Experimental and numerical investigation of water entry impact forces on air-launched AUV

During the launching of the AUV especially in air-launching, the water entry problem is inevitable. The AUV will stir the flow field of surrounding fluid during the moving into water, while the fluid will act a strong force on the AUV in return, which may cause trajectory deflection problems or damage to their structure and inner components in severe cases. The water entry experiment of full-size AUV water highly dependent on the choice of test site and laboratory equipment. Due to the short process of the AUV into the water and the dramatic changes in the mechanical environment, the test system needs to have good dynamic response and high impact resistance. In the experiment, high pressure air was used as the power source of the launcher, and a three-axis accelerometer was installed in the head of the vehicle to collect the changes in acceleration during water entry. Two underwater high-speed cameras are arranged to capture the aerial posture and cavity shape and other information of the AUV, while placed other two high-speed cameras in the air to determine the AUV's aerial trajectory and posture, supplemented by continuous strong light to improve the imaging conditions simultaneously. This way, the experiments of a full-size model in a series of different launching velocities and angles are completed, along with the change laws of the axial and radial forces during the water entry process and the phenomena of cavitation are observed. In order to compare with the experimental results, the numerical model based on a coupled FEM and Smoothed Particle Hydrodynamics (SPH) formulation available in the commercial code Ls-Dyna is established and the water entry process of the AUV in different conditions such as different velocities and different angles were simulated. The obtained numerical results are in good agreement with the experimental, both of which show that both the water entry velocity and entry angle have a great influence on the impact load during the water entry process. The conclusions are expected to be beneficial to forecast the impact forces of water entry about the air-launched AUV, design the structure and select launch condition of the AUV.