Abstract
For the investigation of turbulence and particles, interaction measurement systems are required, which are able to measure velocity and concentration fluctuations simultaneously. Acoustic Doppler Velocimeters (ADV) are widely used for velocity and turbulence measurements in natural and artificial flows. Based on the acoustic sonar theory, a model is presented that correlates the ADVâs Signal-to-Noise Ratio (SNR) and the suspended solids concentration of several natural (Ems Estuary, Lake Eixendorf, Lake AltmĂŒhl) and artificial sediments (Chinafill, quartz powder, bentonite, metakaolin) for the range 0.001 g/Lâ50 g/L. Within the presented method, the sound absorption in water and on particles is considered in a continuous approach for sampling frequencies up to 100 Hz. The widely-used log-linear relation between the SNR and the concentration, which is only valid for low concentrations, was extended for the high concentration regime. Measurement results show a similar behavior of the SNR with respect to varying suspended solid concentrations for different sediments. However, the analysis of the fit parameters shows systematic differences depending on the type of sediment. It is concluded that the proposed model is applicable as well for laboratory use as for measurements in rivers and estuaries. Finally, we discuss the reliability of the results and the methodology with regard to measurements in rivers, lakes, and estuaries.
Highlights
Acoustic Doppler Velocimeters (ADV) probes provide the opportunity to measure in situ the three-dimensional flow velocity in liquids with a high temporal resolution
In order to testify to our methodology for natural sediments, we repeated the measurements for coastal sediments from the Ems Estuary (North Sea, Germany) and for freshwater sediments from Lake AltmĂŒhl and Lake Eixendorf (Bavaria, Germany)
Correlation values between two received signals dropped below 70 %, that is why those measurement results were discarded within the evaluation [31]
Summary
ADV probes provide the opportunity to measure in situ the three-dimensional flow velocity in liquids with a high temporal resolution (up to 200 Hz). This allows evaluating the velocity fluctuations in terms of the turbulent behavior in the flow. The three-dimensional velocity fluctuations ui0 = ui â Ć«i are described as the deviation of the mean value Ć«i. In natural surface waters like rivers, channels, lakes, and estuaries, near-bottom suspended solids of different concentrations are common. In the Ems Estuary in the German Bight, Suspended Solid Concentrations (SSC) were measured up to 300 g/L [2]. It was shown that SSC can lead to turbulence damping and decreasing eddy viscosity [3]
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