Thrust Characterization of a Bioinspired Vortex Ring Thruster for Locomotion of Underwater Robots

A new type of thrusting technology, loosely inspired by the locomotion of cephalopods, offers promising low-speed maneuvering capabilities for a new generation of underwater vehicles and robots. The actuators consist of a small cavity with a moving wall on one side and an orifice on the other side. The net effect of periodic movement of the moving wall is the ingestion of low-momentum fluid inside the cavity and then the expulsion of the fluid as a pulsatile jet from the orifice, with no net mass flux. Continuous operation of the actuator results in a synthetic jet. The actuators provide a net positive momentum flux with zero net mass flux. They are compact with no extruding components to negatively impact the vehicle's drag at cruising speed. Parameters controlling the pulsatile jet and its thrust are identified. The thruster was empirically tested for a large range of frequencies and stroke ratios. The thrust characteristics of the device with respect to frequency was seen to converge to a single thrust coefficient. A model was developed to predict the thrust coefficient. The effect of the stroke ratios on the thrust coefficient is investigated. The model accurately predicts the observed thrust coefficient for stroke ratios up to five where the vortex ring pinchoff occurs. The accuracy of the model degrades for stroke ratios above the formation number where part of the expelled jet is pulled back into the cavity. Additionally, these devices have thrust tracking times faster than those reported for typical propellor-type thrusters, and deliver a fully quantized level of thrust.