Astract: Recent interest in flapping-wing propulsion, in particular for hovering or low-speed flight of micro air vehicles, has led to a renewed interest in the measurement and prediction of unsteady, vortex-dominated flows. The proposed vehicles operate at Reynolds numbers below 20,000, and operate with mildly to fully separated flow throughout the flapping cycle. This paper provides a brief history on the topic, and summarizes research efforts at the Naval Postgraduate School over the last decade, demonstrating the current numerical and experimental capabilities, and indicating areas where further work is required. Specifically, several wind and water tunnel experiments are described where the vortex structures generated by single flapping foils and by two foils arranged in a biplane configuration were visualized and thrust was measured as a function of flapping amplitude and frequency. Additional experiments are reported which show the potential of flapping foils to energize flat-plate boundary layers and to suppress or reduce regions of flow separation. Two- and three-dimensional panel code code and two-dimensional Navier-Stokes computations are also described to analyze these flapping-foil experiments.

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