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When the autonomous underwater vehicle (AUV) enters the water at a small angle, the head of the AUV will be subjected to a torque that causes it to rise, which may cause the AUV to ricochet. The occurrence of ricochet will have an important impact on the trajectory stability of the AUV. In this paper, the finite element method-smoothed particle hydrodynamics (FEM-SPH) coupling algorithm, which absorbs the high efficiency of FEM and the advantages of SPH in dealing with large deformation and meshes distortion, is used to study the small-angle water entry problem of the AUV numerically. In the coupled FEM-SPH algorithm, discrete particles were used to model the zone of water, while the part of the AUV was modeled with finite elements. A contact algorithm couples the finite elements and the particles. Particular attention was paid to the influence of different head hemisphere angles and different initial conditions on the ricochet trajectory of the AUV. The critical conditions and influencing factors of the AUV ricochet phenomenon were given.

This paper presents a systematic work focused on the thrust generation and wake structures of two travelling-wavy foils with tandem, parallel and staggered arrangements, therein presenting essential information about actual schooling fish. The fluid dynamics around two identical foils arranged with a dimensionless horizontal gap, G x , and vertical gap, G y , are investigated at Reynolds number R e = 2 × 1 0 5 . The wake structures are classified into proximity interference, wake interference, proximity and wake interference, and no interference associated with thrust generation. Based on numerical computations, we find that G x significantly affects the re-organisation of the vortex shedding from the leading and following foils, resulting in various propulsive forces acting on the foils. Moreover, G y causes a result whereby the vortex shedding from the leading foil deflects towards the flow behind the following foil, leading to a slight change in the thrust generation for both travelling-wavy foils. This work is helpful for guiding the queue control of multiple bio-inspired underwater robots.

We numerically investigate the force generation and wake structures of a nonsinusoidal periodic pitching foil under a thickness-based Reynolds number of R e D = 255 . We consider various nonsinusoidal periodic trajectories of pitching motion, such as the sawtooth and square trajectories, using an adjustable parameter, K . Similar to previous studies on sinusoidal pitching foils, the present results for the nonsinusoidal pitching foil indicate that the transition from a Benard–von Karman wake to a reverse Be nard–von Karman wake precedes the actual drag–thrust transition. In addition, the symmetry breaking of a reverse Be nard–von Karman wake is observed. As K increases, the drag–thrust transition of the nonsinusoidal pitching foil advances gradually and it becomes easy to destabilize the wake structures produced by the oscillating foil. Since K physically describes how rapidly the foil reverses its pitching direction, we argue that the initial velocity of vortex shedding from the foil increases with K and results in the aforementioned phenomena. The present work provides helpful findings for better understanding the kinematic behaviour of actual animals and for designing bio-inspired robotics.

Air-launched autonomous underwater vehicles (AUVs) are subjected to huge impact loads in the early stage of water entry, which may cause structural damage or failure of electronic components, especially in the case of high speed water entry. Therefore, it is very imperative to carry out experimental and numerical research on the impact loads of air-launched AUVs. An experimental study of the water entry of air-launched AUVs with different launch velocities and angles was conducted using high-speed photography and sensing technology. The axial and radial accelerations of AUVs under different working conditions at the early stage of water entry were obtained. Furthermore, a coupled finite element technique and the smooth particle hydrodynamics method (FEM-SPH) is employed to model the water entry process of air-launched AUVs. Numerical simulation results such as the peak of impact acceleration were compared with the presented experiment results. The good agreement between the experimental results and the numerical simulation results revealed the capability and accuracy of the numerical algorithm in solving the problem of AUV water entry.

We characterised the hydrodynamics of the two staggered swimming fish using vortex dynamic method. Based on the theoretical model, we showed that the horizontal spacing between the two fish plays a crucial role in affecting the fluid dynamics around two fish, while the vertical spacing between the two fish merely affects the value of propulsive force individually generated by the two fish. Furthermore, we revealed that there are two horizontal spacing thresholds for the holding and breaking of the spatial configurations of the two fish, indicating that two fish would swim at the stable status: the synchronous acceleration or deceleration.

The drag–thrust transition of a symmetric foil undergoing pitching oscillation in a uniform flow is numerically studied in the present work. Under a certain thickness-based Reynolds number R e D = 255 , the fluid dynamics around the oscillated foil at a series of values for the chord-based Reynolds number, R e c , are investigated. The various values of R e c are obtained by keeping the maximum thickness of the foil fixed while rhythmically changing the chord length of the foil. Depending on the numerical results, we uncover a scaling relation that links the amplitude-based Strouhal number S r A to the R e c at the drag–thrust transition boundary, which is briefly expressed as S r A ∼ R e c − 1 . Through analysing the velocity field of the foil, it is found that the drag–thrust transition of the pitching foil is closely associated with the critical velocity of the latest vortex produced by the foil. These findings helps in understanding the kinematic behaviour of aquatic animals and designing bio-inspired underwater robots.

In contrast to rigid body autonomous underwater vehicle (AUV) models, elastic AUV models will exhibit hydrodynamic effects due to elastic deformation of the shell during water entry, influencing their load-response characteristics. In this paper, the predictive capability of the selected simulation method is first verified by comparing the simulation results with test data acquired from experiments conducted in a drop-test tank. Then, the effects of the head shape parameters, shell thickness and water-entry state, such as the water-entry velocity and angle, on the acceleration, pressure, stress and structural deformation of an elastic AUV during water-entry impact are investigated. By comparing the characteristics of rigid and elastic bodies, the influence of the hydroelastic effect is revealed. The research results can provide guidance on the design of airborne vehicles and transmedia vehicles.

The skip phenomenon that may occur when a disk impacts water is a subject of perennial fascination for children as well as adults alike; it is also a fairly complex and highly nonlinear fluid-structure interaction problem. In this paper, the coupled FEM-SPH algorithm is used to numerically study the skip characteristics of disk impacting water. The critical conditions of the skip and the relationship between the initial velocity and the maximum skipping times are obtained. Moreover, the dissipation role of energy during the skipping process is discussed. Our conclusions can provide references for other similar issues, such as aircraft ditching, trans-medium vehicles, bouncing projectiles and so on.

Water entry experiments of projectiles with different nose shapes were performed under different entry angles and velocities using high-speed photography technology. The cavity flow characteristics of the near water surface, including splash jet, splash crown, surface seal of cavity, pull away, deep seal of cavity and cavity collapses, were systematically investigated using a high-speed camera. The emphasis of the study is paid on the effect of nose shape, water entry angle and velocity on the evolution of the air entraining cavity. The experimental results demonstrate that the nose shape of projectile has a significant influence on the jet flow, the cavity diameter and trajectory stability in the case of certain other conditions. On the other hand, the splash scale, cavity diameter increase gradually with the increasing of the water entry velocity, as well as the cavitation closed in advance. Furthermore, the water entry angle of the projectile plays an important role in the cavity evolution and the close type.

The optimal aquatic locomotion has previously been associated with a narrow St(= fA/u) number range of 0.2–0.4. We present how animals tune their Strouhal (St) number to this range to reveal the mechanisms influencing efficiency. The self-propelled swimming of a 2D swimmer is simulated using an immersed boundary method. The locomotion kinematics is controlled by two variables, [Formula: see text] and frequency f. We show that only when animals constrain their [Formula: see text] = 0.125–0.25, their St number can fall into the optimal St range. When [Formula: see text] Hz, the St number is independent with frequency. Although different combinations of f and [Formula: see text] can achieve a same cruising velocity, high-f and low-[Formula: see text] motions are more efficient. This can be linked to its larger lateral velocity component in the proto-vortex region and the transition of the tail vortices into small eddies.