Narrow Results

This paper aims to study the aerodynamic and propulsive performance of a plunging airfoil using novel plunging waveform, that creates additional trailing edge vortices, that affects the flow field and, consequently, the aerodynamic loads on the airfoil. This waveform is generated by means of superpositioning of the regular sinusoidal wave with a faster one. This novel waveform not only allows to add another pair of trailing edge vortices but also to control their strength. It is found that as the weight of the faster wave increases, the thrust coefficient increases rapidly while the propulsive efficiency decreases slowly, allowing a window of high thrust with small efficiency reduction. Understanding the effect of generating additional trailing edge vortices within the regular flapping cycle on the propulsive performance is the main objective of this work.

In this study, we illustrate the fractal nature of the wake shed by a periodically flapping filament. Such wake structure is a combination of primary vortex shedding resulting in the von Kármán vortex street, a series of concentrated vortex dipoles formed when the trailing edges of filaments reach their maximum amplitudes and small eddies form along the shear layer connected with the concentrated vortices due to the shear layer instability. The vortex dynamics of the flapping filament are visualized and imaged experimentally using a soap-film flow tunnel with a high-speed camera and a low pressure sodium lamp as a light source. The wake fractal geometry is measured using the standard box-counting method and it is shown that the fractal dimension of the soap pattern boundaries in the wake is D = 1.38 ± 0.05, which agrees well with those measured for fully developed turbulences and other shear flow phenomena. The invariant of the fractality in the wake induced by the flapping filament thus provides another illustration of the geometrical self-similarity and nonlinear dynamics of chaotic fluid flows.