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

A simple and robust device has been developed for measuring three-dime nsional velocities in laboratory waves and other flows. It comprises t wo small perforated balls which have well-defined Morison drag and ine rtia coefficients. Solution of the 'inverse Morison problem' in three dimensions allows the ambient velocity to be computed from measurement s of loading on the balls. This paper describes the calibration of the instrument, the algorithm for computing velocities from forces, and d etails of applications in a wave flume (where results are compared wit h measurements from an adjacent Laser-Doppler Anemometer) and in uni- and multi-directional waves in a basin (with comparisons from an elect ro-magnetic flowmeter and linear wave theory). Measurements are shown to agree very well with the other sources of data where appropriate, a nd suggest that the perforated-ball velocity meter has some advantages over more conventional instruments.