Unsteady Lift on a Fixed-AoA Airfoil in Unsteady Freestream*

Unsteady lift production of an airfoil in a time-varying freestream velocity flow is studied in this work. Various theories such as those of Isaacs (1945), Greenberg (1947), van der Wal & Leishman (1994), and Strangfeld et al. (2016) have been formulated to analytically describe the time-dependent airloads. These theories generally attribute the unsteady response to the effects of unsteady shedding and convection of vorticity in the airfoil wake. However, these theories have been only partially validated on a thick airfoil at low Reynolds number, and the limits of applicability of the theory are presently unknown. This work provides a comprehensive set of experimental data on a range of airfoils at high Reynolds number (> 10⁶), under varying freestream velocity amplitudes, reduced frequencies, and airfoil angles of attack. Surprisingly, the scope of the validity of the theoretical approaches is limited: significant nonlinear aerodynamic effects are found to dominate the lift and moment response in many cases, even at high Reynolds number. This work documents some of those limits and discusses the physical mechanisms underlying the departure of theory from experimental results.