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

Underwater biomimetic propulsion systems that mimic the BCF (Body and/or Caudal Fin) fishes have attracted considerable attention. A miniature caudal fin-like propulsion device, whose design is based on the morphology and kinematics of the koi fish, is proposed in this paper. The propulsion is actuated by flexible fiber-based piezoelectric actuators, Macro Fiber Composite (MFC). A vacuum fabrication procedure is employed to bond the components of the proposed device. The underwater oscillation performances of the proposed propulsion at different steady-state sinusoidal excitations are experimentally investigated. Computational fluid dynamic (CFD) analysis is conducted to investigate the three-dimensional flow generated by the oscillation propulsion system. A simplified model, where the internal bending moment provided by the MFC actuators is replaced by a shear force, is created based on the obtained experimental data. The CFD simulations demonstrate that the maximum instantaneous and mean thrusts generated by the propulsion system during the stable period are 21.5 and 9.5 mN, respectively. These are consistent with the Lighthill’s elongated-body theory. Then, the uneven spatial distributions of the instantaneous vorticity around the propulsion are visualized, using seven two-dimensional planes intersecting at various locations. The CFD results suggest that the vorticity distributions around the trailing and leading edges of the caudal fin considerably differ because of the fin’s unique shape. A reverse Karman vortex street pattern is observed behind the trailing edge of the caudal fin. This indicates a thrust-producing pattern. On the other hand, a Karman vortex street pattern is distinctly observed before the leading edge. This indicates a drag-producing pattern. The geometry and shape effects of the caudal fin-like propulsion device are identified consequently. Those obtain results may be beneficial for the design of underwater vehicles with oscillation fin-like propulsion system.

Underwater oscillation performance and 3D vortex distribution generated by miniature caudal fin-like propulsion with macro fiber composite actuation

Analysis of biomimetic caudal fin shapes for optimal propulsive efficiency

Efficient resolution of natural light and sound intensity is essential for organisms, systems and machines that rely on visual and auditory sensory perception to survive or function effectively in their environment. This resolution obeys Weber’s Law when the smallest resolvable change, a just-noticeable-difference, grows in direct proportion to the stimulus. Here, Weber’s Law is found to be a consequence of attaining the theoretical minimum mean-square error possible, the Cramer–Rao lower bound, in resolving the intensity of naturally scintillating light and sound. The finding is based on statistics from thousands of measurements of naturally scintillating environmental light and sound signals. Remarkably, just-noticeable-differences in light and sound intensity measured over decades of psychophysical experiments with artificial sources are also found to approximately attain the respective Cramer–Rao lower bounds. Human intensity resolution is in this way optimally adapted to the natural scintillation of light and sound. Pattern recognition by simple matched-filter correlation between measured and hypothetical images cancels natural scintillation. For intensity perception obeying Weber’s Law, this is found to be advantageous and statistically optimal because perceived scintillation is independent of the underlying signal pattern. A small visual patch change or acoustic signature truncation is shown to be lost in natural signal-dependent fluctuations if perception with constant intensity resolution is attempted.

Arun Krishnadas

Papers

Analysis of biomimetic caudal fin shapes for optimal propulsive efficiency

Weber’s Law of perception is a consequence of resolving the intensity of natural scintillating light and sound with the least possible error