Thermal boundary condition studies in large aspect ratio Rayleigh-Bénard convection

We study the influence of thermal boundary conditions on large aspect ratio Rayleigh-Benard convection by a joint analysis of experimental and numerical data sets for a Prandtl number Pr = 7 and Rayleigh numbers Ra = 105 - 106. The spatio-temporal experimental data are obtained by combined Particle Image Velocimetry and Particle Image Thermometry measurements in a cuboid cell filled with water at an aspect ratio Gamma = 25. In addition, numerical data are generated by Direct Numerical Simulations (DNS) in domains with Gamma = 25 and Gamma = 60 subject to different idealized thermal boundary conditions. Our experimental data show an increased characteristic horizontal extension scale lambda similar to of the flow structures for increasing Ra , which due to an increase of the convective heat transfer also leads to an increase of the Biot number (Bi) at the cooling plate. However, we find the experimental flow structure size to range in any case in between the ones observed for the idealized thermal boundary conditions captured by the simulations: On the one hand, they are larger than in the numerical case with applied uniform temperatures at the plates. On the other hand, they are smaller than in the case of an applied constant heat flux, the latter of which leads to a structure that grows gradually up to the horizontal domain size. We are able to link this observation qualitatively to theoretical predictions for the onset of convection. Furthermore, we study the effect of the asymmetric boundary conditions on the heat transfer. Contrasting experimental and numerical data reveals an increased probability of far-tail events of reversed heat transfer. The successive decomposition of the local Nusselt number Nuloc traces this effect back to the sign of the temperature deviation Theta similar to, eventually revealing asymmetries of the heating and cooling plate on the thermal variance of the generated thermal plumes.(c) 2023 The Author(s). Published by Elsevier Masson SAS. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).