A wave-following laser Doppler anemometer

A wave-following laser Doppler anemometer was designed to measure two-dimensional water velocity beneath wind-ruffled, mechanically generated progressive water waves. The experimental 35 m long channel (Hsu et al., 1981) contains a horizontal-displacement-type mechanical-wave generating plate at the upstream end, and a sloping beach at the downstream end, with an air inlet for wind simulation located 4.92 m downstream. A two-component modular optical laser Doppler anemometer system, equipped with an argon-ion type laser and a Bragg cell frequency shift, was used for velocity measurements. The wave-following motion allows the velocity probe to sample at points between the wave peak and the wave trough of a progressive water-wave train. The mean velocity profiles obtained in an experiment run in the wave-following frame showed considerably more detail than when the experiment was run in the fixed frame (with the same wind speed and wave amplitude).