Technical note: Turbulence measurements from a Light Autonomous Underwater Vehicle

A self-contained turbulence instrument from Rockland Scientific was installed on a Light Autonomous Underwater Vehicle (AUV) from OceanScan Marine Systems and Technology Lda. We report on the data quality and discuss limitations of dissipation estimated from two shear probes during a deployment in the Barents Sea in February 2021. The AUV mission lasted for 5 hours, operating at a typical horizontal speed of 1.2 ::: 1.1m s−1. The AUV was programmed to find and cross the maximum along-path thermal gradient at 10, 20 and 30m depths along 4 km transects. Although the AUV vibrations contaminate the 5 shear probe records, the noise is mitigated by removing vibration-induced components from shear spectra using accelerometer signal measured in multiple directions. Dissipation rate estimates in the observed transects varied in the range 1× 10−8 and 6× 10−6 W kg−1, with the values from the two orthogonal probes typically in agreement to within a factor of 2. Dissipation estimates from the AUV show good agreement with nearby vertical microstructure profiles obtained from the ship during the transects, indicating that the turbulence measurements from the AUV are reliable for this relatively turbulent environment. 10 However, the lowest reliable dissipation rates are limited to 5× 10−8 W kg−1, making this setup unfit for use in quiescent

. a) Overview map of the study region in the Barents Sea. White isobaths are drawn at 250, 500 and 1000 m depth using IBCAO-v3 (Jakobsson et al., 2012). Ice concentration and ice edge (thick black contour) on 26 February 2021 is from OSI SAF (OSI SAF, 2017).
The initial estimate of the front location x f1 was set by using data from the ship's thermosalinograph. The procedure in 105 Eq. (??) was set to repeat for all the predefined transect depths, where x f k denotes the estimated front position for dive number k. α ∈ [0, 1] denotes the low pass filtering coefficient and was set to 0.5.
2.2 MicroRider ::::::::: Turbulence :::::::: package Turbulence measurements were made using a MicroRider-1000LP (MR) from Rockland Scientific, Canada. The MR was modified to the Tidal Energy (TE) configuration, earlier used in high flow tidal energy channels. The TE configuration includes 110 increasing the sample :::::::: sampling rate to 1024 Hz for fast channels (from the typical 512 Hz), and replacing the ASTP circuit board components with an anti-aliasing filter of 196 Hz (from the typical 98 Hz), and reducing the gain of the shear channel by a factor of 10, from about 1 second to 0.1 second ::::::: seconds. This modification allows reaching wavenumbers high enough to resolve the shear spectrum (reaching 164 cpm at 1.2 ::: 130 :::: cpm :: at ::: 1.5 m s −1 and with 196 Hz anti-aliasing filter). Reduction in the gain is to compensate for the larger signals produced by faster sensor speed through the water (the shear sensor signal 115 increases in proportion to speed-squared).
The MR was equipped with two airfoil velocity shear probes (SPM-38), one fast-response thermistor (FP07), a pressure transducer, a two-axis vibration sensor (a pair of piezo-accelerometers), and a high-accuracy dual-axis inclinometer. The MR samples the signal plus signal derivatives on the thermistor and pressure transducer, and the derivative for shear signals, allowing high resolution measurements. The sampling rate is 1024 Hz for the vibration, shear and temperature sensors, and 128 Hz 130 for pitch, roll and pressure. The accuracy of the measurements is 0.1% for the pressure, 2% for the piezo-accelerometers and 5% for the shear probes. Because of an error in the setup configuration file, the thermistor did not record measurements. Roll, pitch and yaw are clockwise rotations around the x, y and z axis of the AUV or the MR, following the right-hand rule. However, the instrument axis coordinate systems differ: for the MR x points outward from the nose along the instrument's axis, y is to the left (positive toward port) and z is positive upward. For the AUV, the vehicle xyz-frame is aligned with [North,East,135 Down], x is positive in the nominal vehicle direction of motion (forward), y is to the right (starboard) and z is positive in the down direction.
An RPM of 1500 corresponds to 25 Hz, or using a mean speed of 1.1 m s −1 to 23 cycles per meter (cpm). Figure 5a and b show :::: mean : shear spectra in frequency space using 8-s long records (length used for single dissipation estimates), for a moderate and a high value of ε, respectively. Corresponding vibration spectra from the accelerometers are also shown. The

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(green and blue lines) the bin-averaged spectra start deviating from the empirical Nasmyth ::: and ::::::::: theoretical :::::::::::::: Panchev-Kesich Figure 7. Comparison of dissipation estimates from two probes. ϵ1 is from ∂w/∂x measurements, and ϵ2 is from ∂v/∂x. a) Scatter plot of dissipation estimates from each probe, color coded with respect to measurement depth. Gray dashed lines span the agreement within a factor of 2. b) Probability distribution function (PDF) for dissipation rates from each probe, using data from all depths. c), d), e) PDFs for dissipation rates from each probe, using data from :::::: transects :: at 11, 21 and 31 m depth ::::: depths, respectively. spectra significantly for wavenumbers below 4 cpm, especially for ∂v/∂x. While the difference in data quality delivered by the two probes is less than ideal, it is expected that the shear probes oriented orthogonally will sense the vehicle motion differently.
When mounting the MR on the AUV, our main concern was to ensure that the shear sensors protruded outside the region of flow deformation, without modifying the AUV itself. To avoid interfering with the acoustic modem and fluorescence sensor on the upper part of the AUV, our solution was to make brackets and mount :: we :::::::: mounted the MR below the AUV :::: using ::::::: brackets.

6 Summary and Conclusions
A modified MicroRider-1000LP was mounted below a Light AUV and deployed ::::: tested : in the Barents Sea during a cruise in February 2021. The AUV conducted three transects across a surface temperature front at 11, 21 and 31 m depth, while continuously sampling microstructure shear. Dissipation ::: The ::::::::: dissipation : rate of turbulent kinetic energy is estimated from the shear measurements. Although the vibrations of the AUV contaminate the shear probe records, the shear spectra for dissipation 325 levels above 5 × 10 −8 W kg −1 are sufficiently cleaned using the Goodman method (Goodman et al., 2006). Dissipation rates measured from the AUV agree well with the measurements using a loosely-tethered vertical microstructure profiler from the ship. The ::::::: However, ::: the : overall noise level from the AUV is quite large, however. This : ; ::: this setup cannot detect dissipation rates below 5 × 10 −8 W kg −1 reliably, and is unfit for use in quiescent boundary layers. An improved installation of the turbulence probes on the nose of the AUV could reduce some of the limitations reported here and allow acceptable quality dissipation 330 measurements from the AUV in relatively quiet environments. Author contributions. IF, TM-B and EHK collected the data, conceived and planned the analysis. IF and EHK performed the analysis. EHK

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wrote the paper, with advice and critical feedback from IF and TM-B. All authors discussed the results and finalized the paper.
Competing interests. Authors have no competing interests. IF is a member of the editorial board for the Ocean Science