Comparison of turbulence measurements from a SonTek ADV and a nobska MAVS

A series of velocity measurements made beneath a pier on San Clemente Island off the coast of southern California has been used to compare the performance of two high-frequency acoustic current meters in measuring turbulence. The flow under the pier consists of a weak tidal current, periodic motion due to swell and shorter wind waves, and turbulence induced by the interaction of tidal currents and periodic wave motion with the pilings of the pier. Three 10-MHz SonTek ADV´s (Acoustic Doppler Velocimeter) and two Nobska MAVS-3 (Modular Acoustic Velocity Sensor) current meters were attached to a platform at mid-depth (~4 m deep) below the pier. Simultaneous data from all the instruments were recorded for various instrument configurations over a period of several days. During the test period, most of the measured rms velocity of 3-4 cm/s was due to 3-12 s period waves and swell. The tidal current varied from slack up to maximum of ~10 cm/s. Velocity measurements from the two instruments are compared under conditions of both strong and weak current and turbulence level