  This study presents an experimental dataset documenting the evolution of a
turbulent boundary layer downstream of a rough-to-smooth surface transition. To
investigate the effect of upstream flow conditions, two groups of experiments
are conducted. For the \emph{Group-Re} cases, a nominally constant
viscous-scaled equivalent sand grain roughness $k_{s0}^+\approx160$ is
maintained on the rough surface, while the friction Reynolds number $Re_{\tau
0}$ ranges from 7100 to 21000. For the \emph{Group-ks} cases, $Re_{\tau
0}\approx14000$ is maintained while $k_{s0}^+$ ranges from 111 to 228. The
wall-shear stress on the downstream smooth surface is measured directly using
oil-film interferometry to redress previously reported uncertainties in the
skin-friction coefficient recovery trends. In the early development following
the roughness transition, the flow in the internal layer is not in equilibrium
with the wall-shear stress. This conflicts with the common practise of
modelling the mean velocity profile as two log laws below and above the
internal layer height, as first proposed by Elliott (\textit{Trans. Am.
Geophys. Union}, vol. 39, 1958, pp 1048--1054). As a solution to this, the
current data are used to model the recovering mean velocity semi-empirically by
blending the corresponding rough-wall and smooth-wall profiles. The
over-energised large-scale motions leave a strong footprint in the near-wall
region of the energy spectrum, the frequency and magnitude of which exhibit
dependence on $Re_{\tau 0}$ and $k_{s0}^+$ respectively. The energy
distribution in near-wall small scales is mostly unaffected by the presence of
the outer flow with rough-wall characteristics, which can be used as a
surrogate measure to extract the local friction velocity.
