Unsteady Aerodynamics of Pitching Flat Plate Wings.

Abstract

In this study, the aerodynamics of a one-degree-of-freedom wing motion, a constant speed pitch ramp, were investigated to determine unsteady flow dynamics and force generation. This kinematics has discernible regions of constant rotation speed and motion acceleration, which helps isolate several effects. This pitching maneuver is analogous to the perching maneuver by birds and insects; a review of aerodynamics of natural flyers is provided. Maneuverability of natural flyers is difficult to emulate in engineered systems; the unsteady flow field of high degree-of-freedom wing kinematics complicates the analysis of the problem and the simpler one-degree-of-freedom kinematics considered here provides valuable insight for man-made fixed wing systems. In this study, dynamic flow field was investigated over thin finite-aspect-ratio-four wings pitching at several constant pitch rates in constant free-stream flows, giving reduced pitch rate in a range of 0 < K < infinity, in an attempt to understand the interplay of time scale between wing motion and free-stream flow. All work was conducted in free-surface water channel in a chord Reynolds number interval of 0 < Re < 13k by means of flow visualization, force measurement, and particle image velocimetry. A simple linear potential flow theory was implemented to elucidate experimental data; effects of reduced pitch rate, pivot axis location, and wing planform were included in the theory. Moreover, the identification of vortical flow structure was presented in correlation with force generation. The rapid increase of aerodynamic forces is associated with the formation of starting vortex in the wake and reduced-pitch-rate effect at the onset of the wing rotation; the gentle increase of aerodynamic forces is relevant to the simultaneous occurrence of leading-edge vortex formation and trailing-edge vortex shedding during constant rotation rate. Low taper-ratio wing enhances force generation at high reduced pitch rate. The pivot-axis location determines the location of the starting vortex. The leading-edge vortex evolution after the end of the motion is delayed; the time delay is the convection time from the leading edge to the pivot-axis location. Linear potential flow theory with rotation-rate effects gives reasonable estimation of force coefficients.