305,200 research outputs found
Analytic modeling of a size-changing swimmer
No full textCephalopods use large-scale structural deformation to propel themselves underwater, changing their internal volume by 20-50%. In this work, the hydroelastic response of a swimmer comprised of a fluid-filled elastic-membrane is studied via an analytic formulation of two coupled non-linear dynamic equations. This model of the self-propelled soft-body dynamics incorporates the inter- play of the external and internal added-mass variations. We compare the model against recent experiments for a body which abruptly reduces its cross-section to eject a single jet of fluid mass. Using the model we study the impact of size-change excitation on sustained swimming speeds and efficiency
Underwater Soft Robotics: the benefit of body-shape variations in aquatic locomotion
No full textOrganisms travelling in water always exploit some degree of body-shape variations to propel themselves; this may occur in the form of flapping of fins, beating of tails or whole-body ondulatory motions. First systematic observation of the relationship between body kinematics and thrust production were reported for fish and jellyfish and more recently various contribution have addressed the role of body-shape changes in the unsteady propulsion within a rigorous mathematical frame.Less studied, but of greater interest for the soft robotics community, is the case of those organisms which alter their body via volumetric pulsations or iso-volumetric cross sectional modifications. This is the case of squids and octopuses which, being for the most part devoid of prominent rigid parts, can perform extensive inflation and deflation of their bodies. We will discuss how organisms which subject themselves to volume collapse during translation can benefit from burst of speeds. This is achieved by exploiting not only the expulsion of mass from their body, but also from the recovery of kinetic energy, otherwise dissipated by viscosity, via the variation of added mass. This phenomenon has important implications in the design and control of soft-bodied underwater vehicles and marine energy harvesting devices
Underwater soft-bodied pulsed-jet thrusters: actuator modeling and performance profiling
No full textA new kind of underwater vehicle is developed by taking inspiration from cephalopods. Its actuation routine is scrutinized via a suitable model. Similar to octopuses and squids, these vehicles consist of an elastic, hollow shell capable of undergoing sequential stages of ingestion and ejection of ambient fluid, which is driven by the recursive inflation and deflation of the shell. The shell actively collapses, and in this way it expels water through a funnel; then it passively returns to the inflated shape, drawing ambient fluid into the cavity. By doing so, a pulsed-jet propulsion routine is performed that enables the vehicle to propel itself in water. Due to their soft nature, the actuation of these vehicles is largely dependent on the subtle management of the elastic response of the shell throughout the propulsion routine. A kinematic model of the actuation mechanism, thoroughly corroborated by experimental validation, is devised which elucidates the relationship between the active (collapse) and passive (refill) stages of the actuation. Upon association with the dynamics of the vehicle, this model permits the derivation of the generic performance profiles of this new kind of vehicle. It is acknowledged that, for given design specifications, an optimal swimming speed exists in coincidence with the coordinated operation between the crank mechanism driving the shell contraction and the onset of elastic energy, which determines the speed of inflation of the shell. These results are invaluable in the definition of rigorous design criteria and derivation of ad-hoc control laws for a new breed of optimized soft-bodied, pulsed-jet, unmanned underwater vehicles
Forward speed control of a pulsed-jet soft-bodied underwater vehicle
No full textThis paper reports on the development of the control for a new class of soft underwater vehicles. These vehicles exploit their soft-bodied nature to produce thrust by cyclically ingesting and expelling ambient fluid. A forward speed control based on the linearised dynamics of the robot is design. The control succeeds at dealing with the discontinuous thrust by accounting for the shape-change driven actuation
Shape-changing pulsed-jet thruster for new generation AUVs
No full textThe offshore industry is in growing demand of robots capable of dealing with increasingly complex tasks in always more forbidding scenarios. However, standard underwater vehicles are power inefficient and poorly manoeuvrable, making them inadequate for this purpose. Aquatic animals have been looked upon as the perfect autonomous undersea platform and, among aquatic animals, soft-bodied ones may constitute the paradigm of inspiration to design a new breed of working class vehicles. In this talk we argue that the recourse to soft aquatic robots can disclose a solution to the problems commonly encountered in underwater operations. The scope of this talk is to introduce the current state of the art in shape-changing pulsed-jet propulsion and the benefits it can provide in the design of a new generation of AUVs.We start by showing experimental and numerical results which demonstrate how jetting maneuvers produced by volume-changes of a body in water can generate as much as 260% more thrust per pulsation than an equivalent rigid body. This is due to a combination of separation control and recovery of added mass energy. In order for this assets to be employed, specific requirements in terms of design and actuation need to be met. Hence, we present a first series of prototypes characterised by extreme structural compliance and capable of performing a pulsed-jetting routine such as that required to exploit the contribution from hydrodynamic terms. We eventually discuss the limitations of the existing prototypes and illustrate the ongoing developments of this new technology
Octopus-inspired drag cancellation by added mass pumping
No full textRecent work has shown that when an immersed body suddenly changes its size, such as a deflating octopus during rapid escape jetting, the body experiences large forces due to the variation of added-mass energy. We extend this line of research by investigating a spring-mass oscillator submerged in quiescent fluid subject to periodic changes in its volume. This system isolates the ability of the added-mass thrust to cancel the bluff body resistance (having no jet flow to confuse the analysis) and moves closer to studying how these effects would work in a sustained propulsion case by studying periodic shape-change instead of a "one-shot" escape maneuver. With a combination of analytical, numerical, and experimental results, we show that the recovery of added-mass kinetic energy can be used to completely cancel the drag of the fluid, driving the onset of sustained oscillations with amplitudes as large as four times the average body radius. Moreover, these results are fairly independent of the details of the shape-change kinematics as long as the Stokes number and shape-change number are large. In addition, the effective pumping frequency range based on parametric oscillator analysis is shown to predict large amplitude response region observed in the numerics and experiments
Squid-inspired vehicle design using coupled fluid-solid analytical modelling
No full textThe need for enhanced automation in the marine and maritime fields is fostering research into robust and highly manoeuvrable autonomous underwater vehicles. To address these needs we develop design principles for a new generation of soft-bodied aquatic vehicles similar to octopi and squids. In particular, we consider the capability of pulsed-jetting bodies to boost thrust by actively modifying their external body-shape and in this way benefit of the contribution from added-mass variation. We present an analytical formulation of the coupled fluid-structure interaction between the elastic body and the ambient fluid. The model incorporates a number of new salient contributions to the soft-body dynamics. We highlight the role of added-mass variation effects of the external fluid in enhancing thrust and assess how the shape-changing actuation is impeded by a confinement-related unsteady inertial term and by an external shape-dependent fluid stiffness contribution. We show how the analysis of these combined terms has guided us to the design of a new prototype of a squid-inspired vehicle and to the tuning of the natural frequency of the coupled fluid-solid system with the purpose of optimizing its actuation routine
A resonant squid-inspired robot unlocks biological propulsive efficiency
No full textElasticity has been linked to the remarkable propulsive efficiency of pulse-jet animals such as the squid and jellyfish, but reports that quantify the underlying dynamics or demonstrate its application in robotic systems are rare. This work identifies the pulse-jet propulsion mode used by these animals as a coupled mass-spring-mass oscillator, enabling the design of a flexible self-propelled robot. We use this system to experimentally demonstrate that resonance greatly benefits pulse-jet swimming speed and efficiency, and the robot’s optimal cost of transport is found to match that of the most efficient biological swimmers in nature, such as the jellyfish Aurelia aurita. The robot also exhibits a preferred Strouhal number for efficient swimming, thereby bridging the gap between pulse-jet propulsion and established findings in efficient fish swimming. Extensions of the current robotic framework to larger amplitude oscillations could combine resonance effects with optimal vortex formation to further increase propulsive performance and potentially outperform biological swimmers altogether
Are juvenile squid schooling formations hydrodynamically optimal?
No full textRecent studies on the swimming of juvenile oval squid show the development of well-organized schooling formations within the first 6-8 weeks after hatching. Jetting is the primary propulsion mechanism for juvenile squid, and jetting maneuvers produced by volume-change have been show to have enormous hydrodynamic benefits in terms of thrust (260% improvement) and efficiency relative to rigid bodies. This is due to a combination of separation control and recovery of added mass energy. This leads to a simple question: are the oval squid schools hydrodynamically optimal? In other words, are these unsteady hydrodynamic effects enhanced by the presence of other bodies with the same period and phase of motion? In this work, we conduct a simple set of numerical tests on idealized juvenile oval squid to determine the optimal separation distance and schooling arrangement, and compare this to the observed behavior
Drag cancellation by added-mass pumping
No full textA submerged body subject to a sudden shape-change experiences large forces due to the variation of added-mass energy. While this phenomenon has been studied for single actuation events, application to sustained propulsion requires studying periodic shape-change. We do so in this work by investigating a spring-mass oscillator submerged in quiescent fluid subject to periodic changes in its volume. We develop an analytical model to investigate the relationship between added-mass variation and viscous damping and demonstrate its range of application with fully coupled fluid-solid Navier-Stokes simulations at large Stokes number. Our results demonstrate that the recovery of added-mass kinetic energy can be used to completely cancel the viscous damping of the fluid, driving the onset of sustained oscillations with amplitudes as large as four times the average body radius . A quasi-linear relationship is found to link the terminal amplitude of the oscillations , to the extent of size change , with peaking at values from 4 to 4.75 depending on the details of the shape-change kinematics. In addition, it is found that pumping in the frequency range of 1-\frac{a}{2r_0}<\omega^2/\omega_n^2<1+\frac{a}{2r_0} is required for sustained oscillations. These results on the unsteady fluid forces produced by shape-changing bodies provide a foundation for the design and control of soft-bodied underwater vehicles
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