two-dimensional laser Doppler velocimeter. We demonstrate that the profile of the mean velocity strongly differs from that of classical shear flows like the Blasius shape of a laminar flat plate boundary layer or a turbulent logarithmic velocity profile with standard coefficients.