  We present a wind tunnel experiment on a large smooth circular cylinder
mounted between walls. The Reynolds number range of interest is [800 000 : 2
170 000]. This low supercritical regime is often encountered in wind
engineering applications, especially the excitation of flexible circular
structures by vortex shedding. Main measurements are the unsteady wall pressure
distributions around the cylinder by means of synchronized pressure taps with
high resolution in time and space. By using the bi-orthogonal decomposition of
this set of signals, twin Strouhal numbers are detected which correspond to the
second and the fourth terms of the decomposition. These terms are found to
produce the unsteady lift on the cylinder with a main contribution of the
second one for which the associated Strouhal number is close to the one usually
found at subcritical Reynolds numbers.
