Meeting Abstract

S1-2.5  Jan. 4  Thrust augmentation confirmed in self-propelled, tandem flapping foil robots by foil-wake interaction ANDERSON, E.J.*; LAUDER, G.V.; Harvard University

The question of energy “recapture” from the wake of upstream structures or fins of swimming fish by downstream fins has been discussed from a theoretical perspective for many decades. The phenomenon is difficult to confirm experimentally due to the fact that fish are self-propelled, deforming bodies for which the sources of thrust and drag are not easily decoupled. We have confirmed thrust augmentation using a different metric for swimming performance—self-propelled speed. We have constructed a flapping foil robot mounted in a flume on air-bearings that allows for the accurate determination of self-propelled speed. Two flapping foil robots separated by 0.5, 1, or 2 chord lengths in the streamwise direction were connected with a sensitive force transducer. The foils were then programmed to move with a particular pitch and heave. The starting phase of the downstream foil was varied for each trial. The flume speed was tuned until the flapping foil robots were self-propelled, i.e. held station in the flume at the position that they rest when the flume speed was 0. Self-propelled speed and force transducer measurements confirmed the existence of significant maxima in thrust augmentation (and reduction) for particular foil-to-foil spacing, phase differences, and flapping frequencies. Single foil and tandem foil control trials did not exhibit the same effect. Flow visualization shows the mechanism to be related to the effective angle of attack of the downstream foil due to the structure of the wake of the upstream foil. This confirms recent computational work on the phenomenon, the hypotheses by early investigators of fish fluid dynamics, and the suggestions of recent experimental work on bluegill sunfish focusing on the kinematics and flow fields of the in-line fins of swimming fish.