Tidal residual circulation produced by a tidal vortex

The role of the ‘tidal vortex’ in the mechanism of generation of tidal residual circulation is investigated for a bay with a narrow entrance channel. It is shown that the circulation of residual flow is produced not by the vorticity of the inflowing sidewall-boundary layer, but by a tidal vortex formed by rolling up of the discontinuity surface released from the flow separation point at the entrance. This tidal vortex is affected by the circulation of the inflowing water, that is the inflowing tidal vortex. A returning tidal vortex formed in the bay diminishes the circulation of the tidal vortex of the next generation, while the inflowing tidal vortex formed in the open sea increases it. These cases correspond to tidal vortex life-histories of type-II and type-III, respectively (Kashiwai, 1984a). Tidal vortices of each life-history type have different strength and produce residual circulation of different strength, corresponding to each type. The ratio of kinetic energy of residual flow to that of the tidal current through the bay entrance, that is to say the energy gain of the residual circulation, is proportional to the reciprocal of the Strouhal number, and its rate of increase depends on the life-history type of the tidal vortex. This explains the experimental observation reported by Oonishi (1977) and Yanagi (1978) that the energy ratio of residual flow to tidal flow increases with the Reynolds number not monotonously but goes through a maximum and a minimum at intermediate Reynolds number.     

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.     

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.The unstable state of the flow suggested the possibility of controlling the tidal current and tidal exchange in the channel-basin system and this was confirmed experimentally. Under fixed tidal conditions and with fixed basin dimensions, the tidal residual circulation was controlled to form two strong clockwise and anticlockwise circulations or a single circulation by changing the shape of the bay entrance.     

Generation mechanisms of tidal residual circulation

Two new types of mechanism for the generation of tidal residual flow are revealed with the use of a hydraulic model experiment. A remarkable anticlockwise tidal residual circulation is formed in a model bay due to the presence of a tidal current, the Coriolis force and a horizontal boundary. A similar circulation is also formed due to the presence of a bottom slope, a horizontal boundary and a tidal current which flows normal to the inclination of the bottom slope. The residual circulation in the Sea of Iyo in the Seto Inland Sea is considered to be due to a combination of the effects of the Coriolis force, a bottom slope, a horizontal boundary and the tidal current. We classified some of the generation mechanisms of tidal residual flow which have been described to date into seven types on the basis of vorticity considerations.     

Fundamental study on the tidal residual circulation,II

The tidal residual circulation in a bay was investigated by means of a hydraulic model experiment. In addition to the experiment in the previous paper, some other experiments were carried out to study the effect of external parameters to the tidal residual flow. The ratio of the kinetic energy of tidal residual flows to that of tidal currents in the bay depends on the Reynolds number. With the training wall at the mouth, the vorticity transfer from the tidal current to the tidal residual flow sharply decreases. As a result the remarkable tidal residual circulation in the bay does not occur.     

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.     

Inleakage of a vortex-ring bunch onto a plane screen

A method is suggested for simulating axisymmetric laminar or turbulent flows formed during the motion of a vortex-ring bunch of given geometry and circulation toward a plane screen. Earlier, similar problems were simulated with the numerical solution of the Navier-Stokes equations for laminar flows. Turbulent flows have remained unconsidered until now. When a vortex ring approaches the screen, the secondary nonstationary flow is induced near the screen’s surface and this secondary flow causes the formation of the radial boundary layer (provided that air viscosity is taken into account). First, the medium spreads out from the critical point at the screen’s center with the negative pressure gradient along the radial coordinate and then detaches in the region of the positive pressure gradient. This radial wall flow and the corresponding boundary layer are considered in the quasi-stationary approximation. When the boundary layer detaches at successive instances, the flow is replenished with the radially moving secondary vortex rings whose circulations have the sign opposite to that of the circulation of the primary vortex ring. It is the interaction of the primary and secondary vortices that governs process dynamics, which differs substantially from that in the case when the formation of secondary vortices is disregarded. The suggested method is based on the method of discrete vortices (a perfect liquid) and the boundary-layer (laminar or turbulent) theory. During the development of the flow under investigation, the nonstationary ascending flow in the direction perpendicular to the screen’s plane is formed and then this flow decays and dissipates. Simulations for large Reynolds numbers corresponding to the formation of the turbulent boundary layer show that the velocity of ascending vortices in the plane of the initial vortex bunch is less than one-tenth of the initial velocity of the descending vortex ring. The boundary layer is introduced into calculations with the sole goal of determining the parameters of the secondary vortex rings formed during boundary-layer detachments. The interaction of the primary and secondary vortices is then considered within the framework of a perfect medium. Simulations for large Reynolds numbers corresponding to the formation of the turbulent boundary layer on the screen were correlated with the available data obtained in laboratory experiments for small Reynolds numbers. Qualitative agreement between the simulations and experiments is fairly satisfactory. The simulation for one combination of the circulation and vortex-ring geometry takes at most 10–15 min with the use of an average PC.     

Fundamental study on the tidal residual circulation—I

The tidal residual circulation in a bay was experimentally investigated with use of a hydraulic model. The model basin is a square bay of 5 m sides with a one-sided mouth of 1 m wide. The depth of the basin is 0.1 m. The tide of a six-minute period was provided by a tide generator of plunger type through the mouth. Tidal currents in the bay always flow in one direction though its strengths change according to the tidal phase, that is, a strong tidal residual circulation occurs in the bay. A similar flow pattern was observed to occur in a field with a horizontal boundary geometrically similar to the present model. The vorticity transfer from tidal current to residual flow is balanced with the vorticity advection of residual flow and the dissipation due to the viscosity.     

基于船载ADCP观测对罗源湾湾口断面潮流及余流的分析

基于对罗源湾可门水道的25 h连续走航ADCP观测,成功构建了沿走航断面共12个站位的连续海流时间序列,并对这些站位的潮流、余流以及潮通量等进行了分析。结果表明可门水道内的潮流为正规半日潮流,驻波性质明显,涨潮首先出现在水道中下层而退潮则首先发生在水道上层。水道内的潮流为往复流,水道南部M₂分潮流流速较大,并且其倾角自北向南逐渐增加。此外,水道两端的浅水区域内浅水分潮M₄振幅较显著。可门水道内余流呈现出两层结构,20 m以浅余流沿东北向流出海湾,并且出流的核心位置偏南,而20 m以深的余流沿西南向流入湾内,入流的流核位于偏北的近底层区域。对潮通量的积分计算表明通过可门水道进入罗源湾的潮通量约为4.81×108 m3。     

Vertical two-dimensional mechanism of tidal exchange in a bay with a sill-entrance

For a vertical two-dimensional field with a sill at a bay entrance, the tidal exchange mechanism is discussed.The schematic model is proposed as follows. The tidal trapping effect which is detected at the entrance section,i. e., the material transport due to the phase difference between the tidal periodic constituent of material concentration and tidal current at the entrance section, results because the oscillatory tidal flow at the sill entrance induces a gravitational flow along the sill slope inside the entrance. Accordingly, the tidal trapping effect depends largely upon the difference in water density between the bay and open sea and the density stratification in the bay.This model is supported by the observations at Kabira Cove (Okinawa Pref.) and Lake Hamana (Shizuoka Pref.) in 1976 through 1984. In addition, based on this model, in the case of Lake Hamana, the activity of the tidal exchange is inferred to change seasonally.     

Flow past a circular cylinder between parallel walls at low Reynolds numbers

The flow about a circular cylinder placed centrally inside a channel is studied numerically with an unstructured collocated grid finite volume method based on the primitive variable formulation. The distance between the channel walls is allowed to vary to change the blockage ratio. Simulations are carried out over a range of Reynolds numbers that are consistent with the two-dimensional assumption. The study confirms that transition to vortex shedding regime is delayed when the channel walls are close to the cylinder because of the interaction between the vortices from the channel wall and cylinder wake. In the unsteady vortex shedding regime, the wake pattern is opposite to the classic Karman street in respect of the positions of the shed vortices. The cylinder drag coefficient and Strouhal number are considerably increased at smaller gaps while the root-mean-squared lift coefficient is significantly decreased. Several important flow parameters are correlated with the input parameters, namely Reynolds number and blockage ratio.     

Suspended sediment dynamics on a seasonal scale in the Mandovi and Zuari estuaries,central west coast of India

Suspended particulate matter (SPM) collected at regular stations from the Mandovi and Zuari estuaries indicates that the peaks of high SPM coincide with peaks of high rainfall and low salinity and also with peaks of moderate/low rainfall coupled with high salinity during the monsoon. The estuarine turbidity maximum (ETM) is a characteristic feature, it occurs in the channel accompanying spring tide during the monsoon and pre-monsoon, and shifts to the bay on neap tide during post-monsoon. ETM remains at the same position in the Mandovi River, both during the monsoon and pre-monsoon, whereas in Zuari it stretched upstream during monsoon and migrates seaward of the channel during pre-monsoon. The ETM coincides with the freshwater–seawater interface during the monsoon and is formed by the interaction between tidal currents and river flows. The ETM during pre-monsoon is associated with high salinities and is generated by tidal and wind-induced currents. The turbidity maximum on neap tide during post-monsoon may be due to the erosion and resuspension of sediments from the emergent tidal flats and transport of these turbid waters into the bay. Funneling effect of the narrowing bay in the Zuari estuary and associated physical processes effectively enhance the magnitude of the currents and transports sediments to the channel. SPM retention percentage indicates that the estuarine channel is prone to siltation.     

湄洲湾的潮流和余流特征

湄洲湾是福建省中部沿海的港湾，湾内三面为大陆所环抱，湾口朝向东南进入台湾海峡。该湾呈西北向深入内陆约１８ｎｍｉｌｅ，湾口有湄洲岛作为屏障和标志，使整个湾口分成文甲口门（北口宽约０．６５ｎｍｉｌｅ）和剑屿－鹅尾山口门（南口，宽约５．６１ｎｍｉｌｅ）两个口门，距湾口约５ｎｍｉｌｅ的盘屿、大竹等岛屿及距湾口约１２ｎｍｉｌｅ的屿、横屿等岛屿为湾内两道屏障，再往内为内沃。屿和横屿间水面宽约０．５１ｎｍｉｌｅ，水深约２０ｍ，它和北端肖厝－秀屿及南端峰尾－西亭深槽相连接，是该湾的主航道（图１）。该湾航道宽阔水…     

Effect of channel bifurcation on residual estuarine circulation: Winyah Bay, South Carolina

The residual circulation pattern of Winyah Bay, the fourth largest estuary on the eastern coast of the US, is examined using stationary and shipborne current measurements during periods of low freshwater discharge. The estuary has a complex morphology with a single channel and narrow banks at the river entrance and the bay mouth, and a bifurcated channel system (main and western channels, respectively) in the middle part that appears to affect the residual circulation.Overall, the upper (single channel morphology) and middle (dual-channel morphology) parts of the estuary exhibit a baroclinic residual circulation. The presence of bifurcated channels in the middle part of the estuary modifies the typical gravitational circulation. The near-bed landward-directed residual flow is stronger in the deeper main channel than the shallower western channel. This is the result of the fact that the magnitude of residual flow scales with the water depth of the channel and it is also influenced by the opposing patterns of channel alignment in the northern and southern junctions. Analytical modeling confirms that the observed residual currents in the upper and middle estuary are density-induced. In the lower estuary, residual flow is directed seaward throughout the water column of the channel while in the adjacent shoals the residual flow is directed landward, suggesting that in contrast to the upper and middle estuary, the residual flow near the mouth is barotropic, controlled by the tides and the channel-bank morphology.     

The concept of tidal exchange and the tidal exchange ratio

The principal character of the tidal exchange process is neither diffusion nor advection, but a third category of transport, “Massenaustausch”, which appears in the space/time averaged transport. The exchange process can be divided into four fluxes: the flux of standing eddies, the flux of tidal exchange, the flux of tidal eddies and the flux of local eddies. The results of observations at the entrance channel of Kumihama-Bay show a typical example of transport dominated by tidal exchange. The tidal exchange ratio defined by Parkeret al. (1972) applies to the process of exchange between the outflowing watermass and the surrounding watermass outside of the bay mouth, but this should also be considered as being coupled with the ratio for the process of exchange between the inflowing watermass and the surrounding watermass inside of the bay mouth. These two exchange ratios can be combined into a single exchange ratio which describes the exchange process between the outer watermass and the bay water.     

钦州湾潮流特征分析

根据1994年5月和11—12月两个航次的调查及近年有关海湾的海流观测资料，分析了钦州湾的潮流特征，观测资料来自分布于该海湾东中西三个航道及相关区域的8个测站。钦州湾涨落潮流特征：落潮流速大于涨潮流速：东部最大涨落潮流速小于西部：夏季落潮流总是大于冬季：龙门水道附近的流速最大，而其余区域的流速相对较小。外湾(钦州湾)余流，是气旋式环流：水体东进而西出。从湾内来的泥沙和污染物质，主要从西部进入外海；中、东槽是主要“进水”通道，外海低泥沙含量、少污染物质的清洁水从这里潮流而上。因此，与西部相比，东部盐度高，水质清洁，底质重金属含量少。     

基于船载ADCP观测对半封闭海湾湾口断面潮流及余流的分析（英文）

基于对罗源湾可门水道的25小时连续走航ADCP观测,本文成功构建了沿走航断面共12个站位的连续海流时间序列,并对这些站位的潮流、余流以及潮通量等进行了分析。结果表明可门水道内的潮流为正规半日潮流,驻波性质明显,涨潮首先出现在水道中下层而退潮则首先发生在水道上层。水道内潮流为往复流,水道南部M2分潮流流速较大,并且其倾角自北向南逐渐增加。此外,水道两端的浅水区域内浅水分潮M4振幅较显著。可门水道内余流呈现出两层结构,20m以浅余流沿东北向流出海湾,并且出流的核心位置偏南,而20m以深的余流沿西南向流入湾内,入流的流核位于偏北的近底层区域。对潮通量的积分计算表明通过可门水道进入罗源湾的潮通量约为4.81×10^8m^3。     

Circulation and hydrology of Manukau Harbour

Abstract

Current meter and current drogue measurements made over tidal periods show that the circulation in Manukau Harbour is mainly tidal, with strongest flows within the inner harbour in the four main channels. In the entrance channel, peak tidal speeds reach 2.25 m.s^?1 at the surface, and 0.6 m.s^?1 near the bottom. Salinity and temperature observations show that the water is nearly homogeneous with depth in summer. A residence time of 22 d is calculated, assuming the small horizontal salinity contrast is maintained by freshwater inflow and evaporation.     

基于三维调和分离的同安湾口门断面冬季潮流和余流特征的分析

利用同安湾口门断面走航ADCP观测数据,采用基于高斯基函数的Candela空间插值方法,对海流进行空间三维的调和分离,并对分离的各分潮流与余流进行流场的回归检验分析,F(α=0.01)检验显示流场的回归效果显著.观测期间同安湾口门断面平均潮差为5.15 m,最小潮差4.38m,最大潮差5.98 m,断面上涨落潮最大流速分别为92.3、80.3 cm/s,潮流特征分析表明,同安湾口门断面的潮流运动形式为往复流,以半日潮流为主,约占70%潮流信息,潮流流速从海表面向海底递减.余流最大值为12.5 cm/s,空间分布上将同安湾口门断面一分为二形成北进南出的余流进出通道,这与以往余流格局的认识相符.     

Horseshow Vortices at Erodible Boundaries of Nonuniform Flows

The results of experimental studies of the interaction between the horseshoe vortices formed in nonuniform water flows and a sand surface are presented. The central part of the initial cylindrical vortex ascends, driven by the Kutta—Joukowski force. The vortex tails submerged into sand approach each other, grabbing the sand by their ends. Sharp bends are formed at the axes of the vortex tails. If the bends occlude, a ring vortex is formed above the bends. The ring approaches the surface at an angle of 40° and moves along the flow: the angle decreases, and the radius of the ring increases. When the whole vortex reaches the water surface, it breaks, loses the entrapped sand, and forms a ridge on the bottom.