Static measurements of a NACA 0021 airfoil at high Reynolds numbers

Reynolds number effects on the aerodynamics of the moderately thick NACA
0021 airfoil were experimentally studied by means of surface-pressure
measurements. The use of a high-pressure wind tunnel allowed for variation
of the chord Reynolds number over a range of 5.0 × 10^5 ≤ Re_c ≤ 7.9 ×
10^6. The angle of attack was incrementally increased and decreased over a
range of 0° ≤ alpha ≤ 40°, spanning both the attached and stalled regime
at all Reynolds numbers. As such, attached and separated conditions, as
well as the static stall and reattachment processes were studied. A
fundamental change in the flow behaviour was observed around Re_c= 2.0 ×
10^6. As the Reynolds number was increased beyond this value, the stall
type gradually shifted from trailing-edge stall to leading-edge stall. The
stall angle and the maximum lift coefficient increased with Reynolds
number. Once the flow was separated, the separation point moved upstream
and the suction peak decreased in magnitude with increasing Reynolds
number. Two distinct types of hysteresis in reattachment were observed.