Influence of roughness on boundary layer structure and large scale circulation in turbulent thermal convection

With sufficient forcing, measured by the Rayleigh number, Rayleigh-B\'enard convection becomes turbulent. Influence of controlled roughness on the heated bottom plate has been studied from a thermal point of view [1]: a regime transition has been observed corresponding to an increase of the heat flux compared to a smooth plate. With a parallelepipedic convection cell where mean flow can be considered as bi-dimensional and where controlled square-studs roughness have been added, we performed PIV measurements to visualize both large scale circulation and boundary layer close to roughness. We work at Rayleigh number from $10^{9}$ to $10^{10}$, either side of the transition. We show that the boundary layer is thinner above the rougness than between row of square-studs, and a dramatic change of flow structure is observed between rougness. It is in good agreement with previous temperature measurements [2] and brings an explanation to the heat flux increase. Moreover, for large scale circulation, turbulence structure changes: velocity r.m.s. is higher than in the smooth case and presents a large dissymetry.\\[4pt] [1] Tisserand et al., \emph{Physics Of Fluids}, \textbf{23}, 015105 (2011)\\[0pt] [2] Salort et al., \emph{Physics Of Fluids}, \textbf{26}, 015112 (2014)