Zachary J. Taylor

Long-Duration Time-Resolved PIV to Study Unsteady Aerodynamics

A time-resolved particle image velocimetry (PIV) system has been developed at the University of Western Ontario, London, ON, Canada, with long-recording-time capabilities. This system is uniquely suited to the study of unsteady aerodynamics and hydrodynamics, such as avian aerodynamics or bluff-body oscillations. Measurements have been made on an elongated bluff body through the initial build-up phase of flutter. The possibilities to study this instability, which was responsible for the collapse of the Tacoma Narrows Bridge, are significantly broadened by the use of this system. The long-time recording capability of the system allows for novel results since it yields data that are spatially and temporally resolved over a long record length. The buildup of flutter is shown to exhibit complex dynamics that are heavily influenced by the flow-induced motion of the body. Features of the wake turbulence as a function of time are presented and shown to substantially vary.

Estimation of unsteady aerodynamics in the wake of a freely flying European starling (Sturnus vulgaris).

Wing flapping is one of the most widespread propulsion methods found in nature; however, the current understanding of the aerodynamics in bird wakes is incomplete. The role of the unsteady motion in the flow and its contribution to the aerodynamics is still an open question. In the current study, the wake of a freely flying European starling has been investigated using long-duration high-speed Particle Image Velocimetry (PIV) in the near wake. Kinematic analysis of the wings and body of the bird has been performed using additional high-speed cameras that recorded the bird movement simultaneously with the PIV measurements. The wake evolution of four complete wingbeats has been characterized through reconstruction of the time-resolved data, and the aerodynamics in the wake have been analyzed in terms of the streamwise forces acting on the bird. The profile drag from classical aerodynamics was found to be positive during most of the wingbeat cycle, yet kinematic images show that the bird does not decelerate. It is shown that unsteady aerodynamics are necessary to satisfy the drag/thrust balance by approximating the unsteady drag term. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag during flapping flight.

Distribution of spanwise enstrophy in the near wake of three symmetric elongated bluff bodies at high Reynolds number

Three elongated bluff bodies with a chord-to-thickness ratio of seven have been studied experimentally at a Reynolds number based on body thickness of 3 × 104. The defining feature of elongated bluff bodies is the interaction between trailing edge Karman vortex shedding and leading edge separation-reattachment. We have used particle image velocimetry with different body geometries to investigate this interaction for three distinct cases: (i) small leading edge separation-reattachment length; (ii) large leading edge separation-reattachment length; and (iii) one case in between these bounds. The leading edge separation-reattachment is a significant source of spanwise enstrophy. Thus, changes in the wake enstrophy distribution are of particular interest. We have examined the time-averaged distribution and production of both the turbulent kinetic energy and the spanwise enstrophy in the near wake region utilizing proper orthogonal decomposition on the vorticity field to distinguish between turbulence and the ...

Experiments on the vortex wake of a swimming knifefish

The knifefish species propels itself by generating a reverse Kármán street using an anal fin, and the propulsion of this species is known to be highly efficient (Blake in Can J Zool 61:1432–1441, 1983). Previous studies have suggested that there is an optimal swimming range for fish based on the amplitude and frequency of the reverse Kármán street. In the current study, experiments have been performed to measure the ratio between the amplitude and wavelength of vortices in the wake of a knifefish. It is suggested that the wave efficiency can be estimated by optimizing the thrust created by the reverse Kármán street for a given spacing ratio, and present observations have an average value of 0.89. The relationship established between spacing ratio and wave efficiency, in addition to the measured parameters, will be invaluable for bio-inspired designs based on the knifefish.