  We present an experimental study of the aerodynamic forces on a thick and
cambered airfoil at a high Reynolds number (3.6 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. As an attempt to shed some light on the flow physics,
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 the pressure fluctuations are largest for maximum lift
conditions, and that a fluctuation peak along the chord is systematically
located directly upstream of the mean steady separation point, indicating an
intermittent flow separation. For angles of attack corresponding to the onset
of separation, the pressure 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 pressure variance. 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. Moreover, 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.

Fluid Dynamics