Performance Prediction of a Low-Reynolds-Number Airfoil in an Oscillating Freestream

The aerodynamic performance of a NACA0012 airfoil at transitional Reynolds numbers, [Formula: see text], and prestall angles of attack was investigated in a wind tunnel. The major focus was on the development of the suction surface laminar separation bubble (LSB), which affects lift, particularly at low angles of attack. Oscillating (surging) freestream velocity, representing idealized disturbances encountered by micro-aerial vehicles in gusty urban environments, was generated in the wind tunnel. Unsteady aerodynamic forces (lift, drag, and pitching moment) were measured directly using a multicomponent force balance, while the boundary layer and the LSB dynamics were characterized using hot-wire anemometry. In such a surging inflow, flow deceleration promotes turbulent reattachment of the separated laminar boundary layer, whereas acceleration can trigger LSB bursting. These reattachment and bursting events strongly modulate lift and explain the failure of unsteady thin-airfoil theory to predict the lift response under oscillating freestream conditions. Conversely, for large-amplitude velocity oscillations, drag variations are dominated by buoyancy forces, and contributions from boundary-layer unsteadiness become negligible. As a result, drag can be predicted with reasonable accuracy using unsteady thin-airfoil models incorporating added-mass and buoyancy terms when these are complemented by measured quasi-steady forces to account for quasi-steady Reynolds-number effects.