A high resolution near bed coherent acoustic Doppler currentprofiler for measurement of turbulent flow

摘要

The design and performance of a prototype high resolution currentprofiler, using coherent acoustic Doppler techniques for measurement ofturbulent flow in the bottom 1.5 metres of water above the sea bed, isdiscussed. The instrument transmits at 524 kHz and flow is measured in32 range gated bins of 46 mm length, using pulse to pulsecoherent-acoustic Doppler techniques. The instrument has been tested ina laboratory test tank, and in the sea off the leg of a pier. In bothsituations the transducer was deployed looking downwards, and cablelinked to the transmit/receive electronics and thence to the dataprocessing computer. Using the present system of uncoded pulses, themaximum unambiguous detectable velocity is ±0.37 metres/sec.Typically, results from 128 pulses are averaged, giving a velocityreading in each bin every 0.25 sec. Early results show the minimummeasurable averaged velocity to be 5 mm/sec, and the noise level to be±5 mm/sec. Simple anti-aliasing and tracking methods should allowmeasurement of velocities up to 1 metre/second. These methods arereadily implemented in the vertical, or near verticalconfiguration