Experimental Investigation Into the Effect of a Stationary Dorsal Fin for Multiple Configurations - A Bio-Inspired Perspective

Results of experiments aimed at determining the performance of two quasi-two-dimensional hydrofoils arranged in an in-line configuration are presented. This thesis aims at understanding the role of pitching motion of a caudal fin in the presence of a dorsal fin, decoupled from heaving motion. A simplified model has been proposed, which accounts for pitching motion, and explores the role of frequency, amplitude, and separation distance between dorsal and caudal fins. The net thrust generated and the work done by the 2-fin model while undergoing prescribed pitching motions were calculated.

The pitch experiments focus on four parameters to quantify their influence on propulsive performance; (1) frequency, (2) trailing edge amplitude, (3) separation between foils, and (4) flexibility of the fins. The experiments are conducted under both accelerating and free swimming conditions.

It is hypothesized that distance of separation between the static dorsal and pitching caudal fins during free swimming will affect the work done and economy across amplitudes and frequencies due to the weakening of the wake from the leading hydrofoil. It is also expected that the change in flexibility at free-swimming speeds will cause a change in economy, and that the presence of the dorsal fin will amplify this effect. Finally, the thrust is hypothesized to follow trends similar to that of a pitching hydrofoil in terms of amplitude and frequency.

While the increased drag due to the additional hydrofoil reduces the speed of motion, it also reduces the required work to move the lagging hydrofoil. The reduction in speed is ~10%, while the required work decreases by >20% on average, leading to an increase in economy. The effects increase with increasing amplitude and frequency during acceleration. As the distance between the hydrofoils is increased the efficiency and economy of the model reduces. Finally, flexible foils are more efficient in the presence of a stationary fin only at high frequencies and amplitudes.

This study is a valuable preliminary work towards the evolution of design of bio-inspired models and possibly vehicles.

A secondary aim of this thesis is the development of an inexpensive PIV technique to quantify flow fields. A PIV setup has been designed and built that allows the quantification of flows at speeds below 25.4 cm/s. At greater speeds, error margins exceed 10%.