Experimental study of the three-dimensional wakes produced by trapezoidal panels with varying trailing edge geometry and pitching amplitude

The wake behaviors and dynamics associated with bio-inspired propulsion are examined through an investigation of the wake structures produced by five different pitching panels with various trailing edge shapes. The behaviors of the wake structures produced by the panels are captured through the use of stereoscopic particle image velocimetry. The plan-form shapes of the bio-inspired pitching panels are based upon a trapezoidal design, but with the introduction of either a forked, straight, or pointed trailing edge geometry. Each of the five panels are sinusoidally pitched about their leading edge at five different pitching amplitudes, resulting in 25 unique wake scenarios. Results presented in the current work demonstrate that panel geometry and pitching amplitude have significant effects on wake behavior and dynamics. The shape of the trailing edge tends to affect the shape of spanwise vortices as they are shed from the trailing edge. Forked panels tend to shed spanwise vortices that are inflected inwards, although this behavior disappears at larger pitching amplitudes, while pointed panels shed vortices that are inflected outwards at all pitching amplitudes. Increases in pitching amplitude, i.e. Strouhal number, result in greater generation of spanwise and streamwise vorticity, further expansion of the wake in the transverse direction, more pronounced spanwise compression of the wake, and the movement of the wake breakdown location upstream. Additionally, for certain panel geometries at large enough pitching amplitudes, multiple spanwise vortices are observed to be a result of a Kelvin-Helmholtz-like instability. The panels were pitched across a Strouhal number range between 0.09 and 0.66, overlapping with, and extending beyond the range of Strouhal numbers that are employed by various swimming fish and cetaceans.