The leading-edge vortex (LEV) formation on the caudal fin (CF) has been identified as playing a key role in efficient lift-based thrust production of fish-like propulsion. The enhancement of the CF LEV through its interaction with vortices formed upstream due to a median fin with a distinct shape is the focus of this paper. High-speed, high-fidelity videos and particle imaging velocimetry (PIV) were obtained from rainbow trout during steady forward swimming to visualize the undulatory kinematics and two-dimensional flow behavior. Body kinematics are quantified using a traveling-wave formulation that is used to prescribe the motion of a high-fidelity three-dimensional surface model of the fish body for a computational fluid dynamics (CFD) study. The pressure field of the CFD result is compared and validated with the PIV result from the experiment. Using CFD, the vortex forming and shedding behaviors of the anal fin (AF) and their capturing and interaction with the trunk (TK) and the CF are visualized and examined. Coherent AF-bound LEVs are found to form periodically, leading to thrust production of the AF. The vortices subsequently shed from the AF are found to help stabilize and reinforce the LEV formation on the CF by aiding LEV initiation at stroke reversal and enhancing LEV during a tail stroke, which leads to enhancement of lift-based thrust production. The CF is found to shed vortex tubes (VTs) that create backward-facing jets, and the ventral-side VT and the associated backward jets are both strengthened by vortices shed by the AF. An additional benefit of the AF is found to be reduction of body drag by reducing the lateral crossflow that leads to loss of beneficial pressure gradient across the body. Through varying AF-CF spacing and AF height, we find that CF thrust enhancement and TK drag reduction due to the AF are both affected by the position and size of the AF. The position and area of the AF that led to the most hydrodynamic benefit are found to be the original, anatomically accurate position and size. In this paper, we demonstrate the important effect of vortex interaction among propulsive surfaces in fish-like propulsion.

Given growing interest in emulating dolphin morphology and kinematics to design high-performance underwater vehicles, the current research effort is dedicated to studying the hydrodynamics of dolphin-like oscillatory kinematics in forward propulsion. A computational fluid dynamics method is used. A realistic three-dimentional surface model of a dolphin is made with swimming kinematics reconstructed from video recording. The oscillation of the dolphin is found to enhance the attachment of the boundary layer to the posterior body, which then leads to body drag reduction. The flapping motion of the flukes is found to generate high thrust forces in both the downstroke and the upstroke, during which vortex rings are shed to produce strong thrust jets. The downstroke jets are found to be on average stronger than the upstroke jet, which then leads to net positive lift production. The flexion of the peduncle and flukes is found to be a crucial feature of dolphin-like swimming kinematics. Dolphin-inspired swimming kinematics were created by varying the flexion angle of the peduncle and flukes, which then resulted in significant performance variation. The thrust benefits and propulsive efficiency benefits are associated with a slight decrease and slight increase of the flexion of the peduncle and flukes, respectively.

This study investigates the interaction of a two-manta-ray school using computational fluid dynamics simulations. The baseline case consists of two in-phase undulating three-dimensional manta models arranged in a stacked configuration. Various vertical stacked and streamwise staggered configurations are studied by altering the locations of the top manta in the upstream and downstream directions. Additionally, phase differences between the two mantas are considered. Simulations are conducted using an in-house developed incompressible flow solver with an immersed boundary method. The results reveal that the follower will significantly benefit from the upstroke vortices (UVs) and downstroke vortices depending on its streamwise separation. We find that placing the top manta 0.5 body length (BL) downstream of the bottom manta optimizes its utilization of UVs from the bottom manta, facilitating the formation of leading-edge vortices (LEVs) on the top manta’s pectoral fins during the downstroke. This LEV strengthening mechanism, in turn, generates a forward suction force on the follower that results in a 72% higher cycle-averaged thrust than a solitary swimmer. This benefit harvested from UVs can be further improved by adjusting the phase of the top follower. By applying a phase difference of π / 3 to the top manta, the follower not only benefits from the UVs of the bottom manta but also leverages the auxiliary vortices during the upstroke, leading to stronger tip vortices and a more pronounced forward suction force. The newfound interaction observed in schooling studies offers significant insights that can aid in the development of robot formations inspired by manta rays.

As one of few animals with the capability to execute agile yawing maneuvers, it is quite desirable to take inspiration from hummingbird flight aerodynamics. To understand the wing and body kinematics and associated aerodynamics of a hummingbird performing a free yawing maneuver, a crucial step in mimicking the biological motion in robotic systems, we paired accurate digital reconstruction techniques with high-fidelity computational fluid dynamics (CFD) simulations. Results of the body and wing kinematics reveal that to achieve the pure yaw maneuver, the hummingbird utilizes very little body pitching, rolling, vertical, or horizontal motion. Wing angle of incidence, stroke, and twist angles are found to be higher for the inner wing (IW) than the outer wing (OW). Unsteady aerodynamic calculations reveal that drag-based asymmetric force generation during the downstroke (DS) and upstroke (US) serves to control the speed of the turn, a characteristic that allows for great maneuvering precision. A dual-loop vortex formation during each half-stroke is found to contribute to asymmetric drag production. Wake analysis revealed that asymmetric wing kinematics led to leading-edge vortex strength differences of around 59% between the IW and OW. Finally, analysis of the role of wing flexibility revealed that flexibility is essential for generating the large torque necessary for completing the turn as well as producing sufficient lift for weight support.

Biological studies have shown that schooling behavior benefits fish's swimming performance. Particularly, side-by-side, or phalanx, formation, combined with the synchronization of undulation of the neighbors, was shown to allow the group to sustain fast incoming flow. In this work, the hydrodynamic interactions between neighbors in a phalanx school are studied through three-dimensional (3D) flow simulation. Trout-like computational models, composed of trunk (TK), caudal fin (CF), and median dorsal and anal fins (DF and AF), are used with undulatory kinematics prescribed using a traveling-wave equation. An immersed-boundary method direct numerical simulation code is used. Three trout-like models are arranged in a dense phalanx formation so that the middle fish would interact strongly with both its neighbor. The undulation of all the fish are synchronized, with the middle fish undulating either in phase or completely out of phase with its two neighbors, to examine the hydrodynamic interaction in a phalanx school in both in-phase and anti-phase modes. Results show significantly enhanced thrust though at the cost of slightly increased power consumption through anti-phase interaction, which enhanced the 3D pressure fields and momentum of the flow around neighboring CF. The wake of each fish also interacted strongly, producing expanding vortex rings and enhanced wake jets. The addition of median fins to the school additional enhanced the caudal-fin thrust through fin–fin interaction.

The energetic consequences of swimming within a neighboring fish’s vortex street remain a central question in collective locomotion. Recent flume experiments in which a flapping hydrofoil generated a biomimetic wake demonstrated that a trout can station-keep behind the foil while displaying kinematics markedly different from those used in uniform flow. To examine the underlying hydrodynamics, we accurately replicate the fish-foil system by first reproducing the experimentally recorded motions using a joint-based kinematic reconstruction method, and then we simulate the fluid dynamics with three-dimensional computational fluid dynamics. A companion simulation without the foil is also conducted to isolate wake effects. Relative to uniform-flow swimming, the presence of the foil wake reduces the trout’s cycle-averaged hydrodynamic power expenditure by 11.4 ± 0.0003%, a benefit that arises because vortex columns shed by the foil create coherent negative-pressure corridors along the fish’s lateral surface. Power reduction is realized when the trout’s long-wavelength body wave remains phase-locked with the downstream advection of these vortex structures, enabling the fish to harvest pressure-induced thrust while minimizing added-mass losses. These findings provide a mechanistic explanation for wake exploitation in schooling fish, establish phase synchrony as a key control parameter for hydrodynamic benefit, and offer design guidelines for paired biomimetic underwater vehicles that seek to emulate schooling to improve propulsive efficiency.