  Turbulent boundary layers over flat walls in the presence of a hill are
frequently found in nature and industry. Some examples are the air flows over
hills and desert dunes, but also water flows over aquatic dunes inside closed
conduits. The perturbation of a two-dimensional boundary layer by a hill
introduces new scales in the problem, changing the way in which velocities and
stresses are distributed along the flow. When in the presence of sediment
transport, the stress distribution along the hill is strongly related to bed
instabilities. This paper presents an experimental study on the perturbation of
a fully-developed turbulent boundary layer by a two-dimensional hill. Water
flows were imposed over a hill fixed on the bottom wall of a closed conduit and
the flow field was measured by Particle Image Velocimetry. From the flow
measurements, mean and fluctuation fields were computed. The general behaviors
of velocities and stresses are compared to published asymptotic analyses and
the surface shear stress is analyzed in terms of instabilities of a granular
bed.
