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ABSTRACT
New methods have been developed to extract turbulent fluxes of momentum and fine sediments from Acoustic Doppler Velocimeter (ADV) data. The methods were validated with turbidity plume experiments. The ADV's backscatter amplitude signal was used to determine the sediment concentration and its turbulent fluctuations. However, different kinds of noise are found in the backscatter amplitude and the velocity signals, which are polluting the turbulent fluxes results. Therefore, spectral noise correction methods have been developed, which allow more accurate quantification of turbulent velocity and sediment concentration fluctuations. The techniques are applied to two benchmark cases of a vertical sediment-laden jet. Reynolds stresses, turbulent intensity of velocity and sediment concentration as well as turbulent sediment fluxes are shown to agree well with two-fluid plume measurements reported in the literature. The methods presented in this paper can be applied to the processing of measurements in cohesive sediment plumes, turbidity currents or mixing layers in the absence of, or in stable, flocculation.
Acknowledgments
The Authors wish to thank the Agentschap voor Innovatie door Wetenschap en Technologie (IWT) and International Marine and Dredging Consultants (IMDC) for funding this work. The authors are also grateful to the anonymous reviewers and the editors for their valuable comments.
Notation
| a | = | sediment particle radius (m) |
| AMP | = | backscatter amplitude (counts) |
| = | element (i,j) of the beam to Cartesian velocity transformation matrix (–) | |
| = | volume acoustic intensity (dB) | |
| = | plume concentration half-width (m) | |
| = | plume velocity half-width (m) | |
| C | = | time-averaged suspended sediment concentration (mg l |
| = | sediment concentration at the centerline (mg l | |
| = | speed of sound (m s | |
| D | = | plume source pipe diameter (m) |
| = | turbulent mass diffusivity (m | |
| = | amplitude spectrum (unit of Fourier transformed quantity) | |
| f | = | frequency of the acoustic wave (Hz) |
| FP | = | field probe |
| I | = | volume backscatter (dB) |
| = | emitted acoustic intensity (dB) | |
| k | = | acoustic wave number (m |
| = | characteristic momentum length scale for buoyant jets (m) | |
| R | = | acoustic path length (m) |
| = | radial distance in plume, relative to distance from source (–) | |
| SNR | = | signal to noise ratio (dB) |
| = | turbulent Schmidt number (–) | |
| = | factor containing acoustic particle properties (–) | |
| = | factor containing ADV instrument properties (–) | |
| SP | = | side-looking probe |
| SVH | = | sampling volume height ADV (mm) |
| TL | = | pulse transmit length ADV (mm) |
| = | x,y and z direction velocity components (m s | |
| = | fluctuations of u,v,w and c (m s | |
| = | uncorrected fluctuations of u,v,w (m s | |
| = | doppler noise (m s | |
| = | centreline vertical velocity in the plume (m s | |
| z | = | vertical distance from the source (m) |
| = | sediment attenuation coefficient (–) | |
| = | water attenuation coefficient (–) | |
| = | amplification coefficient for Doppler noise in normal Reynolds stresses | |
| = | excess mass density of sediment in ambient fluid (kg m | |
| = | turbulent eddy viscosity (m | |
| ρ | = | mass density of sediment (kg m |
| = | mass density of ambient fluid (kg m | |
| = | noise variance in the amplitude signal (dB) |