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Numerical simulations of fluid-structure interaction (FSI) on an elastic foil heaving with constant amplitude in freestream flow are carried out at a low Reynolds number of 20,000. The commercial software STAR-CCM+ is employed to solve the flow field and the large-scale passive deformation of the structure. The results show that introducing a certain degree of flexibility significantly improves the thrust and efficiency of the foil. For each Strouhal number St considered, an optimal flexibility exists for thrust; however, the propulsive efficiency keeps increasing with the increase in flexibility. The visualisation of the vorticity fields elucidates the improvement of the propulsive characteristics by flexibility. Furthermore, the mechanism of thrust generation is discussed by comparing the time-varying thrust coefficient and vortex structure in the wake for both rigid and elastic foils. Finally, in addition to sinusoidal motions, we also consider the effect of non-sinusoidal trajectories defined by flattening parameter S on the propulsive characteristics for both rigid and elastic foils. The non-sinusoidal trajectories defined by S=2 are associated with the maximum thrust, and the highest values of propulsive efficiency are obtained with S=0.5 among the cases considered in this work.

A bulbous bow is a typical ship structure. Due to the influence of the bulbous bow, complex flow separation and gas capture phenomena may appear during the water entry of ship-like sections. In this paper, experimental and numerical studies on the water entry of a ship-like section with an obvious bulbous bow are carried out. Two thin plates are installed at both ends of the test model to ensure that the flow field during the impact process is approximately two-dimensional. The free-fall drop test is carried out in the test rig equipped with guide rails. By changing drop heights, impact pressure on the model surface with different initial impact velocities is measured. A numerical model for simulating the water entry of the ship-like section is established by using the Computational Fluid Dynamics (CFD) method, based on the Navier-Stokes equations. Reasonable time steps and mesh size are determined by convergence analysis. Four different flow models are used in the numerical analysis. It is found that the K-Epsilon turbulence model can present the most reasonable numerical prediction by comparing numerical results with the experimental data. Furthermore, the influence of the bulbous bow on the impact loads is numerically studied by using the validated numerical model. It suggests that the bulbous bow has little effect on the impact force acting on the bow-flared area but, in the position near the bulbous bow, the pressure will be affected by the second slamming and the air cushion.

Underwater gliders are winged, autonomous underwater vehicles that are broadly applied in physical and biological oceanography. The position of the wing has an important effect on the movement performance of the underwater glider. In this paper, the dynamic motion of a series of underwater glider models with different longitudinal wing positions are simulated, which provides guidance for the design of underwater gliders. The results show that when the net buoyancy is constant, the wing position affects the gliding angle, but does not affect the relationship between the gliding angle and the gliding speed. In addition, the farther the wing position of the glider is from the buoyancy centre, the longer it takes for the attitude of a glider to change, whether the wing is in front of, or behind, the buoyancy centre.

According to the problem of a trimaran cross-deck structure's fatigue strength, taking one trimaran as an example, the responses of ship motion and hydrodynamic pressure on the ship's surface in regular waves are calculated on the basis of the 3D linear potential flow theory. Next, the stress responses of hot-spots in regular waves with different wave angles are evaluated by the finite element analysis of the global trimaran structure. On the basis of linear cumulative damage theories, the fatigue damage is calculated according to the direct calculate method using spectral analysis. Finally, the effect of different sea areas and heading angles' time factors are discussed. The results show that the fatigue damage is greater when using an inshore sea area and considering the time factors of heading angles. The result can offer a reference for the design and development of trimarans.

Launching underwater gliders by aircraft could greatly expand the application of underwater gliders. However, during the process of the glider entry into the water, it will be subjected to significant impact loads, especially during the process of wing entry into the water. In this paper, a foldable wing module is proposed to reduce the water entry impact loads of the glider caused by the wings entering into the water. The effect of the foldable wing module on impact loads reduction and the influence of the foldable wing module on the water entry trajectory are studied by numerical method. The results show that the differences in mass and gravity center position caused by the foldable wing module have little effect on the water entry impact loads of the glider until the wings impact the water. When the glider enters the water obliquely, the wing module reduces the peak radial acceleration of the glider. In addition, the trajectory and time of the glider to reach the horizontal attitude are also reduced. These conclusions will be helpful for the designing of the wings of air-launched underwater gliders. [ABSTRACT FROM AUTHOR]