Acoustic Doppler Current Profiler Measurements Research Articles

Characteristics Analysis of Acoustic Doppler Current Profile Measurements in Northeast Taiwan Offshore

Characteristics Analysis of Acoustic Doppler Current Profile Measurements in Northeast Taiwan Offshore

Reach-Scale Mapping of Surface Flow Velocities from Thermal Images Acquired by an Uncrewed Aircraft System along the Sacramento River, California, USA

An innovative payload containing a sensitive mid-wave infrared camera was flown on an uncrewed aircraft system (UAS) to acquire thermal imagery along a reach of the Sacramento River, California, USA. The imagery was used as input for an ensemble particle image velocimetry (PIV) algorithm to produce near-continuous maps of surface flow velocity along a reach approximately 1 km in length. To assess the accuracy of PIV velocity estimates, in situ measurements of flow velocity were obtained with an acoustic Doppler current profiler (ADCP). ADCP measurements were collected along pre-planned cross-section lines within the area covered by the imagery. The PIV velocities showed good agreement with the depth-averaged velocity measured by the ADCP, with R2 values ranging from 0.59–0.97 across eight transects. Velocity maps derived from the thermal image sequences acquired on consecutive days during a period of steady flow were compared. These maps showed consistent spatial patterns of velocity vector magnitude and orientation, indicating that the technique is repeatable and robust. PIV of thermal imagery can yield velocity estimates in situations where natural water-surface textures or tracers are either insufficient or absent in visible imagery. Future work could be directed toward defining optimal environmental conditions, as well as limitations for mapping flow velocities based on thermal images acquired via UAS.

Anisotropic turbulence modeling for natural channel flow: A numerical approach with finite element method

In this study, we propose a numerical model aimed at improving the understanding and prediction of flow velocities in natural channels. This model specifically focuses on the challenges presented by anisotropic turbulence and bottom roughness. By incorporating the mixing length concept into an algebraic turbulence model and employing the finite element method with Streamline-Upwind/Petrov–Galerkin (SUPG) stabilization, our model seeks to refine the simulation of axial velocity distribution and secondary motions. Validation was achieved through Acoustic Doppler Current Profiler (ADCP) measurements in three sections of the Canal do Rodeador, showing our model’s predictions of discharge and average velocity to have a deviation of approximately 9.34% from experimental data. The results underline the significance of secondary currents and turbulence anisotropy in shaping channel flow behaviors, offering new insights into the interactions between flow characteristics and channel bed features. This model stands out as a robust tool for hydraulic structure design and hydrokinetic potential evaluation, providing a non-intrusive, cost-effective alternative to traditional methods.

Application of coastal acoustic tomography: calibration of open boundary conditions on a numerical ocean model for tidal currents

Coastal acoustic tomography (CAT), which measures path-averaged currents from reciprocal acoustic transmission experiments and reconstructs velocity fields from the multiple path-averaged current data, is useful for monitoring tidal currents in coastal shallow water, especially if data assimilation is employed. Previous CAT data assimilation studies have focused on state estimation problems, i.e., the reconstruction of tidal currents and following dynamical discussion. In this study, we investigate the use of path-averaged currents in a boundary control problem. Specifically, we aim to use the observed path-averaged currents to determine the parameters of a numerical ocean model, which were tidal amplitudes and phases as the open boundary conditions in this study. We investigate two methods: using the ensemble Kalman filter (EnKF) results and a linearization approach called model Green’s function method. Both calibration methods decreased the amplitudes of tidal constituents at the open boundaries. We compare the model performance between the model predictions with and without the calibration of the open boundary conditions. The model predictions with the calibrated open boundary conditions improved the agreement with the observed path-averaged current. We also implemented the sequential updates of EnKF with the two calibrated open boundary conditions. The EnKF results with the independently calibrated two open boundary conditions improved the agreement with the comparison data obtained by acoustic Doppler current profiler measurement compared with the original EnKF result with the initial open boundary conditions.

Spatially quantifying and mapping the riverbed roughness of a tropical river via acoustic Doppler current profiler (ADCP) measurements to develop a 2-D numerical model

Field measurement of the bed roughness of tropical tidal rivers is a long and expensive process. In this study, we quantified the roughness coefficient from the acoustic Doppler current profiler (ADCP) measurements considering flow characteristics along a 240 km length of the Vu Gia Thu Bon River system, in central Vietnam. Our approach entailed the application of the logarithmic velocity distribution trend of turbulent flows to determine the bottom roughness height. Then, we applied the equation suggested by Strickler (1923) to calculate the Manning roughness coefficient. The TELEMAC-2D hydraulic model was then used to simulate the effect of the roughness coefficient on the estimated water level. Despite its uncertainty, the estimated bed roughness was sufficient for the accurate calculation of the hydraulic variables. Compared to those in previous research, the statistical indicators showed good performance in the first simulation process during both the calibration and validation periods along the river system.

Hydro‐Sedimentary Processes of a Plunging Hyperpycnal River Plume Revealed by Synchronized Remote Imagery and Gridded Current Measurements

AbstractThe present knowledge of plunging hyperpycnal river plumes is mainly based on two‐dimensional (confined) laboratory experiments. Several hypotheses on three‐dimensional (unconfined) flow processes have been made, but not tested in situ. In this field study, the dominant three‐dimensional hydro‐sedimentary processes related to unconfined plunging were elucidated by synchronizing autonomous time‐lapse camera images with boat‐towed acoustic Doppler current profiler measurements. It was found that the flow field complies with two‐dimensional conceptualizations along the plume centerline. Perpendicular to the centerline, the plume slumped laterally due to its density excess and simultaneously converged laterally due to its vertical divergence. This combination led to a narrowing of the plume near the surface, resulting in a sediment‐rich triangle‐shaped pattern on the surface near the inflow, and a rather stable width near the bed. The formation of secondary flow cells transporting riverine water and suspended particulate matter (SPM) away from the plume near the bed, up toward the surface and back toward the plume near the surface, was revealed. An increase of the average SPM concentration and the SPM flux in the main flow direction indicates net sediment erosion under the investigated conditions of high discharge and sediment load. This suggests transient storage of sediment during conditions of lower discharge and sediment load, and high morphological activity in the plunging area. These findings allowed extending a classical conceptual plunging model to laterally unconfined sediment‐laden plunging inflows for conditions of well‐mixed ambient water in the plunging region and inflow densimetric Froude numbers exceeding 1, common in nature.

Assessing the impact of an offshore longline mussel farm on local water circulation in a highly hydrodynamic energetic bay

Thought to be a sustainable choice, molluscs are the most consumed aquaculture foods after finfish. The expansion of the mussel aquaculture industry offshore reports lower environmental impacts compared to inshore farms. Although an offshore location has the potential to reduce a farm's ecological impacts, the effect of large developments on water currents is still not fully understood. High hydrodynamic regimes can influence the dispersion of farm biodeposits, organic loading, flow alterations with the potential to impact water residence time, particle and sediment dispersal (including larvae and biodeposits) and seabed sediment resuspension, which in turn, can have ecological impacts. Farm-induced flow changes of the UK's first large scale suspended longline mussel farm were assessed by a combination of oceanographic mooring and vessel-mounted Acoustic Doppler Current Profiler (ADCP) measurements. Data was separated by upstream and downstream according to the direction of the flow in relation to the farm. M-ADCP results showed a 28% decrease current velocities within the farm's boundaries, demonstrating within-farm current attenuation produced by mussel ropes drag. Flow was then redistributed above and beneath the farm showing velocity increases of 66% and 7% respectively, resulting in above-farm flow acceleration and downwelling (with up to 171% increase in near-seabed velocities compared to sea surface velocities). However, overall mean downstream sea surface velocities decreased by 63%. An overall 21% increase in near-seabed velocities showed the farm's effect on near-bed currents, the opposite to what is naturally achieved by seabed friction. This was further measured by VM-ADCP results also showing secondary flow acceleration at the farm's flanks and increased near-headline flow perturbations of up to 80% mean u velocities over a small horizontal scale (0.5 km), further demonstrating the effects of the farm on the local circulation. Through-farm surface current velocities (waves) were reduced by up to 72%. Flow changes were localised and dependent on the different tidal phases, the farm's design and, the abundance of mussel ropes providing drag.

Measurement of backscattering strength of artificial bubbles in the Southern Sea of the Korean Peninsula

Bubbles greatly affect the propagation of sound waves as the acoustic characteristics of a bubble cluster in water are distinct from those of undisturbed water. Therefore, bubble clusters affect the ability of sensors to detect underwater targets by blocking or reflecting sound waves. Additionally, the bubble wakes produced by ships in movement can be detected by wake-homing torpedoes, thus greatly threatening the safety of the ship. Thus, research on bubble dynamics in water is crucial for the development of military technology. Recently, we conducted a quantitative estimation of the acoustic characteristics of artificial bubbles, including their backscattering strength, existence time, population density spectrum level and void fraction in an ideal water-tank environment. Based on our previous findings, the present study sought to measure the acoustic characteristics of artificial bubbles using an acoustic Doppler current profiler (ADCP) in the southern sea of the Korean Peninsula, which is a real marine environment. Additionally, we validated the ADCP measurements by comparing them to those obtained using a scattering strength measurement system (SSMS) developed by our team. Collectively, our findings provide a basis for the development of military technology, as well as for the study of bubble in water.

Morphological Model for Erosion Prediction of India’s Largest Braided River Using MIKE 21C Model

The Brahmaputra River has a dynamic, highly braided channel pattern with frequent river bar formation, making it morphologically very dynamic, especially during the monsoon season with high discharge and sediment load. To understand how the river changes over time, this study focused on two stretches: Palasbari-Gumi and Dibrugarh. Using 2D morphological models (MIKE-21C), the study aimed to predict erosion patterns, plan protective measures, and assess morphological changes over short-term (1 year), medium-term (3 year), and long-term (5 year) periods. Model runs were conducted to predict design variables across these river reaches, encompassing different hydrological scenarios and development-planning scenarios. The coarse sand fraction yielded mean annual sediment load predictions of 257 Mt/year for the 2021 hydrological year and 314 Mt/year under bankfull discharge conditions in the Palasbari-Gumi reach. In the Dibrugarh reach, the corresponding values were 78 Mt/year and 100 Mt/year. Notably, historical records indicate an annual sediment load of 400 Mt/year in the Brahmaputra River. The model results were compared to measurements from Acoustic Doppler Current Profilers (ADCP), showing good accuracy for flow velocities, flood levels, and sediment loads. Discrepancies in peak model velocities compared to ADCP measurements remain consistently below 9% across the majority of recorded data points. The predicted flood levels for the bankfull discharge condition exhibited an outstanding accuracy, reaching nearly 91% at the Palasbari-Gumi site and a notable 95% at the Dibrugarh site. This study has presented a valuable methodology for enhancing the strategic planning and implementation of river training endeavours, particularly within the dynamic and highly braided channels of rivers such as the Brahmaputra River. The approach leverages predictive models to predict morphological changes over a 2–3 years timeframe, contributing to improved river management.

Characteristics of the Kinetic Energy Spectra in the Subpolar North Atlantic

Abstract The subpolar North Atlantic is known to have rich mesoscale and submesoscale variations; however, their spectral characteristics have not been documented in observations. This study documents the kinetic energy (KE) spectra using acoustic Doppler current profiler measurements that cover both the Iceland Basin and the Irminger Sea. The KE spectrum is partitioned into geostrophically balanced motions and unbalanced motions. The results reveal that balanced motions dominate the KE spectra. The unbalanced motions enhance in spring and fall to flatten the spectra and dominate small-scale (<50 km) energy, though uncertainty is high due to measurement noise and method assumptions. In addition, the dynamical framework that drives the balanced motions undergoes distinct seasonal shifts. In the spring and summer seasons of the Iceland Basin, as well as the summer season of the Irminger Sea, the wavenumber spectra of balanced motions exhibit a slope of approximately −3, consistent with the internal quasigeostrophic turbulence theory. Conversely, in the fall season of the Iceland Basin and the spring and fall seasons of the Irminger Sea, the wavenumber spectra of geostrophic balanced motions have a slope close to −2, consistent with surface quasigeostrophic turbulence theory. Additionally, we have found that the intensity of mesoscale eddies in the spring season can modulate both the slope and intensity of the wavenumber spectra of geostrophic balanced flows.

Circulation Dynamics and Dilution in Shallow Estuaries Case Study (1): Dilution Factor Calculations

The ambient conditions of receiving waters such as currents and density stratification influence the dilution of treated wastewater discharged by WWTPs. Hendrix Creek, a semi-enclosed tidal tributary of Jamaica Bay that serves as receiving water of the 26th Ward WRRF, is not well studied. A near-field dilution model was developed and applied to the plant outfall to simulate plume transport and dilution in the initial mixing zone. The long-term ambient field conditions were characterized, and the dilution factor of a tracer was measured to calibrate the model. This was phase one of an integrated dilution and mixing study aimed at understanding the circulation dynamics which contribute to dilution in the creek. Field measurements during the dye release period also aided in the calibration and development of a 3D hydrodynamic model used to accomplish far-field dilution and mixing in phase two of this study. The near-field model results have good agreement with dye survey data and acoustic doppler current profiler (ADCP) measurements in the creek. Under critical conditions, the model predicts a dilution factor of less than 2 in the initial mixing zone, indicating poor dilution. Saline water from Jamaica Bay which served to dilute the treated wastewater appears to be dominated by the high plant discharge resulting in limited dilution in the near-field region of the creek.

Comparison of ADCP and ECCOv4r4 Currents in the Pacific Equatorial Undercurrent

Abstract The Pacific Equatorial Undercurrent (EUC) flows eastward across the Pacific at the equator in the thermocline. Its variability is related to El Niño. Moored acoustic Doppler current profiler (ADCP) measurements recorded at four widely separated sites along the equator in the EUC were compared to currents generated by version 4 release 4 of the Estimating the Circulation and Climate of the Ocean (ECCOv4r4) global model–data synthesis product. We are interested to learn how well ECCOv4r4 currents could complement sparse in situ current measurements. ADCP measurements were not assimilated in ECCOv4r4. Comparisons occurred at 5-m depth intervals at 165°E, 170°W, 140°W, and 110°W over time intervals of 10–14 years from 1995 to 2010. Hourly values of ECCOv4r4 and ADCP EUC core speeds were strongly correlated, similar for the EUC transport per unit width (TPUW). Correlations were substantially weaker at 110°W. Although we expected means and standard deviations of ECCOv4r4 currents to be smaller than ADCP values because of ECCOv4r4’s grid representation error, the large differences were unforeseen. The appearance of ECCOv4r4 diurnal-period current oscillations was surprising. As the EUC moved eastward from 170° to 140°W, the ECCOv4r4 TPUW exhibited a much smaller increase compared to the ADCP TPUW. A consequence of smaller ECCOv4r4 EUC core speeds was significantly fewer instances of gradient Richardson number (Ri) less than 1/4 above and below the depth of the core speed compared to Ri computed with ADCP observations. We present linear regression analyses to use monthly-mean ECCOv4r4 EUC core speeds and TPUWs as proxies for ADCP measurements. Significance Statement Hundreds of scientific papers have used ECCO data products generated with a continually evolving state-of-the-art ocean-model–data synthesis system. We ask, How representative is the latest version of ECCO equatorial ocean currents? We use independent in situ current measurements as the reference dataset to establish the accuracy of ECCO currents in the tropical Pacific. Attention is focused on the Pacific Equatorial Undercurrent (EUC) because it contributes to the formation of El Niño and La Niña events. ECCO EUC core speeds were smaller in magnitude and less variable in time compared to observations. As a consequence, ECCO currents generated smaller vertical mixing in the EUC compared to that inferred from current measurements. We developed a linear regression model to improve representation of monthly-mean ECCO currents.

Optimum Estimation of Coastal Currents Using Moving Vehicles

Abstract Ocean acoustic tomography (OAT) deploys most moored stations on the periphery of the tomographic region to sense the solenoidal current field. Moving vehicle tomography (MVT), an advancement of OAT, not only samples the region from various angles for improving the resolution of mapped currents but also acquires information about the irrotational flow due to the sampling points inside the region. To reconstruct a complete two-dimensional current field, the spatial modes derived from the open-boundary modal analysis (OMA) are preferable to the conventional truncated Fourier series since the OMA technique describes the solenoidal and irrotational flows efficiently in which all modes satisfy the coastline and open boundary conditions. Comparisons of the reconstructions are presented using three different representations of currents. The first two representations explain only the solenoidal flow: the truncated Fourier series and the OMA Dirichlet modes. The third representation, accounting for the solenoidal and irrotational flows, uses all the OMA modes. For reconstructing the solenoidal flow, the OMA representation with the Dirichlet modes performs better than the Fourier series. A large difference appears near the bay mouth, where the OMA-Dirichlet reconstruction shows a better fit to the uniform currents. However, considerable uncertainty exists outside the bay mouth where the irrotational currents dominate. This can be improved by the third representation with the inclusion of the Neumann and boundary modes. The reconstruction results using field data were validated against the acoustic Doppler current profiler (ADCP) measurements. Additionally, incorporating constraints from ADCP measurements enhances the accuracy of the reconstruction. Significance Statement This study contributes toward improving our understanding of accurately measuring oceanic circulation patterns over large areas without relying solely upon stationary sensors or satellite imagery. The study combines multiple sources, such as shipboard ADCP and tomographic techniques, to obtain a complete picture of what is happening beneath surface waters across entire regions under investigation. It has important implications for fields such as climate science, marine biology, and fisheries management, where accurate knowledge of the movement and distribution of water masses is crucial for predicting future trends and making informed decisions.

Detection of suspended macroplastics using acoustic doppler current profiler (ADCP) echo

Plastic pollution has become an enormous environmental problem, endangering ecosystems, livelihoods, safety and human health. Large quantities of plastics are trapped in or transported by rivers. Monitoring methods mostly focus on plastics floating at the surface or deposited on riverbanks, while a substantial part of plastics may be transported below the water surface. Available underwater monitoring methods rely on nets and large equipment, making them labour-intensive, expensive and invasive. The measurements are, therefore, limited to occasional point measurements. In this paper, we explore the potential of echo sounding for the monitoring of suspended macroplastic (plastic items bigger than 5 mm). We performed tests in a controlled (basin), a semi-controlled (harbour) and an uncontrolled (river) environment using the high-end Acoustic Doppler Current Profiler (ADCP). This device is already in use for the estimation of flow velocity and suspended sediment concentrations using the wide network of ADCPs in the Netherlands and other countries. In the undisturbed controlled environment, 25 items varying in size, material, and orientation could be detected up to at least 4.6 m from the ADCP. The semi-controlled experiments showed that most of these items can also be detected among other naturally occurring scatterers, such as aquatic life, organic material and air bubbles. The field tests under natural conditions, combining ADCP and net measurements, showed that ADCP data can be calibrated towards a correct order of magnitude estimate of plastic transport. The coupling of the ADCP data to item characteristics such as size, material and orientation is still challenging, but more research into, for example, the signature of items may enable distinguishing item characteristics. This fundamental knowledge, combined with repetitions of validated field measurements under different flow conditions, is needed for the development of a robust monitoring method. Such a method may enable continuous or cross-sectional monitoring of suspended plastics and give insight into historic and plastic transport through 30-year long datasets. These insights can help improve and determine the effect of current mitigation and cleaning efforts.

Observed interannual variability of the Kuroshio and Luzon Undercurrent associated with tropical and subtropical wind forcing

The interannual variability of the Kuroshio and Luzon Undercurrent (LUC) east of the Luzon Island is examined using observations from a moorings during December 2010–October 2012 and three moorings during January 2018–May 2020 deployed at 18°N. Measurements from acoustic Doppler current profiler (ADCP) equipped on the moorings indicate that the interannual variations of upper-layer Kuroshio and lower-layer LUC were in phase. The interannual variability is strong in the Kuroshio but weakens substantially in the LUC. A proxy of the Kuroshio and LUC based on sea level slope is developed and validated by comparing the ADCP and satellite altimeter measurements. The proxy during 1993–2020 shows that the interannual sea level slope across the Kuroshio is primarily associated with sea level fluctuations on its eastern flank. Diagnostic analysis and numerical sensitivity experiments using a simplified ocean model indicate that the wind forcing in the subtropical northwestern Pacific Ocean played an essential role in the interannual variability of Kuroshio and LUC’s strength and water mass properties during the observation periods in 2011 and 2018–2020, although the tropical winds dominated the interannual variability of the two boundary currents during most time of the past three decades (1993–2020). The mooring observations show that the subsurface water mass in the LUC at the depth of about 650–800 m experienced a considerable freshening event from July 2019 until at least January 2020 due to abnormally strong subtropical wind forcing.

Resource assessment using a combination of seabed mounted and semi-stationary vessel-mounted ADCP measurements

Accurate resource assessment for tidal stream sites is crucial for calculation of the Levelized Cost of Energy (LCOE) of each project or turbine. The main method as recommended by the IEC’s technical specification on tidal stream resource assessment involves using a numerical model as well as several measurements using Acoustic Doppler Current Profilers (ADCP) mounted on the seabed. Ideally, once the locations of the turbines are known precisely, seabed mounted ADCP measurements will be undertaken at all turbine locations. For large tidal arrays this is however challenging, especially with project development constrained by time and budget. The MeyGen project is the largest planned tidal stream energy project in the world, aiming to develop up to 398 MW of installed power in the Pentland Firth, Scotland, with current velocities reaching up to 5 m/s. During Phase 1 of the project, four 1.5 MW turbines were installed. During Phase 2, site characterization campaigns were carried out to plan the deployment of additional turbines, representing 28 MW of tidal power capacity. Based on learning from Phase 1 around the need to accurately calibrate numerical models at a very detailed spatial resolution, and the costs and time to obtain seabed ADCP measurements at many turbine locations, a new measurement methodology was proposed. This method used semi-stationary vessel-mounted ADCP measurements to characterize the flow at the planned turbine locations, rather than relying on a few seabed-mounted ADCP measurements and a numerical model, or relying on seabed-mounted ADCPs at all the locations of the planned turbines.
 This works introduces the method used for calculating the tidal stream velocities at the planned turbine locations. A 35-day seabed mounted ADCP survey was conducted as a baseline within the area of planned turbines. It used a Teledyne SentinelV50 ADCP configured with 2Hz sampling rate, 5-beam velocities and 1 m depth cells. Using harmonic analysis and prediction, the current velocities were obtained for a 18.6 years period.
 Additionally, during seven days of spring tide, water velocities were measured using a vessel mounted ADCP. It used a Teledyne Workhorse 600kHz configured with circa 1Hz sampling rate and 1 m depth cells. The vessel mounted measurements were undertaken at each turbine location during 5 minutes every hour, for a 12-hour tidal cycle. During each 5 minutes the vessel kept its position within 15-meter radius around the nominal position. Similar vessel-mounted measurements were also undertaken above the seabed mounted ADCP, as well as continuously for 12 hours.
 The semi-stationary vessel mounted ADCP measurements were validated against the seabed-mounted ADCP measurements with the co-located 12-hour data. The current velocity magnitude obtained from the vessel mounted measurements at the turbine locations were then correlated with the simultaneous data from the seabed mounted ADCP. This correlation was finally combined with the 18.6 years timeseries predicted at the seabed-mounted ADCP location to obtain long-term times series and statistics for turbine locations. The final presentation and paper will provide the results of this study.

Assessing wave-turbulence separation from ADCP measurements with artifical flow data

Accurate assessment of marine currents is critical for meaningful planning of tidal stream energy deployments. Methods for assessing mean flow speed are well-established, but the issue of assessing phenomena that vary over much shorter time scales than the semidiurnal tide (principally turbulence and waves) is not as settled. This is in part due to the limitations of the standard instruments used for surveying currents at potential tidal stream sites i.e., acoustic Doppler current profilers (ADCPs). Because ADCPs use multiple acoustic beams to sample single components of velocity at widely-separated spatial locations, a reliable analysis of the mean flow can be obtained by a suitable average over measurements from all beams: this approach works well for determining mean flow properties, but is less well-suited for the smaller and faster variations associated with waves and turbulence. In particular, conventional analysis approaches such as the variance method do not meaningfully distinguish between wave-driven and turbulence-driven velocity variations.
 An important consequence of this is that ADCP estimates of turbulent kinetic energy (TKE) are biased high because the TKE calculation method includes wave-driven velocity fluctuations as well as those caused by turbulence. Previous work by the authors has successfully applied a mixed spectral-statistical filter to ADCP estimates of TKE from a tidal stream test site; this filter showed good performance in distinguishing between true TKE and pseudo-TKE due to wave action. However, for field data it is only possible to assess the filter’s performance with respect to wave properties: this is because independent measurements of wave properties can be obtained from a surface buoy, but there is no alternative instrumentation that can yield independent measurements of turbulence.
 The study presented here addresses this shortcoming by generating an artificial flowfield with known turbulence and wave properties, sampling with a virtual ADCP, and applying the spectral-statistical filter to the results. In this way it is shown that the combined filter improves the separation of waves and turbulence over the individual filters by an even greater degree when assessed against turbulence data rather than against independent wave measurements. The study discusses the effects of different artificial turbulence generation schemes, specifically contrasting the spectral or Sandia method with the synthetic eddy method; it also discusses the tuning of spectral filter parameters for optimum separation of wave and turbulence effects.

Measurements of tidal flow variability in Ramsey Sound, Pembrokeshire

The nature of the flow at in-stream tidal energy sites is particularly important for predicting array and device performance, and also for operations and maintenance planning. Previous developers have reported issues such as the choice of vessel, cost of operations, and the limits of operation of deployment vessels. The dynamics of the flow around slack water has been of particular interest at Ramsey Sound in Pembrokeshire (UK) for planning the recovery of an existing turbine, the Tidal Energy Limited ‘Deltastream’.
 This research presents flow characteristics of Ramsey Sound, based on analysis of Acoustic Doppler Current Profiler (ADCP) measurements and tide gauge data from the nearby Standard Port of Milford Haven. The ADCP was located approximately 300 m across the channel, at the northern end of the channel, where the channel width was 1200 m and the mean depth was approximately 33 m. The flow dynamics were examined specifically to look at times potentially suitable for offshore operations Two weeks of data were used in the analysis, spanning a complete spring-neap cycle.
 Results demonstrate that flow velocities exhibited clear asymmetry, with stronger flows on the northerly directed flood tide than on the ebb. There was considerable variation in the measured current speed around the time of the maximum, suggesting large scale bed feature generated turbulence. The flood (northerly) current maximum was approximately in phase with high water at Milford Haven. Cross correlation indicated that the flow generally led the elevation by 20 minutes. In contrast to the expected theory, the current strength at mid-depth was stronger than at the surface on the maximum flood tide. The maximum flow speed in the tide was reasonably predictable from the tide range at Milford. A threshold-based analysis of the ADCP measurements allowed the duration of slow-moving water to be identified for operation planning.
 Operations and planning in light of sound understanding of hydrodynamics at tidal energy sites is critical for future economic success of the tidal energy sector. The results shown here from an ADCP deployment in Ramsey Sound have shown the capability to give useful tools for planning recovery operations.

Comparison of Actuator Line Modelling of Tidal Power Kites with ADCP Measurements

The potential for tidal energy to be a part of future renewable energy systems is expanding. Tidal turbines deployed in open waters or channels are effective methods to harness energy from tidal currents. Sharing similar functionality with wind turbines, horizontally mounted tidal turbines require a minimum tidal current velocity to operate effectively. The Deep Green (DG) power plant which is based on a tethered kite model aims, however, to operate efficiently in tidal current velocities as low as 1.2 m/s. The kite wing is steered in a lemniscate trajectory ( ) almost perpendicular to the tidal current. In the trajectory, the relative flow velocity through the turbine attached to the wing reaches several times the tidal current velocity, enabling efficient operation of the turbine in relatively low-velocity tidal currents. This could reduce geographical limitations in installing large-scale tidal power arrays.
 In a previous project, numerical modelling of the DG was carried out in a tidal flow using Large Eddy Simulations (LES) and Actuator Line Modelling (ALM) implemented in OpenFOAM solver. ALM using momentum sources has been used in modelling wind turbines and has been validated against experimental data and observations. The ALM has been further developed in order to be able to model wings (here the kite) that move in arbitrary paths compared to horizontally mounted turbines with rotational paths. This numerical model for the DG kite in a tidal flow has, however, up to now not been validated against observations, which is beneficial before further analyses are made, e.g., optimization studies.
 In this study, Acoustic Doppler Current Profiler (ADCP) observations taken in the wake of a DG are compared to the results from the numerical model under similar conditions. Comparing the numerical results directly with the observations leads to discrepancies, hence the model data is resampled in a similar way that the ADCP would measure and process data using a virtual ADCP for the model. Flow properties such as the instantaneous and time-averaged stream velocities are in turn compared for both the tidal flow without the DG and with a DG. The effect of the DG on the tidal flow is analysed using the model and the observations. After resampling the model data, the model and observations show good agreement. This suggests 1) that the DG model using ALM can be used for further analysis and 2) that whilst comparing model data with ADCP observations for studying the small-scale effects of tidal turbine wakes, care should be taken to sample the model data consistently with observations.

Scaling the Bubble Penetration Depth in the Ocean

AbstractBubble plume penetration depths have been identified as a key parameter linking subsurface turbulent kinetic energy (TKE) dissipation rates and whitecaps. From data collected in the Atlantic sector of the Southern Ocean, nominally 50°S 40°W, bubble plume penetration depths were estimated from Acoustic Doppler Current Profiler measurements of the acoustic backscatter anomaly. Bubble presence at depth was corroborated using independent measurements of optical scattering. Here, an effective wavelength, observations of significant wave height and atmospheric forcing were used to scale penetration depths of breaking waves under open ocean conditions. The parameterization was developed assuming a correlation between the observed penetration depth and an estimate of the TKE dissipation rate enhancement under breaking waves. The effective wavelength was defined from the effective phase speed based on a momentum and energy balance across the atmospheric wave boundary layer and was considered to be the largest actively wind‐coupled wave and representative of large‐scale breaking for wave ages ranging from 15 to 35 (i.e., 15 ≤ 〈cp/u*〉 ≤ 35). This yields a dimensional penetration depth parameterization in terms of inverse wave age and the length scales under consideration. The parameterization captures the bubble plume penetration depth with stronger forcing leading to deeper injections, reaching up to 9 m. Both length scales are effective at defining the depth of a wave‐affected layer in terms of bubble presence with the effective wavelength better collapsing the data under mixed conditions with deeper plumes associated to larger fractional whitecap coverage.