This paper describes a computational study of the hydrodynamics of a ray-inspired underwater vehicle conducted concurrently with experimental measurementsHigh-resolution stereo-videos of the vehicle’s fin motions during steady swimming are obtained and used as a foundation for developing a high fidelity geometrical model of the oscillatory finA Cartesian grid based immersed boundary solver is used to examine the flow fields produced due to these complex artificial pectoral fin kinematicsSimulations are carried out at a smaller Reynolds number in order to examine the hydrodynamic performance and understand the resultant wake topologyResults show that the vehicle’s fins experience large spanwise inflexion of the distal part as well as moderate chordwise pitching during the oscillatory motionMost thrust force is generated by the distal part of the fin,and it is highly correlated with the spanwise inflexionTwo sets of inter-connected vortex rings are observed in the wake right behind each finThose vortex rings induce strong backward flow jets which are mainly responsible for the fin thrust generation

Thrust producing mechanisms in ray-inspired underwater vehicle propulsion

A fluid─structure interaction solver for the study on a passively deformed fish fin with non-uniformly distributed stiffness

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

In this paper, we numerically investigate the effects of time-varying bending stiffness on the propulsion performance of a flapping foil using a fully coupled fluid-structure interaction model. The flow field is simulated using a Navier-Stokes solver while the structural dynamics is resolved by a nonlinear beam model. The force generation, the passive deformation and the flow field of the flexible foil are significantly affected by the time dependency of flexibility. Here, both the actuation at the leading edge and the stiffness of the foil vary sinusoidally and the phase ϕ between them plays an important role in determining the performance of the foil. At ϕ = 0 degree, the maximum time-averaged thrust coefficient can be increased by approximately 52% whereas the highest propulsion efficiency remains almost the same as that of the foil with a constant flexibility. This is of significance when the size of the wing is often constrained. Additionally, the foil with time-varying stiffness generates considerable lift force, which is attributed to the non-symmetrical deformations and deflected vortex-shedding patterns. Finally, the force generation due to added mass is discussed using a simplified model.

/pdf/effects-of-time-varying-flexibility-on-the-propulsion-4d6ca3thi2.pdf

Effects of time-varying flexibility on the propulsion performance of a flapping foil

Life cycle analysis and cost assessment of a battery powered ferry

In this paper, we present a numerical model capable of solving the fluid-structure interaction problems involved in the dynamics of skeleton-reinforced fish fins. In this model, the fluid dynamics is simulated by solving the Navier-Stokes equations using a finite-volume method based on an overset, multi-block structured grid system. The bony rays embedded in the fin are modeled as nonlinear Euler-Bernoulli beams. To demonstrate the capability of this model, we numerically investigate the effect of various ray stiffness distributions on the deformation and propulsion performance of a 3D caudal fin. Our numerical results show that with specific ray stiffness distributions, certain caudal fin deformation patterns observed in real fish (e.g. the cupping deformation) can be reproduced through passive structural deformations. Among the four different stiffness distributions (uniform, cupping, W-shape and heterocercal) considered here, we find that the cupping distribution requires the least power expenditure. The uniform distribution, on the other hand, performs the best in terms of thrust generation and efficiency. The uniform stiffness distribution, per se, also leads to 'cupping' deformation patterns with relatively smaller phase differences between various rays. The present model paves the way for future work on dynamics of skeleton-reinforced membranes.

/pdf/fluid-structure-interaction-modeling-on-a-3d-ray-3rari0s8px.pdf

Guangyu Shi

Papers

A fluid─structure interaction solver for the study on a passively deformed fish fin with non-uniformly distributed stiffness

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

Life cycle analysis and cost assessment of a battery powered ferry

Effects of time-varying flexibility on the propulsion performance of a flapping foil

Fluid-structure interaction modeling on a 3D ray-strengthened caudal fin.