Turbulent Inertial Subrange Research Articles

Optical fibre sensing of turbulent-frequency motions in the oceanic environment

Observations of turbulence in the oceanic environment are sparse, with very few cases of coherent measurements with significant spatio-temporal extent due primarily to limitations of current observational tools. Here we propose submarine cables with embedded optical fibres as a potential solution to fill this observational gap, and utilise a recent 12-h observational optical fibre data set from a fast-flowing tidal channel to demonstrate such potential. Firstly, the presence of turbulent-scale signals driven by flow-topography interaction is shown at frequencies of 1 Hz and higher. These signals are consistent with the timing of the tidal flow as recorded by a nearby conventional sensor. Secondly, we show the presence of surface gravity waves with periods of 10 s, which are tight in frequency space further offshore but leak energy into the turbulent frequency range on parts of the cable closer to shore. This is compatible with shoreward-propagating surface waves that break in shallow water. Finally, we fit a theoretical spectral structure to the observations to show that much of the collected data (i) has a spectral slope that is consistent with the turbulent inertial subrange, and (ii) has a range of spectral energy consistent with that expected from turbulence generation by bottom drag acting on the tidal flow. In combination, these results highlight the potential for optical fibre sensing of turbulence, and call for a targeted experiment to characterise the fibre’s turbulence-sensing capabilities.

Trailing vortex at drifting ship hull: a numerical study on wandering and turbulence

ABSTRACT Trailing vortices are commonly associated with so-called contrails in the sky. These show instability phenomena which manifest in transverse motions and sudden thickness changes. Aircraft aerodynamics considers this so-called far field whereas for naval hydrodynamics the near-field is of dominating interest. For the latter, vortices are intensively investigated at the propeller, but they emerge also at manoeuvring ship hulls or blunt stern shapes. And this is studied within the current paper: the major trailing vortex next to the KVLCC2 hull at large drift incidence. Numerical simulations with the SST-based IDDES model are conducted with OpenFOAM. The target is to reveal details of the vortex centre flow like the evolution of turbulence and the characteristics and consequences of vortex wandering. The results reveal a low-frequency broadband wandering motion next to a broad turbulent inertial subrange and that about two thirds of resolved TKE can be attributed to wandering.

Rotor-Stator Emulsification in the Turbulent Inertial Regime: Experiments toward a Robust Correlation for the Droplet Size.

The Sauter mean diameter, d32, is a representative parameter in emulsions that indicates the average size of the oil droplets once the emulsion becomes stable. Several mathematical and physical approaches have been employed in the literature to seek expressions for d32 under different conditions. The present work sheds light on this rich literature and emphasizes that the characterization of emulsions is still a fertile field for investigation. In this paper, a new Π-theorem-based model to predict the normalized Sauter mean diameter for the specific case of rotor-stator emulsification is sought by applying a multiple regression analysis on experimental data of oil-in-water (O-W) emulsions produced using three different oils: paraffin, soybean oil, and isopropyl myristate, at different oil-to-water (O/W) ratios and rotor speeds. The proposed model quantifies the roles of the viscous, inertial, and interfacial tension forces, besides the O/W ratio, in the emulsification process within the turbulent inertial subrange. The developed empirical correlation is then contrasted with relevant literature models for reliability assessment; predictions of the present explicit model are proven to be more accurate for the fluid properties and the experimental conditions under study.

Variations of Bottom Boundary Layer Turbulence under the Influences of Tidal Currents, Waves, and Raft Aquaculture Structure in a Shallow Bay

High-frequency measurements of tides, waves, and turbulence were made using the bottom-mounted tripod equipped with the Nortek 6-MHz acoustic Doppler velocimetry during 20–23 February 2016 (winter) and 12–26 June 2017 (summer) in Heini Bay, Yellow Sea. The synchro-squeezed wavelet transform was applied for wave-turbulence decomposition, and an iterative procedure was developed to identify the turbulence inertial subrange in the bottom boundary layer. The analysis results reveal the dependency of the inertial subrange on the tidal current and turbulence intensities. The flood-ebb tidal flows are different between the summer and winter seasons, without and with the presence of dense raft aquaculture for kelp, respectively. In summer, the turbulent kinetic energy (TKE), turbulent Reynolds stress (TRS), and dissipation rate (ε) of TKE increase smoothly with the increasing tidal flow magnitude, and ε is approximately in balance with TKE production related to the vertical shear. The presence of heavy kelp aquaculture in winter causes the reduction in flow speeds and TRS, while keeping TKE and ε at high levels.

Direct numerical simulations of bubble-mediated gas transfer and dissolution in quiescent and turbulent flows

We perform direct numerical simulations of a gas bubble dissolving in a surrounding liquid. The bubble volume is reduced due to dissolution of the gas, with the numerical implementation of an immersed boundary method, coupling the gas diffusion and the Navier–Stokes equations. The methods are validated against planar and spherical geometries’ analytical moving boundary problems, including the classic Epstein–Plesset problem. Considering a bubble rising in a quiescent liquid, we show that the mass transfer coefficient$k_L$can be described by the classic Levich formula$k_L = (2/\sqrt {{\rm \pi} })\sqrt {\mathscr {D}_l\,U(t)/d(t)}$, with$d(t)$and$U(t)$the time-varying bubble size and rise velocity, and$\mathscr {D}_l$the gas diffusivity in the liquid. Next, we investigate the dissolution and gas transfer of a bubble in homogeneous and isotropic turbulence flow, extending Farsoiyaet al.(J. Fluid Mech., vol. 920, 2021, A34). We show that with a bubble size initially within the turbulent inertial subrange, the mass transfer coefficient in turbulence$k_L$is controlled by the smallest scales of the flow, the Kolmogorov$\eta$and Batchelor$\eta _B$microscales, and is independent of the bubble size. This leads to the non-dimensional transfer rate${Sh}=k_L L^\star /\mathscr {D}_l$scaling as${Sh}/{Sc}^{1/2} \propto {Re}^{3/4}$, where${Re}$is the macroscale Reynolds number${Re} = u_{rms}L^\star /\nu _l$, with$u_{rms}$the velocity fluctuations,$L^*$the integral length scale,$\nu _l$the liquid viscosity, and${Sc}=\nu _l/\mathscr {D}_l$the Schmidt number. This scaling can be expressed in terms of the turbulence dissipation rate$\epsilon$as${k_L}\propto {Sc}^{-1/2} (\epsilon \nu _l)^{1/4}$, in agreement with the model proposed by Lamont & Scott (AIChE J., vol. 16, issue 4, 1970, pp. 513–519) and corresponding to the high$Re$regime from Theofanouset al.(Intl J. Heat Mass Transfer, vol. 19, issue 6, 1976, pp. 613–624).

KmT, Kilometer-long mooring of high-resolution Temperature measurements results overview

The global oceans show extensive bottom topography above which intensive turbulent mixing is generated by breaking internal waves. Growing evidence suggests that the mixing is sufficient to maintain the ocean’s stable vertical density stratification, thereby preventing the deep-ocean from becoming a stagnant pool of cold water. Observational details of the extent of turbulent mixing away from topography are still scarce however. In this paper, results are presented from 760 high-resolution temperature sensors that were moored between 8 and 1146.5 m above a 2380-m deep sloping seafloor of Mount Josephine NE-Atlantic for 4 months. The local seafloor-slope is supercritical for semidiurnal internal tides under average stratification. The purpose of the > 1-km-long densely instrumented mooring is to provide insight in the variability of coherent scales, and in the connection between seamount-slope and mixing-effects up to 7-km horizontally into the ocean-interior. Largest four-month mean turbulence values are found near the seafloor, decreasing to half an order of magnitude lower values at 500 m above, and remaining steady-high further up. Local increases of mean turbulence values are observed in bands of 100–200 m thickness. Larger internal wave breaking occurs during baroclinic spring tide, which also affects internal waves and turbulent overturning in the interior. Spectrally, a turbulence inertial subrange dominates part of the internal wave band continuum near the seafloor. Coupling between interior- and near-seafloor-mixing is not only via previously suggested isopycnal transport of mixed waters, but also via strong internal wave interactions. Seamount-flow contributes to absolute vorticity so that blue-shifted near-inertial motions interact with internal tides.

Enhancement of turbulent liquid-liquid dispersions in a round-bottom stirred tank equipped by a flexible impeller

Stirred tanks have been widely used for liquid-liquid mixing in the chemical industry. In the present work, the influences of flexible impeller on the hydrodynamic characteristics and droplet size distribution of oil in water dispersions were investigated using experimental and numerical approaches. The one-way Fluid Structural Interaction (FSI) analysis was carried out to model the geometry of the flexible impeller used in CFD simulations. Computational fluid dynamics (CFD) was coupled with the population balance model (PBM) to simulate the liquid-liquid dispersion process in the stirred tank. The numerical model was validated by the experimental results. Compared with the rigid impeller, the flexible impeller generates a more uniform distribution of turbulent kinetic energy and turbulent dissipate rate in the stirred tank. Compared with the case of rigid impeller, the region indicating the trailing vortices behind the impeller blades was significantly reduced by the flexible impeller. It was found the flexible impeller with a smaller elastic modulus can result in narrower droplet size distributions in turbulent dispersion process. Its peak value of the number fraction frequency was in the range of 244-308μm while that was 308-388μm for the case of rigid impeller. It was found that most of the dispersed phase droplets occur in the turbulent inertial subrange regime. Related correlations between the Sauter mean diameter and local turbulent field were also obtained. The shape factor accounting for the effects of the impeller shape on the Sauter mean diameter was found to be dependent on the material property of the impeller blades, such as the Poisson's ratio and elastic modulus.

A Model for the Spectrum of the Lateral Velocity Component from Mesoscale to Microscale and Its Application to Wind-Direction Variation

A model for the spectrum of the lateral velocity component $$S_v(f)$$ is developed for a frequency range from about 0.2 $$\hbox {day}^{-1}$$ to the turbulence inertial subrange, with the intent of improving the calculation of flow meandering over areas the size of offshore wind farms and clusters. These sizes can correspond to a temporal scale of several hours, much larger than the validity limit of typical boundary-layer models, such as the Kaimal model, or the Mikkelsen–Tchen model. The development of the model is based on observations from one site and verified with observations from another site up to a height of 241 m. The model describes three ranges: (1) the mesoscale from $$0.2\ \hbox {day}^{-1}$$ to about $$10^{-3}\ \hbox {Hz}$$ where a mesoscale spectral model from Larsen et al. (2013: QJR Meteorol Soc 139: 685–700) is used; (2) the spectral gap where the normalized v spectrum, $$fS_v(f)$$ , can be approximated to be a constant; (3) the high-frequency range where a boundary-layer model is used. In order to demonstrate a general applicability of the lateral velocity spectrum model to reproduce the statistics of wind-direction variability, models for both horizontal velocity components, u and v, are used through an inverse Fourier transform technique to produce time series of both components, which in theory could have been the ensemble for calculating the corresponding spectra. The ensemble is then used to calculate directional statistics, which in turn are compared with corresponding statistics from the measured time series. We demonstrate the relevance of the constructed spectral models for calculating meandering effects for large wind farms and wind-farm clusters.

Slow persistent mixing in the abyss

Knowledge about deep-ocean turbulent mixing and flow circulation above abyssal hilly plains is important to quantify processes for the modelling of resuspension and dispersal of sediments in areas where turbulence sources are expected to be relatively weak. Turbulence may disperse sediments from artificial deep-sea mining activities over large distances. To quantify turbulent mixing above the deep-ocean floor around 4000 m depth, high-resolution moored temperature sensor observations have been obtained from the near-equatorial southeast Pacific (7{\deg}S, 88{\deg}W). Models demonstrate low activity of equatorial flow dynamics, internal tides and surface near-inertial motions in the area. The present observations demonstrate a Conservative Temperature difference of about 0.012{\deg}C between 7 and 406 meter above the bottom (hereafter, mab, for short), which is a quarter of the adiabatic lapse rate. The very weakly stratified waters with buoyancy periods between about six hours and one day allow for slowly varying mixing. The calculated turbulence dissipation rate values are half to one order of magnitude larger than those from open-ocean turbulent exchange well away from bottom topography and surface boundaries. In the deep, turbulent overturns extend up to 100 m tall, in the ocean-interior, and also reach the lowest sensor. The overturns are governed by internal-wave-shear and -convection. The turbulence inertial subrange is observed to extend into the internal wave frequency band. The associated mixing is not related to bottom friction processes but to internal wave breaking and near-inertial shear. The mixing facilitates long (hours to day) and high (exceeding 100 mab) dispersal of suspended sediments.

Open-ocean-interior moored sensor turbulence estimates, below a Meddy

A one-year time series of moored high-resolution temperature T-sensor data from 1455 ± 65 m depth on a 3900 m long line in about 5300 m of water in the NE-Atlantic Canary Basin are dominated by salinity (over-)compensated intrusions arising from the effects of Mediterranean outflow waters, which are commonly organized as underwater eddy-lenses ‘Meddies’. During the passage of a Meddy-core above the T-sensors, no intrusions were observed, thereby making it possible to use the temperature records to quantify turbulence parameters. Mean turbulence values confirm previous shipborne profiler estimates of O(10-10) m2 s-3 and O(10-5) m2 s-1 for dissipation rate and eddy diffusivity, respectively. The present data show that these ocean-interior turbulence estimates are from short-lived (<0.5 h) rather intense overturning cells with vertical scales of <5 m. Because the turbulence inertial subrange is found to extend into the internal wave band, the overturns are predominantly driven by shear associated with inertial currents. Kinetic energy, current shear and temperature variance peak at sub-inertial frequencies during the Meddy passage, suggesting wave trapping in the warm anti-cyclonic eddy and/or weakly stratified layers. The observations further show that internal wave displacements are coherent over vertical scales of up to 40 m during the presence of the Meddy compared with vertical coherence scales of <25 m during the more common no-Meddy conditions of double diffusion intrusions.

Turbulent convection and high‐frequency internal wave details in 1‐m shallow waters

AbstractVertically 0.042‐m‐spaced moored high‐resolution temperature sensors are used for detailed internal wave‐turbulence monitoring near Texel North Sea and Wadden Sea beaches on calm summer days. In the maximum 2‐m deep waters, irregular internal waves are observed supported by the density stratification during day‐times’ warming in early summer, outside the breaking zone of &lt; 0.2 m surface wind waves. Internal‐wave–induced convective overturning near the surface and shear‐driven turbulence near the bottom are observed in addition to near‐bottom convective overturning due to heating from below. Small turbulent overturns have durations of 5–20 s, close to the surface wave period and about one third to one tenth of the shortest internal wave period. The largest turbulence dissipation rates are estimated to be of the same order of magnitude as found above deep‐ocean seamounts, whereas overturning scales are observed 100 times smaller. The turbulence inertial subrange is observed to link between the internal and surface wave spectral bands.

Towards uncovering the structure of power fluctuations of wind farms.

The structure of the turbulence-driven power fluctuations in a wind farm is fundamentally described from basic concepts. A derived tuning-free model, supported with experiments, reveals the underlying spectral content of the power fluctuations of a wind farm. It contains two power-law trends and oscillations in the relatively low- and high-frequency ranges. The former is mostly due to the turbulent interaction between the flow and the turbine properties, whereas the latter is due to the advection between turbine pairs. The spectral wind-farm scale power fluctuations Φ_{P} exhibit a power-law decay proportional to f^{-5/3-2} in the region corresponding to the turbulence inertial subrange and at relatively large scales, Φ_{P}∼f^{-2}. Due to the advection and turbulent diffusion of large-scale structures, a spectral oscillation exists with the product of a sinusoidal behavior and an exponential decay in the frequency domain.

Turbulence Measurements from Five-Beam Acoustic Doppler Current Profilers

AbstractTwo new five-beam acoustic Doppler current profilers—the Nortek Signature1000 AD2CP and the Teledyne RDI Sentinel V50—are demonstrated to measure turbulence at two energetic tidal channels within Puget Sound, Washington. The quality of the raw data is tested by analyzing the turbulent kinetic energy frequency spectra, the turbulence spatial structure function, the shear in the profiles, and the covariance Reynolds stresses. The five-beam configuration allows for a direct estimation of the Reynolds stresses from along-beam velocity fluctuations. The Nortek’s low Doppler noise and high sampling frequency allow for the observation of the turbulent inertial subrange in both the frequency spectra and the turbulence structure function. The turbulence parameters obtained from the five-beam acoustic Doppler current profilers are validated with turbulence data from simultaneous measurements with acoustic Doppler velocimeters. These combined results are then used to assess a turbulent kinetic energy budget in which depth profiles of the turbulent kinetic energy dissipation and production rates are compared. The associated codes are publicly available on the MATLAB File Exchange website.

Ocean Speed and Turbulence Measurements Using Pitot-Static Tubes on Moorings

AbstractA low-power (&lt;10 mW), physically small (15.6 cm long × 3.2 cm diameter), lightweight (600 g Cu; alternatively, 200 g Al), robust, and simply calibrated pitot-static tube to measure mean speed and turbulence dissipation is described and evaluated. The measurement of speed is derived from differential pressure via Bernoulli’s principle. The differential pressure sensor employed here has relatively small, but significant, adverse sensitivities to static pressure, temperature, and acceleration, which are characterized in tests in the college’s laboratory. Results from field tests on moorings indicate acceptable agreement in pitot-static speed measurements with independent acoustic Doppler current profiler speeds, characterized as linear fits with slope = 1 (95% confidence), ±0.02 m s−1 bias, and root-mean-square error of residuals (observed minus fitted values) = 0.055 m s−1. Direct estimates of are derived from fits of velocity spectra to a theoretical turbulence inertial subrange. From near-bottom measurements, these estimates are interpreted as seafloor friction velocities, which yield drag coefficients consistent with expected values. Noise levels for , based on 40-min spectral fits, are &lt;10–9 m2 s–3. In comparison to the airfoil (or shear) probe, the pitot-static tube provides the full spectrum of velocity, not just the dissipation range of the spectrum. In comparison to acoustic measurements of velocity, the pitot-static tube does not require acoustic scatters in the measurement volume. This makes the sensor a candidate for use in the deep ocean, for example, where acoustic scatterers are weak.

Noise correction of turbulent spectra obtained from acoustic doppler velocimeters

Velocity spectra are essential in characterizing turbulent flows. The Acoustic Doppler Velocimeter (ADV) provides three-dimensional time series data at a single point in space which are used for calculating velocity spectra. However, ADV data are susceptible to contamination from various sources, including instrument noise, which is the intrinsic limit to the accuracy of acoustic Doppler processing. This contamination results in a flattening of the velocity spectra at high frequencies (O(10)Hz). This paper demonstrates two elementary methods for attenuating instrument noise and improving velocity spectra. First, a “Noise Auto-Correlation” (NAC) approach utilizes the correlation and spectral properties of instrument noise to identify and attenuate the noise in the spectra. Second, a Proper Orthogonal Decomposition (POD) approach utilizes a modal decomposition of the data and attenuates the instrument noise by neglecting the higher-order modes in a time-series reconstruction. The methods are applied to ADV data collected in a tidal channel with maximum horizontal mean currents up to 2m/s. The spectra estimated using both approaches exhibit an f−5/3 slope, consistent with a turbulent inertial sub-range, over a wider frequency range than the raw spectra. In contrast, a Gaussian filter approach yields spectra with a sharp decrease at high frequencies. In an example application, the extended inertial sub-range from the NAC method increased the confidence in estimating the turbulent dissipation rate, which requires fitting the amplitude of the f−5/3 region. The resulting dissipation rates have smaller uncertainties and are more consistent with an assumed local balance to shear production, especially for mean horizontal currents less than 0.8m/s.

Multiscale spectral analysis of temporal variability in evapotranspiration over irrigated cropland in an arid region

The temporal patterns of evapotranspiration (ET) and its biophysical and physiological controls (e.g., soil water content, solar radiation, and canopy conductance) occur over a wide range of time-scales ranging from seconds to decades. Thus, there is increasing interest in understanding at which scales the main temporal correlations between ET and its controlling factors occur across different ecosystems. For this study, we used eddy covariance measurements of ET over 2 years at an irrigated maize field in an arid inland region of northwest China. We applied the wavelet transform as a novel technique to examine spectral characteristics of ET and its controlling factors. The ET power spectra displayed a −1 power law in turbulent inertial subrange at <1-h time-scale, and showed substantial power at daily, seasonal and annual time-scales. The cospectra of ET and soil water content (SWC) showed significant temporal correlation at 5-days, which has implications for calculation of ET using the soil water balance method in this region. We found that ET synchronized with the change of net radiation, and led vapor pressure deficit and air temperature for ∼2h at the 1-day time-scale (i.e., positive lags), but the phase relationship between ET and SWC was influenced by irrigation patterns. Canopy conductance influenced ET variability at the 1-day time-scale, but the effect was not consistent across the growing seasons. Our results show the importance of irrigation practices and its influence on the multi-temporal correlations of ET and its controlling factors. Irrigation can sharply change the phase angle relationship of ET and SWC from −10h to 10h. These results are important for understanding water cycle processes, improving water management, and address food security issues across irrigated croplands in arid regions.

Turbulent transport of a high-Schmidt-number scalar near an air–water interface

AbstractWe measure solute transport near a turbulent air–water interface at which there is zero mean shear. The interface is stirred by high-Reynolds-number homogeneous isotropic turbulence generated far below the surface, and solute transport into the water is driven by an imposed concentration gradient. The air–water interface is held at a constant concentration much higher than that in the bulk of the water by maintaining pure${\mathrm{CO} }_{2} $gas above a water tank that has been initially purged of dissolved${\mathrm{CO} }_{2} $. We measure velocity and concentration fluctuations below the air–water interface, from the viscous sublayer to the middle of the ‘source region’ where the effects of the surface are first felt. Our laboratory measurement technique uses quantitative imaging to collect simultaneous concentration and velocity fields, which are measured at a resolution that reveals the dynamics in the turbulent inertial subrange. Two-point statistics reveal the spatial structure of velocity and concentration fluctuations, and are examined as a function of depth beneath the air–water interface. There is a clear dominance of large scales at all depths for all quantities, but the relative importance of scales changes markedly with proximity to the interface. Quadrant analysis of the turbulent scalar flux shows a four-way balance of flux components far from the interface, which near the interface evolves into a two-way balance between motions that are raising and lowering parcels of low-concentration fluid.

Mesoscale Energy Spectra of Moist Baroclinic Waves

Abstract The role of moist processes in the development of the mesoscale kinetic energy spectrum is investigated with numerical simulations of idealized moist baroclinic waves. Dry baroclinic waves yield upper-tropospheric kinetic energy spectra that resemble a −3 power law. Decomposition into horizontally rotational and divergent kinetic energy shows that the divergent energy has a much shallower spectrum, but its amplitude is too small to yield a characteristic kink in the total spectrum, which is dominated by the rotational part. The inclusion of moist processes energizes the mesoscale. In the upper troposphere, the effect is mainly in the divergent part of the kinetic energy; the spectral slope remains shallow (around −) as in the dry case, but the amplitude increases with increasing humidity. The divergence field in physical space is consistent with inertia–gravity waves being generated in regions of latent heating and propagating throughout the baroclinic wave. Buoyancy flux spectra are used to diagnose the scale at which moist forcing—via buoyant production from latent heating—injects kinetic energy. There is significant input of kinetic energy in the mesoscale, with a peak at scales of around 800 km and a plateau at smaller scales. If the latent heating is artificially set to zero at some time, the enhanced divergent kinetic energy decays over several days toward the level obtained in the dry simulation. The effect of moist forcing of mesoscale kinetic energy presents a challenge for theories of the mesoscale spectrum based on the idealization of a turbulent inertial subrange.

Quality Controlling Surf Zone Acoustic Doppler Velocimeter Observations to Estimate the Turbulent Dissipation Rate

Abstract High-quality measurements of the turbulent dissipation rate ε are required to diagnose field surf-zone turbulence budgets. Quality control (QC) methods are presented for estimating surf zone ε with acoustic Doppler velocimeter (ADV) data. Bad ADV velocity data points are diagnosed with both the ADV signal strength (SS) and correlation (CORR). The fraction of bad SS data points (δSS) depends inversely upon the wave-amplitude-normalized transducer distance below the mean sea surface. The fraction of bad CORR data points δCORR can be elevated when δSS is low. The δCORR depends inversely upon the wave-amplitude-normalized sensing volume distance below the mean sea surface, and also increases with increased wave breaking, consistent with turbulence- and bubble-induced Doppler noise. Velocity spectra derived from both “patched” and “interpolated” time series are used to estimate ε. Two QC tests, based upon the properties of a turbulent inertial subrange, are used to reject bad ε data runs. The first test checks that the vertical velocity spectrum’s power-law exponent is near . The second test checks that a ratio R of horizontal and vertical velocity spectra is near 1. Over all δCORR, 70% of the patched and interpolated data runs pass these tests. However, for larger δCORR &amp;gt; 0.1 (locations higher in the water column), 50% more patched than interpolated data runs pass the QC tests. Previous QC methods designed for wave studies are not appropriate for ε QC. The results suggest that ε can be consistently estimated over the lower 60% of the water column and &amp;gt;0.1 m above the bed within a saturated surf zone.

An Ocean Refractometer: Resolving Millimeter-Scale Turbulent Density Fluctuations via the Refractive Index

Abstract A fiberoptic sensor has been constructed to measure oceanic density fluctuations via their refractive index signature. The resolution (Δz = 1 mm, Δt = 0.2 ms) and precision (Δn &amp;lt; 10−8, Δρ = 3.4 × 10−5 kg m−3) of the device are far better than other methods and are sufficient to resolve the entire turbulent spectrum. Spectra show the salinity Batchelor rolloff at levels undetectable via conductivity measurements. However, the low-wavenumber portion of the spectrum occupied by the turbulent inertial subrange (≈1 m–1 cm scales) is marred by noise resulting from fiber motion in response to turbulent velocity fluctuations. The technique is described, and the first ocean measurements are reported.