An acoustic Doppler velocimeter (ADV) for the characterisation of turbulence in concentrated fluid mud

The paper describes a velocimeter, based on the back-scattering of ultrasonic waves by particles, designed for measuring instantaneous turbulent velocities in a concentrated fluid mud mixture. The acoustic Doppler velocimeter (ADV) needs no calibration and is therefore a potentially useful tool for measuring velocities in the laboratory or in the field. We investigate its reliability for measurements in concentrated cohesive sediment suspensions, where the particle size is usually unknown due to the occurrence of flocculation, and where there is considerable acoustic wave absorption. Measurements in a resonant standing wave demonstrate the ability of the apparatus to measure unsteady velocities. The data validation rate ranges between 20 and 80 Hz for a cohesive sediment concentration in the range 20–140 g l−1. Experiments were performed with two different natural mud mixtures. It is observed that using an ADV does not require prior determination of particle and floc properties. It is furthermore demonstrated that the amplitude of the back-scattered signal received by the transceiver results mainly from a single reflection on particles, whereas echoes experiencing multiple reflections are strongly damped. The use of an ADV for measuring turbulence properties is finally assessed for low Reynolds turbulence, which occurs in Concentrated Benthic Suspension layers.