Velocity fluctuations and boundary layer structure in a rough Rayleigh-Bénard cell filled with water
Abstract
We report particle image velocimetry of the large-scale circulation and the viscous boundary layer in turbulent thermal convection. We use two parallelepipedic Rayleigh-Bénard cells with a top smooth plate. The first one has a rough bottom plate and the second one has a smooth one, so we compare the rough-smooth and the smooth-smooth configurations. The dimensions of the cell allow us to consider a bidimensional mean flow. Many previous heat flux measurements have shown a Nusselt-Rayleigh regime transition corresponding to an increase of the heat flux in the presence of roughness that is higher than the surface increase. Our velocity measurements show that if the mean velocity field is not clearly affected by the roughness, the velocity fluctuations rise dramatically, which is accompanied by a change of the longitudinal velocity structure functions scaling. Moreover, we show that the boundary layer becomes turbulent close to roughness, as it was observed recently in air (O. Liot et al. , J. Fluid Mech. 786 , 275 ( 2016 ) JFLSA7 0022-1120 10.1017/jfm.2015.649 ). Finally, we discuss the link between the change of the boundary layer structure and the changes observed in the velocity fluctuations.