Bistability in the stalling behavior of thick, cambered airfoils under elevated freestream turbulence

Abstract

The aerodynamic performance of a thick, cambered FFA-W3-301 airfoil section near and beyond stall was examined under elevated freestream turbulence using time-resolved surface pressure measurements for chord-based Reynolds numbers from 125,000 to 275,000. Three passive turbulence grids were used to isolate the effects of turbulence intensity and integral length scale, and the results were compared with a clean inflow. The elevated turbulence largely had a positive effect on the mean performance of the airfoil section, eliminating premature flow separation and recovering lift and drag polars more representative of a higher effective Reynolds number; however, resolving the lift in time showed that the time-averaged polars concealed bistable switching. The time-averaged lift was an occupancy-weighted average of two states, and conventional mean and standard-deviation statistics misrepresented the true post-stall unsteady loads. Beyond the maximum-lift angle of attack, the lift alternated repeatedly between two distinct levels with no characteristic frequency. At the highest tested Reynolds number, the switching persisted over an angle-of-attack range of up to nine degrees, more than an order of magnitude wider than that reported for a thin, symmetric section in a low turbulence inflow. The lift levels of the two states were also found to be nearly independent of the inflow turbulence level, whereas the fraction of time spent in each differed between turbulence conditions. conditionally averaged surface pressure distributions and proper orthogonal decomposition of the surface pressure consistently showed that neither state is fully attached; the states correspond to two positions of a single separation front on the suction side, a switching accurately captured by a single pressure mode from the decomposition.