Bias in mean velocities and noise in variances and covariances measured using a multistatic acoustic profiler: the Nortek Vectrino Profiler

Abstract

This paper compiles the technical characteristics and operating principles of the Nortek Vectrino Profiler and reviews previously reported user experiences. A series of experiments are then presented that investigate instrument behaviour and performance, with a particular focus on variations within the profile. First, controlled tests investigate the sensitivity of acoustic amplitude (and Signal-to-Noise Ratio, SNR) and pulse-to-pulse correlation coefficient, R2, to seeding concentration and cell geometry. Second, a novel methodology that systematically shifts profiling cells through a single absolute vertical position investigates the sensitivity of mean velocities, SNR and noise to: (a) emitted sound intensity and the presence (or absence) of acoustic seeding; and (b) varying flow rates under ideal acoustic seeding conditions. A new solution is derived to quantify the noise affecting the two perpendicular tristatic systems of the Vectrino Profiler and its contribution to components of the Reynolds stress tensor. Results suggest that for the Vectrino Profiler: 1. optimum acoustic seeding concentrations are ~3000 to 6000 mg L−1; 2. mean velocity magnitudes are biased by variable amounts in proximal cells but are consistently underestimated in distal cells; 3. noise varies parabolically with a minimum around the ‘sweet spot’, 50 mm below the transceiver; 4. the receiver beams only intersect at the sweet spot and diverge nearer to and further from the transceiver. This divergence significantly reduces the size of the sampled area away from the sweet spot, reducing data quality; 5. the most reliable velocity data will normally be collected in the region between approximately 43 and 61 mm below the transceiver.

Relative Contributions of Bed Load and Suspended Load to Sediment Transport Under Skewed‐Asymmetric Waves on a Sandbar Crest

“…Measured ADPV velocities in the top and bottom 5 bins were discarded due to a known bias with the Nortek Vectrino Profiler (Thomas et al, ), which reduced the vertical sampling range of the ADPV to 21 mm. A time series of cross‐shore velocity profiles, u ( t , z ), from the bed ( z ≈ 1.46 m) to the elevation of ADV 6 ( z ≈ 2.55 m) was constructed for each trial (and site) by merging the instantaneous velocity profiles measured by the ADV array and ADPV.…”

“…Sediment grain saltation occurs in lower energy flow conditions (0.05 < θ < 0.50) and typically results in the development of small-scale bed forms (e.g., ripples). Under strong bed shear stress and strong pressure gradients, typical of skewed-asymmetric waves in the surf zone, small-scale bed forms are planed out with the onset of sheet flow (θ > 0.8; Sumer et al, 1996). Sheet flow is the transport of high concentrations of sediment (ϕ sf~0 .08-0.64 m 3 /m 3 ) supported by intergranular interactions within a relatively thin mobile bed layer.…”

Flow field downstream of individual aquatic plants—Experiments in a natural river with Potamogeton crispus L. and Myriophyllum spicatum L.

“…The velocity records from the VP were filtered to eliminate potential spikes, utilizing parts of the code from MITT open source MATLAB algorithms (MacVicar, Dilling, & Lacey, 2014). First, from each point in a profile, the individual cell that had the best signal accuracy in terms of the highest SNR (signal-to-noise ratio) and the mean velocity consistent with adjacent cells was selected for further processing, usually at a specific distance from the transducer called the sweet spot (as recommended by Brand, Noss, Dinkel, & Holzner, 2016;Koca, Noss, Anlanger, Brand, & Lorke, 2017;Thomas, Schindfessel, McLelland, Creëlle, & De Mulder, 2017). Second, the record from that cell was filtered with regard to the signal-to-noise ratio and correlation values.…”

Scaling Analysis of Multipulsed Turbidity Current Evolution With Application to Turbidite Interpretation

“…These three delay time settings characterize single‐pulsed, short and long delay time flows, respectively. Acoustic Doppler Velocimetry (ADV) was used to capture vertical variations in horizontal velocity (Brand et al, ; Craig et al, ; MacVicar et al, ; Thomas et al, ). The ADV probe was set at 7.1 cm above the flume bed, capturing a flow depth of 2.5 cm.…”

“…Velocity and density sampling were undertaken for three characteristic short lock length flows of L 5 0:125 m; H 5 0:25 m; h 5 0:05 m and Dt 5 0; 4; and 15 s. These three delay time settings characterize singlepulsed, short and long delay time flows, respectively. Acoustic Doppler Velocimetry (ADV) was used to capture vertical variations in horizontal velocity (Brand et al, 2016;Craig et al, 2011;MacVicar et al, 2014;Thomas et al, 2017). The ADV probe was set at 7.1 cm above the flume bed, capturing a flow depth of 2.5 cm.…”