On the evolution of the wake structure produced by a low-aspect-ratio pitching panel

Abstract

Flow visualization is used to interrogate the wake structure produced by a rigid flat panel of aspect ratio (span/chord) 0.54 pitching in a free stream at a Strouhal number of 0.23. At such a low aspect ratio, the streamwise vorticity generated by the plate tends to dominate the formation of the wake. Nevertheless, the wake has the appearance of a three-dimensional von Kármán vortex street, as observed in a wide range of other experiments, and consists of horseshoe vortices of alternating sign shed twice per flapping cycle. The legs of each horseshoe interact with the two subsequent horseshoes in an opposite-sign, then like-sign interaction in which they become entrained. A detailed vortex skeleton model is proposed for the wake formation.

The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel

“…From their measurements and observations, they inferred a distribution of bound vorticity strongly weighted toward the tip, which yields a wake structure composed of interconnected vortex loops with a corresponding distribution of circulation. Despite significant differences in geometry and kinematics, the topology of the wake is consistent with the model proposed by Buchholz & Smits (2006). With increasing Strouhal number, the wake also bifurcates.…”

“…In an earlier stage of the present work, Buchholz & Smits (2006) visualized wakes produced by a rigid panel of aspect ratio 0.54, pitching about its leading edge at Re C = 640. At low Strouhal number (St = 0.23), a chain of structures was observed that was similar in appearance to the low-Strouhal-number wake produced by Guglielmini (2004).…”

Flowfield measurements in the wake of a robotic lamprey

“…At higher Strouhal numbers, where the thrust exceeds the drag, the wake is wider than in steady swimming because the convection velocity of the vortices is lower while their induced velocity is not reduced as much. This is the same behavior Buchholz and Smits (2007) found in the wake of a flat plat at increased Strouhal number. Figure 11 shows the instantaneous velocity and vorticity fields along the body of the robot.…”

“…A recent study by Buchholz and Smits (2007) investigated the flow field in the wake of a pitching flat plate, taken as a simplified tail of a fish without heave. Buchholz and Smits found that the wake consisted of three-dimensional vortex loops linked together in a complicated vortex chain (see Fig.…”

“…Following Williamson and Roshko (1998), this is described as a 2S structure because two single vortices are shed each cycle. As the Strouhal number increases, the wake broadens and two vortices of opposite sign are shed each half cycle, that is, it resembles a 2P structure (Buchholz and Smits 2007). Kern and Koumoutsakos (2006) investigated the thrust production of an anguilliform swimmer by numerical simulation and found that the anterior half of the body had no contribution to the thrust.…”