A numerical study on the tidal residual flow

Characteristics of the tidal residual flow, the steady current induced in the tidal current system, are studied by a numerical method. The model basin has the same topography as that studied byYanagi (1976) and byOonishi (1977) where only the horizontal motion of the residual flow is concerned. In this study, the effect of the vertical motion is investigated as it is associated with the tidal residual flow. To this end, the bottom friction omitted in the previous study (Oonishi, 1977) is included and a two-leveled model is adopted.The first two experiments exclude the earth's rotation and the buoyancy effect on the flow. The results are as follows. The horizontal flow pattern is essentially the same as that obtained in the previous Oonishi study. The bottom friction results in the reduction of the velocity of the residual flow especially in the bottom level. An important result is that vertical velocity is as strong as the horizontal velocity multiplied by the scale ratio and that it remains even in the time-average. Upwelling appears at the center of the residual circulation. This upwelling explains Yanagi's observation in the hydraulic model that the sediment is swept by the flow and accumulates horizontally in the area at the bottom below the center of the residual circulation. The distribution of a tracer, which is simultaneously calculated in these cases, indicates the important role of vertical motion in the material dispersion in the model.The last experiment includes the earth's rotation and the buoyancy effect presuming a more actual sea. It shows another effect of the vertical motion. The Coriolis term, which operates only under the condition that a horizontal divergence of the flow is present, skews the horizontal residual flow pattern.     

太湖夏季环流成因的数值研究

强潮河口盐水入侵对饮用水源地危害极大。基于平面二维水动力盐度模型, 对典型强潮河口—钱塘江的水动力及盐水入侵过程进行了数值模拟研究。结果表明枯水径流时盐度变化与潮位过程曲线类似, 潮差对盐度大小影响显著, 径流量的增加将逐渐减小其相似程度。当流量增加到一定程度后, 继续增加的一定径流量所产生的抑咸效果减弱, 水资源有效利用率降低, 此时允许水源地盐度超标并改从蓄淡避咸水库取水可有效节约水资源。盐度平面分布显示, 盐水入侵在强潮河口弯道处受涨潮流主流线影响明显, 靠近主流线一岸的盐度大于对岸, 单从盐水入侵角度考虑, 强潮河口弯道段的取水口应设置在远离涨潮流主流线一岸。钱塘江河口盐度数值模拟对于研究减轻盐水入侵对水源地危害的措施具有指导意义。     

Vertical structure of the tidal current ellipse in a rotating basin

Tidal current ellipses formed by the Coriolis effect are investigated theoretically, taking account of the effect of horizontal boundaries. This study reveals that even in a narrow bay, the tidal current ellipse appears in the inside of the bottom boundary layer, although the ellipticity cannot be recognized in the outside of the boundary layer. In a wide bay, the ellipticity is observed even in the outside and it is larger in the inside. The rotation direction of these ellipses is counterclockwise in the northern hemisphere. And also, the Coriolis force has an effect to deflect the major axis of the ellipse in the inside of the boundary layer. These nature of the tidal current ellipse is well explained by the consideration of the formation mechanism of the ellipse.     

Horizontal circulations caused by the bottom oscillatory boundary layer in a bay with a sloping bed

The horizontal circulations caused by the combined effect of the bottom oscillatory boundary layer (Stokes layer) and a sloping bed have been investigated both theoretically and experimentally. The generating mechanism is analogous to that for horizontal circulation induced by wind or by density variation. This horizontal circulation can account for a part of the tidal residual current observed in a tidal hydraulic model.     

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.     

一个考虑潮汐、中尺度涡和地形影响的南海底部环流诊断模型

本文构造了一个考虑潮汐、中尺度涡和地形影响下的南海底部环流诊断模型。在该模型中,潮汐混合和涡致混合引起的垂直速率用一个类似的改进参数化方案来表示。该模型结果显示在南海深层吕宋海峡"深水瀑布"和斜压影响最大,潮汐作用和中尺度涡影响次之,风场的影响最小。斜压影响的整体效应与其他因素相反。潮汐混合与涡致混合具有明显的地形依赖性。潮汐混合主要集中在南海北部海盆地形较为陡峭的陆坡区和南海中部海山区,而涡致混合主要集中在海盆西边界区以及中部海山区。在不考虑吕宋海峡"深水瀑布"、潮汐和中尺度涡的情况下(对应吕宋海峡关闭),南海底部环流为反气旋式环流。考虑吕宋海峡"深水瀑布"后,南海底层环流为气旋式环流,而潮汐混合和涡致混合起到加强整个气旋式环流强度的作用。此外,该模型还给出了南海底部环流量级大小与地形坡度之间的密切关系,即地形坡度较大的地方,其流速也大。这对于现场观测有着一定的参考意义。最后,本文用尺度分析的方法从理论上分析了该模型的适用性,证实了该模型具有一定的可靠性。     

Analysis on the Circulation of the Yangtze River Estuary Based on ADCP Measurements

According to analysis on field data obtained by ADCP(Acoustic Doppler Current Profiler),the flow regime of the Yangtze River Estuary is studied by use of a 3-D numerical model.The flow field characteristics,under the influence of Coriolis force,saltwater intrusion and freshwater inflow and tidal current interaction,are depicted in details.The main driving forces and some important effective factors of lateral,longitudinal and horizontal circulation are also analyzed.     

Tidal residual circulation produced by a tidal vortex

“TIDAL VORTEX” is a term for a kind of starting vortex formed as a pair of vortices at the head of a tidal jet emanating from a narrow entrance into a bay. In this study, its formation and movement have been investigated by means of a hydraulic experiment and an analytical model. A tidal vortex is formed as a result of flow separation at the abrupt widening of a channel entrance followed by rolling up of the discontinuity surface around its free end. The vortex shows three types of life-history (type I, II and III), which are characterized by the Strouhal number and the aspect ratio of the horizontal shape of the entrance channel. In the case of type-I, the tidal vortex proceeds toward the inner region of the bay and there amalgamates with successive vortex cores into a core of tidal residual circulation. In the case of type-II, the tidal vortex core flows out into the entrance channel on the ebb but returns back into the bay on the subsequent flood. And, in the case of type-III, the tidal vortex core which was formed on the bay-side opening of the entrance channel flows out to the open sea and never comes back, whereas the core which was formed on the open-sea side of the entrance flows into the bay and never flows out. The circulation of a tidal vortex core is proportional to the reciprocal of the Strouhal number. The movement of the core near the bay entrance is determined by interaction between the cores and transportation due to the irrotational component of the tidal current. There are three types of tidal residual circulation, corresponding to three life-history types of tidal vortices. In the case of type-I, a strong tidal residual circulation is formed, but in type-II a small and weak circulation is formed. While, in type-III, the circulation having an inverse sense of rotation to that of type-I is formed.     

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 finite element model of circulation in shallow water with moving boundary on tidal-flat

To examine the circulation in shallow water with tidal flat, a finite element model for the numerical solution of the shallow water equations was developed by means of standard Galerkin's method. The domain computed was covered with triangular finite elements, and water elevation and velocity were approximated by linear interpolation functions, and the lumped coefficients were used to substitute for solving the high order algebraic equation system. The time-dependent land-water boundary changes are treated mathematically by interrelating the location of the land-water boundary with the instantaneous tidal level. The implicit scheme was adopted for the terms of the bottom friction and the Coriolis effect in the motion equation so that the numerical stability of the model has been improved.The model was applied to the tidal current on shoaling water with large tidal flat off Pikou, and a comparison between observed and calculated values showed good agreement, the flow pattern being reproduced. The result     

Some properties of tidal currents estimated from analytical and LES simulation studies

Some features of tidal current ellipses near the ocean floor are estimated from the bottom-limit solutions of the tidal equations with constant eddy viscosity. It is verified that a clockwise rotating ellipse becomes broad descending through the bottom boundary, whereas a counterclockwise ellipse becomes narrow except near the critical latitude. The major axis of an ellipse near the floor is directed 45° leftward (rightward) against that of the overlying tidal flow ellipse poleward from the critical latitude in the northern (southern) hemisphere. Equatorward from the critical latitude, the major axis near the floor lines up with that of the overlying tidal current ellipse in both hemispheres. The retrograde rotating hodograph against the overlying clockwise tidal hodograph appears near the floor around the critical latitude. These features are confirmed from the numerical studies using the large eddy simulation model. The deflection of the major axis poleward from the critical latitude is a little smaller, about 19°. Furthermore, the effects of nontraditional Coriolis force resulting from the horizontal component of the earth-rotating vector on the development of turbulence were investigated. When the diurnal tidal current is directed westward, the effective vertical eddy viscosity becomes maximum with developing turbulence. While in the case of a semidiurnal tide, a northward tidal current is preferred for turbulence development.     

较大尺度下潮流物模试验理论可行性的数值研究

以渤海及其局部海域为例,通过数值模拟量化地转缺失对水动力过程的影响,评价不同空间尺度下实施潮流物理模型试验的理论可行性。结果表明,对整个渤海、辽东湾这么大尺度的海域实施物模试验理论上是不可行的;对90km尺度的渤海湾,地转缺失使模拟结果存在较大偏差;对莱州湾61km尺度的海域可进行物模试验。研究确定60km为实施潮流物模试验一般的理论允许尺度。     

半封闭海区潮波运动问题的构建

利用海底粘滞性条件,首先导出了海底摩擦与海面坡度之间的简单关系式,在此基础上导出了水平流通量与海面坡度的关系式。然后利用连续性方程进一步得到了海面升高即潮位所满足的二阶椭圆型方程,进而导出了以潮位形式给出的半封闭海区岸壁不可穿透条件。最终在给定半封闭海区开边界水位分布的条件下构建了完整的关于潮位分布函数的微分方程边值问题。初步讨论表明,海底摩擦矢量与水平流通量矢量并不在相反的方向上。     

A Lagrangian mean theory on coastal sea circulation with inter-tidal transports Ⅱ. Numerical Experiments

The results of the new concept of coastal sea circulation are demonstrated by numerical simulations for the first time. The numerical experiments in three types of rectangular model seas illustrate the dependence of circulation on tidal phases due to the convectively nonlinear effect which is estimated by a newly defined drift dispersion index. Then, the present theory is applied in the Bohai Sea of China. At the Bohai Straits and the Huanghe River mouth area the circulation direction even reverses owing to different initial tidal phases which shows that the theory copes with nonlinearity well. The calculated M2 tideinduced residual circulation shows that a clockwise gyre exists in the center of an anticlockwise gyre in the central Bohai Sea due to the topographic features. In the Bohai Gulf the tide induced circulation shows a 3D structure with outflow at the surface and the inflow at the bottom which can partly explains the spread of the Huanghe River fresh water out of the Bohai Gulf and the inflow of the sediment from the Huanghe River.     

A Lagrangian mean theory on coastal sea circulation with inter-tidal transports Ⅱ. Numerical Experiments

The results of the new concept of coastal sea circulation are demonstrated by numerical simulations for the first time. The numerical experiments in three types of rectangular model seas illustrate the dependence of circulation on tidal phases due to the convectively nonlinear effect which is estimated by a newly defined drift dispersion in-dex. Then, the present theory is applied in the Bohai Sea of China. At the Bohai Straits and the Huanghe River mouth area the circulation direction even reverses owing to different initial tidal phases which shows that the the-ory copes with nonlinearity well. The calculated M2 tide-induced residual circulation shows that a clockwise gyre exists in the center of an anticlockwise gyre in the central Bohai Sea due to the topographic features. In the Bo-hai Gulf the tide induced circulation shows a 3D structure with outflow at the surface and the inflow at the bottom which can partly explains the spread of the Huanghe River fresh water out of the Bohai Gulf and the inflow of the sediment from the Huanghe River.     

带丁坝型半封闭矩形海湾中M_2分潮潮波和潮流特性分析

将黄渤海海域概化为矩形海湾,山东半岛概化为垂直于海岸的巨型丁坝,形成一个带丁坝型半封闭矩形海湾。利用DELFT3D-FLOW计算模块在上述海湾中进行了M2分潮数值模拟,对其无潮点和辐射状潮流场的特性进行了分析。研究发现:考虑科氏力影响和巨型丁坝反射作用,坝前形成了明显的无潮点,但在等水深条件下并未形成辐射状潮流场,而叠加上倾斜海底地形后其得以形成;无潮点和辐射状潮流场顶端位置受水深影响明显,随着平均水深的增大,无潮点将向湾口和湾中轴线方向偏移,而辐射状潮流场顶端则向湾口方向偏移。研究结果有助于加深对带丁坝型半封闭矩形海湾中驻潮波系统形成机制和动力特性的理解和认识。     

Some numerical experiments of the dynamics of the wintertime circulation in the Northern Huanghai Sea and the Bohai Sea

Some numerical experiments have been developed to explain dynamically the mechanism of the formation of the wintertime circulation in the Northern Huanghai Sea and the Bohai Sea. From the results of the numerical experiments with Coriolis parameter having positive and negative values respectively, it is found that the current pattern characterized by flowing into the Bohai Sea in the northern part of the Bohai Straits and flowing out of the Bohai Sea in the southern part of the Bohai Straits is controlled basically by the bottom topography with "the deepness on the northern side and shallowness on the southern side'in the Bohai Straits, and is not related to Coriolis parameter having positive and negative values. It also shows that the Huanghai Warm Current regarded as the inllow condition passing through the in-ports at the water boundary of the numerical experiments only influences the region near the water boundary and the eastern region of the Northern Huanghai Sea. In addition, the mechanism of the     

Combined tidal and wind driven flows and residual currents

The effect of a residual current on the combined tidal and wind driven flow and the resulting bedload sediment transport in the ocean has been investigated, using a simple one dimensional two-equation turbulence closure model. Predictions of the combined tidal and wind driven flow with given residual currents are presented, showing that the residual current has a substantial effect on both the depth averaged mass transport and the mean bedload transport directions; in some cases the effect of the residual current is to almost reverse the mean bedload transport direction. The residual current affects the rotation of the flow due to the Coriolis effect in the lower part of the water column (the near-surface flow is wind dominated), causing a larger or smaller clockwise rotation of the depth averaged mass transport, depending on the direction of the residual current.     

渤海海冰漂移过程的数值模拟和试验

建立了一个包含潮流作用的准定常海冰动力学模式，利用实测风资料和计算的潮流场对辽东湾中部的冰块漂移过程进行数值模拟，模拟的冰块漂移过程和实况基本一致。表明模式具有反映冰漂移过程动力特征的能力。通过对各动力因子的数值试验，说明引入潮流作用的必要性，并分析了各动力因子在冰漂移过程中的作用。     

Formation mechanism of the cold-water belt formed off the soya warm current

The cold-water belt (CWB) is frequently formed off the Soya Warm Current (SWC) during summer and autumn. The detailed distribution of the flow and temperature fields observed by the R/V Sinyo-maru in the summer of 2001 captured the structures of the SWC and the CWB. The temperature and density distributions showed that the vertical distribution of the CWB is associated with the upwelling formed off the SWC. Numerical experiments using a two-layer model with realistic bottom topography have been performed to understand the formation mechanism of CWB and the upwelling structure off the current. In the experiment, the sea level difference between the Japan Sea and the Okhotsk Sea, and baroclinic flow assuming the Tsushima Warm Current were given along the open boundary. The numerical model well reproduces the current system of the SWC and upwelling region off it. The upwelling region is formed at the Soya Strait first, and then it spreads on the offshore side along the SWC as a developing current system. Analysis of the model data indicated that the geostrophic balance mainly dominates in the current system, while convergence of the bottom Ekman transport due to the SWC forms the upwelling region as the secondary circulation. In addition, the advection effect due to the SWC is found to strengthen the upwelling.