Ambient Currents Research Articles

Experimental studies on performances of flexible floating oil booms in coupled wave-current flow

Floating oil booms are commonly-adopted facility to collect spilled oil on sea surface, or to protect specific areas against oil slick spreading. In this study, 931 runs of laboratory test were carried out under wave-current coupling conditions to investigate hydrodynamic performances of the flexible floating oil boom. The tests first conducted a comparison on motion responses between the flexible floating boom and the rigid one to indicate the necessity of taking the flexibility of boom into consideration. Then a comprehensive analysis was carried out to investigate the effects of the ambient currents, waves and the boom characteristics of material stiffness, diameter of floater, length of skirt, and B/W (Buoyancy/Weight) ratio on the motion responses of the flexible floating booms. Finally, by taking the water blockage effect in front of the boom into consideration in the definition of boom effectiveness, the effective draft and freeboard were compared between the flexible boom and rigid one under fixed current and wave conditions. The effects of currents, waves, skirt lengths and B/W ratios on the effective draft and effective freeboard are assessed.

Pneumatocysts provide buoyancy with minimal effect on drag for kelp in wave-driven flow

Many seaweeds have buoyant gas-filled bladders (pneumatocysts) that hold fronds upright in the water column and enhance their access to light for photosynthesis. However, ambient water currents bend flexible seaweeds, pushing fronds closer to the substratum where light is lower, so the hydrodynamic drag on pneumatocysts may counteract their buoyancy in flowing water. The effects of pneumatocysts on frond hydrodynamic drag were investigated in this study, as well as how the positions of pneumatocysts along fronds affect their motion and height in wave-driven water flow. The kelp Egregia menziesii was used as a model organism because it is abundant on wave-swept rocky shores and because our field surveys revealed that this species shows great variation in pneumatocyst size, number, and location on fronds. In laboratory towing-tank studies, it was found that drag on pneumatocysts was reduced when they were bent over by flowing water. The drag due to pneumatocysts was small compared to the drag on a whole frond. At flow speeds up to 0.58ms−1, the buoyant force exerted by a pneumatocyst was greater than the drag it experienced. In wave-tank experiments using models of fronds with pneumatocysts at different positions, the pneumatocysts were most effective at lifting fronds high in the water column when they were located at the distal tips of the fronds, both in small and large waves. However, if fronds had pneumatocysts that were not at the tip, an increase in the peak velocities of waves led to an increase in the heights of the fronds in the water column. In the field, pneumatocysts did not affect the back-and-forth horizontal motion of E. menziesii exposed to waves, but fronds with pneumatocysts were higher in the water column than fronds with no pneumatocysts, even when the number of pneumatocysts on a frond was low. Our results indicate that pneumatocysts can exhibit great variability in size, number, and location with only a small effect on hydrodynamic forces on a kelp, that pneumatocysts at frond tips are most effective at holding kelp high in the water column, but that only a few pneumatocysts at any location along a frond can enhance the frond's height in waves.

Numerical modelling for quantitative environmental risk assessment for the disposal of drill cuttings and mud

To investigate the fate of drilling waste and their impacts towards surrounding environment, numerical models were generated using an environmental software; MIKE by DHI. These numerical models were used to study the transportation of suspended drill waste plumes in the water column and its deposition on seabed in South China Sea (SCS). A random disposal site with the model area of 50 km × 25 km was selected near the Madalene Shoal in SCS and the ambient currents as well as other meteorological conditions were simulated in details at the proposed location. This paper was focusing on sensitivity study of different drill waste particle characteristics on impacts towards marine receiving environment. The drilling scenarios were obtained and adapted from the oil producer well at offshore Sabah (Case 1) and data from actual exploration drilling case at Pumbaa location (PL 469) in the Norwegian Sea (Case 2). The two cases were compared to study the effect of different drilling particle characteristics and their behavior in marine receiving environment after discharged. Using the Hydrodynamic and Sediment Transport models simulated in MIKE by DHI, the variation of currents and the behavior of the drilling waste particles can be analyzed and evaluated in terms of multiple degree zones of impacts.

Monitoring and analysis of electronic current transformer’s field operating errors

The energy metering accuracy and relay protection reliability in power grid systems critically depends on the phase current measurement accuracy. Commonly manufactured electronic current transformers (ECT) may exhibit unacceptable errors during field operation. Our research platform targets to remotely monitor ECT’s errors caused by ambient temperature variation, load current, and adjacent phase interference. A 0.2% accuracy, 110kV, Rogowski coil-based ECT prototype was selected for field operation in over 2years of monitoring tests. Correlated field and laboratory tests showed ambient temperature-related errors of less than 0.1% in current ratio and 2′ in phase, findings consistent with the type tests. The load current and adjacent phase interference introduce negligible errors for load currents larger than 0.2In (where In represents the phase rated current). However, the phase error exceeds 20′ for load currents less than 0.2In, possibly leading to false conclusions. Based on the above influencing factors, standardize the 0.2% accuracy ECT’s error less than half of the error limit ensures acceptable field error levels.

The fate of human remains in a maritime context and feasibility for forensic humanitarian action to assist in their recovery and identification

The number of annual maritime fatalities reported in the Mediterranean has more than doubled in the last two years, a phenomenon closely linked to the increase of migrants attempting to reach Europe via the Mediterranean. The majority of victims reportedly never gets recovered, which in part relates to the fact that the mechanisms and interaction of factors affecting marine taphonomy are still largely not understood. These factors include intrinsic factors such as whether the individual was alive or dead at the time of submergence, the individual’s stature and clothing, as well as extrinsic factors such including ambient temperature, currents, water depth, salinity and oxygen levels. This paper provides a compilation of the current literature on factors influencing marine taphonomy, recovery and identification procedures for submerged remains, and discusses the implications for the retrieval and identification of maritime mass fatalities as part of the humanitarian response, specifically humanitarian forensic action, to the consequences of the current migration phenomenon.

An analytical model for preliminary assessment of dredging-induced sediment plume of far-field evolution for spatial non homogeneous and time varying resuspension sources

In recent years, increasing attention has been paid to assess the dispersal of resuspended sediments and related water quality problems due to dredging operations. This paper presents an analytical model aimed to predict the temporal evolution and spatial distribution in the far field of the suspended sediments concentration increase related to dredging activities or open water sediments disposal. In particular, whatever the dredging source strength and geometry can be considered to define the suspended sediments concentration leaving the immediate vicinity of the resuspension source. Indeed, a feature of the model is the removing of the hypotheses of continuous source and steady state, peculiar to the majority of available theoretical models. Hence, the proposed model is able to describe different dredging resuspension sources and to provide the temporal and spatial picture of the resulting plume. Of course, some hypotheses have to be assumed in order to make possible to achieve the analytical solution of the governing equation: the model is two dimensional in the horizontal plane; the ambient currents are assumed to be homogeneous in space and slowly time varying; the turbulent diffusion coefficients and flocculent settling velocity are homogeneous in space; the water depth is constant; the domain is infinite. Even with its limitations, the model is still able to provide a worst case preliminary assessment of sediments plume migration very useful to guide more detailed numerical analysis and to select the more appropriate simulation scenarios. The analytical model is detailed in order to be used for numerical model testing purposes. A series of practical applications is described through the paper (i) to catch the general features of the involved far field phenomena, (ii) to compare the model results to those of previous researches and (iii) to provide a series of benchmark cases useful for the testing of numerical models. The proposed model may be also used as a first rough prediction of the area affected by plume dispersion by considering different dredging scenarios (i.e. different equipment and operational techniques and forced by site-specific environmental conditions), and thus to provide a basis for more sophisticated modeling aimed to support dredging projects planning and management.

A new model of respiration in blastoid (Echinodermata) hydrospires based on computational fluid dynamic simulations of virtual 3D models

AbstractHydrospires are internal structures in blastoids that primarily served a respiratory function. Historically, hydrospires have been modeled as passive-flow respiratory structures with a vertical orientation. This project constructed virtual 3D models of blastoids from legacy acetate peel collections at the Naturalis Museum in the Netherlands. Computational fluid dynamic (CFD) simulations of the blastoid models reconstructed in living position indicated that hydrospires likely were oriented horizontally when the blastoid was in feeding mode in current velocities>0.5 cm/s to 10 cm/s. In this range of current velocities, passive water flow through the hydrospires did not produce conditions optimized for efficient gas exchange. However, optimal water flow through the hydrospires could be achieved if the excurrent velocity of water exiting the hydrospire through the spiracle was approximately one-half the velocity of ambient environmental currents. Maintaining such a ratio in the dynamic current systems in which blastoids lived suggests that cilia-driven active water flow through the hydrospires is a better model for optimizing respiratory effectiveness.

Potential Vorticity Dynamics of Coastal Outflows

AbstractA simple quasigeostrophic model is used to examine the outflow from a river, estuary, or strait into a coastal ocean. As shown by Johnson et al., these quasigeostrophic outflows are accurately described by analytical long-wave solutions. This paper first uses these solutions and contour dynamics simulations to discuss the behavior of coastal outflows. Second, it extends the model and the long-wave theory to consider the effects of ambient currents, tides, winds, or a variable source flux. Third, consideration of the momentum flux at the source is used to understand the turning of the current, showing that steady solutions conserve momentum, hence resolving the momentum imbalance paradox of Pichevin and Nof. Finally, a new numerical scheme to compute steady outflow boundaries is developed. The model focuses on the key dynamics driven by the source velocity and the generation of vorticity as the buoyant fluid adjusts. The simplicity of the model, and insight given by the long-wave solutions, enables a full understanding of the dynamics. The outflows display a range of behaviors, including indefinitely growing near-source bulges, steady boundary profiles with varying offshore width, bidirectional currents, and rarefying or eddy-like leading heads, all of which can be understood with the long-wave theory. Despite the simplicity of the model, the results show good agreement in comparison with observations, experiments, and numerical models.

Estuarine Plume: A Case Study by Satellite SAR Observations and <italic>In Situ</italic> Measurements

Estuary plumes on the western Louisiana continental shelf in the Gulf of Mexico, outside of constricted channels of the Sabine and Calcasieu Lakes were observed on two RADARSAT-1 synthetic aperture radar (SAR) images. The in situ data showed a change in salinity of ~2–3 PSUs across the front on the northwest edge of the plume, corresponding to locations of vertical salinity variation of 5.5–7.5 PSU. Velocity magnitude outside of the inlet reached more than 2.3 ms $^{{\mathrm {-1}}}$ seaward and a brackish lens in the upper 4.5 m of the water column, the spread of which contributed to the development of a plume. The volume of brackish water within the plume was estimated an order of magnitude larger than the volume output from the Calcasieu River within an entire ebb tidal period. This implies that the plume was formed during multiple tidal cycles and it sustained during the flood tides because only weak diurnal tides exist. Both satellite and in situ observations showed an interesting western intensification of the plume. An analysis to the momentum/vorticity equations suggests that the western intensification is due to the asymmetry of the vorticity-divergence relationship under the Coriolis force. This can also be enhanced by an ambient coastal current that strengthens the convergence at the front. Estuary plumes are shown as bright and dark features in two SAR images. Results from a radar model simulation and in situ water temperature data showed that the SAR observed plume features were dominated by the water temperature differences.

Explicit wave action conservation for water waves on vertically sheared flows

This paper addresses a major shortcoming of the current generation of wave models, namely their inability to describe wave propagation upon ambient currents with vertical shear. The wave action conservation equation (WAE) for linear waves propagating in horizontally inhomogeneous vertically-sheared currents is derived following Voronovich (1976). The resulting WAE specifies conservation of a certain depth-averaged quantity, the wave action, a product of the wave amplitude squared, eigenfunctions and functions of the eigenvalues of the boundary value problem for water waves upon a vertically sheared current. The formulation of the WAE is made explicit using known asymptotic solutions of the boundary value problem which exploit the smallness of the current magnitude compared to the wave phase velocity and/or its vertical shear and curvature; the adopted approximations are shown to be sufficient for most of the conceivable applications. In the limit of vanishing current shear, the new formulation reduces to that of Bretherton and Garrett (1968) without shear and the invariant is calculated with the current magnitude taken at the free surface. It is shown that in realistic oceanic conditions, the neglect of the vertical structure of the currents in wave modelling which is currently universal might lead to significant errors in wave amplitude. The new WAE which takes into account the vertical shear can be better coupled to modern circulation models which resolve the three-dimensional structure of the uppermost layer of the ocean.

Vertical dense jet in flowing current

The discharge of brackish water, as a dense jet in a natural water body, by the osmotic power plants, undergoes complex mixing processes and has significant environmental impacts. This paper focuses on the mixing processes that develop when a dense round jet outfall perpendicularly enters a shallow flowing current. Extensive experimental measurements of both the salinity and the velocity flow fields were conducted to investigate the hydrodynamic jet behavior within the ambient current. Experiments were carried out in a closed circuit flume at the Coastal Engineering Laboratory (LIC) of the Technical University of Bari (Italy). The salinity concentration and velocity fields were analyzed, providing a more thorough knowledge about the main features of the jet behavior within the ambient flow, such as the jet penetration, spreading, dilution, terminal rise height and its impact point with the flume lower boundary. In this study, special attention is given to understand and confirm the conjecture, not yet experimentally demonstrated, of the development and orientation of the jet vortex structures. Results show that the dense jet is almost characterized by two distinct phases: a rapid ascent phase and a gradually descent phase. The measured flow velocity fields definitely confirm the formation of the counter-rotating vortices pair, within the jet cross-section, during both the ascent and descent phases. Nevertheless, the experimental results show that the counter-rotating vortices pair of both phases (ascent and descent) are of opposite rotational direction.

A periodic freshwater patch detachment process from the Block Island Sound estuarine plume

AbstractPrevious observations suggest periodic freshwater patches separating from the Block Island Sound (BIS) estuarine plume. In this study, the dynamics of the separation process is investigated through a series of numerical experiments using the Regional Ocean Modeling System (ROMS). In addition, we explore the seasonal variability of the freshwater patches and their response to river discharge and ambient current. The model results indicate that episodic freshwater patches are triggered by small changes in tidal currents over the spring‐neap tidal cycle. The spring‐neap variation in tidal currents causes significant, monthly fluctuations in turbulent mixing and vertical stratification in BIS, modulating the freshwater discharge thereby generating episodic freshwater patches that move both downstream along the southern shore of Long Island and toward Rhode Island Sound (RIS). The realistically configured model shows that the freshwater patches experience strong seasonal variability. They are largest in spring when the river discharge peaks, and smallest in summer due to the weak river discharge and a robust upstream ambient current from RIS. According to the analysis of the freshwater transport out of BIS, we conclude that such detachment occurs at tidal mixing fronts.

Turbulence-particle interactions under surface gravity waves

Abstract The dispersion and transport of single inertial particles through an oscillatory turbulent aquatic environment are examined numerically by a Lagrangian particle tracking model using a series of idealised test cases. The turbulent mixing is incorporated into the Lagrangian model by the means of a stochastic scheme in which the inhomogeneous turbulent quantities are governed by a one-dimensional k- ε turbulence closure scheme. This vertical mixing model is further modified to include the effects of surface gravity waves including Coriolis-Stokes forcing, wave breaking, and Langmuir circulations. To simplify the complex interactions between the deterministic and the stochastic phases of flow, we assume a time-invariant turbulent flow field and exclude the hydrodynamic biases due to the effects of ambient mean current. The numerical results show that the inertial particles acquire perturbed oscillations traced out as time-varying sinking/rising orbits in the vicinity of the sea surface under linear and cnoidal waves and acquire a non-looping single arc superimposed with the high-frequency fluctuations beneath the nonlinear solitary waves. Furthermore, we briefly summarise some recipes through the course of this paper on the implementation of the stochastic particle tracking models to realistically describe the drift and suspension of inertial particles throughout the water column.

Characteristics of Microstructure Turbulence Measurements With a Moored Instrument in the South China Sea

AbstractA newly designed autonomous instrument was evaluated for long-term ocean turbulence measurements in the South China Sea from October 19, 2013, to February 10, 2014 at 21°09.90′N, 117°42.03′E. A shear velocity time series was collected continuously for 115 days at a single depth of 250 m using two orthogonal shear probes (PNS06). The mechanical design of wings is developed for an observation platform to achieve characteristics of stability and flexibility of current direction tracking. The depth and pitch in comparison with ambient current speed are analyzed, and the results indicate that the platform has satisfied stability requirements for turbulence measurement. The comparison between the instrument heading and current direction shows that the instrument flexibly responds to variations of ambient currents. Throughout the observation period, 62% of the data sets is of significant correlation with correlation coefficients above 0.5, and 4% is of strong correlation. The shear velocity spectra correspond well to the theoretical Nasmyth spectrum, and the magnitude of dissipation rates estimated from shear is approximately Ο(10−8) W kg−1. The moored instrument is functional as designed and provides a reliable observation platform for extended turbulent measurements.

Optimized Generation and Absorption for Three-Dimensional Numerical Wave and Current Facilities

AbstractThis article presents an optimized setup for a numerical wave/current facility that allows for fast and efficient generation and absorption of random directional waves, in the presence of uniform noncollinear ambient currents. In this study, the generation of waves was optimized, in terms of computational effort, by replacing the built-in sine and cosine functions with a Taylor series approximation. The mesh in the absorption zone was also optimized with the use of a mesh grading that resulted in larger cells toward the outflow boundary. Results show that the numerical cost is reduced by 70% for the wave generation and by 90% for wave absorption. Results compare very well for wave generation with and without currents with respect to regular wave/current and random wave theories. Wave reflection was generally less than 1% for linear waves, less than 4% for nonlinear waves, and less than 5% for random waves.

The energetic cost of filtration by demosponges and their behavioural response to ambient currents.

Sponges (Porifera) are abundant in most marine and freshwater ecosystems, and as suspension feeders they play a crucial role in filtering the water column. Their active pumping enables them to filter up to 900 times their body volume of water per hour, recycling nutrients and coupling a pelagic food supply with benthic communities. Despite the ecological importance of sponge filter feeding, little is known about how sponges control the water flow through their canal system or how much energy it costs to filter the water. Sponges have long been considered textbook examples of animals that use current-induced flow. We provide evidence that suggests that some species of demosponge do not use current-induced flow; rather, they respond behaviourally to increased ambient currents by reducing the volume of water filtered. Using a morphometric model of the canal system, we also show that filter feeding may be more energetically costly than previously thought. Measurements of volumetric flow rates and oxygen removal in five species of demosponge show that pumping rates are variable within and between species, with the more oxygen consumed the greater the volume filtered. Together, these data suggest that sponges have active control over the volume of water they process, which may be an adaptation to reduce the energetic cost of filtration in times of high stress.

Large sediment drifts on the upper continental rise west of the Antarctic Peninsula

Large sediment mounds on the continental rise around Antarctica have been interpreted as sediment drifts formed through interaction of downslope transport of mainly glacially derived sediments with alongslope bottom currents (Rebesco et al. 1996). Large sinuous channels between drifts represent turbidity current pathways that are probably most active during glacial periods. Drift sediments are mainly silt and clay, interpreted as fine-grained components of turbidity currents, entrained and redistributed by ambient bottom currents (Barker et al. 1999). ‘Drift 5’, west of the Antarctic Peninsula, is described here as an example of a large sediment drift (Fig. 1a). Fig. 1. Multibeam bathymetry, sea floor photographs and acoustic profiles of Drift 5, on the upper continental rise west of the Antarctic Peninsula. ( a ) Location of study area (red box; map from IBCSO v. 1.0). ( b ) Multibeam image compilation overlaid on interpolated IBCSO v 1.0 regional bathymetry, with shaded-relief illumination from NE. Grid-cell size 80 m. ( c ) Multibeam image compilation over the proximal part of the drift with shaded relief from NW. Acquisition systems various, but mostly Kongsberg EM120, 12 kHz. Grid-cell size 80 m. ( d ) Acoustic sub-bottom profile across the drift summit plateau. Acquisition system TOPAS PS018 parametric profiler. Secondary beam frequency 1.5–5 kHz. VE×25. ( e ) Photograph taken from ROV Isis at 2937 m water depth of cliff face on the proximal lower part of the NE flank of the drift exhibiting stratified sediments containing coarse ice-rafted debris. The two red dots …

Triad resonance between gravity and vorticity waves in vertical shear

Weakly nonlinear theory is used to explore the effect of vertical shear on surface gravity waves in three dimensions. An idealized piecewise-linear shear profile motivated by wind-driven profiles and ambient currents in the ocean is used. It is shown that shear may mediate weakly nonlinear resonant triad interactions between gravity and vorticity waves. The triad results in energy exchange between gravity waves of comparable wavelengths propagating in different directions. For realistic ocean shears, shear-mediated energy exchange may occur on timescales of minutes for shorter wavelengths, but slows as the wavelength increases. Hence this triad mechanism may contribute to the larger angular spreading (relative to wind direction) for shorter wind-waves observed in the oceans.

Fluid Motions Associated with Engineered Area Bubble Plumes

AbstractCurrent bubble oil booms (BOBs) create bubble curtains to protect fixed installations in calm waters such as harbors, but fail in more energetic environments. Field and lab studies reveal that area bubble plumes enable BOB applications in such environments. Area bubble plumes exhibit dynamics distinct from point- or line-source plumes, including thicker and more robust outwelling flows and relatively smaller turbulence scales. Area bubble plume studies broaden our understanding of natural systems, where currents and waves are ubiquitous, leading to a variety of marine applications. Laboratory studies in a large flume have investigated the fluid dynamics of shallow line and area bubble plumes. These studies focused on area-plume application to oil spill response, which, based on preliminary tests, featured improved outflows with a downstream barrier. A range of ambient currents, current–plume angles, and airflows were tested. Key to understanding current–plume interactions was the upstream rotor or...

Numerical Thermal Analysis of Fuse Mounted on Fuse Holder

This paper presents a three-dimensional thermal model of an electric fuse mounted on its fuse holder. The proposed model can be used in steady-state conditions but also during transient operating conditions. A comparison at the same ambient temperature and load current, between the thermal model of the fuse with and without its fuse holder, has been performed. In the case of load current and ambient temperature variation, a thermal analysis at different voltage drop values on the fuse contacts has been done. For different thermal load on fuse contacts and current variation, the temperature distribution along the fuse link, has been investigated. The experimental tests led to the validation of the proposed three-dimensional thermal model of the fuse mounted on its fuse holder.