WAKE OF A LOW ASPECT RATIO PITCHING PLATE, St = 0.23
J. H. J. Buchholz and A. J. Smits
A rigid plate of aspect ratio S/C=0.54 (S=span, C=chord)
is pitched in a uniform flow about its leading edge. The
double-amplitude to span ratio is A/S=0.31. The
leading edge is hinged to the trailing edge of a
stationary symmetric 2-D airfoil. Fluorescent dyes are
introduced through a series of holes on each side of the
airfoil support. The Reynolds number based on the
chord of the plate and the freestream velocity U is 640.
Different flow regimes can be identified based on the
Strouhal number, St=fA/U (f=pitching
frequency). Twice in each flapping cycle, a horseshoe
vortex is created, consisting of vorticity shed by the top,
bottom, and trailing edges. At St=0.23 (this image, top and side views) the legs
of each vortex loop are entrained by the tip of the
subsequent loop forming a chain-like structure.
Studying such wakes is believed to be important for
understanding the mechanisms of thrust production in
fish-like swimming. See also St = 0.43 and St = 0.64.
Buchholz, J. H. J. and Smits, A. J., "On the evolution of the wake structure produced by a low aspect-ratio pitching
panel." Journal of Fluid Mechanics, 546, pp. 433-443, 2006.