Justin T. King

Momentum Distribution in the Wake of a Trapezoidal Pitching Panel

The oscillation of bioinspired fin-like panels in a uniform freestream flow creates chains of vortex rings, including streamwise segments that induce significant threedimensional effects. With increasing Strouhal number, this wake structure induces flow with increasing nondimensional momentum, defined relative to the freestream velocity, in the downstream direction. This increase in relative momentum with increasing Strouhal number is consistent with greater nondimensional thrust production, which has been shown previously in the literature. These results were obtained via stereoscopic particle image velocimetry water tunnel experiments at Strouhal numbers ranging from 0.17 to 0.56 downstream of a continuously pitching trapezoidal panel. Features of the wake dynamics including spanwise compression, transverse expansion, transverse wake splitting or bifurcation, and wake breakdown are elucidated through analyses of phase-averaged as well as timeaveraged velocity fields, in addition to common vortex identification methods.

Experimental study of the three-dimensional wake of a trapezoidal pitching panel

Aquatic animals can maneuver and propel themselves through a variety of means, including the oscillation and undulation of flukes and fins. The motions of these can species develop thrust-producing, highly three-dimensional wakes. Experiments have shown that bio-inspired propulsors can operate with large propulsive efficiencies, with some efficiencies being greater than those of a screw-propeller propulsion system. In the current work, stereoscopic particle image velocimetry (SPIV) was used to characterize the wake produced by a rigid, bio-inspired trapezoidal pitching panel. Detailed analysis in terms of Strouhal number is the focus of the current work, and the Strouhal number range tested was from 0.17 to 0.56. The results show that a highly three-dimensional wake structure develops, which is composed of interconnected streamwise and spanwise vortex tubes. The streamwise vortex tubes induce spanwise flow toward the midspan that drives a spanwise compression of the wake. An increase in Strouhal number results in the movement of the location of wake breakdown upstream and more exaggerated spanwise compression of the wake.

Experimental study of the spanwise wake compression of a trapezoidal pitching panel

Aquatic animals can maneuver and propel themselves through a variety of means. Among these means, are the oscillation and undulation of the flukes and fins of different cetaceans and fishes. The motions of these species can be employed to develop thrust-producing, highly three-dimensional wakes. Recently, a great deal of interest in incorporating certain biological propulsion schemes into engineering designs has been generated. Experiments have shown that bio-inspired propulsors can develop large efficiencies, with some efficiencies being greater than those of a screw-propeller propulsion system.In the current work, stereoscopic particle image velocimetry (PIV) was used to characterize the wake produced by a rigid, trapezoidal pitching panel. Prior work has shown that one of the dominant parameters governing wake structure is the Strouhal number. Detailed analysis in terms of Strouhal number is the focus of the current work, and the Strouhal number range tested was from 0.17 to 0.56.Copyright

Three dimensional unsteady wake of a trapezoidal pitching panel

Stereoscopic particle image velocimetry (SPIV) was used to investigate the wake of a rigid, pitching, trapezoidal panel, used as a model of thrust-producing fish caudal fins. Experiments were conducted for a range of Strouhal numbers from 0.17 to 0.56. A reverse von Karman vortex street was observed along the mid-span of the wake whereas complexity and three-dimensional effects increased towards the tip. The phenomena of wake breakdown was investigated and shown to depend on Strouhal number. Spanwise velocity was measured in order to investigate the interaction of streamwise vortices and the vorticity created by the swept edge. Assumption of two dimensional flow at the midspan made in previous work by Green et al.1 in order to compute FTLE fields was validated by showing that spanwise velocity at the midspan is small relative to the other velocity components prior to wake breakdown. Downwash and sidewash effects were measured at quarterspan and tip location. Previously unknown upwash effects were observed to investigate the contraction of the wake in the spanwise direction. Planar laser induced fluorescence showed that large streamwise vortices create the up, down, and sidewash effects observed in the experimental results.