  We present the synchronized unsteady wall pressure measurements performed at
the circumference of a full scale 35 m circular chimney submitted to natural
wind. The atmospheric turbulent boundary layer generates a turbulence intensity
of 15 % at the altitude of the measurements and the corresponding Reynolds
number is 1.13 106. For the first time, the steady and unsteady pressure
distributions around the circumference are presented, despite large
difficulties in the measuring process. The analysis of the pressure
distribution using the bi-orthogonal decomposition (BOD) is a key factor in the
success of the processing. By comparison with wind tunnel similar measurements,
with 2D cylinders, the time-averaged pressure distribution is found between the
one of a subcritical regime and a supercritical regime, slightly closer to the
subcritical regime notably because the base pressure coefficients are similar.
The atmospheric turbulence is an essential ingredient for the unsteady pressure
response on the chimney, which generates high levels of RMS pressure, much
higher than all those measured in wind tunnel, even under the turbulent flow
generated by a grid. The analysis by BOD allowed the extraction of a term which
is mainly generated by alternate vortex shedding. Despite a very noisy signal,
the Strouhal number is determined to be 0.20-0.24. The corresponding shape of
the pressure distribution is compared with those found in wind tunnel: it is
shown that the full scale field measurements are close, but not identical, to
the small scale measurements in wind tunnel on a rough cylinder submitted to a
grid generated turbulent flow.

Fluid Dynamics