An experimental investigation of the propulsive thrust of oscillating foils of different planforms

Luznik, Luksa (1997) An experimental investigation of the propulsive thrust of oscillating foils of different planforms. Masters thesis, Memorial University of Newfoundland.

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Abstract

A second design loop in the development of the computer controlled oscillating motion apparatus was completed as a part of this research. Specifically, work was concentrated in two areas: first, modifications to the existing motion control software with the aim to produce a continuous sinusoidal motion, and second, design of a dynamometer to measure more accurately the thrust developed by the oscillating foil. -- Bench tests conducted with the changes in the motion control indicated that the limitations to produce true sinusoidal oscillatory motion were due to the oscillating apparatus drive system. A single degree of freedom dynamometer consisted of two flexible vertical sheets to which a rigid plate was connected forming a U frame. This design allowed for sensitivity in the fore and aft direction, but was very rigid in the transverse direction. The oscillating apparatus was supported by the frame, and thrust or drag forces developed by the oscillating foil were transmitted through this frame to the load cell. -- An experimental study to measure developed thrust from a series of rigid foils subjected to unsteady, oscillatory motion has been conducted in the towing tank at Memorial University of Newfoundland. The foils tested consisted of a rectangular planform, a straight tapered and swept back planform, as well as the planform with curved leading edge, similar to the flukes of a fin whale. The foils had the same aspect ratio of 6. Two foils were fabricated out of brass, and another was made out of wood. The brass foils had a span of 200 mm, while the wooden foil had a span of 400 mm. In the experiment the heave amplitude to chord ratio was large (of order 1) and the pitch amplitudes tested were :6, 13, 17.5 and 21 deg. The pitching axis position was fixed at chord/2, and pitch led heave by 90 deg. -- Results of developed thrust from different planforms are presented and examined with respect to four areas of interest: large amplitude motion, effects of different planforms on developed thrust, foil performance at large angles of attack, and a comparison of an existing 3D unsteady flow panel code to predict developed thrust with the experimental results. -- Comparison of experimental results with the panel method showed that for small reduced frequencies and angles of attack, agreement in the magnitude of the thrust coefficient was good. At large reduced frequencies and angles of attack results from the panel method underpredicted the developed thrust. This discrepancy in results was due to separation and dynamic stall which occurred in the range of 33 to 40 deg. angle of attack. The combination of a leading edge sweep and taper of the foil's planform has a marked difference in developed thrust compared to the foil with a straight leading edge. This was due to different stall behaviour over foils with leading edge sweep compared to the stall of foils with a straight leading edge.

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