Current Velocity Distribution Research Articles

Statistical models and distributions of current velocities with application to the prediction of extreme events

A multiple input system cross-spectral analysis method for ocean current velocities, involving the major meteorological and hydrographical forcing terms, is described and analyzed. The model uses joint observed data sets of forcing and current velocities to calculate initially unknown transfer functions that correlate each forcing term to the current velocity in transform space. From further forcing data sets, spectral estimates of the current velocity can be calculated with a higher level of significance than from a shorter, observed, series. The method is applied to data from the Öresund, between Sweden and Denmark, and the Gullmar Fjord at the Swedish west coast. The results show that this method is suitable to estimate the statistical parameters of a current in a specific area, using the transfer functions, if the forcing is known for a long time and the current for a briefer period. It has advantages over purely statistical models, as it accounts for the coupling between the statistical parameters of the current and the forcing. It also has advantages over complex hydrodynamic model, as it is simple to use and interpret. The paper also contains a brief analysis of statistical distributions of current velocities sampled with various instruments in different hydrographic regimes. It is concluded that currents can be regarded as normally distributed when dominating tidal components have been removed by filtration.

A vertically moving grid finite‐element modelling of tidal flow in the Changjiang Estuary, China

AbstractAn estuarine two‐dimensional vertical finite‐element model of tidal flow has been established by laterally integrating Navier–Stokes equation. To this end, a moving grid finite‐element method has been used. An arbitrarily shaped quadrilateral element has been selected. This model has been validated by using field data from two monitoring stations at the North Passage of the Changjiang Estuary. Using this numerical model, two types of modelled results were obtained: (1) vertical distributions of tidal current velocities at the North Passage of the Changjiang Estuary; (2) longitudinal distributions of tidal current velocities at maximum flood tide, at high slack water, at maximum ebb tide and at low slack water tide at the North Passage of the Changjiang Estuary. The conclusion is that the model provides a reasonable agreement with observed data. Copyright © 2003 John Wiley & Sons, Ltd.

Modelling hydrodynamics and sediment flux within a macrotidal estuary: problems and solutions

A model of estuarine circulation and sediment transport is described. The model uses a 2-D longitudinal and vertical grid to predict the distribution of tidal elevation, current velocity, density and sediment concentration. It has been developed for relatively narrow estuaries that have a large tidal range and potentially high river flow. The advantage over simple 1-D and depth-averaging 2-D models is that it can give reasonable representation of both gravitational circulation and the vertical distribution of suspended sediment concentrations, thereby providing better estimates of the bottom stress and sediment flux, particularly in the deep channel. The model has been developed as a management tool to provide some of the advantages of a full 3-D model, but with smaller development and running costs. It can be used to predict concentrations of contaminants in estuaries that have a significant load of suspended cohesive sediment, and can differentiate between dissolved contaminants and those that are attached to suspended sediments. It is a potentially useful tool for predicting the distribution of relatively short-lived contaminants that are affected by sediment concentration, such as bacteria and some radionuclides.

Swimming performance of blacknose dace (Rhinichthys atratulus) mirrors home-stream current velocity

Flowing waters may represent a force that structures the locomotor capacity of stream fishes. We used a modified critical swimming speed (Ucrit) procedure to investigate the relationship between base-flow conditions and locomotor performance of blacknose dace (Rhinichthys atratulus) from five sites within three watersheds of Baltimore County, Maryland. Our modified test used 5-min intervals between incremental increases of 5 cm·s–1in swim-tunnel current velocity. This time increment represented a realistic transit time across riffles found in the home streams of dace. To characterize current velocity conditions of the streams, we measured current velocity at 55 evenly spaced points per site during base-flow conditions. Swimming performance varied greatly among 32 individual fish from the five sites (55Ucritfrom 26.33 to 69.00 cm·s–1) and was positively correlated (r2= 0.38, p = 0.002) with mean base-flow current velocities at the site of collection. Additionally, among fish from the site with the widest and most even distribution of current velocities (from 0 to 54 cm·s–1), we observed the largest range of swimming performances. Our results suggest that variation in flow conditions among streams influences swimming ability of blacknose dace and can result in heretofore-unappreciated intraspecific variation in swimming performance.

Comparison of CFD and EFD for the Series 60 C B = 0.6 in steady drift motion

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.

Modelling of Re-Working Carbonate Sediments under Strong Tidal Current Setting

A numerical model has been developed which successfully simulated not only carbonate precipitation but also re-sedimentation of carbonate material under a strong tidal current environment. Carbonate reservoirs in oil fields, offshore Abu Dhabi, were deposited under such environments, so that there is difficulty to predict the reservoir heterogeneity in the exploitation stage where no more appraisal wells would be available. A numerical process model is the only solution for predicting reservoir heterogeneity under such conditions; however, most of the carbonate models that have been developed in recent years take account only of the generation of carbonate material under specified depositional environments. Sedimentation by tidal currents, which is similar to clastic sedimentation, was also neglected in the models because of its lesser importance for carbonate reservoir characterization. The present model estimates in detail the distribution of tidal current velocity in the field by using a commercial flow model with multi-layered systems. For a simplification, only the bed load transportation was taken into account for transportation of carbonate sediments. It was possible to formulate the effect by bed load transportation as a function of water depth, grain size and current velocity at each point for a given grid system. For simulation of a longer period of ancient sedimentation, the ‘Mean Field Method’ and ‘Error Tolerance Level’ are introduced for effective calculation of the equivalent result using longer time conditions than daily tidal cycles. The model developed was tested against the present-day geologic situation from the Bahamas where many of the input parameters can be observed. Then it is applied to the real oil field, offshore Abu Dhabi. We could reproduce the geologic sequences that are conceptually similar to the observations in both cases. The model thus has the potential to predict ancient reservoir quality at the petroleum exploitation stage.

Full Band Monte Carlo Simulation of Short Channel MOSFETs in 4H and 6H-SiC

ABSTRACTThis is a presentation of a full band Monte Carlo (MC) study, which compares electron transport and device performance for 4H and 6H-SiC 100 nm n-channel MOSFETs. The model used for the electrons is based on data from a full potential band structure calculation using the Local Density Approximation (LDA) to the Density Functional Theory (DFT). For the holes the transport is based on a three band k-p model including spin orbit interaction. The two polytypes are compared regarding surface mobilities obtained with the program, as well as transconductance, unit current gain frequency, carrier velocity, I-V characteristics and energy distribution in the channel for the MOSFETs.

Approaches to quantifying long-term continental shelf sediment transport with an example from the Northern California STRESS mid-shelf site

Modeling shelf sediment transport rates and bed reworking depths is problematic when the wave and current forcing conditions are not precisely known, as is usually the case when long-term sedimentation patterns are of interest. Two approaches to modeling sediment transport under such circumstances are considered. The first relies on measured or simulated time series of flow conditions to drive model calculations. The second approach uses as model input probability distribution functions of bottom boundary layer flow conditions developed from wave and current measurements. Sediment transport rates, frequency of bed resuspension by waves and currents, and bed reworking calculated using the two methods are compared at the mid-shelf STRESS (Sediment TRansport on Shelves and Slopes) site on the northern California continental shelf. Current, wave and resuspension measurements at the site are used to generate model inputs and test model results. An 11-year record of bottom wave orbital velocity, calculated from surface wave spectra measured by the National Data Buoy Center (NDBC) Buoy 46013 and verified against bottom tripod measurements, is used to characterize the frequency and duration of wave-driven transport events and to estimate the joint probability distribution of wave orbital velocity and period. A 109-day record of hourly current measurements 10 m above bottom is used to estimate the probability distribution of bottom boundary layer current velocity at this site and to develop an auto-regressive model to simulate current velocities for times when direct measurements of currents are not available. Frequency of transport, the maximum volume of suspended sediment, and average flux calculated using measured wave and simulated current time series agree well with values calculated using measured time series. A probabilistic approach is more amenable to calculations over time scales longer than existing wave records, but it tends to underestimate net transport because it does not capture the episodic nature of transport events. Both methods enable estimates to be made of the uncertainty in transport quantities that arise from an incomplete knowledge of the specific timing of wave and current conditions. © 1997 Elsevier Science Ltd

A laboratory study of longshore currents over barred and non-barred beaches

A detailed description is given of the results of laboratory experiments on wave-driven longshore currents on both barred and non-barred beaches. The objective is to examine the cross-shore distribution of the longshore current velocities for purely wave-driven currents, with emphasis on the position of maximum current velocity with respect to areas where wave energy is dissipated. Unidirectional obliquely incident waves, both regular and random, were used. The measurements were performed in a large wave basin with a pump recirculation system to create spatially homogeneous longshore currents. The experiments yielded information on wave transformation, set-up of the mean water level and the cross-shore distribution of wave-driven longshore current velocity. A number of cases are presented and compared with each other. The measurements show that in the case of purely wave-driven longshore currents, the maximum current velocities occur close to areas where wave breaking is most intense. The effect of mixing, bottom friction and wave rollers on the longshore current velocity profile are examined in more detail with help of numerical modelling. Existing model equations, based on the assumption of alongshore uniformity, are used. The results for the mean longshore current profile on a barred beach are in close agreement with the measurements.

Numerical Analysis of Arc Plasma Temperature in EDM Process Based on Magnetohydrodynamics.

In this paper, the temperature of the arc plasma and the arc extinguishing time in the electrical discharge machining (EDM) process are calculated based on the magnetohydrodynamics (MHD)approximation. The temperature of the arc plasma determines the heat Flux at the surface of a workpiece which serves as the main heat source in melting and vaporizing of the workpiece. The arc extinguishing time, which must be shorter than the pulse interval, is closely related to the stabiliy of the machining process and hence the machining rate of the process.In cases for which local thermodynamic equilibrium can be assumed, the plasma is described as continuous matter, and the MHD equations are solved by the finite difference method to detemine the distributions of current density, Fluid velocity and temperature. The calculated results show that almost all of the discharge energy is equally distributed into anode and cathode, and that the energy transfer is mostly determined by the heat conduction but not affected by convection or radiation. Further more it is also found that the plasma is extinguished within a few microseconds after the current supply is ceased for each discharge pulse.

Bubbles in the Near-Surface Ocean: Their Various Structures

Abstract Air is entrained by breaking waves to produce bubbles. A highly transient macrobubble cloud is first generated under the breaker, with larger bubbles returning sooner to the sea surface. Those remaining smaller bubbles are then dispersed by near-surface turbulent shear flows in a shallow layer immediately below the undulating sea surface. Bubbles in this layer are shown to be nearly suspended; moreover, vertical distributions of current velocity and bubble concentration within this layer are found to be similar. Both of these features are used to demonstrate that the observed bubble layer, being on the order of 1 m thick, is formed through the longitudinal dispersion process. In addition, isolated microbubble plumes that penetrate several times deeper than the bubble layer appear to be produced also by breaking waves; however, in this case breaking waves temporarily deflect the bubble layer.

溶融池内の電磁対流に関する実証実験

The authors have developed a mathematical model of heat and mass transfer in the stationary TIG arc weld pool considering four possible forces which induce streaming. They are two kinds of body forces, i.e., Electro-magnetic and buoyancy forces, and two shear stresses, i.e., surface tension and aerodynamic drag force. However, every mathematical modeling is always made under the appropri ate assumption and boundary conditions as well as some simplification. Also, one encounters problems on the lack of physical properties of material, especially at elevated temperature. Therefore, it is essentially necessary to verify the model by experiments.The present authors have conducted various model experiments for the effects of electro-magnetic force and aerodynamic drag force to verify the validity of developed model. In this paper, it was revealed the experimental results of the individual effect of electromagnetic force on the convection using a mercury pool contained in a basin with current flow and velocity distribution of liquid was measured.Also, another model experiment was performed using a wood's metal surface and electric current was flown from the soldering iron to wood's metal and the resultant penetration shape was obserbed. In both model experiments, the coincidence of calculated and experimental model was experimentally verified.

Field Investigation of the Thermal Bar in Lake Ladoga, Spring 1991

The thermal bar in Lake Ladoga was studied during one week in May 1991. Temperature and current velocity distributions were measured. The movement of the thermal bar and the horizontal heat transport were examined. The temperature off-shore the thermal bar was found to be constant within a vertical. The velocity distributions were complex and dependent on wind conditions. Calculations of changes in heat content showed that there was a horizontal heat transport from the near-shore warm zone towards the thermal bar. The observed off-shore progression of the thermal bar was compared with analytically computed progression rates. The progression rate is underestimated if it is assumed that surface heating and vertical mixing are the only mechanisms contributing to the thermal bar progression. Better agreement is obtained if mixing of warm and cold water near the thermal bar zone is considered.

Alongshore Sediment Transport Rate

This paper describes a study of alongshore sediment transport rate based on three‐dimensional hydraulic model experiments performed with regular and irregular waves. Deep‐water wave conditions, wave heights through the surf zone, wave‐breaking angles, longshore current velocity distribution, and the bed‐load and suspended‐load sediment transport distributions are measured simultaneously to form a coherent data set. An expression is developed linking sediment transport rate to wave steepness, beach slope, relative grain size, and breaking angle. The expression compares well with published field data.

Laboratory measurements of uniform longshore currents

Experiments on uniform longshore currents in a wave basin are described. The measurements were performed in a basin with a pumped recirculation through openings in the wave guides. Minimal return flows in the offshore region of the basin are found to be accompanied with longshore currents which are virtually uniform alongshore. Three-dimensional longshore current velocity distributions were measured with much attention to quality control. Detailed experimental results are presented for different wave fields, two beach slopes and two beach roughnesses.

STUDY ON THE FORMATIONAL MECHANISM OF THE NORTHERN YELLOW (HUANGHAI) SEA COLD WATER MASS(I)——SOLUTION OF THE MODEL

A hydrodynamic-thermodynamic equation set was set up to reflect the formational mechanism and evolution of the Northern Yellow (Huanghai) Sea cold water mass (NYSCWM) and its density circulation. Appropriate mathematical physical models were established by using some physical postulations. An approximate analytic solution to expound the distributions of temperature and three-dimensional current velocity, which can be used to expound the formational mechanism of the NYSCWM and its density circulation is obtained by using the theory of boundary layer and perturbational analyses.

内浦湾における放出放射能の挙動評価

It is difficult at present to verify the distribution of radioactivities and to estimate their accumulation trends only from the results of environmental monitoring because of quite low level of effluent from nuclear power plants. Therefore, a mathematical model was developed to support the environmental monitoring as well as to predict the accumulation trends of radioactivities discharged into the environment. Then the model was simulated by applying on the behavior of 60Co released from the nuclear power station located in the coast of Uchiura Bay, a part of Wakasa Bay.The mathematical expression is composed of an estimation model for distribution of current velocities based on the results of field observations, a diffusion equation including the terms of advection, diffusion, appearance and disappearance, and a box model applied for some reaches. Adsorption on sediment and effect of scavenging are also taken into consideration. Details of the model are described in this paper and the validity is checked and discussed. The results of verification draw the conclusions as follows:(1) On the concentration of 60Co in the sediment only detectable near the discharge point, the values estimated and observed were agreed within a factor of three.(2) It was estimated that the half amount of 60Co released during 8yr after 1974 is accumulated in the bay sediment, whereas the other is lost away from the bay.(3) The results of calculation showing no increase of accumulation at 1982 suggest nonprogressive accumulation in the future.

Distribution of water discharges and current velocities in a braided channel

1. As a result of the investigations, the distribution of the discharge and current velocities in braided channels of a flatland river under natural conditions and with the implementation of various engineering measures was obtained. The effective measures on regulating the distribution of discharges in a braided channel for activating dying channels were determined. 2. The regularities of fluid flow at the point of separation were established, which have a substantial effect on the results of calculating the distribution of the discharge in the channels. Detailing of the calculations of the distribution of the discharge in channels by means of the main equation of uniform motion without consideration of the regularities of fluid motion at the point of separation leads to an increase of the deviation of the results obtained from the actual data. 3. The results of the investigations have importance for developing optimal variants of structures being designed and increasing the reliability of existing structures, for improving the navigation conditions on rivers with an analogous scheme of braiding of the flow, since they reflect the characteristics of the distribution of discharges and velocities in a braided channel under various conditions which are not being taken into account by the existing hydraulic calculation methods.

Flow field, swimming velocity and boundary layer: parameters which affect the stimulus for the lateral line organ in blind fish.

The data presented support the hypothesis that the flow field supplies the stimulus to the lateral line organ (LLO) in blind cave fish (Anoptichthys jordani). Two basic predictions from the theoretical analysis of the flow field were confirmed: (i) individual blind cave fish prefer particular swimming velocities, (ii) the velocity preferred depends on the cross-sectional area (CSA) of the fish, i.e. the smaller the CSA the higher the swimming velocity. This relationship was found also in experimentally blinded fish of other species. Furthermore, when placed in unfamiliar surroundings, blind cave fish swim at higher velocities than in familiar surroundings for a certain habituation period. The boundary layer which surrounds the fish attenuates the amplitude of the hydrodynamic stimulus because of its damping properties. Computations of the current velocity distribution within the boundary layer indicate that the stimulus for freestanding neuromasts is considerable even during swimming in open water.

Habitat Selection by Smallmouth Bass in Response to Physical Characteristics in a Natural Stream

Habitat selection by smallmouth bass Micropterus dolomieui in shallow pools of the Flat River, Michigan, during summer 1981 and 1982 was related to the distribution of substrate type, depth, and current velocities. When current velocity was increased through manipulation, smallmouth bass reduced time spent in a habitat, foraged less, and moved more. Manipulation of light intensity between sunlit and shaded habitats did not influence use of habitats by smallmouth bass. When smallmouth bass behavior and habitat choice were observed without habitat manipulation, the fish chose gravel- to boulder-sized substrate and they avoided sand and silt substrates. Areas deeper than 45 cm were preferred by Flat River smallmouth bass; however, preference for deep areas was directly related to fish length in 1982. Areas of reduced current velocity, generally less than 15 cm/s, also were preferred. Smallmouth bass spent much of their time in foraging-related activities; up to 40% of fish between 10 and 18 cm long were observed feeding in association with northern hog suckers Hypentelium nigricans. In contrast to laboratory experiments on habitat choice by smallmouth bass that lacked prey, Flat River smallmouth bass selected specific substrate types and were more active than fish in the laboratory, probably in response to prey distribution.