This paper describes the evolution of an incompressible turbulent boundary layer on
the flat wall of an ‘S’-shaped wind tunnel test section under the influence of changing
streamwise and spanwise pressure gradients. The unit Reynolds number based on the
mean velocity at the entrance of the test section was fixed to 106 m−1, resulting in Reynolds numbers Reδ2, based on the
streamwise momentum thickness and the local freestream velocity, between 3.9 and
11 × 103. The particular feature of the experiment
is the succession of two opposite changes of core flow direction which causes a sign
change of the spanwise pressure gradient accompanied by a reversal of the spanwise
velocity component near the wall, i.e. by the formation of so-called cross-over velocity
profiles. The aim of the study is to provide new insight into the development of the
mean and fluctuating flow field in three-dimensional pressure-driven boundary layers,
in particular of the turbulence structure of the near-wall and the cross-over region.Mean velocities, Reynolds stresses and all triple correlations were measured with a
newly developed miniature triple-hot-wire probe and a near-wall hot-wire probe which
could be rotated and traversed through the test plate. Skin friction measurements
were mostly performed with a wall hot-wire probe. The data from single normal wires
extend over wall distances of y+ [gsim ] 3 (in wall units), while the triple-wire probe covers
the range y+ [gsim ] 30. The data show the behaviour of the mean flow angle near the
wall to vary all the way to the wall. Then, to interpret the response of the turbulence
to the pressure field, the relevant terms in the Reynolds stress transport equations are
evaluated. Finally, an attempt is made to assess the departure of the Reynolds stress
profiles from local equilibrium near the wall.
Near-wall velocity measurements by particle-shadow tracking