The mechanism of bubble capture in a vortical flow is investigated using a Lagrangian bubble tracking method. The motion of bubbles and the factors influencing their movement are examined. Detailed analysis is conducted on the roles played by each force component, such as the lift, added mass, and centrifugal forces, in the bubble capture process. An interesting finding is the identification of the stabilizing effect of the azimuthal lift force on the bubble capture mechanism. Furthermore, a model for capture time based on the radial force balance is also developed, and validated with existing experimental data. These findings, including the force mechanism and capture time model, provide a foundation for understanding the bubble capture process and can potentially inform future studies on tip vortex cavitation inception such as determining the cavitation hotspot.

Inflow turbulence is relevant for many engineering applications relating to noise generation, including aircraft wings, landing gears, and non-cavitating marine propellers. While modelling of this phenomenon is well-established for higher Mach number aerospace problems, lower Mach number applications, which include marine propellers, still lack validated numerical tools. For this purpose, simplified cases for which extensive measurement data are available can be used. This paper investigates the effect of inflow turbulence on a circular cylinder at a Reynolds number of 14,700, a Mach number of 0.029, and with inflow turbulence intensities ranging between 0% and 22%. In the present work focus is put on the hydrodynamics aspect, with the aim of addressing radiated noise in a later study. The flow is simulated using the partially averaged Navier Stokes equations, with turbulence inserted using a synthetic inflow turbulence generator. Results show that the proposed method can successfully replicate nearfield pressure variations and relevant flow features in the wake of the body. In agreement with the literature, increasing inflow turbulence intensity adds broadband frequency content to all the presented fluctuating flow quantities. In addition, the applied variations in inflow turbulence intensity result in a major shift in flow dynamics around a turbulence intensity of 15%, when the dominant effect of von Kármán vortices on the dominant flow dynamics becomes superseded by freestream turbulence.

Capability of a pulsed-jetting, aquatic soft robot to perform turning manoeuvres by means of a steerable nozzle is investigated experimentally for the first time. Actuation of this robot is based on the periodic conversion of slowly-charged elastic potential energy into fluid kinetic energy, giving rise to a cyclic pulsed-jet resembling the one observed in cephalopods. A steerable nozzle enables the fluid jet to be deflected away from the vehicle axis, thus providing the robot with the unique ability to manoeuvre using thrust-vectoring. This actuation scheme is shown to offer a high degree of control authority when starting from rest, yielding turning radii of the order of half of the body length of the vehicle. The most significant factor affecting efficiency of the turn has been identified to be the fluid momentum losses in the deflected nozzle. This leads, given the current nozzle design, to a distinct optimum nozzle angle of 35°.

The design, calibration and testing of an experimental rig for measuring 2-DOFs unsteady loads over aquatic robots is discussed. The presented apparatus is specifically devised for thrust characterization of a squid-inspired soft unmanned underwater vehicle, but its modular design lends itself to more general bioinspired propulsion systems and the inclusion of additional degrees of freedom. A purposely designed protocol is introduced for combining calibration and error compensation upon which force and moment measurements can be performed with a mean error of 0.8% in steady linear loading and 1.7% in unsteady linear loading, and mean errors of 10.2% and 9.4% respectively for the case of steady and dynamic moments at a sampling rate of the order of 10 Hz. The ease of operation, the very limited cost of manufacturing and the degree of accuracy make this an invaluable tool for fast prototyping and low-budget projects broadly applicable in the soft robotics community.

A soft hydraulic actuator is presented which uses elastic energy storage for the purpose of pulsed-jet propulsion of soft unmanned underwater vehicles. The actuator consists of a flexible membrane that can be inflated using a micropump and whose elastic potential energy may be released on demand using a controllable valve, in a manner inspired by the swimming of squids and octopuses. It is shown that for equivalent initial elastic energy, the drop in peak thrust is linearly proportional to the decrease in nozzle cross section. Peak hydraulic power amplification of the soft actuator of approximately 75% is achieved with respect to that of the driving pump, confirming that passive elasticity can be exploited in aquatic propulsion to replicate the explosive motion skills of agile sea-dwelling creatures.