Observations of turbulence under weakly and highly stratified conditions in the Ariake Sea

Observations of turbulence, stratification, and mean current were made using a microstructure profiler and an acoustic Doppler current profiler (ADCP) during four cruises at a central location in the Ariake Sea, under weakly and strongly stratified conditions. Continuous measurements of the dissipation rate of turbulent kinetic energy (TKE), ε, were made. These revealed that frictional bed turbulence with quarterdiurnal variation in the bottom boundary layer (BBL) was one of the most energetic sources of vertical mixing in the sea. Thickness of the BBL was strongly confined by the stable stratification. We investigate a relationship between the BBL height h and the Ozmidov scale. We present a systematic argument that describes the vertical structure and characteristic scales of velocity and turbulence inside the frictional BBL, where the stratification persisted. Considerable deviation of observed vertical shear from the law of the wall indicated a modification of turbulent scales by the stratification. Shear stress calculated from the velocity data using vertical integration of the equation of motion was found to decrease approximately linearly with height. The TKE production rate P, estimated using the shear stress, was highly correlated with the dissipation rate. The buoyancy contribution to TKE balance in the BBL was quantified in terms of the flux Richardson number R _f as R _f?=?0.12.     

珠江河口底边界层憩流动力特征

通过在珠江黄茅海河口进行的25 h定点连续观测,分析了潮流底边界层在憩流时刻的动力特征。结果表明,憩流期非恒定性增强,改变了平均流和湍流结构。主要表现:1)平均流结构出现三种不同类型,分别是对数分布(摩擦力主导)、垂向均匀分布(惯性主导)和过渡状态(摩擦力和惯性力平衡);2)湍流各向异性增强,湍谱惯性子区带宽减小。各湍流特征参数在转流时刻达到最小值,表明惯性作用抑制了湍流的发育。     

Estimation of turbulent kinetic energy dissipation rate in the bottom boundary layer of the Pearl River Estuary

A structure function approach is applied to estimate the turbulent kinetic energy(TKE) dissipation rate in the bottom boundary layer of the Pearl River Estuary(PRE).Simultaneous measurements with an acoustic Doppler velocimeter(ADV) supplied independent data for the verification of the structure function method.The results show that,1) the structure function approach is reliable and successfully applied method to estimate the TKE dissipation rate.The observed dissipation rates range between 8.3×10 4 W/kg and 4.9×10 6 W/kg in YM01 and between 3.4×10 4 W/kg and 4.8×10 7 W/kg in YM03,respectively,while exhibiting a strong quarter-diurnal variation.2) The balance between the shear production and viscous dissipation is better achieved in the straight river.This first-order balance is significantly broken in the estuary by non-shear production/dissipation due to wave-induced fluctuations.     

Estimation of Bed Shear Stresses in the Pearl River Estuary

Mean and fluctuating velocities were measured by use of a pulse coherent acoustic Doppler profiler(PC-ADP) and an acoustic Doppler velocimeter in the tidal bottom boundary layer of the Pearl River Estuary.The bed shear stresses were estimated by four different methods:log profile(LP),eddy correlation(EC),turbulent kinetic energy(TKE),and inertial dissipation(ID).The results show that(a) all four methods for estimating bed stresses have advantages and disadvantages,and they should be applied simultaneously to obtain reliable frictional velocity and to identify potential sources of errors;(b) the LP method was found to be the most suitable to estimate the bed stresses in non-stratified,quasi-steady,and homogeneous flows;and(c) in the estuary where the semi-diurnal tidal current is dominant,bed shear stresses exhibit a strong quarter-diurnal variation.     

夏季长江河口层化与湍流混合特征分析

利用2016年夏季长江河口现场水文特性与湍流微结构观测资料, 分析了长江河口水体温盐结构、层化发育、湍流与混合特征。结果表明: 1)夏季长江河口水体密度层化结构明显, 根据各层水体密度梯度差异, 可将水体分为底部混合层和上层密度跃层, 两部分的密度层化界限与浮力频率等值线lg N ² = - 4.0接近。2)底部混合层湍动能耗散率大, 层化结构弱, 水体分层稳定性弱; 上层密度跃层湍动能耗散小, 层化结构强, 水体分层稳定性强, 这有利于河口内波的发育与传播。3)在密度层化的作用下, 水体的湍动能耗散率、湍动能剪切生成及浮力通量的能量关系在一定范围内符合湍动能局部能量平衡方程。不同层之间的湍流弗劳德数Fr_t和湍流雷诺数Re_t在Fr_t-Re_t平面上呈现明显的分区, 与经典的分层剪切流理论基本吻合。     

Modeling of the Turbulence in the Water Column under Breaking Wind Waves

Past studies have shown that there is a wave-enhanced, near-surface mixed-layer in which the dissipation rate is greater than that derived from the “law of the wall”. In this study, turbulence in water columns under wind breaking waves is investigated numerically and analytically. Improved estimations of dissipation rate are parameterized as surface source of turbulent kinetic energy (TKE) for a more accurate modelling of vertical profile of velocity and TKE in the water column. The simulation results have been compared with the experimental results obtained by Cheung and Street (1988) and Kitaigorodskii et al. (1983), with good agreement. The results show that the numerical full model can well simulate the near-surface wave-enhanced layer and suggest that the vertical diffusive coefficients are highly empirical and related to the TKE diffusion, the shear production and the dissipation. Analytical solutions of TKE are also derived for near surface layer and in deep water respectively. Near the surface layer, the dissipation rate is assumed to be balanced by the TKE diffusion to obtain the analytical solution; however, the balance between the dissipation and the shear production is applied at the deep layer. The analytical results in various layers are compared with that of the full numerical model, which confirms that the wave-enhanced layer near the surface is a diffusion-dominated region. The influence of the wave energy factor is also examined, which increases the surface TKE flux with the wave development. Under this region, the water behavior transits to satisfy the classic law of the wall. Below the transition depth, the shear production dominantly balances the dissipation. This revised version was published online in August 2006 with corrections to the Cover Date.     

A numerical study of swash flows generated by bores

The dynamic processes of bore propagation over a uniform slope are studied numerically using a 2-D Reynolds Averaged Navier–Stokes (RANS) solver, coupled to a non-linear k − ε turbulence closure and a volume of fluid (VOF) method. The dam-break mechanism is used to generate bores in a constant depth region. Present numerical results for the ensemble-averaged flow field are compared with existing experimental data as well as theoretical and numerical results based on non-linear shallow water (NSW) equations. Reasonable agreement between the present numerical solutions and experimental data is observed. Using the numerical results, small-scale bore behaviors and flow features, such as the bore collapse process near the still-water shoreline, the ‘mini-collapse’ during the runup phase and the ‘back-wash bore’ in the down-rush phase, are described. In the case of a strong bore, the evolution of the averaged turbulence kinetic energy (TKE) over the swash zone consists of two phases: in the region near the still-water shoreline, the production and the dissipation of TKE are roughly in balance; in the region farther landwards of the still-water shoreline, the TKE decay rate is very close to that of homogeneous grid turbulence. On the other hand, in the case of a weak bore, the bore collapse generated turbulence is confined near the bottom boundary layer and the TKE decays at a much slower rate.     

Application of Two Turbulence Closure Schemes in the Modelling of Tidal Currents and Salinity in the Hooghly Estuary

This paper has two purposes. The first is to study the circulation and salinity in Hooghly Estuary, along the east coast of India and the second is to compare the performance of two turbulence closure schemes by modelling it. A breadth averaged numerical model using a sigma co-ordinate system in the vertical is briefly described. Vertical diffusion of momentum and salt are parameterized by a simple first-order turbulent closure or by a one equation model for turbulent kinetic energy (TKE) which uses a specified mixing length. The results are compared with the available neap and spring tide observations along the estuary for both low and high discharge periods.The computed elevations and currents are in reasonable agreement with the observations showing no major differences in vertical current profiles by both the turbulent schemes. However, there is a slight under-prediction of bottom currents. The salinity profiles predicted by TKE model show better matching with observations. Statistical tests are also conducted to study the comparative performance of the turbulent closure schemes. The maintenance of two layer structure in residual currents and salt variability are also studied by using the model.     

珠江河口底边界层湍流特征量研究

通过高频流速仪ADV和脉冲相干声学多普勒剖面仪PC-ADP对珠江河口崖门底边界层进行了三个测次潮内(25 h)顶点连续观测,利用观测数据计算分析了潮流底边界层内的湍流特征量及其时空变化.结果表明:1)对于半日潮流占优的河口,各湍流特征量均具有明显的四分之一周日的变化规律;2)湍流强度、床底应力和摩阻流速在潮内的平均值以位于河口湾的测点所测值最大,依次向上游递减,而湍动能耗散率则沿河口湾至上游逐渐增大;3)三个测次边界层内涡动粘滞系数的平均值分别为2.42×10 -3 m 2/s、2.20×10 -4m 2/s和6.16×10 -4 m 2/s,拖曳系数的均值为7.89×10 -3、1.63×10 -3和1.99×10 -2,两者潮内的变化均非常显著,相差可达一到两个数量级;4)在充分混合的潮流底边界层内,湍动能生成与耗散基本处于局部平衡状态,三个测次湍动能耗散率均值在8.89×10 -5 W/kg～7.43×10 -6 W/kg之间.     

Effects of time dependence in unstratified tidal boundary layers: results from large eddy simulations

A three-dimensional Large Eddy Simulation (LES) model is used to simulate oscillating tidal boundary layers and test previous results obtained from one-dimensional boundary layer models and turbulence measurements in tidal channels. The LES model produces low-order turbulence statistics in agreement with the semi-analytic theory and observations. It shows a logarithmic layer in the mean velocity profile and a linear distribution of Reynolds stress with water depth. However, the eddy viscosity profile predicted by the LES model is not parabolic but better matches a parabolic profile modified by wake effect observed in the outer part of depth-limited steady boundary layers. Low-order turbulence statistics can be scaled by the instantaneous friction velocity at the bottom boundary. Although turbulence intensities in three directions fluctuate over a tidal cycle, their normalized values are in good agreement with those determined from laboratory experiments of steady open-channel flows. The LES model confirms that tidal turbulence is in quasi-equilibrium. However, it also demonstrates the importance of flow acceleration/deceleration term in the depth-integrated momentum balance for the mean flow. Phase differences are found between flows at different heights above the bottom boundary.     

Once again: once again—tidal friction

Topex/Poseidon (T/P) altimetry has reopened the problem of how tidal dissipation is to be allocated. There is now general agreement of a M₂ dissipation by 2.5 Terawatts (1 TW = 10¹² W), based on four quite separate astronomic observational programs. Allowing for the bodily tide dissipation of 0.1 TW leaves 2.4 TW for ocean dissipation. The traditional disposal sites since (1920) have been in the turbulent bottom boundary layer (BBL) of marginal seas, and the modern estimate of about 2.1 TW is in this tradition (but the distribution among the shallow seas has changed radically from time to time). Independent estimates of energy flux into the marginal seas are not in good agreement with the BBL estimates.T/P altimetry has contributed to the tidal problem in two important ways. The assimilation of global altimetry into Laplace tidal solutions has led to accurate representations of the global tides, as evidenced by the very close agreement between the astronomic measurements and the computed 2.4 TW working of the Moon on the global ocean. Second, the detection by and (1996) of small surface manifestation of internal tides radiating away from the Hawaiian chain has led to global estimates of 0.2 to 0.4 TW of conversion of surface tides to internal tides. Measurements of ocean microstructure yields 0.2 TW of global dissipation by pelagic turbulence (away from topography). We propose that pelagic turbulence is maintained by topographic scattering of barotropic into baroclinic tidal energy, via internal tides and internal waves. Previous estimates by (1974); , (1982)) of this conversion along 150,000 km of continental coastlines gave a negligible 0.02 TW; evidently the important conversion takes place along mid-ocean ridges.The maintenance of the abyssal global stratification requires a much larger expenditure of power. 2 TW versus 0.2 TW. This is usually attributed to wind forcing. If tidal power is to play a significant role here, then the BBL estimates need to be reduced. The challenge is to estimate dissipation from the energy flux divergence in the T/P adjusted tidal models, without prior assumptions concerning the dissipation processes.     

Estuarine salt conservation: a Lagrangian approach

Understanding tidal and mean flow transport mechanisms that maintain the estuarine salt balance against seaward transport by river flow is one of the fundamental questions of estuarine oceanography. Previous salt transport studies have failed to resolve this problem for two principal methodological reasons, in addition to the inherent variability of estuaries. First, salt transport expansions used to represent the salt balance have included a large number of terms that could not be related to any theory of estuarine circulation and whose physical meaning was thus unclear. Second, it has proven difficult to measure small mean velocities in the presence of much larger tidal variations. A new analysis method that expresses Lagrangian scalar property transport in terms of time and space variations of Eulerian variables is much simpler than expansions previously used and yields new insights into estuarine transport processes. This approach (like previous expansions) is valid for narrow systems in which cross-channel transport processes are weak and the ratio of tidal amplitude to mean depth is small. A surface boundary condition is used to eliminate one class of troublesome terms. The equivalence of the tidal-cycle average transport of tidal variations in salinity and the Stokes transport of mean salinity is then employed to show that the vertically integrated tidal flow plays no role in the overall salt balance. That is, seaward transport of salt caused by the river flow can only be balanced by landward transport resulting from correlations between shear and stratification in the mean, tidal and overtide flows. In a hypothetical inviscid estuary, tide and overtide flows would be vertically uniform, and outward fluvial salt transport could only be balanced by shear and stratification in the mean gravitational circulation. In a more realistic example with strong friction, the gravitational circulation would be severely damped, and inward transport could only be accomplished by correlations of shear and stratification in the tide and overtide flows.     

Salt-wedge propagation in a Mediterranean micro-tidal river mouth

The dynamics of a seasonally formed salt-wedge propagating along the micro-tidal channel of Strymon River estuary, Northern Greece, and its consequences on river water quality, are thoroughly studied through intensive sampling campaigns. The wedge is developed at the downstream river part, under the summer limited freshwater discharge conditions (Q < 30 m³/s). The geometric features of the wedge (length and thickness) appeared directly related to Strymon River discharge. A maximum intrusion length of 4.7 km along Strymon River estuary was observed under minimum river discharge of almost 6 m³/s. Relations produced from in situ data illustrate that limited river flow expands the wedge horizontally, reducing its vertical dimension, while higher flows lead to increased wedge thickness. Estuarine flushing time ranges between 0.2 and 1.5 days, exponentially dependent on Strymon River discharge. Wedge velocities depicted tidal asymmetry between tidal phases, with consistent inward motion, even under the ebb tidal stage. Strong vertical stratification prevails throughout the tidal cycle, proving the limited vertical mixing between the two layers, although higher interfacial stresses are produced in ebb. Bottom topography plays an interesting role in wedge propagation, as the presence of an underwater sill either prevents saline intrusion during flood or isolates the front of the wedge from its core at the ebb. Ecological consequences of salt-wedge propagation in Strymon River estuary are the frequent evidence of bottom hypoxic conditions and the increased TSS levels, leading to the occurrence of a turbidity maximum at the tip of the salt-wedge. Higher BOD and ammonium levels were mostly observed at the river end, associated to point and non-point pollution sources. Nitrates and silicates were found associated with freshwater fluxes, while ammonia levels were related to saline intrusions. The reduced phosphorus freshwater fluxes, resulting from phosphorus uptake at the upstream reservoir (Kerkini Lake) and the increased bottom turbidity induced by the salt-wedge seem responsible for the limited chlorophyll-a levels along Strymon River estuary.     

Dissipation Rate of Turbulence in a Water Layer under Wind Waves Based on Data of a Laboratory Experiment

Izvestiya, Atmospheric and Oceanic Physics - The aim of this work is to obtain estimates and parameterization of the dissipation rate of the turbulence kinetic energy (TKE dissipation), ε,...     

海浪破碎对海洋上混合层中湍能量收支的影响

海浪破碎产生一向下输入的湍动能通量,在近海表处形成一湍流生成明显增加的次层,加强了海洋上混合层中的湍流垂向混合。为了研究海浪破碎对混合层中湍能量收支的影响,文中分析了海浪破碎对海洋上混合层中湍流生成的影响机制,采用垂向一维湍封闭混合模式,通过改变湍动能方程的上边界条件,引入了海浪破碎产生的湍动能通量,并分别对不同风速下海浪破碎的影响进行了数值研究,分析了混合层中湍能量收支的变化。当考虑海浪破碎影响时,近海表次层中的垂直扩散项和耗散项都有显著的增加,该次层中被耗散的湍动能占整个混合层中耗散的总的湍能量的92.0%,比无海浪破碎影响的结果增加了近1倍;由于平均流场切变减小,混合层中的湍流剪切生成减小了3.5%,形成一种存在于湍动能的耗散和垂直扩散之间的局部平衡关系。在该次层以下,局部平衡关系与壁层定律的结论一致,即湍动能的剪切生成与耗散相平衡。研究结果表明,海浪破碎在海表产生的湍动能通量影响了海洋上混合层中的各项湍能量收支间的局部平衡关系。     

A test of the ADV-based Reynolds flux method for in situ estimation of sediment settling velocity in a muddy estuary

Under conditions common in muddy coastal and estuarine environments, acoustic Doppler velocimeters (ADVs) can serve to estimate sediment settling velocity (w _s) by assuming a balance between upward turbulent Reynolds flux and downward gravitational settling. Advantages of this method include simple instrument deployment, lack of flow disturbance, and relative insensitivity to biofouling and water column stratification. Although this method is being used with increasing frequency in coastal and estuarine environments, to date it has received little direct ground truthing. This study compared in situ estimates of w _s inferred by a 5-MHz ADV to independent in situ observations from a high-definition video settling column over the course of a flood tide in the bottom boundary layer of the York River estuary, Virginia, USA. The ADV-based measurements were found to agree with those of the settling column when the current speed at about 40 cm above the bed was greater than about 20 cm/s. This corresponded to periods when the estimated magnitude of the settling term in the suspended sediment continuity equation was four or more times larger than the time rate of change of concentration. For ADV observations restricted to these conditions, ADV-based estimates of w _s (mean 0.48±0.04 mm/s) were highly consistent with those observed by the settling column (mean 0.45±0.02 mm/s). However, the ADV-based method for estimating w _s was sensitive to the prescribed concentration of the non-settling washload, C _wash. In an objective operational definition, C _wash can be set equal to the lowest suspended solids concentration observed around slack water.     

Effect of roughness formulation on the performance of a coupled wave,hydrodynamic, and sediment transport model

A variety of algorithms are available for parameterizing the hydrodynamic bottom roughness associated with grain size, saltation, bedforms, and wave–current interaction in coastal ocean models. These parameterizations give rise to spatially and temporally variable bottom-drag coefficients that ostensibly provide better representations of physical processes than uniform and constant coefficients. However, few studies have been performed to determine whether improved representation of these variable bottom roughness components translates into measurable improvements in model skill. We test the hypothesis that improved representation of variable bottom roughness improves performance with respect to near-bed circulation, bottom stresses, or turbulence dissipation. The inner shelf south of Martha’s Vineyard, Massachusetts, is the site of sorted grain-size features which exhibit sharp alongshore variations in grain size and ripple geometry over gentle bathymetric relief; this area provides a suitable testing ground for roughness parameterizations. We first establish the skill of a nested regional model for currents, waves, stresses, and turbulent quantities using a uniform and constant roughness; we then gauge model skill with various parameterization of roughness, which account for the influence of the wave-boundary layer, grain size, saltation, and rippled bedforms. We find that commonly used representations of ripple-induced roughness, when combined with a wave–current interaction routine, do not significantly improve skill for circulation, and significantly decrease skill with respect to stresses and turbulence dissipation. Ripple orientation with respect to dominant currents and ripple shape may be responsible for complicating a straightforward estimate of the roughness contribution from ripples. In addition, sediment-induced stratification may be responsible for lower stresses than predicted by the wave–current interaction model.     

河口底边界层湍流观测后处理技术方法分析

河口底边界层过程是河口海岸研究与工程应用中的重要内容。三维点式高频流速仪(ADV)已经成为湍流现场观测的最有效的工具之一,然而受测量状态、复杂的波流环境、底床几何结构等因素的影响,湍流观测的后处理目前还不成熟。在前人工作的基础上,提出了河口底边界层湍流观测后处理的综合技术方法,包括测量状态判断、数据质量检测、坐标系旋转、去除毛刺及滤波,探讨了这些处理方法中的某些步骤及处理顺序对湍流参数估算可能产生的影响,提出了综合后处理技术的准确性评估方法。该研究对于近岸海洋湍流混合、泥沙输运等重要问题的解决可以提供较为扎实的技术支持。     

Rainfall effect on wind waves and the turbulence beneath air-sea interface

Rainfall effects on wind waves and turbulence are investigated through the laboratory experiments in a large wind-wave tank. It is found that the wind waves are damped as a whole at low wind speeds, but are enhanced at high wind speeds. This dual effect of rain on the wind waves increases with the increase of rain rate, while the influence of rainfall-area length is not observable. At the low wind speed, the corresponding turbulence in terms of the turbulent kinetic energy （TKE） dissipation rate is significantly enhanced by rain- fall as the waves are damped severely. At the high wind speed, the augment of the TKE dissipation rate is suppressed while the wind waves are enhanced simultaneously. In the field, however, rainfall usually hin- ders the development of waves. In order to explain this contradiction of rainfall effect on waves, a possibility about energy transfer from turbulence to waves in case of the spectral peak of waves overlapping the inertial subrange of turbulence is assumed. It can be applied to interpret the damping phenomenon of gas trans- fer velocity in the laboratory experiments, and the variation of the TKE dissipation rates near sea surface compared with the law of wall.     

Features of Profiles for Currents,Momentum Flux,and a Turbulence Dissipation Rate in Wind-Wave Channel

Izvestiya, Atmospheric and Oceanic Physics - Vertical profiles of the mean horizontal currents U(z), the vertical momentum fluxes τ(z), and the turbulence kinetic-energy dissipation rates (TKE...