A perforated-ball velocity meter for underwater kinematics measurement in waves and current

A perforated-ball velocity meter (PVM) has been developed for measuring all three components of water particle velocities at five different vertical locations simultaneously in regular, long crested as well as short crested free surface wave field in laboratory scale. The device measures the wave induced forces on a cantilevered ball-tube assembly. The wave kinematics is computed from these forces by solution of an inverse problem given by the Morison equation. The drag and inertia coefficients of both the ball and the tube are experimentally determined and the forces on both the ball and the tube are used in the inverse Morison problem. Measurements in wave flume and wave basin have been conducted using the multistage PVM and the measured kinematics has been successfully compared with theory using wave elevation measurements. The device can be used to measure current speed as well as wave kinematics in a wave-cum-current field.