Flow past flapping foils: Surface texture and unsteadiness effects

Evolution has lead to extremely efficient biological swimmers and flyers. Animals are able to use the surrounding flow to obtain performance gains, like in the fish-schooling operation. Although nowadays their way of traveling is being replicated in mechanical systems, such as Micro Air Vehicles or energy harvesters, we are still far from achieving their efficiency. In this thesis we aim to advance towards more efficient propulsive methods. We mimic the fish-schooling operation by the use of tandem flapping foils, and we explore real-life aspects that could influence the hydrodynamics performance. Do animals present surface texture for propulsive performance reasons? How does the unsteadiness in the surrounding flow affect the performance of a flapping foil? In this thesis we aim to answer those questions by conducting numerical simulations and experiments. We find that the addition of surface texture (36\% and 70\% coverage area) is prejudicial for a pure pitching foil. As for the second question, we explore the importance of adding physically relevant inputs to recover the dynamics of a flapping foil submerged inside unsteady incoming flow, and executing transitions in its motion. We later use the recovered force model to predict the path that maximises its propulsive performance. We finish by focusing on more efficient ways of developing the transition motion, finding that the optimal manoeuvre is associated with a more efficient evolution of the transition velocities