Self-propelled fish locomotion in an otherwise quiescent fluid

Since the deep observations by Leonardo da Vinci, understanding fish locomotion in water has always attracted the attention of scientists in many fields, from fluid mechanics to other disciplines concerning environmental sciences. The complexity of this problem is mainly given by the non-linear interaction between the fish body and the surrounding fluid otherwise at rest, leading to the desired forward locomotion and to the unavoidable angular and lateral recoil reactions, which are essential for a correct evaluation of the swimming performance. Despite many advances have been obtained for the study of fish self-propulsion in recent years, from simple mathematical models up to complex numerical solutions, the main mechanisms underlying fish locomotion are not fully clarified and still require further investigations. In this thesis free swimming conditions is deeply analyzed for both steady swimming and fast maneuvers by a theoretical approach which considers the full body-fluid system to obtain the ex- changed internal forces. The focus is on the added mass and the vortex shedding contributions to the locomotion performance and on the role of recoil motions which, together with the prescribed body deformation, define the free swimming behavior. To this purpose, the impulse formulation allows for an easy isolation of the potential contri- bution, related to the added mass, and of the vortical contribution related to bound and released vorticity and a simple two-dimensional numerical model with concentrated vorticity is adopted for the numerical simulations to generate meaningful results able to clarify these physical phenomena. The aim is a unified procedure for both undulatory and oscillatory swimming to obtain valid an- swers for cruising speed, expended energy and kinematics, hence for the swimming performance in terms of the cost of transport and propulsive efficiency. The same model is also able to give new insights on the impressive performance characterizing fish fast maneuvers. The extreme turning capability and the large acceleration, so essential to fish survival along pray-predator encounters, are studied by highlighting the potential and the vortical impulses and their interplay induced by recoil motions, to show their relevance for the realization of the maneuver.