A Coherent Doppler Profiler for High-Resolution Particle Velocimetry in the Ocean: Laboratory Measurements of Turbulence and Particle Flux

Abstract

A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution particle velocimetry in the ocean, in particular for remote measurements of suspended sediment flux and turbulence in the nearshore and continental shelf bottom boundary layer. Acoustic backscatter estimates of suspended particle concentration and velocity are determined simultaneously from the phase and amplitude of the backscattered signal over an O(1 m)-long profile with subcentimeter resolution, at an ensemble-averaged rate of O(25 Hz). To characterize the performance of the profiler as a remote turbulent flux sensor, laboratory experiments were carried out in a particle-laden high-Reynolds number round jet. The results include comparisons between the acoustic Doppler and standard laboratory sensors, and to previous experimental and theoretical results for turbulent jets. The observed mean radial particle flux profile is found to be consistent with the computed mean flux profile for a turbulent jet. The measured entrainment rate is within 10% of the accepted value for turbulent round jets. Energy spectra of the turbulent motions demonstrate the Ϫ5/3 slope characteristic of the inertial subrange. The kinetic energy spectral densities obtained with the Doppler profiler match observations with a Sontek acoustic Doppler velocimeter and are in qualitative agreement with hotfilm measurements within the jet. Space-time domain images of particle flux exhibit well-defined coherent structures. Cospectral analysis demonstrates that these larger structures dominate the particle flux.

Application of Coherent ADCP for Turbulence Measurements in the Bottom Boundary Layer

“…Besides instrumental noise, the high-wavenumber response of the ADCP is limited by the system spatial resolution (Zedel and Hay 1999). The maximum wavenumber resolved by the instrument is 2/(2d), with d being the width of the acoustic beam in the particular depth bin.…”

Acoustic backscatter inversion for suspended sediment concentration and size: A new approach using statistical inverse theory

“…A series of laboratory experiments were conducted in the Dalhousie University jet tank (Hay, 1991;Zedel and Hay, 1999), with the goal of obtaining a baseline data set for testing backscatter inversion with a recently developed multi-frequency acoustic instrument (MFDop; Hay et al 2012a,b). The experimental setup is shown in Fig.…”

Turbulence measurements in a jet: Comparing the vectrino and vectrinoii

“…This flow has well defined spatial structure that exhibits self similarity and it has been thoroughly studied ([1], [7]). Importantly, the combination of significant mean velocities with high levels of turbulence provide a challenging environment for coherent sonar based systems [8] and the jet is therefore ideal for the present instrument comparison. The measurements are compared both between the two instruments and with known jet characteristics as reported by [1].…”