Residual Current Flow Research Articles

Hydrodynamic Processes Controlling Sand Bank Mobility and Long-Term Base Stability: A Case Study of Arklow Bank

Offshore sand banks are an important resource for coastal protection, marine aggregates, and benthic habitats and are the site of many offshore wind farms. Consequently, a comprehensive understanding of the baseline processes controlling sand bank morphodynamics is imperative. This knowledge will aid the development of a long-term robust marine spatial plan and help address the environmental instability arising from anthropogenic activities. This study uses a validated, dynamically coupled, two-dimensional hydrodynamic and sediment transport model to investigate the hydrodynamic processes controlling the highly mobile upper layer of Arklow Bank, while maintaining overall long-term bank base stability. The results reveal a flood and ebb tidal current dominance on the west and east side of the bank, respectively, ultimately generating a large anticlockwise residual current eddy encompassing the entire bank. This residual current flow distributes sediment along the full length of the sand bank. The positioning of multiple off-bank anticlockwise residual current eddies on the edge of this cell is shown to influence east–west fluctuations of the upper slopes of the sand bank and act as a control on long-term stability. These off-bank eddies facilitate this type of movement when the outer flows of adjacent eddies, located on both sides of the bank, flow in a general uniform direction. Whereas they inhibit this east–west movement when the outer flows of adjacent eddies, on either side of the bank, flow in converging directions towards the bank itself. These residual eddies also facilitate sediment transport in and out of the local sediment transport system. Within Arklow Bank’s morphological cell, eight morphodynamically and hydrodynamically unique bank sections or ‘sub-cells’ are identified, whereby a complex morphodynamic–hydrodynamic feedback loop is present. The local east–west fluctuation of the upper slopes of the bank is driven by migratory on-bank stationary and transient clockwise residual eddies and the development of ‘narrow’ residual current cross-flow zones. Together, these processes drive upper slope mobility but maintain long-term bank base stability. This novel understanding of sand bank morphodynamics is applicable to bedforms in tidally dominated continental shelf seas outside the Irish Sea.

Impacts of tidal flat reclamation on suspended sediment dynamics in the tidal-dominated Wenzhou Coast, China

Reclamation of tidal flats is one of the engineering measures of expanding land area and developing the economy in coastal areas; however, this process disturbs the natural processes of the tidal flat system. Taking the flood-tidal dominant Oufei tidal flat at the Wenzhou coast as a study area, the influences of tidal flat reclamation on tidal and suspended sediment dynamics were comprehensively investigated via numerical modeling. Before the reclamation, the Eulerian residual current flows alongshore and the residual sediment transport on-shore in the tidal flat area. The reclamation reduces the tidal flat area, shifting the M4 co-phase line toward the coast and squeezing the flood-dominated area. Consequently, the tidal current is weakened and the suspended sediment concentration (SSC) is reduced, but the residual sediment transport change insignificantly. The residual sediment transport in the Feiyunjiang Estuary is controlled by the tidal pumping effect. The upper estuary shows a net sediment transport landward, while a seaward transport is observed in the lower estuary, which further splits into two circulation outside the estuary. The south Oufei dike construction interferes the northern sediment circulation, resulting in the alteration of local SSC and enhancing landward sediment transport inside the estuary. The methodology and understandings arising from this study could be a good reference for the analysis of suspended sediment transport under tidal flat reclamation effects at other sites.

Analysis of Residual Current Flows in Inverter Based Energy Systems Using Machine Learning Approaches

Faults and unintended conditions in grid-connected photovoltaic systems often cause a change of the residual current. This article describes a novel machine learning based approach to detecting anomalies in the residual current of a photovoltaic system. It can be used to detect faults or critical states at an early stage and extends conventional threshold-based detection methods. For this study, a power-hardware-in-the-loop approach was carried out, in which typical faults have been injected under ideal and realistic operating conditions. The investigation shows that faults in a photovoltaic converter system cause a unique behaviour of the residual current and fault patterns can be detected and identified by using pattern recognition and variational autoencoder machine learning algorithms. In this context, it was found that the residual current is not only affected by malfunctions of the system, but also by volatile external influences. One of the main challenges here is to separate the regular residual currents caused by the interferences from those caused by faults. Compared to conventional methods, which respond to absolute changes in residual current, the two machine learning models detect faults that do not affect the absolute value of the residual current.

Bedforms controlled by residual current vortices in a subtropical estuarine tidal channel

The objective of the present work was to analyze the bottom morphology, the genesis of the bedforms, and the patterns of current and sediment transport of the internal portion of the south mouth in Paranaguá Bay, Paraná - Brazil. This region was used as a laboratory field to study the geomorphology of a dune field on a tide channel, with features of flood delta in the inner portion and ebb delta in the direction of the continental shelf. A sediment hydrodynamic model was used to calculate the direction and magnitude of the sediment transport and the residual current flows. A map of apparent roughness was used in the numerical model, obtained through the soundings of a sidescan sonar (Starfish Sonar) and estimated from the classification of the dune field, so it is possible to obtain simulations with higher correspondence with reality. It was observed two vortices of residual circulations near the shores of Pontal do Sul and Ilha do Mel, with directions towards the continental shelf. The residual circulation vortex is responsible for the triangular shape of the large dunes near the Pontal do Sul margin. In the flood, the sediment material is conducted inward forming small ripples and accumulating sediment material on large dunes. During the ebb, these ripples are supposedly destroyed and attached to the large dunes or to the water column. On the other hand, the bedforms of Saco do Limoeiro are formed due to the effects of the tide when entering the mouth and the confinement of the water mass in the region until the next moment of padding.

Laboratory experiment on the 3D tide-induced Lagrangian residual current using the PIV technique

The 3D structure of the tide-induced Lagrangian residual current was studied using the particle image velocimetry (PIV) technique in a long shallow narrow tank in the laboratory. At the mouth of the tank, a wave generator was used to make periodic wave which represents the tide movement, and at the head of the tank, a laterally sloping topography with the length of one fifth of the water tank was installed, above which the tide-induced Lagrangian residual current was studied. Under the weakly nonlinear condition in the present experiment setup, the results show that the Lagrangian residual velocity (LRV) field has a three-layer structure. The residual current flows inwards (towards the head) in the bottom layer and flows outwards in the middle layer, while in the surface layer, it flows inwards along the shallow side of the sloping topography and outwards along the deep side. The depth-averaged and breadth-averaged LRV are also analyzed based on the 3D LRV observations. Our results are in good agreement with the previous experiment studies, the analytical solutions with similar conditions and the observational results in real bays. Moreover, the volume flux comparison between the Lagrangian and Eulerian residual currents shows that the Eulerian residual velocity violates the mass conservation law while the LRV truly represents the inter-tidal water transport. This work enriches the laboratory studies of the LRV and offers valuable references for the LRV studies in real bays.

Study of 2D Residual Current Density Profiles of BSCCO and YBCO HTS Tapes by Means of 3D Hall Probe System

The residual current distribution in the high-temperature superconducting (HTS) tapes effects the operation of power cables, superconducting magnets, and other devices built using these tapes. This effect is crucial for the restarting procedure of the devices after either emergency or scheduled shutdown or after the short circuit. Furthermore, the current distribution along and across the HTS tape provides us with information on the quality of the tape performance. In this paper, we examine a two-dimensional (2D) residual current density distribution in different BSCCO and YBCO tapes by means of a three-dimensional Hall probe system after the emergency shutdown of the 100-A direct current. After the experimental measurement of the self-magnetic field near the tape inducted by residual current, the inverse problem solver, which aims at calculating the current density distribution throughout the tape, is applied. The results of the calculation show the strong asymmetry and vortex-like structure of the 2D residual current flow in the BSCCO tape, whereas the current flow in YBCO tapes has a tendency toward symmetry about the center of the HTS tape. The significant difference in shape and amplitude of the residual current density profiles is observed as well.

In situ study of the sintering of a lead phosphovanadate in an Environmental Scanning Electron Microscope

The in situ sintering of a powder of Pb 3(VO 4) 1.6(PO 4) 0.4 composition was performed in an Environmental Scanning Electron Microscope. The electric current induced by the electron beam was found to reduce the effective temperature of sintering as well as to accelerate the kinetics of shrinkage of a cluster composed of sub-micrometric grains of material. The presence of the residual current flow in the cluster during observation for in situ experiments helps to reduce the apparent sintering temperatures from 50 to 150 °C compared to conventional heating conditions without current.

Bulk particulate organic carbon in the East China Sea: Tidal influence and bottom transport

Five oceanographic surveys were carried out in the East China Sea (ECS) and Yellow Sea from 1999 to 2003. In all, seven different sections were surveyed, but one section (the PN section) was observed on every cruise. Two time-series stations were also surveyed, one located at the Changjiang River mouth, the other over the continental shelf in the PN section. We identified biogeochemical characteristics for waters close to the Changjiang Estuary and in the Kuroshio waters (KW), respectively. Resuspension is a strong feature near bottom over the ECS continental shelf, with suspended matter values 13 times higher than that for the surface. A model of particulate organic carbon (POC) dynamics based on a rectangle equation reveals that POC concentration close to the Changjiang Estuary varies with a semidiurnal period of ∼13 h, coinciding with the tidal period. The upper limit for POC residence times in the seasons we covered over the shelf are estimated to be on the order of weeks and generally increase seaward from near the Changjiang Estuary to the KW. Short POC residence times suggest that POC in the ECS is rapidly exported from euphotic waters. A nepheloid layer, observed as elevated suspended matter in near the bottom of the water column, is important in particle transfer over the shelf, especially in winter when the residual current flows mainly eastward. Cross-shelf transport of POC via the nepheloid layer is estimated to be 0.22 × 10 12 g yr −1. Comparison with other work indicates that POC transport is ∼2% of the Changjiang POC input.

Determination of denitrification in the Chesapeake Bay from measurements of N2 accumulation in bottom water

This study demonstrates the feasibility of using direct N2 measurements in an estuary for determination of denitrification. High precision measurements of dinitrogen: argon ratios (N2∶Ar) were made by membrane inlet mass spectrometry on water samples taken along the length of the Chesapeake Bay in July and October 2004. The N2∶Ar ratio in low salinity surface water was elevated relative to air saturation by 0.3–0.5% with no systematic change along the length of the Bay. N2∶Ar in high salinity bottom water exhibited a linear increase in the landward direction along a 144-km longitudinal section. In this section of the Bay covering 20% of the main stem, the bottom water salinity was statistically uniform and the increase in N2∶Ar was in the direction of net residual current flow. The system was analyzed as a capped river with the assumption that N2 entered the water from the underlying sediment where denitrification is known to take place. The rate of denitrification needed to support the measured increase in N2 was calculated using an average residual current velocity and water column depth. The increase in N2 with distance (0.046μmol N l−1 km−1) equated to an average denitrification flux of 73 μmol N m−2 h−1. N2 fluxes determined on sediment cores taken from the source and terminus regions of the delineated water mass were 45±23 and 83±39 μmol N m−2 hr−1, respectively, which were not statistically different from the whole system estimate. The measured change in oxygen concentration within the bottom water was used to estimate nitrogen remineralization and the efficiency of denitrification. Denitrification efficiency (nitrogen denitrified/nitrogen remineralized) was estimated to be in the range of 22–28% for the bottom water sediment system and 30–37% considering the sediment zone alone.

Tidal-induced Lagrangian and Eulerian mean circulation in the Bohai Sea

Long-term transport processes in coastal seas with time scales from weeks to seasons time scale are controlled by residual circulation. In the Bohai Sea, an ultrashallow shelf sea of China, tidal residual is almost the dominant factor to the circulation due to slight stratification and weak wind in summer. In order to establish an adequate hydrodynamic base to the ecosystem dynamics of the Bohai Sea, the differences of tide-induced Lagrangian and Eulerian mean circulation are discussed and calculated in this contribution. The Stokes drift is analyzed theoretically. According to Longuet-Higgins [Deep-Sea Res. 16 (1969) 431], the Lagrangian flow is the sum of the Eulerian flow and the Stokes drift that is induced by the mean kinetic energy and coastal nonlinear interaction. Stokes drift is large in the coastal sea and in areas where the vorticity and/or divergence are large. Vertical mass transports by Stokes drift are also the result of nonlinear interaction of current, water level and topography. Hamburg Shelf Ocean Model (HAMSOM) is applied in the Bohai Sea to simulate the tides and tidal currents. The tide-induced Lagrangian mean circulation and the Eulerian one are calculated at the same time. In the area where the Stokes drift is in the same direction as the Eulerian residual, the Lagrangian one is stronger than the Eulerian one. Where they are pointing in opposite directions, the Lagrangian one is small, like in the southwest of the Bohai Bay, Liaodong Bay and Bohai Strait. The Lagrangian residual current flows into the Bohai Bay along its southern bank causing deposition of Huanghe River sediments. This is in agreement with observations.

Residual current flows off North East England

The elucidation of the general residual flow of water along the North East Coast has been attempted using data from current meter records taken between 1971 and 1975. Meteorological data has been used to assess the effects of wind and pressure gradients.

Some mechanisms for the generation of residual current flow off the Scottish East Coast

A number of current meter records obtained from positions within 40 km off the Scottish East Coast have been examined in order to determine the nature of the residual current flow in this area. It is shown that currents are generally a consequence of the local wind and externally derived forces which may be storm surge or the effect of wind outwith the region. Depending on which driving forces are dominant on a particular occasion, residual current at inshore and offshore positions may be similar or completely opposed to one another. The effects of stratification on tidal and residual flow is demonstrated.