Position detection method and hydrodynamic characteristics of the water entry of a cylinder with multidegree motion

This study experimentally investigates the hydrodynamic characteristics of a cylinder with multidegree motion during water entry with different initial horizontal velocities and inclined angles. A mechanical device is designed to provide certain initial conditions for the cylinder, and the cavity evolution and cylinder position are recorded with a high-speed camera. To obtain precise hydrodynamic forces, a new image-processing method is established to detect the center and attitude angle of the cylinder based on a step edge model and the Grubbs criterion. Additionally, an algorithm is employed to correct the refraction effects in the raw data. Using the above methods, the phenomena of asymmetric cavities, cavity pinch-off, double cavities and cavity ripples are observed in the wake region of the cylinder. Before the cavity pinches off, the hydrodynamic force driving the cylinder motion is generated from the pressure difference between the wet and dry parts. After pinch-off, the hydrodynamic force is mainly dominated by the induced lift, which depends on the angular speed, and a high-pressure region created by the pinch-off cavity causes an instantaneous impulse force that causes quick decreases in the cylinder accelerations. In addition, for the cylinder lift coefficient, the average rate of change increases almost linearly with an increasing initial angle of attack during water entry. For the drag coefficient, the average rate of increase rises nearly linearly with increasing absolute value of the initial angle of attack before pinch-off. After cavity pinch-off, this rate increases almost linearly with increasing |v(0)|/alpha L-0. [GRAPHICS] .