VELOCITY-MEASUREMENTS IN MULTIDIRECTIONAL WAVES USING A PERFORATED-BALL VELOCITY METER

A simple and robust device has been developed for measuring three-dimensional velocities in laboratory waves and other flows. It comprises two small perforated balls which have well-defined Morison drag and inertia coefficients. Solution of the 'inverse Morison problem' in three dimensions allows the ambient velocity to be computed from measurements of loading on the balls. This paper describes the calibration of the instrument, the algorithm for computing velocities from forces, and details of applications in a wave flume (where results are compared with measurements from an adjacent Laser-Doppler Anemometer) and in uni- and multi-directional waves in a basin (with comparisons from an electro-magnetic flowmeter and linear wave theory). Measurements are shown to agree very well with the other sources of data where appropriate, and suggest that the perforated-ball velocity meter has some advantages over more conventional instruments.