  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.
