Three-Dimensionality Effects due to Change in the Aspect Ratio for the Flow around an Impulsively Pitching Flat Plate

Three-dimensionality effects have been investigated for a flat plate undergoing an impulsive pitch-up motion in a large-scale, free-surface water channel with the presence and absence of an endplate to represent a symmetry plane and to prevent free-surface effects. To obtain the same aspect ratios without using the endplate, appropriately sized wings were also used in a fully submerged arrangement. The velocity fields were acquired in the near wake of the flat plate using a two-dimensional (2D) two-component digital particle image velocimetry system, and simultaneous force measurements were performed. The wing had an aspect ratio ranging from 2 to 8 and pitches up to 45° in 1 or 6 convective times. The force measurement results revealed that major difference occurred after the motion was completed in which the lift and drag forces exhibited a local increase and decrease. Interactions between leading edge, trailing edge, and tip vortices were investigated; flow structures were correlated with the force-time histories, and variations with aspect ratio were also addressed.