  This paper presents an experimental study on the transition from smooth to
rough walls, and back to the smooth one, in turbulent closed-conduit flows.
These transitions cause a shift on flow velocity profiles that changes their
parameters when compared to the flow over a smooth wall. Different water flow
rates were imposed in a closed conduit of rectangular cross section, where
rough elements consisting of cavities of d- and k-type were positioned covering
a part of the bottom wall of the test section. Reynolds numbers based on the
channel half-height were moderate, varying between 7800 and 9600, and the
regime upstream of the rough elements was hydraulically smooth. Experimental
data for this specific case remain scarce and the involved physics rests to be
understood. The flow field was measured by low frequency PIV (particle image
velocimetry) and by flow visualization, the latter using a continuous 0.1 W
laser, a high-speed camera, and scripts written by the authors. From the
instantaneous fields measured with PIV, the mean velocities, fluctuations,
shear stresses and turbulence production were computed. The results show the
presence of oscillations in Reynolds stress and turbulence production, that are
higher for the k-type roughness and were not shown in previous experimental
works. From the high-speed movies, the angular velocities and frequencies of
vortices in the cavities were computed, and the occurrence of fluid ejection
from the cavities to upper layers of the flow was observed. A relation between
the angular velocities of inner-cavities vortices and the oscillations in
Reynolds stress and turbulence production is proposed.
