Motion induced unsteady aerodynamic loads with development of vortical flow

Abstract

The development of vortical flow significantly changes the properties of motion induced aerodynamic loads. Tests in the Transonic Wind Tunnel G\"ottingen with a half wing model of a Lambda wing are analyzed. Pitch oscillations were imposed by a hydraulic actuator with amplitudes (0.08°-1.5°) and frequencies (up to 40Hz) relevant for aeroelastic analyses. Unsteady surface pressure and flow field measurements were performed. With increasing angle of attack, a leading edge vortex emerges and moves inboard. The unsteady loads show a strong dependency on the effects of the vortical flow which can be described based on the measurements at constant angles of attack. The effects are lagging the motion. In the rear outboard part of the wing, the unsteady pressures are small, but the amplitude of the motion is high. Hence, a significant amount of energy - force integrated over motion - is transferred from the fluid to the motion. For a free motion or deformation of the wing this would cause a destabilization. A significant effect of the vortical flow is the strong discrepancy between the position where it emerges and is affected and the position, e.g. further outboard, where the flow may generate unsteady, potentially destabilizing, loads