Water entry characteristics of an air-droppable underwater glider in waves

In the complex marine environment, the water entry process of air-droppable underwater gliders (ADUGs) faces numerous challenges. In particular, wave loads significantly affect the water entry characteristics and reduce the deployment success rate. This research builds a scaled-down ADUG model and a numerical wave tank through smoothed particle hydrodynamics - finite element method (SPH-FEM) to explore the water entry characteristics of ADUGs in waves. An improved recursive digital filtering algorithm is employed to calculate the shock response spectrum of the ADUG under wave conditions. The results show that the water entry impact acceleration varies with multiple parameters, including water entry point, wave height, water entry speed, and water entry angle. When the wave phase angle is π, the ADUG is more prone to exhibit the ‘ricochet’ behavior, which is most likely to happen when the wave height is 0.3 m. The inflection point in the impact response spectrum for water entry of the ADUG is mainly in the frequency band of 200 - 400 Hz. Under the same conditions, the angle of attack has little influence on the impact response spectrum. Tank experiments are performed based on a scaled-down model to investigate the water entry impact of ADUGs in waves. The experimental results show good consistency with the simulation results, with a maximum error of 13.58%. The research findings can guide the structural design and deployment planning of ADUGs and also offer theoretical references for the water entry research of other air-droppable equipment with complex geometries.