Three-dimensional structure of the thermal boundary layer in turbulent Rayleigh-Bénard convection: A Lagrangian perspective
1Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, P.O. Box 49, Budapest H-1525, Hungary
In turbulent Rayleigh-Bénard convection, it is generally acknowledged that the thermal boundary layer (BL) governs heat transfer, although its scaling with the Rayleigh number is still debated. Various methods have been developed to measure the characteristic thickness of the BL, which are mostly employed in a time-averaged manner. Among them, only the slope method can be applied in an instant of time, being therefore capable of time-resolved analysis; however, it provides no further insight when the Nusselt number is known. Accordingly, the average properties of the BL are thoroughly studied, mainly in the context of heat transfer; its time-dependent structural dynamics and roughness remain largely unexplored. Here, we propose a Lagrangian method to characterize both the time-averaged and the spatio-temporal evolution of the BL, that marks the edge of the BL where convective and diffusive transports overlap. The characteristic thickness of the BL, that is defined by this method, is not a trivial function of the Nusselt number and can be considered a potential tool to analyze the BL structure while varying the Nusselt number. It is also demonstrated using 3D direct numerical simulations, that the injection of heat is extremely inhomogeneous in space and time; the vast majority of heat accumulates in narrow domains, that is governed by the local plume dynamics.