Velocity fluctuations and boundary layer structure in a rough Rayleigh-B{\'e}nard cell filled with water

TL;DR

This study uses Particle Image Velocimetry to compare velocity fluctuations and boundary layer structures in turbulent Rayleigh-Bénard convection with rough and smooth plates, revealing turbulence onset near roughness and changes in flow dynamics.

Contribution

It provides detailed velocity measurements showing how roughness affects boundary layer turbulence and flow fluctuations, advancing understanding of heat transfer regimes in turbulent convection.

Findings

Velocity fluctuations increase dramatically with roughness.

Boundary layer becomes turbulent near roughness.

Flow structure functions change with roughness.

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-B{\'e}nard 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.