Experimental study on the appearance of kelvin-helmholtz-like instabilities in the wakes produced by bio-inspired pitching panels

Stereoscopic particle image velocimetry is used to investigate the unsteady, three-dimensional wakes produced by bio-inspired pitching panels with varying trailing edge geometries. The planform shapes of the five panels were selected as rudimentary approximations of the shapes of caudal fins found in nature. The panels are sinusoidally pitched about their leading edges through five different pitching amplitudes within a constant freestream flow, generating five unique Strouhal numbers for each specific panel. Results demonstrate that a Kelvin-Helmholtz-like instability develops in the separated shear layer that emanates from the trailing edge of the panel. The wavelength of this instability decreases as the strength of the vortex sheet shed from the trailing edge increases. The Kelvin-Helmholtz-like instability arises when a relatively high magnitude velocity gradient forms near the moving trailing edge of the panel. The velocity gradient arises as two parallel streams of transverse flow, directed opposite each other, develop near the trailing edge of the panel as the panel retreats from a location of maximum pitching amplitude. As pitching amplitude, and therefore Strouhal number, increases, this vortex sheet becomes stronger. The instability occupies an appreciable portion of the spanwise extent of the wake, but ultimately disappears when the spanwise distance away from the plane of the midspan becomes large enough.