Velocity fluctuations and boundary layer structure in a rough Rayleigh-Bénard cell filled with water

Olivier Liot (1) , Quentin Ehlinger (2) , Elonore Rusaouën (1) , Thibaut Coudarchet (1) , Julien Salort (1) , Francesca Chillà (1)

1 Phys-ENS - Laboratoire de Physique de l'ENS Lyon
2 ILM - Institut Lumière Matière [Villeurbanne]

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 to consider a bi-dimensional mean flow. Lots of previous heat flux measurements have shown a Nusselt–Rayleigh regime transition corresponding to an increase of the heat flux in presence of roughness which 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. It 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 the air [Liot et al., JFM, vol. 786, pp. 275-293]. Finally we discuss the link between the change of the boundary layer structure and the ones observed on the Large Scale Circulation.

Mots clés: Natural convection; 4727-i Turbulent flows; numbers: 4755pb Thermal convection; 4425+f Natural convection; Thermal convection; Turbulent flows

Domaines: Mécanique des fluides [physics.class-ph]; Dynamique des Fluides [physics.flu-dyn]

Olivier Liot, Quentin Ehlinger, Elonore Rusaouën, Thibaut Coudarchet, Julien Salort, et al.. Velocity fluctuations and boundary layer structure in a rough Rayleigh-Bénard cell filled with water. Physical Review Fluids, 2017, 2 (4), ⟨10.1103/PhysRevFluids.2.044605⟩. ⟨hal-01391492v2⟩