Shallow and Deep Dynamic Stall for Flapping Low Reynolds Number Airfoils

We consider a combined experimental (based on flow visualization, direct force measurement and phase-averaged 2D particle image velocimetry in a water tunnel), computational (2D Reynolds-averaged Navier-Stokes) and theoretical (Theodorsen's formula) approach to study the fluid physics of rigid-airfoil pitch-plunge in nominally two-dimensional conditions. Shallow-stall (combined pitch-plunge) and deep-stall (pure-plunge) are compared at a reduced frequency commensurate with flapping-flight in cruise in nature. Objectives include assessment of how well attached-flow theory can predict lift coefficient even in the presence of significant separation, and how well 2D velocimetry and 2D computation can mutually validate one another. The shallow-stall case shows promising agreement between computation and experiment, while in the deep-stall case, the computation's prediction of flow separation lags that of the experiment, but eventually evinces qualitatively similar leading edge vortex size. Dye injection was found to give good qualitative match with particle image velocimetry in describing leading edge vortex formation and return to flow reattachment, and also gave evidence of strong spanwise growth of flow separation after leading-edge vortex formation. Reynolds number effects, in the range of 10,000-60,000, were found to influence the size of laminar separation in those phases of motion where instantaneous angle of attack was well below stall, but have limited effect on post-stall flowfield behavior. Discrepancy in lift coefficient time history between experiment, theory and computation was mutually comparable, with no clear failure of Theodorsen's formula. This is surprising and encouraging, especially for the deep-stall case, because the theory's assumptions are clearly violated, while its prediction of lift coefficient remains useful for capturing general trends. © 2009 US Government

Angle of Attack and Planform Effects on Flat Plate Vortices at Low Reynolds Numbers

Tip vortices of low aspect ratio wings were studied in a water tunnel at Reynolds numbers of 8,000 and 24,000, with dye injection and digital particle image velocimetry in crossflow planes in the near wake, for rectangular, semi-elliptical and delta-wing planforms. The velocity data were used to calculate lift via circulation, and the results were compared with direct force measurements. The objectives of the study were to assess how low vortex circulation, how well the measurement of lift coefficient from tip-vortex circulation, how well the measurements, fit the slender wing theory lift curve slope, and the extent to which planform shape affects lift coefficient while aspect ratio and area are kept constant. All models were thin flat plates with square edges. For the range of streamwise locations and angles of attack studied, apparent viscous effects on vorticity in the wake were small, resulting in good agreement between lift coefficient values inferred from the measurements, the force balance data, and the classical inviscid formulas

PIV on Simple Mechanical Flapping Wings for Hover-like Kinematics

A Particle Image Velocimetry (PIV) study on aspect ratio effects was performed to highlight the role of three-dimensional aerodynamic contributions relative to inertial contributions to lift and power for a hover-like passive pitch rotation flapping kinematic motion. Interest in the research and development of flapping wing micro air vehicles (MAVs) continues to grow. Despite the large body of work performed recently, an abundance of unanswered questions still exist. The approach taken in this study is to begin by reducing the complexity of the problem through parametric variation of a single geometric parameter, aspect ratio. The present PIV research has a parallel ongoing force acquisition experiment led by the US Air Force Research Labs (AFRL/RBAL). When the AFRL study has been completed, the lift and inertial force results will be used in conjunction with the PIV results to provide more insight into the physics behind the resulting forces. One important finding for the kinematics studied is that an intermediate aspect ratio (rectangular flat plate) wing will result in the greatest lift efficiency from the standpoint of both aerodynamics and power