NACA0012 and 0018 pitching airfoils in an oscillatory relative free-stream are examined in a water tunnel with an electric motion-rig to oscillate the test article, and in a wind tunnel with moving-louvers that vary flow speed in the test section, at reduced frequency k=0.074. History of lift and pitching-moment is compared between the two facilities and with the classical attached-flow theoretical treatment, for (1) combined pitch and free-stream oscillating, (2) pure pitch, and (3) free-stream oscillation at constant incidence. The pitch-motion takes the airfoil into and out of static stall, with consequent dynamic-stall effects, and departure from attached-flow theory on the downstroke. Relative streamwise oscillation at 50% of mean-value gives departure from quasi-steady lift, and also from attached-flow unsteady theories, depending on the phase of motion. However, coupling between the dynamic-stall from pitching and the relative free-stream oscillation is weak, suggesting that the two respective nonlinearities do not mutually couple. These broad trends were shown to be larger than discrepancy between wind-tunnel and water-tunnel results.
展开▼
