While flat plates constitute the canonical geometry in aerodynamics, many complex variations of the flat plate problem still remain to be fully characterized. Inspired by nature's designs, this study explores two different mechanisms that make use of plate geometries, and investigates the principles that govern their behavior. The presence of compliance, abundant in plant structures, and the use of unsteady three-dimensional motions, preferred by nature's fliers and swimmers, are considered.
The first part of this presentation is dedicated to investigating an inverted flag; an unactuated flexible cantilever plate that is clamped at its trailing edge and submerged in a flow. The resonance between solid motion and fluid forcing generates large-amplitude unsteady deformations of the structure that are used for energy harvesting purposes. Flags of very small aspect ratio are shown to present dynamics significantly different to those of their large aspect ratio counterparts, undergoing a saddle-node bifurcation instead of a divergence instability. The angle-of-attack of the flag is then modified to reveal the existence of dynamical regimes additional to those present at zero angle-of-attack. A side-by-side flag configuration is finally explored, highlighting the presence of an energetically favorable symmetric flapping mode among other coupled dynamics.
The second part of this talk delves into the analysis of underwater flapping propellers and the optimization of their three-dimensional motion to generate desired maneuvering forces, with the objective of obtaining an appendage for use in autonomous underwater vehicles that can perform both fast maneuvering and efficient propulsion. An experimental optimization procedure is employed to obtain the most efficient trajectory that generates a specified side force. The effect of increasing the fin's aspect ratio is examined, and a highly efficient trajectory, that makes use of high three-dimensionality and rotation angles, is obtained for a fin of AR=4. The use of a flexible fin is then analyzed and shown to be detrimental to the maneuvering efficiency of the system.