  The boundary layer structure of the velocity and temperature fields in
turbulent Rayleigh-Benard flows in closed cylindrical cells of unit aspect
ratio is revisited from a transitional and turbulent viscous boundary layer
perspective. When the Rayleigh number is large enough, the dynamics at the
bottom and top plates can be separated into an impact region of downwelling
plumes, an ejection region of upwelling plumes and an interior region away from
the side walls. The latter is dominated by the shear of the large-scale
circulation (LSC) roll which fills the whole cell and continuously varies its
orientation. The working fluid is liquid mercury or gallium at a Prandtl number
Pr=0.021 for Rayleigh numbers between Ra=3e+5 and 4e+8. The generated turbulent
momentum transfer corresponds to macroscopic flow Reynolds numbers with values
between 1800 and 46000. It is shown that the viscous boundary layers for the
largest Rayleigh numbers are highly transitional and obey properties that are
directly comparable to transitional channel flows at friction Reynolds numbers
Re_tau slightly below 100. The transitional character of the viscous boundary
layer is also underlined by the strong enhancement of the fluctuations of the
wall stress components with increasing Rayleigh number. An extrapolation of our
analysis data suggests that the friction Reynolds number Re_tau in the velocity
boundary layer can reach values of 200 for Ra beyond 1e+11. Thus the viscous
boundary layer in a liquid metal flow would become turbulent at a much lower
Rayleigh number than for turbulent convection in gases and gas mixtures.
