Nature in engineering for monitoring the oceans: towards a bio-inspired flexible autonomous underwater vehicle operating in an unsteady flow

It has long been understood that swimming marine animals have evolved capabilities in terms of speed, manoeuvrability, and efficiency which are desirable for underwater vehicles. Despite this, solutions inspired by nature, or bio-inspiration, are very rarely applied to solve engineering challenges. In particular, it is understood that fish have the ability to alter their mode of swimming to interact with naturally produced vortices as a method of conserving energy and in certain instances extracting energy from a flow. This paper considers whether a bio-inspired flexible autonomous underwater vehicle (AUV) could exploit unsteady flow features to reduce its cost of transport. An analytical model is developed which allows an AUV designer to predict which flow frequencies excite the natural vibration modes of a flexible cylinder. It is demonstrated that by placing a flexible cylinder in an unsteady flow, such as downstream of a bluff body, a similar mechanism to that used by fish may be exploited to move the cylinder upstream with no power input except that extracted from the flow.