Fluid-structure interaction simulations on skeleton-reinforced biomimetic fin propulsion

  • Guangyu Shi

Student thesis: Doctoral Thesis


This study is inspired by the key features possessed by the fins of ray-finned fish (e.g.,soft membrane supported by bony rays, anisotropic flexibility, individual actuation of fin rays, active curvature and stiffness control). A better understanding of the effects of these characteristics will provide inspirations and guidelines for the design ofbio-inspired underwater locomotion systems, which are playing an increasingly important role as the growing activities in ocean engineering.Due to the complicated structures of fish fins, it is of great challenge to numerically model such bio-membrane systems, which involve the fluid-structure interaction (FSI)between the flexible fin and the surrounding flow, and the modelling and active control of individual fin rays. It is therefore preferable to develop a compact and handy FSI solver, which allows to be tailored for specific problems, rather than using commercial software, which have no direct model capable of handling skeleton-strengthened bio-membrane systems and provides little freedom to betailored. To elucidate the effects of the aforementioned main characteristics on the performance of biomimetic fin propulsion, a fully coupled FSI solver capable of simulating the dynamics of skeleton-reinforced bio-membranes is established in thepresent work. Specifically, a flow model, which solves the 3D unsteady compressible Navier-Stokes equations on an overset, multi-block, structured grid system with a finite-volume method, is coupled with a structural model, which solves a nonlinear Euler-Bernoulli beam equation with a finite-difference method, within a partitioned framework. The developed FSI solver is thoroughly validated against benchmark cases available in literature and good agreements are obtained.Firstly, the established FSI model is applied to investigate the effects of different spanwise deformations on the propulsion performance of a simplified 3D ray-supported caudal fin. The rays are modelled as nonlinear beams. Kinematically,the leading edge of the fin undergoes a sinusoidal sway motion while the rest part deforms passively. 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 interms 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.;Subsequently, the effect of active curvature control on the performance of aray-strengthened caudal fin is examined. Kinematically, the fin is activated by a uniform sway motion at the basal ends of the rays, and distributed time-varying forces along each ray individually, which imitate effects of tendons that actively change the curvatures of the rays. The dynamics of the fin is closely associated with the exact distribution of phase lags (between the sway motion and external forces) among the rays. We find that the fin's performance can be significantly enhanced by active control when the mean phase lag is less than 90 degree. Among different deformation patterns,the cupping deformation (C-mode) produces the best propulsion performance. The underlying physical mechanism is found to be areas with increased pressure attributed to three-dimensional fin deformations. W-shape deformations (W-mode) have a similar(yet less pronounced) effect. In addition to symmetric fin deformations, asymmetric deformations such as heterocercal mode (H-mode) and undulation mode (S-mode) are reproduced in the present work. Both of which are able to generate vertical forces.Compared with the H-mode, the S-mode creates less thrust force but it significantly reduces the transverse force, making it more suitable in cases when there is no other mechanism to balance the transverse force (e.g. during the braking maneuver).Finally, the propulsion performance of a skate-inspired underwater robot with a pair of ray-supported undulating pectoral fins is numerically investigated with the fully coupled FSI solver. Each pectoral fin is activated independently via individually distributed time-varying forces along each fin ray, which imitate effects of tendons that can actively curve the fin rays. We find that the propulsion performance of the bio-inspired robot is closely associated with the phase difference between the leading edge ray and the trailing edge ray of the pectoral fin. The results show that with asymmetrical kinematics, the highest thrust is created when the phase difference is 90 degree while the point maximizing the propulsion efficiency varies with the motion frequency. It is also found that there is a minimum frequency of generating net thrust for a specific parameter setup, which rises as the increase of phase difference.Compared with the symmetrical kinematics, the non-symmetrical kinematics generates more complicated hydrodynamic forces and moments which may be beneficial for maneuvering.
Date of Award23 Apr 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorQing Xiao (Supervisor) & Sandy Day (Supervisor)

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