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1.
《中国科学:地球科学(英文版)》2015,(11)
In situ observations and numerical simulations of turbulence are essential to understanding vertical mixing processes and their dynamical controls on both physical and biogeochemical processes in coastal embayments. Using in situ data collected by bottom-mounted acoustic Doppler current profilers(ADCPs) and a free-falling microstructure profiler, as well as numerical simulations with a second-moment turbulence closure model, we studied turbulence and mixing in the Xiamen Bay, a freshwater-influenced tidal bay located at the west coast of the Taiwan Strait. Dynamically, the bay is driven predominantly by the M2 tide, and it is under a significant influence of the freshwater discharged from the Jiulong River. It is found that turbulence quantities such as the production and dissipation rates of the turbulent kinetic energy(TKE) were all subject to significant tidal variations, with a pronounced ebb-flood asymmetry. Turbulence was stronger during flood than ebb. During the flooding period, the whole water column was nearly well mixed with the depth-averaged TKE production rate and vertical eddy viscosity being up to 5?10?6 W kg?1 and 2?10?2 m2 s?1, respectively. In contrast, during the ebb strong turbulence was confined only to a 5?8 m thick bottom boundary layer, where turbulence intensity generally decreases with distance from the seafloor. Diagnosis of the potential energy anomaly showed that the ebb-flood asymmetry in turbulent dissipation and mixing was due mainly to tidal straining process as a result of the interaction between vertically shared tidal currents and horizontal density gradients. The role of vertical mixing in generating the asymmetry was secondary. A direct comparison of the modeled and observed turbulence quantities confirmed the applicability of the second-moment turbulence closure scheme in modeling turbulent processes in this weakly stratified tidally energetic environment, but also pointed out the necessity of further refinements of the model. 相似文献
2.
The present experimental investigation focuses on the characteristics of near bed turbulence in a fully rough, uniform open-channel
flow over a gravel-type bed. Due to bed topography small scale heterogeneity, the flow is not uniform locally in the near
bed region and a double averaging methodology is applied over a length scale much larger than the gravel size. The double-averaged
Turbulent Kinetic Energy (TKE) budget derived in the context of the present flow over a gravel bed differs from the TKE budget
written for flow over a vegetation canopy. The non-constant shape of the roughness function measured in our gravel bed leads
to an additional bed-induced production term which is null for vertical roughness elements, such as simplified vegetation
elements.
The experimental estimation of the terms of the TKE budget reveals that the maximum turbulent activity takes place away from
the reference plane, near the roughness crests. However, within the interface sublayer the work of the bed induced velocity
fluctuations against the Reynolds stress is of the same magnitude as the main turbulence production term. Consequently, the
characteristics of the TKE budget have similarities with uniform flows over canopies and strongly differ from uniform flows
over smooth and transitionally rough flows over sedimentlike beds. 相似文献
3.
This paper investigates the phenomena of turbulent bursting events,and to make a comparative study among the events in the scour geometry developed by short circular cylinders of fixed length with different diameters placed on the sand bed transverse to the flow.The distributions of turbulent kinetic energy(TKE),turbulent diffusion in both longitudinal and vertical directions at different locations of scour marks,and the contributions of burst-sweep cycles to the Reynolds shear stress are presented. 相似文献
4.
Measurements of turbulent fluctuations of horizontal and vertical components of velocity, salinity and suspended particulate matter are presented. Turbulent Prandtl numbers are found to increase with stratification and to become larger than 1. Consequently, the vertical turbulent mass transport is suppressed by buoyancy forces, before the turbulent kinetic energy (TKE) and vertical turbulent momentum exchange are inhibited. With increasing stratification, the buoyancy fluxes do not cease, instead they become countergradient. We find that buoyantly driven motions play an active role in the transfer of mass. This is in agreement with trends derived from Monin–Obukhov scaling. For positive Richardson flux numbers (Ri f ), the log velocity profile in the near-bed layer requires correction with a drag reduction. For negative Ri f , the log velocity profile should be corrected with a drag increase, with increasing |Ri f |. This highlights the active role played by buoyancy in momentum transfer and the production of TKE. However, the data do not appear to entirely follow Monin–Obukhov scaling. This is consistent with the notion that the turbulence field is not in equilibrium. The large stratification results in the decay of turbulence and countergradient buoyancy fluxes act to restore equilibrium in the energy budget. This implies that there is a finite adjustment timescale of the turbulence field to changes in velocity shear and density stratification. The energy transfers associated with the source and sink function of the buoyancy flux can be modeled with the concept of total turbulent energy. 相似文献
5.
6.
定常情况下,本文利用球坐标系（λ,φ,r）的动力、热力学方程导出三维速度场（vλ,vφ,vr）的动力系统.这种包括摩擦力和热传导的不可压缩大气运动的动力系统,无论从定性上还是从定量上都能描述由赤道和极地间的加热不均匀造成的三圈环流.定性上表明,在北半球经向速度vφ和纬向速度vλ同符号,地表刮北风（vφ<0）和刮东风（vλ<0）相对应,刮南风（vφ>0）和刮西风（vλ>0）相对应.在南半球,经向速度vφ和纬向速度vλ符号相反,刮北风（vφ<0）和刮西风（vλ>0）相对应,刮南风（vφ>0）和刮东风（vλ<0）相对应.定量分析表明球面上的压力场p可以用球调和函数plm（sinφ）cosmλ来表达.当取l=6,m=0时即可导出三圈环流.在经圈剖面（φ,r）上,地表的φ1=±56°和φ2=±28°左右,以及赤道是速度场的奇点,它们都是鞍点,说明在副热带是下沉运动,在中纬度是上升运动,这正是三圈环流中的Ferrel环流的特征.这样经向速度vφ和纬向速度vλ相联系,经向速度vφ又和垂直速度相联系,那么三圈环流的三维速度场就构成了一个整体. 相似文献
7.
Near‐bed shear stress,turbulence production and dissipation in a shallow and narrow tidal channel 
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下载免费PDF全文 Near‐bed, highly resolved velocity profiles were measured in the lower 0.03 m of the water column using acoustic Doppler profiling velocimeters in narrow tidal channels in a salt marsh. The bed shear stress was estimated from the velocity profiles using three methods: the log‐law, Reynolds stress, and shear stress derived from the turbulent kinetic energy (TKE). Bed shear stresses were largest during ebbing tide, while near‐bed velocities were larger during flooding tide. The Reynolds stress and TKE method gave similar results, while the log‐law method resulted in smaller bed shear stress values during ebbing tide. Shear stresses and turbulent kinetic energy followed a similar trend with the largest peaks during ebbing tide. The maximum turbulent kinetic energy was on the order of 1 × 10? 2 m2/s2. The fluid shear stress during flooding tide was approximately 30% of the fluid shear stress during ebbing tide. The maximum TKE‐derived shear stress was 0.7 N/m2 and 2.7 N/m2 during flooding and ebbing tide, respectively, and occurred around 0.02 m above the bed. Turbulence dissipation was estimated using the frequency spectrum and structure function methods. Turbulence dissipation estimates from both methods were maximum near the bed (~0.01 m). Both the structure function and the frequency spectrum methods resulted in maximum dissipation estimates on the order of 4 × 10? 3 m2/s3. Turbulence production exceeded turbulence dissipation at every phase of the tide, suggesting that advection and vertical diffusion are not negligible. However, turbulence production and dissipation were within a factor of 2 for 77% of the estimates. The turbulence production and dissipation decreased quickly away from the bed, suggesting that measurements higher in the water column cannot be translated directly to turbulence production and dissipation estimates near the bed. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
8.
In recent years, various attempts have been made to estimate the amount of numerical mixing in numerical ocean models due
to discretisation errors of advection schemes. In this study, a high-resolution coastal model using the ocean circulationmodel
GETM is applied to the Western Baltic Sea, which is characterised by energetic and episodic inflows of dense bottom waters
originating from the Kattegat. The model is equipped with an easy-to-implement diagnostic method for obtaining the numerical
mixing which has recently been suggested. In this diagnostic method, the physical mixing is defined as the mean tracer variance
decay rate due to turbulent mixing. The numerical mixing due to discretisation errors of tracer advection schemes is defined
as the decay rate between the advected square of the tracer variance and the square of the advected tracer, which can be directly
compared to the physical variance decay. The source and location of numerical mixing is further investigated by comparing
different advection schemes and analysing the amount of numerical mixing in each spatial dimension during the advection time
step. The results show that, for the setup used, the numerically and physically induced mixing have the same orders of magnitude
but with different vertical and horizontal distributions. As the main mechanism for high numerical mixing, vertical advection
of tracers with strong vertical gradients has been identified. The main reason for high numerical mixing is due to bottom-following
coordinates when density gradients, especially for regions of steep slopes, are advected normal to isobaths. With the bottom-following
coordinates used here, the horizontal gradients are reproduced by a spurious sawtooth-type profile where strong advection
through, but not along, the vertical coordinate levels occurs. Additionally, the well known relation between strong tracer
gradients and high velocities on the one and high numerical mixing on the other hand is approved quantitatively within this
work. 相似文献
9.
Eddy flux corrections for CO2 exchange in broad-leaved Korean pine mixed forest of Changbai Mountains 总被引：1，自引：0，他引：1
WU Jiabing Guan Dexin SUN Xiaomin YU Guirui Zhao Xiaosong HAN Shijie JIN Changjie 《中国科学D辑(英文版)》2005,48(Z1)
Based on analysis of mechanisms causing energy no-closure and nocturnal low fluxes issues for CO2 exchange studies by eddy covariance method, corrections were done with the raw data sets obtained from Changbai Mountains forest flux site, to evaluate the impacts of sonic anemometer tilt, frequency response limitations and advection on estimation of CO2 exchange, respectively. The results show that the planar fit coordinate transforming method is superior to the streamline coordinate transforming method in tilt correction. The latter could cause a systematical underestimation of eddy fluxes relating with the angle of sensor and terrain tilt. The underestimation of CO2 and energy fluxes for frequency response limitations average 3.0% and 2.0% during daytime, respectively, which increase by 9.0% and 5.5% during nighttime, respectively. The corrections of frequency response limitations are closely related to atmospheric stability. The advection loss of CO2 fluxes is dominated by nocturnal vertical advection, which is at least 18% when the horizontal advection is neglected. It is suggested that more work be done to understand the characteristics of horizontal advection and turbulent eddies under a complexcircumstance. 相似文献
10.
《Advances in water resources》2003,26(3):277-294
Two subgrid-scale modeling techniques––Smagorinsky’s postulation for the horizontal eddy viscosity and the Mellor–Yamada level-2 model for the vertical eddy viscosity––are applied as turbulence closure conditions to numerical simulations of resolved-scale baroclinic lake circulations. The use of the total variation diminishing (TVD) technique in the numerical treatment of the advection terms in the governing equations depresses numerical diffusion to an acceptably low level and makes stable numerical performances possible with small eddy viscosities resulting from the turbulence closure parameterizations. The results show that, with regard to the effect of an external wind stress, the vertical turbulent mixing is mainly restricted to the topmost epilimnion with the order of magnitude for the vertical eddy viscosity of 10−3 m2 s−1, whilst the horizontal turbulent mixing may reach a somewhat deeper zone with an order of magnitude for the horizontal eddy viscosity of 0.1–1 m2 s−1. Their spatial and temporal variations and influences on numerical results are significant. A comparison with prescribed constant eddy viscosities clearly shows the importance of subgrid-scale closures on resolved-scale flows in the lake circulation simulation. A predetermination of the eddy viscosities is inappropriate and should be abandoned. Their values must be determined by suitable subgrid-scale closure techniques. 相似文献
11.
Vertical mixing and elements of mesoscale dynamics over North Carolina shelf and contiguous Gulf Stream waters 总被引：2，自引：1，他引：1
Iossif Lozovatsky Jesus Planella-Morato Kipp Shearman Qing Wang Harindra Joseph S. Fernando 《Ocean Dynamics》2017,67(6):783-798
Results of microstructure measurements conducted in October–November of 2015 as a part of the Coupled Air Sea Processes and Electromagnetic Ducting Research (CASPER) project are discussed. The measurements were taken on the North Carolina shelf and across the Gulf Stream front. On the shelf, the oceanic stratification was influenced by highly variable surface salinity and along-bottom advection. Vertical mixing was mostly governed by variable winds. The vertical eddy diffusivity was estimated using the VMP-based dissipation measurements, and the diffusivity values obtained during calm periods and stormy winds were compared. Parameterization of the diffusivity for various mesoscale dynamical conditions is discussed in terms of shear instabilities and internal wave-generated turbulence based on data obtained in deep waters of the Gulf Stream and on the continental slope. 相似文献
12.
Summary The results of an investigation of the time variations in the vertical distribution of charged nuclei (large ions) in the atmosphere in fair weather were presented. These experiments were carried out with instruments installed in aB-17 aircraft for measuring and recording simultaneously the concentration and mobility distribution of particles with mobility between 0.7 and 2.0×10–2 cm2/sec-volt, electrical conductivity, temperature, pressure, humidity and air speed.Approximately 25 series of flights were carried out. Each series consisted of three to seven flights over an eight to twenty-four hour period. All flights in a given series were carried out over a fixed flight path. The average time interval between flights was four hours. On each flight constant altitude measurements were made between 700 and 15 000 feet above the surface at altitude intervals of 1000 feet.From shortly after sunrise to early afternoon all flights show a systematic increase with time in the charged nuclei concentration up to the level to which turbulent and convective mixing extends. Analysis of the meteorological data obtained simultaneously shows this is due primarily to the daily turbulent cycle.The late afternoon and night results are more complicated. It was found necessary to separate this data into two groups corresponding to periods of low and high wind velocity. During periods of low advection a systematic decrease with time was observed in the nuclei concentration from the middle of the afternoon to sunrise. Analysis indicates that these changes are the result of nuclei coagulation and subsidence in the atmosphere. Non-systematic changes with time occurred during periods of intense advection which were found to be the result of continued instability in the lowest layers of the atmosphere and to inhomogeneities in the air mass. Above the exchange layer the changes in nuclei concentration in time at a given level were found to be non-systematic.The time variations in the electrical conductivity were summarized and shown to be primarily the result of changes in the nuclei concentration. Further support for this interpretation of the results is given by the nature of the changes with time in the horizontal variations of charged nuclei, electrical conductivity and humidity. 相似文献
13.
Rachel M. Allen Julian A. Simeonov Joseph Calantoni Mark T. Stacey Evan A. Variano 《Ocean Dynamics》2018,68(4-5):627-644
Turbulence measurements were collected in the bottom boundary layer of the California inner shelf near Point Sal, CA, for 2 months during summer 2015. The water column at Point Sal is stratified by temperature, and internal bores propagate through the region regularly. We collected velocity, temperature, and turbulence data on the inner shelf at a 30-m deep site. We estimated the turbulent shear production (P), turbulent dissipation rate (ε), and vertical diffusive transport (T), to investigate the near-bed local turbulent kinetic energy (TKE) budget. We observed that the local TKE budget showed an approximate balance (P?≈?ε) during the observational period, and that buoyancy generally did not affect the TKE balance. On a finer resolution timescale, we explored the balance between dissipation and models for production and observed that internal waves did not affect the balance in TKE at this depth. 相似文献
14.
On 15 January 2010, Thumba (8.5°N, 76.9°E) witnessed one of the longest known noontime annular solar eclipses (ASEs) spanning
a period of about 7 min, centered at 1314 hours local time. In this research article, we present a case study on the behaviour
of the atmospheric boundary layer characteristics and its vertical structure in response to this rare celestial event by making
use of a suite of different in-situ instruments. During the peak period of the ASE, the incoming solar irradiance was dimmed
by about 87% of its normal values, resulting in a significant reduction in the magnitudes of turbulent kinetic energy and
surface-layer turbulent fluxes of heat and momentum. The intensity and vertical thickness of the sea/land breeze circulation
cell over the study domain also weakened. However, the mixed layer heights determined from balloon-borne GPS Radiosonde did
not show any appreciable changes. Analysis of vertical profiles of thermodynamic parameters in association with the wind direction
during ASE indicated the formation of a double mixed layer between 700 and 1500 m and is attributed to horizontal advection
of a different airmass at those altitudes. 相似文献
15.
基于中国气象局提供的气象站点月值资料,NOAA、CMAP降水格点月值资料,NDVI卫星资料及再分析资料,利用统计方法分析了1961-2014年青藏高原感热与中国东部季风雨带关键区夏季降水的年代际变化,并根据热动力平衡方程结合CESM模式试验解释了21世纪初高原感热异常对关键区夏季降水的影响机理.结果表明:21世纪初,黄淮、江淮地区降水增加,而长江以南地区降水减少.同时,高原感热也发生年代际增强,当春季感热增强后,大气热能上传导致夏季高原近地面产生气旋性环流异常,大气辐合;高层产生反气旋性环流异常,大气辐散.黄淮、江淮地区在对流层中低层受异常偏南风控制,高层受高原上空的大尺度反气旋环流影响产生异常偏北风.此外,高原感热增强通过影响黄淮、江淮地区产生暖平流输送和非绝热加热正异常,该区域产生异常的上升运动,降水量增加.长江以南地区在对流层中低层存在一个异常的反气性环流,有来自海洋的冷平流输送,同时大气非绝热加热在该地区为负异常,产生异常的下沉运动,降水量减少.模式敏感性试验的结果证实了当高原感热发生年代际增强,黄淮、江淮地区水平温度平流及非绝热加热为正异常,而在华南地区为负异常,从而导致黄淮、江淮地区大气上升运动增强,降水增加;而华南地区下沉运动增强,降水减少. 相似文献
16.
The kinematic characteristics of three-dimensional turbulent open flow were studied in experiments with the use of an up-to-date measurement technique. Relationships between these characteristics and the vertical and horizontal distribution of flow velocity are shown. 相似文献
17.
D. Y. Liu S. J. Niu J. Yang L. J. Zhao J. J. Lü C. S. Lu 《Pure and Applied Geophysics》2012,169(5-6):809-819
Comprehensive fog field observations were conducted during the winters of 2006–2009 at the Nanjing University of Information Science and Technology to study the macro and micro-physical structures and the physical–chemical processes of dense fogs in the area. The observations included features of the fog boundary layer, characteristics of fog water, the particle spectrum, the chemical composition of atmospheric aerosols, radiation and heat components, turbulence, meteorological elements (air temperature, pressure, wind speed, wind direction), and environmental monitoring. The fogs observed were divided into four types: radiation fog, advection–radiation fog, advection fog, and precipitation fog, according to the mechanisms and primary factors of the fog processes. Fog boundary-layer structures of different types and their corresponding characteristics were then studied. Fog boundary-layer features, temperature structures, wind fields, and fog maintenance are discussed. The results show that radiation fog had remarkable diurnal variation and formed mostly at sunset or midnight, and lifted after sunrise or at noon, and that advection–radiation fog and advection fog were of very long duration. Extremely dense fogs occurred only in radiation-related cases. Inversion in radiation fog was short-lived, disappearing 1 or 2 hours after sunrise or at noon, faster than that in advection–radiation fog. When wind direction reversed from easterly to westerly or from southerly to northerly, the fog became an extremely dense fog. Low-level jet at times impeded fog development, whereas at other times it encouraged fog continuance. The deep inversion was merely an essential condition for a thick fog layer; sufficient vapor supply was advantageous to the formation and maintenance of a deep fog layer. 相似文献
18.
by Tom Clemo 《Ground water》2010,48(1):68-78
A model coupling fluid hydraulics in a borehole with fluid flow in an aquifer is developed in this paper. Conservation of momentum is used to create a one-dimensional steady-state model of vertical flow in an open borehole combined with radially symmetric flow in an aquifer and with inflow to the well through the wellbore screen. Both laminar and turbulent wellbore conditions are treated. The influence of inflow through the wellbore screen on vertical flow in the wellbore is included, using a relation developed by Siwoń (1987) . The influence of inflow reduces the predicted vertical variation in head up to 15% compared to a calculation of head losses due to fluid acceleration and the conventional Colebrook-White formulation of friction losses in a circular pipe. The wellbore flow model is embedded into the MODFLOW-2000 ground water flow code. The nonlinear conservation of momentum equations are iteratively linearized to calculate the conductance terms for vertical flow in the wellbore. The resulting simulations agree favorably with previously published results when the model is adjusted to meet the assumptions of the previous coupled models. 相似文献
19.
Characterization of hydraulic suitability of Chinese sturgeon (Acipenser sinensis) spawning habitat in the Yangtze River 总被引：1，自引：0，他引：1
Hydrodynamic flow fields affect the ecological processes such as the water diversity and the distribution of organisms. Understanding the hydrodynamic and ecological processes is critical for the restoration and protection of especially fragile ecological habitats in river systems. This study uses turbulent flow to characterize the ecological behaviour of Chinese Sturgeon (Acipenser sinensis) in the Yangtze River. The Delft3D‐Flow model, which is first validated with field‐measured data, is used to simulate the flow field within spawning habitats of Chinese Sturgeon, downstream of Gezhouba Dam. The model‐simulated turbulent kinetic energy (TKE) and its distribution pattern are then used to characterize the hydraulic environment of the fish's spawning habitat. For the spawning habitat, downstream of Gezhouba Dam, the lower limit of TKE for the Chinese Sturgeon egg mass field is 0.025 m2/s2, which occurs at a velocity less than 1.7 m/s. Chinese Sturgeon prefers habitats with TKE range of 0.010 ~ 0.015 m2/s2 for resting. This suggests that discharge regimes provide the basis for ecological regulation of the Three Gorges Reservoir and the scientific reference for river management. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
20.
A.P. Nicholas 《地球表面变化过程与地形》2001,26(4):345-362
Results from a series of numerical simulations of two‐dimensional open‐channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open‐channel flow over two contrasting gravel beds. Boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra‐grid‐scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at y/h = 0·1–0·3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra‐grid‐scale roughness also allows a reduction in the height of the near‐bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near‐bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel‐bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh‐independent solutions. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献