The effect of standing biomass on flow velocity and turbulence in Spartina alterniflora canopies

Plant-flow interactions on the surface of tidal wetlands result in flow characteristics that are profoundly different from non-vegetated flows. Reductions in mean flow velocity and turbulence, especially the vertical components, limit vertical mixing and may impact a wide range of processes including geochemical exchanges at the sediment water interface, larval recruitment and dispersion, and sediment deposition and retention. The goal of this paper is to quantify horizontal and vertical components of velocity, turbulence intensity and total turbulent kinetic energy in Spartina alterniflora canopies in southeastern North Carolina and to relate flow characteristics to particulate transport on the marsh surface. Another aim of this paper is to assess the extent to which the distribution of standing biomass affects mean flow and turbulence by comparing S. alterniflora data to other canopy types and through a series of canopy manipulations which altered canopy height and stem densities.The results of this study indicate that flow velocity, turbulence intensity, and total turbulent kinetic energy (TKE) are significantly reduced within the vegetated canopy and that this reduction is inversely related to the amount of biomass present in the water column. Within the canopy, approximately 50% of the initial mean velocity and TKE is reduced within 5 m of the canopy edge. Within the canopy, mean velocity and TKE_horiz usually exceeded vertical velocity or TKE_vert and the vertical components of flow were attenuated more strongly than the horizontal. These results suggest that within the vegetation, turbulence contributes more to lateral advection than to vertical mixing. As a result, total suspended solid concentrations were shown to decrease logarithmically with distance from the canopy edge and to decrease at a faster rate in more densely vegetated regions of the canopy (i.e. lower TKE_vert) as compared to areas of sparser vegetation (i.e. higher TKE_vert).     

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.     

长江口湍流剖面的观测与分析

小尺度湍流过程对河口物质输运与能量交换至关重要。受传统观测方法的限制, 河口浅水区域的剖面观测资料至今较为匮乏, 进而限制了湍流过程的研究。为此, 采用新型5波束声学多普勒流速剖面仪(Nortek Signature 1 000 kHz AD2CP)在长江口开展湍流剖面观测, 获取高频、低噪的高质量湍流剖面数据, 并与声学多普勒点式流速仪(acoustic doppler velocimeters, ADV)同步观测的数据进行对比。结果表明, 通过AD2CP与ADV获得的近底部边界层摩阻流速u_*、拖曳系数C_d、雷诺应力S_R等特征参数基本一致, 底摩擦与波浪能量为河口区域湍动能的主要输入源。湍流垂向结构存在显著的非局地平衡, 即温盐等斜压作用引起的浮力通量、对流项以及强波浪作用影响的脉动压力做功、黏性输运等因素可能为长江口湍流非局地平衡的主因。     

Vertical exchange in the ocean

We analyze the expressions for vertical turbulent flows and the coefficients of turbulent exchange in the ocean and their dependences on the stratification of density. It is shown that, in the case of high density gradients, turbulent mixing is localized in separate bounded relatively small zones (turbulized spots or layers). Formulas for typical vertical sizes of turbulent spots and local coefficients of vertical turbulent exchange depending on the Väisälä-Brunt frequency are presented. We consider a model of vertical turbulent exchange for the case of spot-like intermittent turbulence (a similar model was independently developed at the Marine Hydrophysical Institute of the Ukrainian Academy of Sciences with certain differences in the statement the problem and methodology). We emphasize the priority of A. G. Kolesnikov in the investigations of vertical turbulent exchange in the ocean by the direct method, in the development and construction of quick-response measuring devices with detectors of pulsations of the components of velocity and temperature, and in the first determination of the characteristics of turbulence in lakes, seas, and oceans performed by using this equipment.     

挡潮闸矩形孔口式鱼道紊流结构及放鱼试验研究

对挡潮闸枢纽中矩形中孔、底孔鱼道中紊流结构进行了较为系统的试验研究,并做了放鱼试验。选择了一种鱼类偏爱流速所对应的流量作为典型流量,考虑了不同的孔口位置(中孔和底孔),用声学多普勒测速仪(ADV)量测了测点的三维瞬时流速及流向,分析了矩形孔口鱼道的三维时均流速分布特征、断面最大流速沿程变化规律、流速矢量场、紊动强度分布及雷诺应力分布。此外,还通过放鱼试验,利用在鱼体植入T形标签和高速摄影机观察了过鱼对象对中孔、底孔的反应情况,分析了过鱼对象与矩形孔口鱼道紊流结构的关系。试验结果表明:水流经中孔形成三维紊动自由射流,经底孔形成三维壁面射流,中孔纵向流速呈高斯分布,而底孔纵向流速则近似为高斯分布,流速由孔口向两侧逐渐减小;中孔和底孔横向流速在位于孔口范围内的纵剖面上沿程减小,孔口之外则变化较小;中孔和底孔垂向流速分布特征表现为在铅垂方向上均存在旋涡;在中孔水平面和纵剖面上,纵向最大流速均沿程衰减;中孔和底孔情形孔口处紊动强度和雷诺应力比非孔口处大得多,而非孔口处不同水深平面上紊动强度和雷诺应力变化趋于平缓;过鱼对象喜爱在紊动强度分布的峰值区和雷诺应力较大变幅区溯游。     

江苏近海潮流湍流切应力研究

潮流的流速分布和湍流切应力的分布密切相关。为了研究潮流湍流切应力的特征和变化规律,本文从潮流运动方程推导出随潮流呈周期性变化的湍流切应力随深度的分布表达式。从江苏近海现场实测流速剖面中计算得到一个潮周期内的湍流切应力的变化过程。通过将实测值与本文理论解的比较发现,计算值能很好地反映出实测值的变化特征,尤其是能正确地反映潮流在加速和减速过程中,湍流切应力偏离线性分布呈现上凹和下凹的现象,以及只有在加减速转换的时候切应力才呈现线性分布的特点。最后,通过分析湍流切应力振幅及相位沿水深的变化情况,认为反映潮流周期、涡粘性和水深之间关系的参数Ri是决定湍流切应力弯曲程度及分布形态的重要参数。对于近海潮流,水深是影响湍流切应力偏离线性程度的主要因素。     

Large-scale turbulence under a solitary wave

The structure of large-scale turbulence under a broken solitary wave on a 1 in 50 plane slope was studied. Three-component velocity measurements were taken at different heights above a smooth bed in the middle surf zone using an acoustic Doppler velocimeter. The measured data showed that turbulent velocity components were well correlated in the middle part of the water column. The velocity correlations could be produced by an oblique vortex similar to the obliquely descending eddy observed previously by other investigators. The vertical distributions of the relative values of the components of the Reynolds stress tensor showed that the structure of turbulence evolved continuously between the free surface and the bottom. The evolution was related to transition from two-dimensional to three-dimensional flow structures and the effect of the solid bottom on flow structures. Time histories of measured turbulent kinetic energy and turbulence stresses showed episodic turbulent events near the free surface but more sporadic turbulence in the lower layer. Large or intense turbulent events were found to have short duration and time lag relative to the wave crest point. These events also maintained good correlations between the turbulence velocity components close to the bottom.Instantaneous turbulent velocity fields were measured near the bottom at the same cross-shore location by using a stereoscopic particle image velocimetry system. These measurements showed that the near-bed flow field was characterized by large-scale, coherent flow structures that were the sources of most of the turbulent kinetic energy and turbulence stresses. The types of organized flow structures observed included vortices and downbursts of turbulence descending directly from above, lateral spreading of turbulent fluid along the bed, and formation of vortices in shear layers between fluid streams. A common feature of the organized flow structures near the bed was the large turbulence velocities in the longitudinal and transverse directions, which reflected the influence of a solid bottom on the breaking-wave-generated turbulence arriving at the bed.     

Laboratory experiments for wave motions and turbulence flows in front of a breakwater

In this paper, laboratory data for free surface displacements and velocity fields in front of a caisson breakwater covered with wave-dissipating blocks, together with wave pressures acting on the caisson, are presented and discussed. The core of the breakwater is made of a concrete caisson with a vertical front wall. The caisson is protected by a thick layer of tetrapods and is supported by a rubble mound. The breakwater is placed on the 1/25 impermeable slope. Two types of incident waves are used in the experiments: nonbreaking waves and spilling-type breaking waves. In the breaking wave case, the incident wave breaks offshore before it reaches the breakwater. The velocity data are obtained by using both the Laser Doppler Velocimeter (LDV) and the Electromagnetic Current Meter (EMCM). The raw data are analyzed using a numerical-filtering scheme so that turbulent fluctuations are separated from the phase-dependent wave motions. The vertical profiles of the time-averaged (over a wave period) turbulent velocity components at several vertical cross-sections in front of the breakwater are then analyzed. The spatial variations of the time-averaged turbulence velocity suggest that turbulence is generated inside the protective armor layer and transported into the flow region in front of the breakwater. The wave pressures on the vertical face and on the bottom of the caisson are also reported.     

Laboratory study of wave and turbulence velocities in a broad-banded irregular wave surf zone

The characteristics of wave and turbulence velocities created by a broad-banded irregular wave train breaking on a 1:35 slope were studied in a laboratory wave flume. Water particle velocities were measured simultaneously with wave elevations at three cross-shore locations inside the surf zone. The measured data were separated into low-frequency and high-frequency time series using a Fourier filter. The measured velocities were further separated into organized wave-induced velocities and turbulent velocity fluctuations by ensemble averaging. The broad-banded irregular waves created a wide surf zone that was dominated by spilling type breakers. A wave-by-wave analysis was carried out to obtain the probability distributions of individual wave heights, wave periods, peak wave velocities, and wave-averaged turbulent kinetic energies and Reynolds stresses. The results showed that there was a consistent increase in the kurtosis of the vertical velocity distribution from the surface to the bottom. The abnormally large downward velocities were produced by plunging breakers that occurred from time to time. It was found that the mean of the highest one-third wave-averaged turbulent kinetic energy values in the irregular waves was about the same as the time-averaged turbulent kinetic energy in a regular wave with similar deep-water wave height to wavelength ratio. It was also found that the correlation coefficient of the Reynolds stress varied strongly with turbulence intensity. Good correlation between u′ and w′ was obtained when the turbulence intensity was high; the correlation coefficient was about 0.3–0.5. The Reynolds stress correlation coefficient decreased over a wave cycle, and with distance from the water surface. Under the irregular breaking waves, turbulent kinetic energy was transported downward and landward by turbulent velocity fluctuations and wave velocities, and upward and seaward by the undertow. The undertow in the irregular waves was similar in vertical structure but lower in magnitude than in regular waves, and the horizontal velocity profiles under the low-frequency waves were approximately uniform.     

海州湾水流紊动强度和含沙量对沉降速度的影响研究

选取海州湾近岸潮流和含沙量实测资料,分析水体紊动强度与含沙量对近岸絮凝体沉降速度的影响,提出了新的沉降速度确定方法。研究表明：淤海州湾近岸泥沙沉降速率大部分在 0.05 ~ 2.50 mm/s 之间,潮周期内泥沙絮凝体的沉降速度 有明显变化。于含沙量较小时,泥沙絮凝体的沉降速度基本随含沙量的增加而增加;含沙量较大时,含沙量与沉降速度呈现出负相关,无论是大潮还是中潮,当含沙量达到 0.7 kg/m3左右时,絮团沉降速度最大,而随着含沙量的增大,絮团沉降速度开始减小。盂在涨落潮垂线平均流速最大时刻,紊动强度达到峰值,含沙量较低时,随着紊动强度增加,沉降速度也随之增加,大潮期间紊动强度对泥沙沉降速度的影响高于中潮。榆新的泥沙沉降速度计算公式不仅考虑了含沙量,还计入了紊动强度 G,大大提高了沉降速度计算值与实测值的相关性。     

The wind field over wind-waves

The velocity fluctuations of wind over wind-waves in a wind tunnel are measured with a X-type hot-wire anemometer at some heights over the water surface.The observed vertical profiles of the wave-induced velocity fluctuations and the wave-induced Reynolds stress at the wave spectral peak frequency are different from those expected from the inviscid quasi-laminar model;i.e., the observed vertical profiles of the power spectral density of the wave-induced horizontal or vertical velocity fluctuations of wind have the minimum value at the height much heigher than the critical layer, and the value of the wave-induced Reynolds stress is negative at several heights over the water surface. From the comparison between the experimental results and the numerical solutions of a linear model of the turbulent shear flow over the wavy boundary, it is shown that the discrepancy described above can be attributed to the atmospheric turbulence.     

Turbulence in the ocean: Development of investigations, evolution of ideas, and prospects

We present an analytic survey of a series of works devoted to the direct methods for the investigation of turbulence in natural basins originated under the guidance of A. G. Kolesnikov in the 50s. We describe the principal results of the early stage which formed the basis for future investigations. The main scientific directions of the latest period are also considered from the viewpoint of the development of Kolesnikov's ideas: investigation of the processes of generation and energy supply of small-scale turbulence, relationship between turbulence and the fine vertical structure and internal waves, and the mechanisms of vertical turbulent exchange in the ocean. A model of vertical exchange under the conditions of intermittent “source” turbulence is distinguished. We also discuss the directions of development and prospects of the investigation of turbulence. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

不同湍流模式下钱塘江涌潮水流三维模拟

钱塘江涌潮具有动力作用强和流速变化快等特点。涌潮水流紊动复杂,流速的垂向分布和紊动强度息息相关。通过涌潮水流实测资料的分析可以发现,涌潮作用下流速垂向分布在底部和上层存在差异。为研究涌潮作用下流速垂向分布的特征,应用基于非结构网格下有限体积法模型FVCOM对钱塘江涌潮河段水流运动进行三维数值模拟。考虑到涌潮紊动作用复杂且对流速的垂向分布起着重要影响,采用不同的湍流模式对涌潮传播过程中水流的运动特征开展研究。通过与涌潮河段实测资料的验证,复演涌潮到达前后水流运动特征,给出涌潮水流湍动能的变化过程。研究成果有助于深入认识涌潮水流紊动特征和流速的分布规律,为涌潮作用下物质输运的研究提供基础。     

Estimating the characteristics of the vertical turbulent viscosity in the upper 200-m layer of the Black Sea

The influence of high vertical velocity gradients in the Black Sea Rim Current on the intensity of the vertical turbulent exchange is demonstrated on the basis of numerical modeling based on CTD data. The vertical turbulent exchange is confirmed by the anomalous distribution of the hydrochemical parameters in the redox layer. A system of equations for the kinetic energy of the turbulence and dissipation rate (k-? model) is used for the calculation of the coefficient of the vertical turbulent viscosity (diffusivity).     

垂直分辨率对长江口海域M2分潮模拟的影响

基于EFDC(Environmental Fluid Dynamics Code)模式建立了长江口及其邻近海域的三维水动力学模型, 研究模型的垂直分辨率对该海域M₂分潮模拟的影响。结果表明:垂直分辨率的变化对M₂分潮传播方向的模拟结果影响较小, 但其可通过底摩擦和湍流耗散两个计算过程来影响潮能通量的模拟结果, 最终对长江口和杭州湾内的M₂分潮振幅产生显著的影响。最底层厚度较大时, 上层自由水体的高流速特征在最底层过于明显, 进而导致计算的底摩擦应力偏高, 此时提高底层的垂直分辨率会降低底摩擦对能量的耗散。另一方面, 垂直湍流混合作用会随垂直分辨率的增加而增强, 所以垂直分辨率增加到一定程度后, 上层自由水体的高流速会经由增强的湍流混合而更多的传入底层, 使计算的底摩擦应力随垂直分辨率的提高而有重新增加的趋势, 进而又增强底摩擦对潮能的耗散。     

Micro-scale profiler (MSP) for measurement of small-scale turbulence in the ocean

This paper describes a free-fall micro-scale profiler (MSP) which can measure vertical profiles of microscale velocity shear, temperature gradient, and conductivity gradient of both fine and microscale ranges, together with the vertical profiles of temperature and conductivity. The measurable vertical scales of the microscale shear range from about 50 cm to 2 cm. The MSP was designed to perform profiling to depths of 500 m with a nominal fall speed of about 70 cm sec⁻¹. Retrieval of the instrument is accomplished by a 700 m Kevlar string of 1.5 mm in diameter wound on a portable winch, after ballast has been released at a present depth by a mechanical rupture disk. The results of sea trials show that the microscale shear spectrum nearly corresponds with the theoretical one derived on the assumption of homogeneous turbulence.     

大气湍流引起飞机颠簸的理论分析和数值试验

基于飞机载荷因数变量方程、无量纲的大气垂直运动方程和热力学方程,建立起描述大气湍流和飞机颠簸的湍流模型,并对该模型进行理论分析和数值试验.研究指出,负的理查森数(Ri)对应着静力不稳定和动力不稳定流动,在不稳定大气层结条件下,当Ri<-Ra/PrRe2时,对流状态的大气运动必将由对流转变成湍流,并对运行于其中的飞机产生飞机颠簸.但在稳定大气层结条件下,当理查森数大于临界理查森数时(Ri>Ric=-Ra/PrRe2),正的Richardson数是动力稳定的,非对流状态的大气运动表现为重力波.当理查森数小于临界理查森数(O相似文献   

沿岸海域三维斜压场的数值模拟 Ⅱ、渤海湍流能量数值计算

研究建立渤海海域三维斜压场的湍流能量模型。对计算网格无法分辩的湍流运动，引入湍流能量求得垂向动量、质量及能量交换系统，从而形成封闭的湍流运动方程组。定量地计算了湍流能量、湍流速度及有风、无风状况下湍流能量的垂直分布等，探讨了湍流运动对潮波运动的影响。     

Turbulence and energy dissipations of surf-zone spilling breakers

A laboratory study on the turbulence and wave energy dissipations of spilling breakers in a surf zone is presented. Instantaneous velocity fields of propagating breaking waves on a 1/20 slope were measured using Particle Image Velocimetry (PIV). Due to the large region of the evolving wave breaking generated turbulent flow, seven PIV fields of view (FOVs) were mosaicked to form a continuous flow field in the surf zone. Mean and turbulence quantities were extracted by ensemble averaging 25 repeated instantaneous measurements at each FOV. New results for distribution and evolution of turbulent kinetic energy, mean flow energy, and total energy across the surf zone were obtained from analyzing the data. The turbulence dissipation rate was estimated based on several different approaches. It was found that the vertical distribution of the turbulence dissipation rate decays exponentially from the crest level to the bottom. The resulting energy budget and energy flux were also calculated. The calculated total energy dissipation rate was compared to that based on a bore approximation. It was found that the ratio of turbulence dissipation rate to total energy dissipation rate was about 0.01 in the outer surf zone and increased to about 0.1 after the breaking waves transformed into developed turbulent bores in the inner surf zone.