  We present an experimental study of the aerodynamic forces on a thick and
cambered airfoil at a high Reynolds number 3.6 x 10^6, which is of direct
relevance for wind turbine design. Unlike thin airfoils at low chord-based
Reynolds numbers, no consistent description currently exists for the stall
process on such airfoils. We consider two chord-wise rows of instantaneous wall
pressure measurements, taken simultaneously at two spanwise locations over a
range of angles of attack. We show that around maximum lift conditions, a
strong asymmetry is observed in the statistics of the normal force on each
chord. In this range of angles of attacks, the pressure fluctuations are
largest in the adverse pressure gradient region, and the fluctuation peak along
the chord is systematically located directly upstream of the mean steady
separation point, indicating intermittent flow separation. Moreover, the
fluctuations are characterized by bi-stability in both space and time: for each
spanwise location, large excursions of the local wall pressure between two
different levels can be observed in time (jumps), and these excursions are
highly anti-correlated between the two spanwise locations (spatial switches).
The characteristic time scale for the switches is found to be well correlated
with the amplitudes of the fluctuations. Application of Proper Orthogonal
Decomposition (POD) analysis to each row of sensors confirms that the flow
separation is an inherently local, three-dimensional and unsteady process that
occurs in a continuous manner when the angle of attack increases. The
correlation between the dominant POD mode amplitudes is found to be a good
indicator of bi-stability. Most of the fluctuations can be captured with the
two most energetic POD modes. This suggests that force fluctuations near the
maximal lift could be modelled by a low-order approach, for monitoring and
control purposes.
