Tidal Vortices Research Articles

Ports of Call: A Maritime Study of Korean Modernist Poetry

Abstract: This essay undertakes a survey of Korean modernist poetry in miniature by means of a singular motif—the seaport (hanggu)—in whose tidal vortex the politics and poetics of the Korean Peninsula were recalibrated at the outset of the modern age, specifically the years submerged in the depths of Japanese colonial rule (1910–45). While the essay's comparison between Kim Ch'ang-sul's (1903–50) and Chŏng Chi-yong's (1902–50) 1920s-era modernist aesthetics of dockside labor and harbor construction foregrounds an oscillation between proletarian collectivity and depoliticized solipsism, the pairing of Im Hwa (1908–53) and O Chang-hwan (1918–51) elucidates the divergent vectors of utopian deferral and fatalistic anomie constituting two incommensurable critical standpoints from which to confront the objective upsurge in seaport modernization and industrialization in the late 1930s.

Numerical Simulation of the Kuroshio Flowing over the Hirase Seamount in the Tokara Strait in Autumn: Tidal Vortex Shedding in a Baroclinic Jet

Abstract Idealized numerical simulations of the Kuroshio western boundary current flowing over the Hirase seamount were conducted to examine the mechanisms of phenomena observed by shipboard and mooring measurements. Along the Kuroshio, enhanced mixing [vertical diffusivity, Kρ = O(10−2) m2 s−1] was observed in a low-stratification layer between high-shear layers around low tide, and a V-shaped band of the negative vertical component of relative vorticity (ζz) was also observed. Those features were reproduced in simulations of the Kuroshio that included the D2 tide. In the simulation, a streak of negative ζz detached from the Hirase turned into vertically tilted 10-km-scale vortices. The buoyancy frequency squared (N2) budget at the mooring position showed that the low stratification was caused by vertical and horizontal advection and horizontal tilting. The Kρ tended to increase when the Ertel potential vorticity (PV) < 0, as expected given the inertial instability. However, the magnitude of Kρ also depended on the tidal phase near Hirase, and Kρ was increased in the high vertical shear zones at the periphery of vortices where a strain motion is large. These results indicate that not only inertial instability but also tidal and vertical shear effects are important for driving turbulent mixing. Significance Statement A basin-scale distribution of wind stress drives a strong surface-intensified current in the western part of each ocean basin, such as the Gulf Stream and the Kuroshio. This western boundary current is regarded as a place where the kinetic energy and vorticity generated by winds are dissipated, allowing the basin-scale circulation to keep a steady state, but its dissipation mechanisms are not well understood. To understand the mechanisms, we conducted idealized numerical simulations that isolate the interactions between a seamount and the current as well as tidal currents, and compared results with observations. Our findings provide insights into how the current transfers kinetic energy to smaller scales when it flows over a seamount.

The coastal macro-vortices dynamics in Hong Kong waters and its impact on water quality

Large-scale macro-vortices could be formed in the lee or around natural obstacles such as islands and headlands during the interaction between geographical features and coastal circulations induced by tides and winds. Tidal and wind-induced vortices play a fundamental role in transport processes of sediments and nutrients and the effects are then reflected on water quality of estuaries and coastal areas. In the present study, the coastal hydrodynamics of the Pearl River Estuary and Hong Kong waters was analyzed in detail using a high-resolution model based on the Finite-Volume Coastal Ocean Model (FVCOM), forced by tides and seasonal winds. The results showed that the local dynamics was strongly affected by the continuous shedding of macro-vortices. A classification of these coherent structures was suggested based on their typical evolution to distinguish between tidal vortices and wake vortices using four non-dimensional parameters. The spatial distribution, the seasonality and the probability density functions of their geometrical characteristics were presented and discussed in detail. The results suggested that tidal currents played a dominant role in most of the Pearl River Estuary and the Hong Kong waters, generating for the entire year a high and constant number of macro-vortices that occupied most of the north channels of Hong Kong. Finally, we linked the vortices distribution and their typical features with several important water quality parameters, suggesting possible explanations of the observed trends.

Evidence of Transient Energy and Enstrophy Cascades in Tidal Flows: A Scale to Scale Analysis

AbstractTidal currents are predominant in coastal areas, causing the generation of vortices at different scales. We reproduce the main process of vortex shedding generated in tidal systems with inlets and channels using a laboratory large‐scale model. A filter‐space technique is implemented to analyze nonlinear energy/enstrophy transfer rates and map out the energy pathways through the flow scales of the measured velocity fields. We provide sound evidence of the transitional character of the energy cascades during a tidal period. The periodic generation and destruction of tidal vortices plays a relevant role in the transition from an inverse to a direct energy cascade within a tidal period. The period‐averaged energy budget shows the coexistence of multiple cascades. Small scales follow a direct energy cascade, whereas a split‐energy cascade is found at intermediate and large scales, where part of the injected energy goes to small scales and part to a larger flow scale.

Study on characteristics of tidal dynamics and vortex in Zhoushan Archipelago sea area

Numerical modelling is used to study the characteristics and mechanisms of tidal dynamics and vortex in Zhoushan Archipelago sea area. Results show that amplitudes and current speeds of the semidiurnal tides decrease significantly when passing through the islands. Conversion of tidal energy, archipelago topography and formation of shallow-water tides are important reasons for the evolution of tidal waves. The ebb tide has a longer duration in the archipelago sea area, particularly in narrow channels. The M2-M4 constituents play a major role in tidal asymmetry. The flow paths of tidal currents change much, causing the high speed flooding/ebbing near the north/south bank. The tidal vortex is mainly distributed on both sides of and behind the islands, which is related to the shape of islands and the flow velocity. Referring to the current directions, positive/negative vorticity generally appears on the right/left side of the islands.

Experimental Study on the Propagation and Stability of Tidal Vortices

AbstractLaboratory studies of tidal vortex formation and propagation through an idealized inlet configuration are presented for various water depths over flat bathymetry. Using surface particle ima...

Simulation of laboratory experiments for vortex dynamics at shallow tidal inlets using the fine resolution environmental hydrodynamics (Frehd) model

In this paper, we present the results of numerical simulations matched to laboratory experiments of tidal, starting-jet vortices forming at idealized, barotropic inlets. The laboratory experiments are for sinusoidally varying inflow/outflow along a wide, flat basin. Inlet configurations include simple inlets with negligible channel length as well as a longer inlet channels through a long barrier island and jettied inlet. Laboratory observations are made using particle image velocimetry, and we analyze the laboratory and numerical model velocity fields to compare starting-jet trajectory, diameter, and total circulation. The numerical simulations use the Fine Resolution Environmental Hydrodynamic model, solving the three-dimensional, hydrostatic, barotropic equations of motion. Our goal is to identify the minimum model attributes necessary to match the vortex properties. This is accomplished using a third-order upwind scheme for advection, a constant bottom friction drag coefficient, and a one-equation turbulence model for transport of turbulent kinetic energy. The optimal value of the bottom drag coefficient matches the time-average value at the basin inflow/outflow boundaries, and the optimal dissipation coefficient, $$C_{me}$$, was an order of magnitude smaller than literature values for a steady open-channel flow. We show that the drag coefficient mainly affects the advection speeds of the outer coastal waters, which control the penetration of the tidal vortices away from the inlet, and the lower turbulence dissipation rate results from the low turbulent dissipation in the large, shallow starting-jet vortices.

Using surface drifter observations to measure tidal vortices and relative diffusion at Aransas Pass, Texas

Tidal vortices play an important role in the flushing of coastal regions. At the mouth of a tidal inlet, the input of circulation by the ebb tide may force the formation of a starting-jet dipole vortex. The continuous ebb jet current also creates a periodic sequence of secondary vortices shed from the inlet mouth. In each case, these tidal vortices have a shallow aspect ratio, with a lateral extent much greater than the water depth. These shallow vortices affect the transport of passive tracers, such as nutrients and sediment from the estuary to the ocean and vice versa. Field observation of tidal vortices primarily relies on ensemble averaging over several vortex events that are repeatable in space and can be sampled by a fixed Eulerian measurement grid. This paper presents an adaptive approach for locating and measuring within tidal vortices that propagate offshore near inlets and advect along variable trajectories set by the wind-driven currents. A field experiment was conducted at Aransas Pass, Texas to measure these large-scale vortices. Locations of the vortices produced during ebb tide were determined using near real-time updates from surface drifters deployed near or within the inlet during ebb tide, and the paths of towed acoustic Doppler current profiler (ADCP) transects were selected by analysis of the drifter observations. This method allowed ADCP transects to be collected within ebb generated tidal vortices, and the paths of the drifters indicated the presence of both the starting-jet dipole and the secondary vortices of the unstable ebb tidal jet. Drifter trajectories were also used to estimate the size of each observed vortex as well as the statistics of relative diffusion offshore of Aransas Pass. The field data confirmed the starting-jet spin-up time (time until the vortex dipole begins to propagate offshore) measured in the laboratory by Bryant et al. [6] and that the Strouhal condition of \(St=0.2\) predicts the shedding of secondary vortices from the inlet mouth. The size of the rotational core of the vortex is also shown to be approximated physically by the inlet width or by \(0.02UT\), where U is the maximum velocity through the inlet channel and T is the tidal period, and confirms results found in laboratory experiments by Nicolau del Roure et al. [23]. Additionally, the scale of diffusion was approximately 1–15 km and the apparent diffusivity was between 2–130 \(m^2/s\) following Richardsons law.

Mapping Tidal Current Structures in Zhitouyang Bay, China, Using Coastal Acoustic Tomography

The first Chinese coastal acoustic tomography (CAT) experiment for mapping the tidal currents in Zhitouyang Bay near Zhoushan Island was successfully performed with seven acoustic stations from July 12 to 13, 2009. Using CAT, the horizontal distributions of the tidal currents in the tomography domain were calculated by the inverse analysis, in which the travel time differences for sound traveling reciprocally between the station pairs are used as data. The specified tidal current structures, such as the strong east-west oscillation of the tidal current, the branched current, and the tidal vortices, were reconstructed as snapshots at the successive tidal phases. The relative vorticity calculated from the inverted current fields served to specify the current structures, such as tidal vortices. The inversion-estimated uncertainty of (0.02-0.08) m s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> narrowed the root-mean-square difference (RMSD) of (0.00-0.11) m s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> between the 3-min interval original data and the hourly mean data for all the sampled data, which may be regarded as a measure of error. Throughout the tidal phases, the divergence from the inverted current showed a positive (negative) distribution in the shallow (deep) region as an overall view. However, the divergence for the entire tomography domain was nearly equal to zero, corresponding to no net transport. This result implies that the observational errors are quite small for the present experiment. This experiment reaffirms that coastal acoustic tomography is an accurate and efficient observational method for continuously mapping tidal current structures in coastal regions that are characterized by heavy shipping traffic and active fishing.

Cover Picture: Chiral Centers in the Side Chains of α‐Amino Acids Control the Helical Screw Sense of Peptides (Angew. Chem. Int. Ed. 40/2004)

A tidal vortex (helix) is formed, both right‐ and left‐handed, where a violent tidal current meets a peaceful sea (the picture from Naruto City Tourist Association shows an example at Naruto channel, Japan). In contrast, α‐helices of proteins almost always show a right‐handed helical screw sense because of the stereogenic α‐carbon center of α‐amino acids. M. Tanaka et al. now demonstrate on page 5360 ff. how the left‐handedness of a α‐helical peptide is controlled by the side‐chain stereogenic centers.

Development of a Semi-Spectral Coastal Ocean Model and Its Application to the Neko Seto Sea in the Seto Inland Sea

For studying coastal ocean processes, a new ocean model has been developed based on the semi-spectral method using three-dimensional primitive equations. The basic idea is that the vertical mode is used as a basis. As a first step in the development of the model, the Fourier cosine series is used as a basis by assuming that the reference density is homogeneous. To test the performance of the model, it has been applied to the regional model of the Neko Seto Sea in the Seto Inland Sea. The model result has been compared intensively with the observational result of acoustic Doppler current profiler (ADCP) on the eastern side of the Meneko Seto Strait. The model reproduced the realistic horizontal and vertical structure of the tidal current and vortices. The tidal current in Nigata Bay has been compared with a previous observational study. The generation process of the clockwise vortex at eastward flow is found to be different from the previous observational description. By using the vorticity analysis, it was found that the vortex stretching associated with the sloping bottom topography is essential to the generation of the counterclockwise vortex during eastward flow and the clockwise vortex during westward flow in Nigata Bay. On the other hand, the advection of vorticity from the edge-like coastal geometry is essential for the generation of the vortices on the eastern side of the Strait. However, the positive vorticity supply from tilting enhances the growth of the counterclockwise vortex, and is indispensable for reproducing its realistic position.

Coastal acoustic tomography system and its field application

The coastal acoustic tomography system (CATS), composed of five moored acoustic stations, has been constructed to measure current fields. The system is developed with special considerations in mind, including the use of Global Positioning System clock signals in the synchronization of the system clock timing among the multiple acoustic stations, and the use of the differently coded Gold sequences to identify the acoustic signals corresponding to individual stations from a received signal. The CATS was successfully applied to map the structure of strongly nonlinear tidal currents in the coastal sea. In spite of the limited spatial resolution caused by inadequate sound transmission data, the two-dimensional tidal vortices features of growth, translation, and decay processes are reconstructed through an inverse analysis of the acoustic travel time obtained among the station pairs. It is evident that the CATS is a powerful tool for measuring variable current fields generated in the coastal seas.

Computer Simulation of Coastal Acoustic Tomography by a Two-Dimensional Vortex Model

Computer simulation of coastal acoustic tomography with four, five, seven and nine stations was applied to two-dimensional vortex fields of horizontal domain 5 km × 5 km. Travel time data obtained in reciprocal directions between all pairs of acoustic stations were analyzed to reconstruct the vortex fields by the stochastic inverse method, reduced to the damped least squares method. The weighting factor appearing in the inverse analysis was determined by applying the L-curve method, in which a point making both the size of estimated error ‖y - Ex‖ and solution ‖x‖ as small as possible in a balance is specified as an optimum. The performance of this method was examined using two-dimensional vortex models which have different wavenumber spectra and adding the random error of different levels to the travel time difference data y. This study suggests that in the selection of the optimal weighting factor the horizontal section of the simulated tidal vortex fields can be well reconstructed by the coastal acoustic tomography system composed of five to seven acoustic stations located in the periphery of the vortex fields.

Assimilation of coastal acoustic tomography data into a barotropic ocean model

A data assimilation technique was applied to analyze the first coastal acoustic tomography data, obtained in the Neko‐Seto Channel of the Seto Inland Sea, Japan on March 2–3, 1999. The ensemble Kaiman filter technique accompanied by the forecast of error covariances by means of the Monte Carlo method is found to be best fitted to the present problem because currents in the tomography region are dominated by the strongly nonlinear tidal vortices. Furthermore, the analyzed results are in better agreement with the shipboard ADCP results than those obtained by the inverse analysis of the travel time difference data for the pairs of acoustic stations.

A Method to Enhance the Circulation of a Enclosed Bay by Strengthening Tidal Vortices Occured at a Bay Mouth

A Method to Enhance the Circulation of a Enclosed Bay by Strengthening Tidal Vortices Occured at a Bay Mouth

The role of tidal vortices in material transport around straits

The strong tidal current (tidal jet) in straits generates tidal vortices with a scale of several kilometers. The role of the vortices in material transport was investigated in the Neko Seto Sea, located in the western part of the Seto Inland Sea of Japan. A clockwise vortex with a diameter of about 0.8 km was observed in Nigata Bay (lying between two straits, the Neko Seto Strait and the Meneko Seto Strait). It was concluded that the clockwise vortex was the tidal vortex which was generated by the tidal jet in the Meneko Seto Strait. The vortex moved into the bay with the tide, but tended to stay on the sand bank in the bay. It was confirmed by current measurement with an ADCP and turbidity measurement that the secondary convergent flow was generated in the bottom layer of the vortex. This secondary flow seemed to contribute to the formation of the sand bank. It was suggested that tidal vortices may play an important role in the sediment transport and formation of topography in and around straits.

Formation of sand banks due to tidal vortices around straits

Sand banks around straits are used as a commercial fishing ground. In order to clarify the mechanism of sand bank formation, the Lagrangian method was used to measure currents and turbidity around the banks in the Neko Seto Sea in the Seto Inland Sea of Japan. A neutrally buoyant float released in the Neko Seto Strait at the maximum tidal flow stage was engulfed in a pair of tidal vortices and moved around one of the sand banks. The vertical distribution of turbidity, which was measured by the vessel moving with the neutral float, showed an extremely high turbidity in the bottom layer of this bank area. According to the analysis of these observational data, the process of sand bank formation around straits is as follows. The tidal vortex transports water mass with suspended materials (including sand) which are whirled up at the bottom by the tidal jet. In the decaying stage of the vortex, the materials in the bottom layer are gathered in the central part of the vortex by the secondary convergent flow in the vortex. Among these materials, a large-size sand particle with a high critical erosion velocity accumulates at the bottom and forms banks. The distribution of bottom sediment and the thickness of alluvium support this result.

Scale dependence of divergence and vorticity of near-surface flows in the sea

To area-average the horizontal divergence and the vertical component of vorticity, three methods are proposed and examined. The polygon method is based on deformations of a polygon made by connecting drifters. They are deployed at widely different scales of 1 cm− 1,000 km and tracked using various procedures. The loop method is adopted when a drifter completes at least two loops of trajectory in a tidal vortex, a ring or a gyre. Even if data for a drifter completing only one loop is available, the vorticity can be calculated. The crossing method is applied to the GEK data on the circumference of a Kuroshio ring. The data which will be used to calculate them in Part 2 are summarized in tables. Offset dispositions of positive and negative divergences or vorticities on a horizontal plane and in a water column are shown. Probably, the vertical offset of vorticities does not occur in general. The area-turnover of a polygon of drifters are discussed. Sampling time-intervals, appropriate to the scale of the area, for the polygon and loop methods are examined. A first impression of Rhines' (1979) sketch has produced a misunderstanding that the polygon method would be useless because a limited number of drifters cannot follow such a complicated deformation of the material line over a long period. It is shown that adopting a short time-scale appropriate to the length scale furnishes a practical solution to the problem.

TIDICS—Control of tidal residual circulation and tidal exchange in a channel-basin system

A self-excited oscillation of tidal flow occurs in a tidal channel-basin system with a narrow and high speed inflowing tidal jet. The mechanism of this oscillation was examined through hydraulic experiments. Results show that the oscillation is caused by alternation in the strength of asymmetric residual circulation caused by a biased supply of vorticity by asymmetric tidal vortex pairs, and that the mechanism of alternation also depends on negative feed-back between the residual circulation and the vorticity supply.

A hydraulic experiment on tidal exchange

Tidal exchange through a narrow entrance channel was studied experimentally with the use of a simplified hydraulic model. The inflowing water mass, visualized with dye solution, exhibits the shape of a starting plume with a starting vortex pair at its head. Because of their periodical formation by the tide, these are called the “tidal plume” and “tidal vortex pair”. The axis of the tidal plume deflects and undulates with a period 2 to 9 times that of the tide. Together with this undulation, the vortex pair becomes asymmetric. A circulating flow is formed in the bay which affects the shape of the inflowing and outflowing water masses. A part of the inflowing water mass flows out during the subsequent ebb, and this outflowing portion can be divided into two parts. One is the water remaining in the entrance channel at high water which flows out during the first half of the subsequent ebb and the other is the water flowing round the bay in the circulating flow during flood that flows out during the latter half of the subsequent ebb. Both contribute to the exchange ratio, but we can estimate an upper limit for the exchange ratio by neglecting the latter outflow. This neglected portion is considered in the concept of the age composition of outflowing water. The age composition of the bay water shows the existence of intermittent effluence superposed on a trend in the age composition that is similar to that of the well-mixed case. From the analysis of a model consisting of a number of mixing tanks connected in series with a recycle flow, it is concluded that this intermittent effluence occurs in the case of weak mixing due to the effect of circulating flow in the bay but is negligible in the case of strong mixing.