The effects of chordwise deformation and the half-amplitude asymmetry on the hydrodynamic performance and vortex dynamics of batoid fish have been numerically investigated, in which the two parameters were represented by the wavenumber ( $W$ ) and the ratio of the half-amplitude above the longitudinal axis to that below ( $HAR$ ). Fin kinematics were prescribed based on biological data. Simulations were conducted using the immersed boundary method. It was found that moderate chordwise deformation enhances the thrust, saves the power and increases the efficiency. A large $HAR$ can also increase thrust performance. By using the derivative-moment transformation theory at several subdomains to capture the local vortical structures and a force decomposition, it was shown that, at high Strouhal numbers ( $St$ ), the tip vortex is the main source of thrust, whereas the leading-edge vortex (LEV) and trailing-edge vortex weaken the thrust generation. However, at lower $St$ , the LEV would enhance the thrust. The least deformation ( $W=0$ ) leads to the largest effective angle of attack, and thus the strongest vortices. However, moderate deformation ( $W=0.4$ ) has an optimal balance between the performance enhancement and the opposite effect of different local structures. The performance enhancement of $HAR$ was also due to the increase of the vortical contributions. This work provides a new insight into the role of vortices and the force enhancement mechanism in aquatic swimming.

The wake of a pump-jet propulsor (PJP) with a pre-swirl stator is investigated using stress-blended eddy simulations. The flow field is analyzed in detail through a systematic comparison of the wake morphology under different loading conditions, allowing the destabilization process and the mechanism of wake instabilities to be inspected. To further examine the evolution of the vortices, as well as their interaction and destabilization, the pressure fluctuations and spectra of turbulent kinetic energy are considered. The mean loads are in good agreement with experimental results. The PJP flow field has a complex vortical system, the evolution of which determines the wake instabilities. The tip clearance leakage vortex first exhibits short-wave instabilities, and the destabilization process then accelerates under the effects of duct shed vortices, which promote the generation of secondary vortices. The secondary vortices further enhance the destabilization process and lead to chaotic evolution. The stator blade root vortices are strongly affected by the rotor blade root vortices, causing an exchange of vorticity that depends on the relative intensity of the two sets of root vortices. The instability of the hub vortices is apparently related to the upstream vortices. The correlation between the tip clearance leakage vortices and the instability of hub vortices is very weak.

A numerical analysis based on detached eddy simulations is conducted to investigate vortex instabilities in the wake of a preswirl pumpjet propulsor. Three models are established to separate the roles that the rotor, stator, and duct play in the vortex structure of the pumpjet propulsor. In this paper, only the vortex structure of the rotor is considered. The results show that the vortex system of the rotor is mainly composed of the tip vortices, a hub vortex, the trailing tip vortices, and the trailing root vortices. The trailing tip vortices are generated by the premature shedding of the tip vortices in the rotor model compared with a normal single propeller. The existence of trailing root vortices increases the stability of the hub vortex. Furthermore, a unique multi-inductance instability mode of the tip vortex, called the “overlap–forward” phenomenon, is found for low values of the advance coefficient J. It is found that the instability of the tip vortex depends not only on the spiral-to-spiral distance but also on the highest-efficiency point of the propeller. The instability inception point of the tip vortex moves farther downstream with increasing J, whereas when J is greater than the highest-efficiency point of the propeller, the stable length of the tip vortices drops sharply. The energy transfer process from blade harmonics to shaft harmonics of the tip vortices depends on J and is related to the spatial evolution of the tip vortices.

Bionic underwater robots are the intersection of biology and robotics; they have the advantages of propulsion efficiency and maneuverability. A novel vehicle that combines a gliding and flapping propulsion inspired by a manta ray is presented in this article. The outstanding character of the robot is that its integrated maneuverable flapping propulsion relies on two bionic flexible pectoral fins and long-range efficient gliding propulsion, which is based on a buoyancy-adjustment system and a mass-adjustment system. We designed the biomimetic manta ray robot and analyzed the principle of the gliding and flapping system in this paper. The gliding propulsion capability and the flapping propulsion performance are verified through gliding and swimming experiments. In conclusion, the designed bionic manta robot provides a platform with practical application capabilities in marine environment detection, concealed reconnaissance, and aquaculture.

A numerical study based on detached eddy simulations is conducted to investigate the effects of the odd and even number of rotor/stator blades, that is, n_r/n_s, on the hydrodynamic performance of a pre-swirl pumpjet propulsor (PJP). In this paper, six PJPs, the PJP 6-4 (n_s–n_r), 8-6, 10-8, 7-5, 9-7, and 11-9, are created. The hydrodynamic performance, the unsteady force of blades, and the wake structure of the PJPs are compared. The results show that the frequency of the fluctuating force of the whole rotor highly depends on the number or, more specifically, the parity of n_r. When the parameter n_r is the even number, it can be found that the total unsteady force of the rotor blades will be strengthened at the k-order stator-blades-passing frequency ($k=1/2n_r$ ). Moreover, it indicates that the superposition-enhancement coefficient (is defined as $A^{*}$ ) at $k=1/2n_r$ equals to n_r, at least from the present tests. In terms of both the rotor and stator numbers are even, a phenomenon of the rotor–stator resonance occurs at $f=1/2n_s{n}_r{f}_n$ , where f_n represents the hub rotational frequency. This work is expected to give some insight in the design of a PJP.

In this study, numerical simulations are employed to investigate the hydrodynamic performance and wake topology of a swimming Rhinoptera javanica. The study is motivated by the quest to understand the hydrodynamics of median and paired fin(MPF) mode with both spanwise and chordwise flexibility. The simulations employ an immersed boundary(IB)-simplified sphere function-based gas kinetic scheme(SGKS) method that allows us to simulate flows with complex moving boundaries on fixed Cartesian grids. A computational model is constructed based on biological data. The evolution of the hydrodynamic force and 3D vortex structures are presented. Besides, the effect of frequency and amplitude are also discussed to explain some behaviors of the actual Rhinoptera javanica. This work can provide a baseline for the design of a bio-inspired underwater vehicle.