Dependence of wake structure evolution on the frequency of a pitching wing: A numerical investigation using LES

Abstract

The flapping kinematics have revealed a new paradigm of locomotion which is highly maneuverable and efficient compared to traditional propulsion systems. This work numerically investigates a three-dimensional wing undergoing pitching oscillations using Large eddy simulation (LES). The wake structure is presented using the [Formula: see text]-function, and the influence of pitching frequency on the structure of the wake is discussed. The vortices appear to travel as interconnected vortex rings in the bifurcated wake and are completely different from the two-dimensional investigations. The increase in pitching frequency [Formula: see text] generates the interlinking structures between the two limbs of the bifurcated wake. The interlinking between the two limbs increases with [Formula: see text] and will get disrupted at sufficiently high [Formula: see text]. The increase in [Formula: see text] increased the force generated by the pitching wing, and the periodic dynamics between the force coefficients slowly transformed to quasi-periodic dynamics. The transition to quasi-periodic dynamics has not reduced the force generation of the pitching wing. The disruption of interlinking structures between the two limbs enhanced the lift generated by the pitching wing. The present investigation shows that the wake vortex structure strongly depends on [Formula: see text]. The generation of a jet in the wake is visible in the mean velocity profile indicating the thrust developed by the wing. The formation of multiple peak profiles indicated the formation of multiple jets, which resulted in enhanced thrust production. The Reynolds stress field generated from the velocity fluctuations appears to be influenced by both the pitching oscillations and vortex interactions. This study can guide the selection of better kinematic parameters to enhance the propulsive performance.