Current Velocity Distribution Research Articles

The system of currents above a complex bottom relief and its influence on the transfer of admixtures

This article describes the results of natural and theoretical investigations of the fields of the current velocity and water composition parameters in a system of stratified currents above a complex bottom relief. Current velocity distributions were obtained and verified, taking into account all the flows included in the system. The dependencies of the distributions of the concentrations of diluted oxygen and chlorophyll-a upon the parameters of jet and near-bottom density currents were revealed.

Two-dimensional horizontal modeling of fine-sediment transport at the South Channel–North Passage of the partially mixed Changjiang River estuary, China

Tidal flow and fine-sediment transport at the South Channel–North Passage of the partially-mixed Changjiang River estuary were studied using a two-dimensional horizontal (2DH) numerical model. This 2DH model was achieved by depth-integrating the momentum and convection–diffusion equations. The Alternating Direction Implicit scheme was used to solve the governing equations. The iterative method was adopted for the calculation of convection and diffusion terms of momentum equation. Comparisons between calculated and measured results (tidal elevations and depth-averaged velocities) have shown reasonable agreement. Horizontal distributions of tidal current velocity and suspended sediment concentration were qualitatively consistent with observations. Those modeled results were analyzed to elucidate the mechanisms for the formation of the turbidity maximum and intratidal variations in fine-sediment transport processes.

Numerical analysis of effect of asymmetric stern of ship on its screw propeller efficiency

Numerical analysis of effect of asymmetric stern of ship on its screw propeller efficiency During designing the ship its designer tends to achieve as-high-as possible efficiency of ship's propulsion system. The greatest impact on the efficiency is introduced by ship's screw propeller whose efficiency depends not only on its geometry but also distribution of wake current velocity. To change wake current distribution and improve propeller efficiency an asymmetric form is usually applied to stern part of ship hull. This paper presents results of numerical analysis of wake current velocity distribution, performed by using a CFD method for a B 573 ship of symmetric stern and the same ship of an asymmetric stern. Next, the mean values of screw propeller efficiency in non-homogenous water velocity field were calculated for both the hull versions of B 573 ship.

Modelling Potential, Current and Gas Velocity Distributions in Micro-Tubular Hollow Fiber SOFC Stacks

Micro-tubular solid oxide fuel cells of ca.1 mm diameter offer high volumetric power densities and tolerance to rapid thermal cycling, making them attractive as micro-power sources. Mathematical models accounting for charge, mass, energy and momentum balances, developed to predict single hollow fiber (HF)-SOFC behavior, have been extended to micro-tubular fuel cell stacks, mainly to study the effects of stack configuration on potential, current and velocity distributions. A stack design based on embedding of cells in a cathode matrix was predicted to perform well, producing 0.3 W for a 2Xseries by 2Xparallel stack of 5 mm long cells (1.79 W cm-3), performance being very sensitive to material conductivity and cross-sectional area of the series interconnect. A novel stack design with equal active surface area proposed herein, was predicted to perform less well, producing ca. 0.2 W for a 2Xparallel stack of 10mm long cells, primarily due to axial electrode potential losses.

Modelling Potential, Current and Gas Velocity Distributions in Hollow Fiber SOFC Stacks

not Available.

Wave Forces on a Large Structure in the Presence of a Current

The wave forces acting on a large structure in the presence of a current have been investigated numerically. It is noted that the velocity potential in a wave-current coexisting field can be separated into an unsteady wave potential and a steady current potential. The unsteady wave potential with effects of both a uniform current and a large structure was calculated by using boundary element method. The steady current potential can be expressed as the sum of a uniform current and a steady disturbance due to the existence of the structure. This means that current velocity will be changed in the vicinity of the structure. The variation of current velocity around the structure was then calculated by using a surface vorticity method. Substituting both unsteady wave potential and current velocity distribution into the first-order dynamic pressure equation and integrating the pressure over the body surface, the first-order wave forces on a large structure in the presence of a current then can be obtained. Comparison of the present results with those of Matsui et al. (1991) are made. The agreement is satisfactory. The numerical model proposed here has its adaptability to the problem of waves, current and a large structure interactions.

Characteristics and hydrodynamics of boulders transported by storm waves at Kudaka Island, Japan

This study investigates the size, position and the long axis orientation of 210 boulders at Kudaka Island, Japan. These boulders were deposited from the reef crest to the slope of the back reef moat, distributed within 275 m from the reef edge. Most boulders were rectangular to ellipsoidal, without sharp broken edges. They are reef rock fragments estimated as < 63 m 3 (< 127 t). The second largest boulder (54 t) was not observed in aerial photographs taken in 1977 and 1993, although it appears in photographs taken in 2005 and 2007. Considering that no large tsunami event occurred during 1993–2005, the second largest boulder is expected to have been emplaced by typhoon-generated storm waves. Moreover, the positions of many boulders were found to have shifted after 1977. These boulders were highly likely to have been repositioned by the storm waves. Results showed that boulders' motion follows an exponential fining trend shoreward. This trend fits well with the distribution of the height of the storm wave after breaking on the reef flat. The largest storm waves after 1977 (typhoon 0704 in 2007) were probably responsible for the current boulder distribution. Using the relation between the distributions of boulders and the significant wave height of typhoon 0704, the approximate transport distance of boulders by an arbitrary storm wave at the island can be estimated. The storm wave boulders' distribution is also useful to estimate the storm wave properties: we estimated the maximum current velocity distribution of waves generated by typhoon 0704 on the reef flat as up to 6.5 m/s using the boulder distribution.

Modelling ultrafine structure in dark matter haloes

Various laboratory-based experiments are underway attempting to detect dark matter directly. The event rates and detailed signals expected in these experiments depend on the dark matter phase space distribution on sub-milliparsec scales. These scales are many orders of magnitude smaller than those that can be resolved by conventional N-body simulations, so one cannot hope to use such tools to investigate the effect of mergers in the history of the Milky Way on the detailed phase-space structure probed by the current experiments. In this paper we present an alternative approach to investigating the results of such mergers, by studying a simplified model for a merger of a sub-halo with a larger parent halo. With an appropriate choice of parent halo potential, the evolution of material from the sub-halo can be expressed analytically in action-angle variables, so it is possible to obtain its entire orbit history very rapidly without numerical integration. Furthermore by evolving backwards in time, we can obtain arbitrarily-high spatial resolution for the current velocity distribution at a fixed point. Although this model cannot provide a detailed quantitative comparison with the Milky Way, its properties are sufficiently generic that it offers qualitative insight into the expected structure arising from a merger at a resolution that cannot be approached with full numerical simulations. Preliminary results indicate that the velocity-space distribution of dark matter particles remains characterized by discrete and well-defined peaks over an extended period of time, both for single and multi-merging systems, in contrast to the simple smooth velocity distributions sometimes assumed in predicting laboratory experiment detection rates.

ADCP measured flow current of the middle-lower Changjiang River channel

The water column flow velocity of 36 river sections in the river reach between Hankou (Wuhan) and Wuxue of the middle-lower Changjiang River. Their cross sectional distribution patterns in relation to the river channel morphologies were examined by using ship-mounted ADCP (Acoustic Doppler Current Profiler) instrument. The results indicate four (I-VI) types of river channel morphology associated flow patterns: I—laterally deepening riverbed topographic pattern; II—symmetrical to asymmetrical riverbed topographic pattern; III—relative flat riverbed topographic pattern, and IV—sandbar supported riverbed topographic pattern. All these correspond to the different patterns of flow velocity distribution. The maximum flow velocity is usually related to the deeper water depth, but irregular water column distribution of flow current velocity results often from the vortices’ current associated with river knots. Deeper river water depth is usually identified in the river reach located slightly downstream to the river knot, where faster flow velocity occurs. Downward change in flow velocity fits semi-log law, showing an exponential decreasing flow current with the maximum flow velocity near the water surface. However, in the river reach near the river knots, the water column distribution of flow current velocity does not fit the semi-log law, showing the irregular flow current pattern. This study, in context of river catchment management, highlights the controls of riverbed morphology to the flow current structure, which will shed light on the post study of Three Gorges damming in 2009.

静岡県馬込川河口域における非対称な流況場と堆砂特性

This paper investigates asymmetric current fields and resulting sedimentation characteristics observed around the Magome River mouth. Field survey was first conducted to obtain detailed bathymetry of the river mouth and to capture spatial current patterns by tracing wooden chips floated on the water surface. Image rectification technique was utilized to track the actual coordinates of wooden tracers. Time-varying water level and flow velocity near the bottom were also measured and, not only yielded time-varying tidal characteristics, did these data support the validity of estimated current patterns using wooden tracers. Circulation current patterns induced by broken waves were clearly observed in the area where the distance between two jetties was widened and local sedimentation was observed in the landward edge of this circulation current. This circulation current also caused asymmetric distributions of tidal current velocity across the narrow river mouth. Numerical analysis was finally conducted to investigate the influence of observed asymmetric current on the sand discharge rate at the river mouth.

Field studies of river flow dynamics in the lower pool of a hydrosystem

Results of field studies of Istra River current dynamics over a 35-km reach in the lower pool of a hydrosystem are analyzed. Data on the distribution of current velocities in cross-sections with different morphometry and the characteristics of bottom deposits at significantly different water discharges, and data on variations in the water surface slope are presented. The coefficients of hydraulic friction are assessed.

導流堤を含む浜名湖今切口周辺地形と漂砂移動特性

This paper explores the characteristics of sediment transport around Imagire-guchi, the opening of Hamana-Lake. Special focus was put on the spatial distributions of tidal current velocity, which should be the primary force that causes sediment transport and resulting local sedimentation characteristics in the lake. We first performed field survey and obtained the distribution of tidal current velocity across Imagire-guchi as well as suspended sediment concentration. The measured current velocity had significant variation across the tidal flow direction and, in the eastside of the opening, time-varying velocity profile was asymmetry, i. e., the landward high-tide current velocity was larger than the ebb-tide seaward current velocity. Similar asymmetric feature was observed in the measured suspended sediment concentration. As a result, we deduced that certain amount of sand may still flow into the lake in the east-side of the opening while vice versa in the other west-side of the opening. Finally, we performed numerical analysis that surely explained the observed feature and somehow supported our deduction.

Methodical aspects of the application of acoustic Doppler current profilers in the Black Sea

For the first time, an acoustic Doppler current profiler (ADCP) produced by the RDI is used in the water area of the Black Sea in the lowered mode. This direction in the application of acoustic Doppler current profilers is now in the stage of development and verification of various procedures of measurements with subsequent data processing and enables one to get the distributions of current velocities down to depths of 1000–6000 m depending on the modification of the instrument. We describe the procedures of measurements performed with the help of the ADCP in the lowered mode in the course of an expedition and the stages of processing of the primary data based on the experience of application of similar acoustic current profilers at the Marine Hydrophysical Institute of the Ukrainian National Academy of Sciences accumulated in the 1980s. We generalize the experience of application of ADCP under the hydrological conditions of the Black Sea, propose the algorithms of data processing, present the profiles of absolute current velocity at several stations, compare these profiles with the geostrophic velocities, and determine the deep-water structure of the field of currents in a shelf-open-sea section made along 31.17°E.

A dynamic model for cause of formation of Peru Current

Abstract The generation mechanism of Peru Current (PC) induced by Antarctic Circumpolar Current (ACC) near the eastern boundary of South Pacific is investigated by decomposing the ACC deformation into basic loop current and eastern boundary current (western boundary current) before (after) ACC flows through Drake strait. In order to simulate this dynamic process, starting from the linearly quasi-geostrophic vorticity equation, we formulate the boundary value problems of differential equation separately at the eastern boundary area and the western boundary area; and then give the solving process of the boundary value problems using corrected Fourier method; finally obtain the Perturbation solutions of the eastern and western boundary areas which include a same set of unknown parameters relating closely to the distribution of current velocity at Drake strait. In addition, the two-sided current of Drake strait should satisfy smooth and linked conditions at Drake strait, and these matching conditions determine this set of parameters. As a result, we present the complete solutions of the ACC deformation at Drake strait; consequently obtain the dynamic explanation for cause of formation of PC.

Interactions among waves, current, and mud: Numerical and laboratory studies

Interactions between waves, current, mud and turbulence are very complicated in the coastal and estuarine turbid waters. It is still necessary to improve our understanding of the fundamental physical processes governing the cohesive sediment transport in the coastal and estuarine waters. A numerical model is developed to study the interactions among waves, current, and mud. An eddy viscosity model for wave and current is proposed in order to close the equations of wave motion or of current motion in a combined flow, respectively. The equations of mud transport are derived based on the visco-elastic properties of mud. Coupling the equations of wave motion or of current motion for water layer with those of mud layer can give (1) wave height; (2) distributions of current velocities in the water layer; (3) distributions of transport velocities at the water–mud interface; and (4) distributions of mass transport velocities within the mud layer. These modeled results are in a reasonable agreement with experimental results. Results suggest that (1) the rate of wave attenuation increases in the opposing currents (currents against in the direction in which the waves propagate) and decreases in the following currents (currents in the same direction as that in which the waves propagate); (2) the opposing currents would have more significant effects on the rate of wave height attenuation than the following currents; (3) the effect of current on the rate of wave attenuation on the muddy bottom is larger than that on the rigid bottom; (4) mud transport rate increased in the following currents but decreased in the opposing currents; and (5) the rate of wave height attenuation on the mud bottom is one order of magnitude larger than that on the rigid bottom.

Distribution and diffusion of suspended sediment in the offshore area of Changjiang estuary,China

The Changjiang estuary located in the coastal area of East China is the largest estuary of the country.The offshore area is the main place where sediments spread and pile up in the Changjiang estuary,and is also the remarkable area of land-ocean interaction.This paper takes the offshore area of the Changjiang estuary as a study area,utilizes the abundant in-situ data to study the characteristic of suspended sediment under the different water conditions and calculates the vertical diffusion coefficients. The difference of suspended sediment concentration is very remarkable in this area,the transient suspended sediment concentration is low,ranging from less than 0.1kg/m~(3) to over 8 kg/m~(3),the different runoff,tidal current,salinity and topography determine the complexity of the spatio-temporal distribution of the sediment concentration.The study result shows that the main features of the suspended sediment are in an asymmetric distribution which presents high concentration in the west and south and low in the east and north.This pattern of the suspended sediment concentration distribution proves that the suspended sediment spreading course is obviously influenced by the runoff and ocean dynamics,the topography of relative function and restriction in the study area. The difference in longitudinal distribution is very remarkable in the offshore area of the Changjiang estuary,the sediment concentration reduces gradually from the west to the east,and it is obviously higher in the spring tide than in the neap tide.Salinity is greater than the density of the silt-contained water,as the diluted water from the upper reaches float above the high-density salt water,and fine sediment deposits in the mouth of the salt wedging. Generally,the suspended sediment concentration increases gradually from surface to bed,because of asymmetric distribution and fluctuation of tidal current velocity,the vertical instantaneous velocity is always changing.So it is very difficult to divide various kinds of vertical distribution.The vertical diffusion coefficient is relatively huge in the west and middle water area,and is irregularly distributed in the vertical mixing,so the suspended sediment concentrations increase from the surface to the bottom equably in the area.Along with the strengthening of the degree of the vertical mixing,the suspended sediment concentrations also increase remarkably.As a result,two high suspended sediment concentration zones come into being in the south and the north of the offshore area of the Changjiang estuary.

Sediment distribution study in the Gulf of Kachchh, India, from 3D hydrodynamic model simulation and satellite data

A fully 3D finite difference baroclinic hydrodynamic model ‘COSMOS’ is applied to the Gulf of Kachchh (GoK) to study tidal variation, ocean currents, residual tidal current, sea surface temperature distribution, etc. The ‘COSMOS’ is based on the hydrostatic and Boussinesq approximations and uses a vertical double sigma coordinate with a step-like grid. In addition to the momentum and continuity equations, the model solves two-transport equations for salinity and temperature and an equation of state to include baroclinic effects. The other objectives are quantitative assessment of suspended sediments by analyzing SeaWiFS data and understand the relation between residual tidal currents and erosion, movement and deposition of sediments. The model results showed that the dominant current system within GoK is controlled by the simulated tidal and residual tidal currents variation. The residual tidal current velocity reduces from the surface towards the bottom. The residual surface current velocity distribution displays cyclonic eddy continuity regions within the GoK. It is concluded that bottom topography variation plays an important role in determining the distribution of residual current velocity and thereby sediment distribution. The residual current velocity distribution map showed conformity with the gross geomorphic subdivisions and the sediment distribution map of Gulf of Kachchh. The suspended sediment plume pattern followed the current pattern obtained from modeling. The model results of the sea surface temperature of the December month showed good agreement with the temperature structure and the pattern obtained form NOAA/AVHRR data.

High-frequency observations of wind-forced onshore transport at a coastal site in Baja California

We investigated the effect of the daily sea breeze on the surface flow at Bahia Salsipuedes, Baja California, Mexico. Drifter-tracking experiments were conducted over a 2-week period in July, 1999. Surface drifters were deployed daily and their trajectories tracked from a small boat for 3–7h. Wind speed and direction, as well as the vertical distribution of temperature and current velocities were measured throughout the observation period. Our results indicate that strong winds associated with the daily sea breeze at the study area can produce onshore surface currents with velocities of up to 10cms−1 (net transport of up to 2km in a diurnal cycle). There was a hyperbolic relationship between onshore wind stress and net onshore transport of the drifters. A decrease in stratification and more consistent pattern of onshore transport were associated with sustained winds >6ms−1 towards the end of the study. Progressive vector diagrams computed from near-surface currents failed to predict the trajectory of drifters, especially on days where wind forcing was more consistent in terms of direction and magnitude. We discuss the implications of these results regarding the collection of settlement data and their interpretation in terms of larval supply to coastal populations.

Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of 210Pb fluxes, calculated from inventories of sedimentary excess 210Pb, with 210Pb input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers ≥0.23, a fractional seamount height of ∼0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived 210Pb fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived 210Pb flux is lower than the water-column-derived 210Pb flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years (∼5 210Pb half lives) northward currents prevailed within the lowest ∼1000 m of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean.

Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of 210Pb fluxes, calculated from inventories of sedimentary excess 210Pb, with 210Pb input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers ≥0.23, a fractional seamount height of ∼0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived 210Pb fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived 210Pb flux is lower than the water-column-derived 210Pb flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years (∼5 210Pb half lives) northward currents prevailed within the lowest ∼1000 m of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean.