Plane-layer Rayleigh-Bénard convection up to Ra=10¹¹: Near-wall fluctuations and role of initial conditions

We study temperature and velocity fluctuations in turbulent Rayleigh-Bénard convection through direct numerical simulations in a three-dimensional plane layer of height H with horizontal periodic boundary conditions and aspect ratio 4H× 4H× H. The analysis is performed for Rayleigh numbers 10⁵ ≤ Ra ≤ 10¹¹ and Prandtl number $Pr=0.7$. First, the study systematically summarizes the height-dependent statistics of velocity and temperature fluctuations and reports corresponding scalings with the Rayleigh number. Secondly, we include an analysis on the role of coherent and incoherent flow regions near the wall for the global heat transfer. Thirdly, we investigate the dependence of turbulent heat and momentum transfer on additional finite-amplitude shear flow modes in the initial condition at time $t=0$, which either decay unforced with respect to time in a long transient or remain existent when a steady volume forcing in the momentum balance of the Boussinesq model is added. In the latter case, logarithmic near-wall layers for mean velocity and temperature are formed. In all cases, no impact on the global turbulent heat and momentum transfer within the error bars is detected, even though the shear mode amplitude is of the order of the characteristic free-fall velocity.