Describes the development and verification of a six degree of freedom, non-linear simulation model for the REMUS AUV, the first such model for this platform. In this model, the external forces and moments resulting from hydrostatics, hydrodynamic lift and drag, added mass, and the control inputs of the vehicle propeller and fins are all defined in terms of vehicle coefficients. The paper briefly describes the derivation of these coefficients. The equations determining the coefficients, as well as those describing the vehicle rigid-body dynamics, are left in non-linear form to better simulate the inherently non-linear behavior of the vehicle. Simulation of the vehicle motion is achieved through numeric integration of the equations of motion. The simulator output is then verified against vehicle dynamics data collected in experiments performed at sea. The simulator is shown to accurately model the motion of the vehicle. The paper concludes with recommendations for future model validation experiments.

Development of a Six-Degree of Freedom Simulation Model for the REMUS autonomous Underwater Vehicle

Biological fish can create high forward swimming speed due to change of thrust/drag area of pectoral fins between power stroke and recovery stroke in rowing mode. In this paper, we proposed a novel type of folding pectoral fins for the fish robot, which provides a simple approach in generating effective thrust only through one degree of freedom of fin actuator. Its structure consists of two elemental fin panels for each pectoral fin that connects to a hinge base through the flexible joints. The Morison force model is adopted to discover the relationship of the dynamic interaction between fin panels and surrounding fluid. An experimental platform for the robot motion using the pectoral fin with different flexible joints was built to validate the proposed design. The results express that the performance of swimming velocity and turning radius of the robot are enhanced effectively. The forward swimming velocity can reach 0.231 m/s (0.58 BL/s) at the frequency near 0.75 Hz. By comparison, we found an accord between the proposed dynamic model and the experimental behavior of the robot. The attained results can be used to design controllers and optimize performances of the robot propelled by the folding pectoral fins.

Dynamic Analysis of a Robotic Fish Propelled by Flexible Folding Pectoral Fins

The work in this paper focuses on the examination of the effect of variable stiffness distributions on the kinematics and propulsion performance of a tuna-like swimmer. This is performed with the use of a recently developed fully coupled fluid-structure interaction solver. The two different scenarios considered in the present study are the stiffness varied along the fish body and the caudal fin, respectively. Our results show that it is feasible to replicate the similar kinematics and propulsive capability to that of the real fish via purely passive structural deformations. In addition, propulsion performance improvement is mainly dependent on the better orientation of the force near the posterior part of swimmers towards the thrust direction. Specifically, when a variable body stiffness scenario is considered, the bionic body stiffness profile results in better performance in most cases studied herein compared with a uniform stiffness commonly investigated in previous studies. Given the second scenario, where the stiffness is varied only in the spanwise direction of the tail, similar tail kinematics to that of the live scombrid fish only occurs in association with the heterocercal flexural rigidity profile. The resulting asymmetric tail conformation also yields performance improvement at intermediate stiffness in comparison to the cupping and uniform stiffness.

https://iopscience.iop.org/article/10.1088/1748-3190/abb3b6/pdf

The effect of variable stiffness of tuna-like fish body and fin on swimming performance

In this paper, we numerically investigate the propulsive performance of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes. We employ our recently developed body-conforming fluid-structure interaction solver for our high-fidelity numerical study. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45 ° (concave plate), 90 ° (rectangular plate) to 135 ° (convex plate) while maintaining the constant area of the flexible plate. For a wide range of flexibility, three distinctive flapping motion regimes are classified based on the variation of the flapping dynamics: (i) low bending stiffness K B low, (ii) moderate bending stiffness K B moderate near resonance, and (iii) high bending stiffness K B high. We examine the impact of the frequency ratio f * defined as the ratio of the natural frequency of the flexible plate to the actuated pitching frequency. Through our numerical simulations, we find that the global maximum mean thrust occurs near f * ≈ 1 corresponding to the resonance condition. However, the optimal propulsive efficiency is achieved around f * = 1.54 instead of the resonance condition. While the convex plate with low and high bending stiffness values shows the best performance, the rectangular plate with moderate K B moderate is the most efficient propulsion configuration. To examine the flow features and the correlated structural motions, we employ the sparsity-promoting dynamic mode decomposition. We find that the passive deformation induced by the flexibility effect can help in redistributing the pressure gradient, thus, improving the efficiency and the thrust production. A momentum-based thrust evaluation approach is adopted to link the temporal and spatial evolution of the vortical structures with the time-dependent thrust. When the vortices detach from the trailing edge, the instantaneous thrust shows the largest values due to the strong momentum change and convection process. Moderate flexibility and convex shape help to transfer momentum to the fluid, thereby improving the thrust generation and promoting the transition from drag to thrust. The increase in the trailing edge angle can broaden the range of flexibility that produces positive mean thrust. The role of added mass effect on the thrust generation is quantified for different pitching plates and the bending stiffness. These findings are of great significance to the optimal design of propulsion systems with flexible wings.

A high-fidelity numerical study on the propulsive performance of pitching flexible plates

Numerical study on the hydrodynamics of C-turn maneuvering of a tuna-like fish body under self-propulsion

Analysis of biomimetic caudal fin shapes for optimal propulsive efficiency

Jacket-type steel members are widely used in near and offshore structures wherein tubular members are welded together to either form or protect the load-carrying member. Tubular joints are subject to damage as a result of fatigue, marine growth and corrosion from the environment. These structures are conventionally inspected for loss of wall thickness and pitting to prevent catastrophic damage and improve failure prediction systems using the conventional ultrasonic testing (UT). However, especially in the case of marine structures, direct access to the structure is hindered by marine growth, insulation or coating. Surface preparation is an essential step before conventional nondestructive testing modalities can be used. Marine growth is removed using powered brushes, high-pressure water jets or in some cases, manually using chisels causing the procedure to be time consuming and expensive. An alternative technology which can be used for wall thickness estimation without removing marine growth (that is thicker than 10 mm) is pulsed eddy current (PEC) which uses a stepped input signal to detect wall-thinning areas. In this paper, the authors present a methodology of rapidly estimating thickness of the steel members in the splash-zone and deeper underwater zones using PEC without removing marine growth or insulation on a remotely operated robotic vehicle (ROV). The results are compared to the conventional ultrasonic testing methodology performed both by professional divers and an ROV using a commercially available 2.25 MHz ultrasonic transducer. Key advantages and limitations of the ROV-based PEC system are discussed in detail.

Thickness Estimation of Marine Structures Using an ROV-Based Pulsed Eddy Current Technique

Set in the context of the development of bioinspired robotics systems, this paper seeks to understand the influence of the choice of the flapping orientation of fins on the propulsive performance of small underwater vehicles. In particular, the thunniform mode of Body and/or Caudal Fin (BCF) propelled systems is studied. This research is motivated by the fact that not much literature is available on the influence of flapping orientation of marine organisms and a number of mechanisms are found in nature. Dorso-ventral flapping with a positive metacentric height is shown to yield better self-stabilizing effects and lesser energy consumption compared to sideways flapping. Moreover, with dorso-ventral flapping, the choice of metacentric height could lead to the possibility of adjusting the body's rotational oscillation amplitudes to positively affect the downstream fluid interactions for the caudal fin. This is not possible with sideways flapping where the designer would be forced to change the flapping kinematics or the body shape in the sagittal plane, to adjust the body oscillation amplitudes. While the main body of results are obtained using simulations for underwater vehicle dynamics with coefficients of the REMUS underwater vehicle, stability analysis for a generalised case is also presented.

Numerical Study on Swimming Performance Based on Flapping Orientations of Caudal Fins for Bio-robotic Systems

Aquatic animals and mammals in nature, in particular, the Body and/or Caudal Fin (BCF) swimmers swim either by flapping their fins in the sideways direction or the dorso-ventral direction. Not much literature is available on the effects of the performance of these robots based on the choice of its flapping orientation. In this research, it is found that dorso-ventral flapping could lead to better self-stabilizing effects and lesser energy consumption compared to sideways flapping. It is also found that the choice of dorso-ventral flapping offers the possibility of controlling the body’s oscillation amplitude while flapping. This is an appealing advantage for underwater surveying robots carrying cameras and sensors as controlled body oscillations could yield better results from its payloads. The main body of results is obtained with simulations for underwater vehicle dynamics with the coefficients of the REMUS underwater vehicle, while stability analysis for a generalised case is also presented.

Effect of flapping orientation on caudal fin propelled bio-inspired underwater robots

Maneuverability and propulsive efficiency are of much interest in autonomous underwater robots. In this paper, we present a novel underwater robot design with two reconfigurable and detachable swimming modules that would be capable of offering both maneuverability and propulsive efficiency. They are also capable of reconfiguring automatically to take two different orientations favoring reduced drag in the swimming direction. A key feature of this design is that the reconfigurability is achieved without additional actuators - helpful in the development of autonomous swarm robots with good maneuverability and efficiency.

Santhosh Ravichandran

Papers

Analysis of biomimetic caudal fin shapes for optimal propulsive efficiency

Thickness Estimation of Marine Structures Using an ROV-Based Pulsed Eddy Current Technique

Numerical Study on Swimming Performance Based on Flapping Orientations of Caudal Fins for Bio-robotic Systems

Effect of flapping orientation on caudal fin propelled bio-inspired underwater robots

Bio-inspired Underwater Robot with Reconfigurable and Detachable Swimming Modules