Improving the accuracy of four‐receiver acoustic Doppler velocimeter (ADV) measurements in turbulent boundary layer flows

Abstract

Acoustic Doppler instrument measurements suffer from random spikes and Doppler noise. Using a fourreceiver ADV (acoustic Doppler velocimeter; Vectrino manufactured by Nortek) that allows recording beam velocities, we combine a spike-removal procedure on the beam velocities with a noise-reduction method on the flow velocities to improve turbulence measurements. We compare the results with those obtained from ADVP (acoustic Doppler velocity profiler) measurements under the same conditions, i.e., in turbulent open-channel flow over a coarse-grained bed. It is shown that spikes are best removed from ADV beam velocity data before calculating flow velocities, thereby correcting all three flow velocity components at the source. Spikes in beam velocities do not correlate with low correlation values. ADVP data generally have few spikes and do not need spike removal treatment, showing that spikes are instrument related. The noise reduction method is based on the decorrelation of the Doppler noise terms contained in two vertical velocities redundantly sampled in the same volume. The combined ADV data treatment is sufficient to significantly extend the resolved frequency range in the velocity spectra. It reduces RMS values by up to a factor of 2, and the corrected values agree with ADVP results and theoretical predictions, indicating that both treatments are needed. Owing to spatial averaging effects over the ADV sample volume, a sampling frequency limit of close to 50 Hz is determined by the deviation of the spectra from the -5/3 slope.

Enhanced sedimentation beneath particle‐laden flows in lakes and the ocean due to double‐diffusive convection

“…Due to their sampling geometry, the Vectrino+ ADVs provide very accurate measurements of vertical velocity ( w ), compared to the horizontal components. Theoretically, the noise level of the vertical velocity components is 13.9 times lower than the noise level of the horizontal velocity component in a four‐beam ADV [ Doroudian et al ., ]. While the accuracy of horizontal velocities is quoted as ± 1 mm s −1 [ Nortek, ], the vertical velocity has been estimated to have an accuracy of at least 10 times better, i.e., ±0.1 mm s −1 [ Devi and Kumar, ].…”

A Froude‐scaled model of a bedrock‐alluvial channel reach: 1. Hydraulics

“…The ADV data were initially filtered using a correlation threshold of 0.4 [Martin et al, 2002;Strom and Papanicolaou, 2007], a signal-to-noise (SNR) ratio of 10 [Wahl, 2000;Strom and Papanicolaou, 2007], and the expected measurement range. Further removal of spikes caused by aliasing was achieved by removing all velocity measurements that fell outside 3 standard deviations of the mean and then recalculating and repeating this step once [Buffin-Bélanger et al, 2006;Doroudian et al, 2010].…”

Noise in Turbulence Measurements Using Acoustic Doppler Velocimetry

“…Turbulence measurements are not only affected by Doppler noise, but also by random spikes. In contrast with Doppler noise, random spikes are not inherent to the measurement technique and occur mostly in poor measurement environments (Doroudian et al, 2010). To reduce the effect of noise resulting from random spikes, several post-processing filters have been proposed with the aim of improving the turbulence statistics measured by ADVs.…”

“…It was shown that the modified method improved the spectrum over all frequencies compared to the original despiking method. Spike-removal, however, depends on the flow conditions and caution should be taken when proposing universal guidelines (Doroudian et al, 2010). Chanson et al (2008) used an ADV for high-frequency velocity measurements in a small estuary.…”