Experimental Characterization of Structural Traveling Wave-Induced Thrust

Using nature-inspired solutions for propulsion, this work investigates the use of traveling waves to generate thrust in water. A design based on a slender cantilever beam similar to flagella in bacteria is submerged in water and excited with a sinusoidal motion to study the impact of frequency and amplitude of the oscillation on the thrust generation. Structural measurements combined with advanced flow diagnostic techniques are used to characterize the behavior of the fluid–structure system. The structural response and the induced traveling waves are first studied in air and characterized through laser vibrometry and high-speed digital image correlation. This demonstrated the possibility of inducing traveling waves in the structure and permitted to identify the conditions that maximize the traveling versus the standing wave contribution. The characterization of the fluid–structure interaction has been done using Laser Doppler Anemometry (LDA). LDA measurement was carried out downstream from the beam at a fixed distance to measure the velocity of the induced flow at different excitation conditions (amplitude and frequency). The results showed that the coupling between the structural motion and the thrust generated is nonlinear in nature and depends on the tip displacement of the beam. Empirical laws that relate the amplitude and frequency of excitation to the generated thrust are here proposed.