Effect of Pivot Point on Aerodynamic Force and Vortical Structure of Pitching Flat Plate Wings

This paper reports aerodynamic forces and 2D PIV velocity measurements on a rectangular wing with an effective aspect ratio 4 during a pitch ramp-hold wing motion at reduced frequencies from 0.022 to 0.39. The parameter space in terms of the Stokes number and the Reynolds number is used to correlate pitch rate and free stream velocity effects. The maximum angle of attack is 45 degrees and pivot locations, such as leading edge pivot, midchord pivot, and trailing edge pivot, are considered. The evolution of aerodynamic forces as functions of convective time, pitching time, and angle of attack are discussed. Noncirculatory effects are observed for reduced frequency higher than 0.13. For leading edge pivot axis non-circulatory effects produce a positive force spike at the start of rotation and a negative spike at the end of rotation. The opposite is found for trailing edge pivot. For midchord pivot the non-circulatory force spikes are not present. Rotation rate effects are observed during the constant rate part of the motion for reduced frequency higher than 0.065. For leading edge pivot axis non-circulatory effects and rotation rate effects combine to produce very large lift and drag coefficients. Incipient vortex shedding is observed at high reduced frequency during the hold phase for convective times in the range 4-20. For reduced frequency lower than 0.022 the flow is quasi steady and lifting line theory provides good estimates of lift for angle of attack below the steady stall angle. Although significant leading edge suction was measured at the final steady state condition, projection of the normal force in the drag direction is the main contribution to the drag. PIV measurements show the flow topology for different pivot axes. For leading edge pivot axis a starting vortex is formed at the trailing edge which promotes development of the LEV and enhances aerodynamic forces. For mid-chord pivot axis starting vortices are formed at the leading and trailing edges. For trailing edge pivot axis the starting vortex is formed at the leading edge only. Formation of a starting vortex at the leading edge delays development of the LEV and inhibits aerodynamic force generation.