Numerical investigation on the water entry impact characteristics of autonomous underwater vehicles

Water entry of autonomous underwater vehicles (AUVs) is an unsteady and complex process accompanied by a huge hydrodynamic impact force which consequently affects the structure globally and locally. Therefore, precise modeling of this phenomenon is indispensable for the structure design of the vehicle. In this article, numerical model employing an Arbitrary-Lagrangian Eulerian (ALE) formulation is used to study the water entry impact of AUV. A penalty coupling algorithm will be employed which allows the interaction between the solid and the fluids. The feasibility and precision of the numerical technique is validated by the experimental data of the water entry of a decelerating object. After validation, the proposed numerical method is employed to examine the hydrodynamic behavior of AUV water entry under various launch parameters at the initial stage of impact. Numerical results from ALE method are also compared with smooth particle hydrodynamics (SPH) method. This reveals that ALE method can accurately simulate large deformation problems with less computational cost. The analysis results indicate that the time period at which the impact acceleration reaches its maximum value decreases as the launch velocity of the AUV increases. Axial and radial impact loads are calculated at various launch angles for fixed impact velocity of the vehicle. It is shown that oblique water entry of AUV is more sensitive to the radial impact load. It is concluded that water entry angle and launch velocity are the crucial parameters greatly influencing the impact characteristics of the AUV. Quantitative comparison between numerical and experimental data proves that the proposed numerical algorithm can reliably be used for water entry impact problems at high velocities.