Performance of biologically inspired flapping foils

Abstract

Flapping foil propulsion is thought to provide AUVs with greater maneuverability than propellers. This thesis seeks to simplify the design process for this type of propulsion system by identifying thrust and wake characteristics for flapping foils and determining how these factors scale with certain parameters. First, the wake of a flapping NACA 0030 foil was studied qualitatively using fluorescent dye visualizations. The foil was heaved and pitched in a sinusoidal fashion. The effects of varying Reynolds number, Strouhal number, maximum pitch angle, and the phase shift between heave and pitch were studied. It was determined that at very low Strouhal numbers the wake was 'S' like and at moderate Strouhal numbers the wake contained discreet horseshoe-like vortices. Next, the wake was studied quantitatively using particle image velocimetry (PIV). Through this technique, numerical thrust coefficients and vorticity strengths were obtained as well as qualitative information regarding the morphology of the wake. The coefficient of thrust peaked at a different Strouhal number for each Reynolds number studied. This trend was compared to natural phenomenon.

(cont.) Impulsively started maneuvers were also studied using PIV. Both single flaps and half flaps of the foil were studied. It was determined that impulsively started single flaps produced a much larger maximum coefficient of thrust than the impulsively started half flaps. Many of the experiments were repeated using a biologically inspired trout tail shaped foil.