  The effect of the wind of Rayleigh-Benard convection on the boundary layers
is studied by direct numerical simulation of an L/H=4 aspect-ratio domain with
periodic side boundary conditions for Ra={10^5, 10^6, 10^7} and Pr=1. It is
shown that the kinetic boundary layers on the top- and bottom plate have some
features of both laminar and turbulent boundary layers. A continuous spectrum,
as well as significant forcing due to Reynolds stresses indicates undoubtedly a
turbulent character, whereas the classical integral boundary layer parameters
-- the shape factor and friction factor (the latter is shown to be dominated by
the pressure gradient) -- scale with Reynolds number more akin to laminar
boundary layers. This apparent dual behavior is caused by the large influence
of plumes impinging onto and detaching from the boundary layer. The
plume-generated Reynolds stresses have a negligible effect on the friction
factor at the Rayleigh numbers we consider, which indicates that they are
passive with respect to momentum transfer in the wall-parallel direction.
However, the effect of Reynolds stresses cannot be neglected for the thickness
of the kinetic boundary layer. Using a conceptual wind model, we find that the
friction factor C_f should scale proportional to the thermal boundary layer
thickness as C_f ~ lambda_Theta, while the kinetic boundary layer thickness
lambda_u scales inversely proportional to the thermal boundary layer thickness
and wind Reynolds number lambda_u ~ lambda_Theta^{-1} Re^{-1}. The predicted
trends for C_f and \lambda_u are in agreement with DNS results.
