Boundary layers in thermal convection are fluctuation-dominated

We study the dynamics of thermal and momentum boundary layers in three-dimensional direct numerical simulations of Rayleigh-B\'enard convection for the Rayleigh number range 10⁵ ≤ Ra ≤ 10¹¹ and $Pr=0.7$. Using a Cartesian slab with horizontal periodic boundary conditions and an aspect ratio of 4, we obtain statistical homogeneity in the horizontal x- and y-directions, thus approximating an infinitely extended system. We observe that upon canonical use of long-time and area averages, a coherent mean flow is practically absent. Instead, the velocity field close to the wall is a collection of differently oriented local shear patches interspersed by shear-free, incoherent flow regions. These shear patches occupy an area fraction of approximately $40%$ for all $Ra$. Rather than resulting in a pronounced mean with small fluctuations about it, the velocity field is dominated by strong fluctuations of all three components around a non-existent or weak mean. This feature is particularly pronounced for Ra ≥ 10⁹. We discuss the consequences of these observations for convection layers with larger aspect ratios, including boundary layer instabilities and the resulting turbulent heat transport.