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.

The evolution of starting jet circulation, impulse and kinetic energy are derived in terms of kinematics at the entrance boundary of a semi-infinite axisymmetric domain. This analysis is not limited to the case of parallel jet flows; and the effect of non-zero radial velocity is specifically identified. The pressure distribution along the entrance boundary is also derived as it is required for kinetic energy modelling. This is done without reliance on an approximated potential function (i.e. translating flat plate), making it a powerful analytical tool for any axisymmetric jet flow. The pressure model indicates that a non-zero radial velocity is required for any ‘over-pressure’ at the nozzle exit. Jet flows are created from multiple nozzle configurations to validate this model. The jet is illuminated in cross-section, and velocity and vorticity fields are determined using digital particle image velocimetry (DPIV) techniques and circulation, impulse and kinetic energy of the jet are calculated from the DPIV data. A non-zero radial velocity at the entrance boundary has a drastic effect on the final jet. Experimental data showed that a specific configuration resulting in a jet with a converging radial velocity, with a magnitude close to 40 % of the axial velocity at its maximum, attains a final circulation which is 90–100 % larger than a parallel starting jet with identical volume flux and nozzle diameter, depending on the stroke ratio. The converging jet also attains a final impulse which is 70–75 % larger than the equivalent parallel jet and a final kinetic energy 105–135 % larger.

A new type of underwater thruster was designed to provide high-accuracy, low-speed maneuvering to underwater robots. Located internal to the vehicle surface, these thrusters have a minimal effect on the forward-drag profile of the vehicle. These thrusters, whose inspiration comes from the natural propulsion of cephalopods and jellyfish, generate control forces by successive ingestion and expulsion of jets of water from a cavity mounted in the hull of the vehicle. The jetting process has no net mass flux but results in a positive momentum flux. A time-dependent thrust model was developed, which predicted the thruster dynamics as a function of time, actuation frequency, and thruster-driving parameters. A linear transfer-function model was developed to approximate both the thruster and vehicle dynamics, which led to maneuver categorization into three regimes: Cruising, Docking, and Transition. The predicted frequency response was verified through hybrid simulation to be accurate for predicting general trends and cutoff frequency.

Inspired by the natural locomotion of jellyfish and squid, a series of compact thrusters series is developed for propulsion and maneuvering of underwater vehicles. These thrusters successively ingest and expel jets of water in a controlled manner at high frequencies to generate propulsive forces. The parameters controlling the performance of the thrusters are reviewed and investigated to achieve higher thrust levels. The thrusters are compact and can be placed completely inside a vehicle hull providing the desired maneuvering capability without sacrificing a sleek hydrodynamic shape for efficient cruising. The system design of a prototype hybrid vehicle, called CephaloBot, utilizing these thrusters, is also presented. A compact and custom-developed embedded system is also designed for the CephaloBot. Key features of the system include a base set of navigational sensors, an acoustic system for localization and underwater communication, Xbee RF transceiver for communication above water, and a LabVIEW programmed processing board.

This study lays out a methodology whereby conditions necessary for a vortex ring to separate from the shear flow can be identified by a relationship between characteristic velocities of the jet and the vortex ring along the axis of symmetry. This criterion identifies pinch-off to take place when the velocity induced at the origin of the forming vortex ring surpasses the maximum feeding velocity along the centerline, defined to be twice the piston velocity. A strategy for determining these characteristic velocities purely from the jet driving programs (i.e., without direct knowledge of the state of the leading vortex ring) is presented. A variety of jet driving conditions, including different nozzle geometries (converging radial velocity) and different jet velocity programs, are examined to validate the relationship between pinch-off and the characteristic velocities. These parameters are examined and adjusted independently of each other so that the effect of each jetting parameter can be observed independently. Nozzles which induce a converging radial velocity decrease the jet formation number to approximately two (for nearly constant velocity programs), due to the large increase in vorticity flux without increasing volume flux. Accelerating the jet velocity to compensate for the growing vortex ring substantially increases the formation number of both parallel and converging jet flows. The new centerline velocity criterion coincided very closely with vortex ring pinch-off for all cases tested, validating this criterion as a predictive tool.