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Gymnarchus niloticus efficiently maneuvers at low speeds in complex environments, using its undulating dorsal fin for propulsion. This study utilized a three-dimensional numerical undulating fin model in conjunction with the differential element method to investigate the propulsion efficiency, thrust, and power dynamics (input/output) of the undulating fin across varying flow velocities. Results indicate that as inflow velocity rises, average efficiency peaks before rapidly declining, while instantaneous efficiency amplitudes increase. Thrust is notably lower at the fin’s edges than in central regions. These findings offer critical insights for optimizing undulating fin designs.

deeply analyzing the influence of ocean currents on the swimming mechanism of fish is of great significance to the study of bionic fish movement behavior. A bionic fish with a three-joint rudder driving the tail fin was chosen as the research object, the current speed and the swimming speed of the bionic fish were synthesized using the vector method, and the swinging of the fish body was controlled by the udf program of the dynamic mesh technology, and the swimming behaviors of the bionic fish were simulated using fluent software. Then, the force and vortex distribution of the bionic fish were investigated at different current sizes and different flow angles under the action of the ocean current. The simulation was carried out using fluent software to simulate the swimming behavior of the bionic fish. The calculation results provide a reference for reducing the influence of currents on the bionic fish and improving the propulsive efficiency and propulsive stability.

In the present study, a three-dimensional numerical squid model was generated from a computed tomography images of a longfin inshore squid to investigate fluid flow characteristics around the squid. The three-dimensional squid model obtained from a 3D-printer was utilized in digital particle image velocimetry (DPIV) measurements to acquire velocity contours in the region of interest. Once the three-dimensional numerical squid model was validated with DPIV results, drag force and coefficient, required jet velocity to reach desired swimming velocity for the squid and propulsion efficiencies were calculated for different nozzle diameters. Besides, velocity and pressure contour plots showed the variation of velocity over the squid body and flow separation zone near the head of the squid model, respectively. The study revealed that viscous drag was nearly two times larger than the pressure drag for the squid's Reynolds numbers of 442500, 949900 and 1510400. It was also found that the propulsion efficiency increases by 20% when the nozzle diameter of a squid was enlarged from 1 cm to 2 cm.

Fish in schooling formations navigate complex flow fields replete with mechanical energy in the vortex wakes of their companions. Their schooling behavior has been associated with evolutionary advantages including energy savings, yet the underlying physical mechanisms remain unknown. We show that fish can improve their sustained propulsive efficiency by placing themselves in appropriate locations in the wake of other swimmers and intercepting judiciously their shed vortices. This swimming strategy leads to collective energy savings and is revealed through a combination of high-fidelity flow simulations with a deep reinforcement learning (RL) algorithm. The RL algorithm relies on a policy defined by deep, recurrent neural nets, with long–short-term memory cells, that are essential for capturing the unsteadiness of the two-way interactions between the fish and the vortical flow field. Surprisingly, we find that swimming in-line with a leader is not associated with energetic benefits for the follower. Instead, “smart swimmer(s)” place themselves at off-center positions, with respect to the axis of the leader(s) and deform their body to synchronize with the momentum of the oncoming vortices, thus enhancing their swimming efficiency at no cost to the leader(s). The results confirm that fish may harvest energy deposited in vortices and support the conjecture that swimming in formation is energetically advantageous. Moreover, this study demonstrates that deep RL can produce navigation algorithms for complex unsteady and vortical flow fields, with promising implications for energy savings in autonomous robotic swarms.

Enabling technology permits the naval architect to do more with fewer resources, increasing output, decreasing cost and improving productivity, with the resulting benefits being widely distributed in a worldwide economy. For example a bulk carrier’s energy consumption per ton-mile today is less than 3% of what it was a century and half ago – due to more efficient machinery, larger hulls with lower resistance per ton and improved propulsive efficiency, yet with higher speed and shorter port times.

Hydrodynamic effects are severely affecting the performance of a Marine Propeller. Basically, manuvering of an Unmanned Aquatic Vehicle's is controlled by its propeller, which is drastically depends on the fluid. Therefore the study about fluid behaviour and its effects are mandatory to enhance the efficiency of the marine propeller. In this work deals, the hydrodynamic force estimations on the marine propeller by using both theoretical formulae and numerical analysis. The aim of this work is to obtain the fine tuned hydrodynamic forces of marine propeller for its performance enhancement. Fundamentally, complexities involved in the force estimation approaches are represent the fluid behaviour and environmental conditions. Standard formulae are used for estimations of hydrodynamic forces. CATIA is used for the generation of conceptual design on the marine propeller. ANSYS Fluent 16.2 is used for numerical simulation, in which fluid is provided the ocean water properties. Finally, the hydrodynamic forces are compared for future work.

Energetic benefit and enhanced performance are considered among the most fascinating achievements of collective behaviours, e.g. fish schools and flying formations. The collective locomotion of two self-propelled flapping plates initially in a side-by-side arrangement is investigated numerically. Both in-phase and antiphase oscillations for the two plates are considered. It is found that the plates will spontaneously form some stable configurations as a result of the flow-mediated interaction, specifically, the staggered-following (SF) mode and the alternate-leading (AL) mode for the in-phase scenario and the moving abreast (MA) mode and the AL mode for the antiphase scenario. In the SF mode, the rear plate follows the front one with a staggered configuration. In the AL mode, the plates chase each other side-by-side alternately. In terms of propulsive speed and efficiency, the performance of the plates in the SF mode with small lateral spacing $H$ is found to be better than those in the tandem following case ( $H=0$ ) and the side-by-side case (i.e. the AL mode). To achieve higher propulsive efficiency, no matter in-phase or antiphase oscillations, the two plates with moderate bending stiffness, e.g. $K\\approx O(1)$ , are preferred and they should be close enough in the lateral direction. For the side-by-side configuration, the performance of each plate in the antiphase and in-phase scenarios is enhanced and weakened in comparison with that of the isolated plate, respectively. Besides the pressure and vorticity contours, the normal force and thrust acting on the plates are also analysed. It is revealed that the thrust is mainly contributed by the normal force at moderate bending stiffness. The normal force and thrust are critical to the propulsive speed and efficiency. For two self-propelled plates, in view of hydrodynamics, to achieve higher performance the in-phase SF mode and antiphase flappings in the side-by-side configuration are preferred.

A fish may gain hydrodynamic benefits from being a member of a school. Inspired by fish schools, a two-dimensional simulation was performed for flexible fins propelled in tandem, diagonal, triangular and diamond configurations. The flow-mediated interactions between the flexible fins were analysed by using an immersed boundary method. A transverse heaving motion was prescribed on the leading edge of each fin, and other posterior parts passively adapted to the surrounding fluid as a result of the fluid–flexible-body interaction. The flexible fins were allowed to actively adjust their relative positions in the horizontal direction. The four basic stable configurations are spontaneously formed and self-sustained purely by the vortex–vortex and vortex–body interactions. The hydrodynamic benefits depend greatly on the local positions of the members. For the same heaving motion prescribed on the leading edge, the input power of the following fin in the stable tandem and diagonal configurations is lower by 14 % and 6 %, respectively, than that of the leading fin. The following fin in the diagonal formation can keep pace with the leading fin even for reduced heaving amplitudes because of the help of the leader via their shared fluid environment, where its required input power is reduced by 21 %. The heaving amplitudes of the trailing fins are reduced to optimize the propulsive efficiency, and the average efficiencies in the triangular and diamond configurations increase by up to 14 % and 19 %, respectively, over that of the isolated swimmer. The propulsive efficiencies are enhanced by 22 % for the fins in the second row and by 36 % for the fin in the third row by decreasing the heaving amplitude in the diamond formation.

We use small-amplitude inviscid theory to study the swimming performance of a flexible flapping plate with time-varying flexibility. The stiffness of the plate oscillates at twice the frequency of the kinematics in order to maintain a symmetric motion. Plates with constant and time-periodic stiffness are compared over a range of mean plate stiffnesses, oscillating stiffness amplitudes and oscillating stiffness phases for isolated heaving, isolated pitching and combined leading-edge kinematics. We find that there is a profound impact of oscillating stiffness on the thrust, with a lesser impact on propulsive efficiency. Thrust improvements of up to 35 % relative to a constant-stiffness plate are observed. For large enough frequencies and amplitudes of the stiffness oscillation, instabilities emerge. The unstable regions may confer enhanced propulsive performance; this hypothesis must be verified via experiments or nonlinear simulations.

Swimming animals need to generate propulsive force to overcome drag, regardless of whether they swim steadily or accelerate forward. While locomotion strategies for steady swimming are well characterized, far less is known about acceleration. Animals exhibit many different ways to swim steadily, but we show here that this behavioral diversity collapses into a single swimming pattern during acceleration regardless of the body size, morphology, and ecology of the animal. We draw on the fields of biomechanics, fluid dynamics, and robotics to demonstrate that there is a fundamental difference between steady swimming and forward acceleration. We provide empirical evidence that the tail of accelerating fishes can increase propulsive efficiency by enhancing thrust through the alteration of vortex ring geometry. Our study provides insight into how propulsion can be altered without increasing vortex ring size and represents a fundamental departure from our current understanding of the hydrodynamic mechanisms of acceleration. Our findings reveal a unifying hydrodynamic principle that is likely conserved in all aquatic, undulatory vertebrates.