共查询到20条相似文献，搜索用时 62 毫秒
1.
The boundary layer formed under the footprint of an internal solitary wave is studied by numerical simulation for waves of
depression in a two-layer model of the density stratification. The inviscid outer flow, in the perspective of boundary-layer
theory, is based on an exact solution for the long wave-phase speed, yielding a family of fully nonlinear solitary wave solutions
of the extended Korteweg–de Vries equation. The wave-induced boundary layer corresponding to this outer flow is then studied
by means of simulation employing the Reynolds-averaged Navier–Stokes (RANS) formulation coupled with a turbulence closure
model validated for wall-bounded flows. Boundary-layer characteristics are computed for an extensive range of environmental
conditions and wave amplitudes. Boundary-layer transition, identified by monitoring the eddy viscosity, is correlated in terms
of a boundary-layer Reynolds number. The frictional drag is evaluated for laminar, transitional, and turbulent cases, and
correlations are presented for the friction coefficient plus relevant measures of the boundary-layer thickness. 相似文献
2.
Abstract Non-Markovian closure theories are compared with ensemble averaged direct numerical simulations (DNS) for decaying two-dimensional turbulence at large scale Reynolds numbers ranging from ≈ 50 to ≈ 4000. The closures, as well as DNS, are formulated for discrete wave numbers relevant to flows on the doubly periodic domain and are compared with the results of continuous wave number closures. The direct interaction approximation (DIA), self-consistent field theory (SCFT) and local energy-transfer theory (LET) closures are also compared with cumulant update versions of these closures (CUDIA, CUSCFT, CULET). The cumulant update closures are shown to have comparable performance to the standard closures but are much more efficient allowing long time integrations. The discrete wave number closures perform considerably better than continuous wave number closures as far as evolved energy and transfer spectra and skewness are concerned. The discrete wave number closures are in reasonable agreement with DNS in the energy containing range of the large scales for Reynolds numbers ranging from ≈ 50 to ≈ 4000. The closures tend to underestimate the enstrophy flux to high wave numbers, increasingly so with increasing Reynolds number, resulting in underestimation of small-scale kinetic energy. 相似文献
3.
Large-scale zonal flow driven across submarine topography establishes standing Rossby waves. In the presence of stratification,
the wave pattern can be represented by barotropic and baroclinic Rossby waves of mixed planetary topographic nature, which
are locked to the topography. In the balance of momentum, the wave pattern manifests itself as topographic formstress. This
wave-induced formstress has the net effect of braking the flow and reducing the zonal transport. Locally, it may lead to acceleration,
and the parts induced by the barotropic and baroclinic waves may have opposing effects. This flow regime occurs in the circumpolar
flow around Antarctica. The different roles that the wave-induced formstress plays in homogeneous and stratified flows through
a zonal channel are analyzed with the BARBI (BARotropic-Baroclinic-Interaction ocean model, Olbers and Eden, J Phys Oceanogr 33:2719–2737, 2003) model. It is used in complete form and in a low-order version to clarify the different regimes. It is shown that the barotropic
formstress arises by topographic locking due to viscous friction and the baroclinic one due to eddy-induced density advection.
For the sinusoidal topography used in this study, the transport obeys a law in which friction and wave-induced formstress
act as additive resistances, and windstress, the effect of Ekman pumping on the density stratification, and the buoyancy forcing
(diapycnal mixing of the stratified water column) of the potential energy stored in the stratification act as additive forcing
functions. The dependence of the resistance on the system parameters (lateral viscosity ε, lateral diffusivity κ of eddy density advection, Rossby radius λ, and topography height δ) as well as the dependence of transport on the forcing functions are determined. While the current intensity in a channel
with homogeneous density decreases from the viscous flat bottom case in an inverse quadratic law ~δ
–2 with increasing topography height and always depends on ε, a stratified system runs into a saturated state in which the transport becomes independent of δ and ε and is determined by the density diffusivity κ rather than the viscosity: κ/λ
2 acts as a vertical eddy viscosity, and the transport is λ
2/κ times the applied forcing. Critical values for the topographic heights in these regimes are identified. 相似文献
4.
Development of closures and parameterizations for subgrid scale effects is a significant and longstanding problem in the numerical
simulation of environmental flows. The model described herein uses a rigorous approach for developing double-averaged governing
equations — first a traditional Reynolds averaging to derive the Reynolds averaged Navier-Stokes equation (RANS), then a volume
average to derive a set of double-averaged equations (DANS). An existing finite element flow model is then modified to accommodate
these equations. This process gives rise to several new terms that require closures, as well as a new equation for free surface
elevation. This paper is directed toward model development and uses several existing closure schemes as test cases. 相似文献
5.
Micro- and macro-dispersive fluxes in canopy flows 总被引：3，自引：0，他引：3
Resolving every detail of the three-dimensional canopy morphology and its underlying topography remains untenable when modeling
high Reynolds number geophysical flows. How to represent the effects of such a complex morphological variability and any concomittant
topographic variability into one-dimensional bulk flow representation remains a fundamental challenge to be confronted in
canopy turbulence research.
Theoretically, planar averaging to the scale of interest should be applied to the time-averaged mean momentum balance; however,
such averaging gives rise to covariance or dispersive terms produced by spatial correlations of time-averaged quantities that
remain ‘unclosed’ or require parameterization. When the averaging scale is commensurate with few canopy heights, these covariances
can be labeled as ‘micro-dispersive’ stresses. When averaging is intended to eliminate low-wavenumber topographic variations,
we refer to these covariances as ‘macro-dispersive’ terms. Two flume experiments were used to explore the magnitude and sign
of both micro- and macro-dispersive fluxes relative to their conventional Reynolds stresses counterparts: a rod-canopy with
variable roughness density and a dense rod canopy situated on gentle hilly terrain. When compared to the conventional momentum
flux, the micro-dispersive fluxes in the lowest layers of sparse canopies can be significant (∼50%). For dense canopies, the
dispersive terms remain negligible when compared to the conventional momentum fluxes throughout. For the macro-dispersive
fluxes, model calculations suggest that these terms can be neglected relative to the Reynolds stresses for a deep canopy situated
on a narrow hill. For the region in which topographic variations can interact with the pressure, both model calculations and
flume experiments suggest that the macro-dispersive fluxes cannot be neglected, and their value can be 20% of the typical
Reynolds stresses. 相似文献
6.
Acoustic Doppler current profilers (ADCPs) have been used to measure Reynolds stresses in tidally dominated environments where wave action was minimal. In this paper, we examine observations from a microtidal estuary where the effects of wind stress and surface waves dominate the velocity variance. Reynolds stress measurements in this setting require a technique for addressing surface gravity wave contamination. We present here a method of reducing the effect of wave motion on Reynolds stresses by subtracting coincident observations along the axis of the ADCP beam. Linear wave theory is used to account for the attenuation of wave orbital velocities with depth. Using this method, Reynolds stress values are brought in line with those predicted by drag laws at the surface and bottom. The apparent Reynolds stress that is removed by the along-axis subtraction is shown to be largely due to the interaction of a slight tilt (1°) in the ADCP and the wave orbital velocity. During periods of stronger wind and waves, there is evidence of enhanced near-surface turbulence and momentum flux, presumably due to breaking waves. During these events, our calculated Reynolds stress magnitudes still appear reasonable, although the directions are suspect. We develop a diagnostic technique that clearly demarcates this region when it occurs. Coincident density profile measurements are used with the ADCP data to compute gradient Richardson numbers throughout the water column. Enhanced Reynolds stresses appear to correspond to Richardson numbers less than one.
Responsible editor: Alejandro Souza 相似文献
7.
We report on an experimental study conducted to investigate the influence of small-scale wind waves on the airflow structure
in the immediate vicinity of the air–water interface. PIV technique was used to measure the two-dimensional velocity fields
at wind speeds of 3.7 and 4.4 m s−1 and at a fetch of 2.1 m. The flow structure was analyzed as a function of wave phase. In the near-surface region, significant
variations were observed in the flow structure over the waveform. The phase-averaged profiles of velocity, vorticity, and
Reynolds stress showed different behavior on the windward and leeward sides of the wave in the near-surface region. The influence
of wave-induced velocity was restricted within a distance of three significant wave heights from the surface, which also showed
opposite trends on the windward and leeward sides of the crest. The results also show that the turbulent Reynolds stress mainly
supports downward momentum transfer whereas the wave-induced Reynolds stress is responsible for the upward momentum transfer
from wave to wind. In the immediate vicinity of the air–water interface, the momentum is transferred from waves to wind along
the windward side, whereas, the momentum transfer is from wind to waves along the leeward side. 相似文献
8.
Fangli Qiao Yeli Yuan Tal Ezer Changshui Xia Yongzeng Yang Xingang Lü Zhenya Song 《Ocean Dynamics》2010,60(5):1339-1355
A theoretical framework to include the influences of nonbreaking surface waves in ocean general circulation models is established
based on Reynolds stresses and fluxes terms derived from surface wave-induced fluctuation. An expression for the wave-induced
viscosity and diffusivity as a function of the wave number spectrum is derived for infinite and finite water depths; this
derivation allows the coupling of ocean circulation models with a wave number spectrum numerical model. In the case of monochromatic
surface wave, the wave-induced viscosity and diffusivity are functions of the Stokes drift. The influence of the wave-induced
mixing scheme on global ocean circulation models was tested with the Princeton Ocean Model, indicating significant improvement
in upper ocean thermal structure and mixed layer depth compared with mixing obtained by the Mellor–Yamada scheme without the
wave influence. For example, the model–observation correlation coefficient of the upper 100-m temperature along 35° N increases
from 0.68 without wave influence to 0.93 with wave influence. The wave-induced Reynolds stress can reach up to about 5% of
the wind stress in high latitudes, and drive 2–3 Sv transport in the global ocean in the form of mesoscale eddies with diameter
of 500–1,000 km. The surface wave-induced mixing is more pronounced in middle and high latitudes during the summer in the
Northern Hemisphere and in middle latitudes in the Southern Hemisphere. 相似文献
9.
Dye plumes were generated at three depths in the seasonal thermocline between 7 and 11 m, 22 km south of Key West on 21 August 1980 and photographed at about 10 second intervals with an underwater camera system. Eleven pairs of consecutive pictures are analyzed to determine the mean current vertical shear and the width of the plumes by positioning reference points relative to the rod attached to the camera system. The relative distances of reference points are calibrated with the stereophotogrammetric method for one pair. The eddy diffusivity is calculated by use of a model of turbulent diffusion developed byTaylor (1921). Its values range from 5 to 25 cm2s–1 for the plume widths ranging from 33 to 132 cm. The Richardson number is calculated for each pair of pictures with the vertical density gradient estimated from temperature profiles. Its values are higher than the critical value of 0.25 except for one case. The diffusivity was higher by orders of magnitude than the molecular one and indicates the presence of turbulence together with billow like features of the plumes in spite of high Richardson numbers. This suggests that the billow turbulence might be caused by effects of surface gravity waves and not by the Kelvin-Helmholtz instability. 相似文献
10.
Marcos E. C. Bernardes Mark A. Davidson Keith R. Dyer Ken J. George 《Ocean Dynamics》2006,56(3-4):186-197
The main objective of this paper is to address the principal mechanisms involved in the medium-term (order of months to years) morphodynamic evolution of estuaries through the application of a process-based numerical modelling. The Teign estuary (Teignmouth, UK) is the selected site. The system is forced by the macrotidal semi-diurnal tide in the English Channel and is perturbed to a minor extent by high river discharge events (freshets). Although waves have a definite influence on the adjacent coastal area, Wells (Teignmouth Quay Development Environmental Statement: Changes to Physical Processes. Report R.984c:140. ABP Marine Environmental Research Ltd., Southampton, 2002b) suggested that swell waves do not enter the estuary. Hence, wave effects are neglected in this study, as only tides and the river discharge are taken into account. The sediment grain size is highly variable, but mainly sandy. Within the frame of the COAST3D project (), four bathymetric surveys of the adjacent coastal area were carried out at a nearly weekly intervals. The outer estuary and the adjacent coastal area were also surveyed every 6 months as part of the COASTVIEW project (). Based on these data and on continuously measured parameters, such as water level, waves, wind and river discharge, numerical modelling of the morphodynamic processes can be tested. To replicate the morphological changes in the medium-term within a feasible simulation time, forcing conditions are reduced through the use of an input reduction method (called ensemble technique). In this study, simulations are based on the coupling between Telemac-2D and its non-cohesive sediment transport module, Sisyphe (version 5.3 for both modules). Three different sediment transport formulae were tested: (1) Engelund and Hansen (A monograph on sediment transport in alluvial streams, 3rd edn. Technological University of Denmark, Copenhagen, 1967) including the modifications proposed by Chollet and Cunge (J Hydraul Eng 17(1):1–13, 1979); (2) Bijker (Mechanics of sediment transport by the combination of waves and current. In: Design and reliability of coastal structures. 23rd international conference on Coastal Engineering, pp 147–173, 1968) and (3) Soulsby (Dynamics of Marine Sands. A manual for practical applications. HR Wallingford, Wallingford, p 142, 1997) modified version of van Rijn [J Hydraul Eng 110(10):1431–1456, 1984a, J Hydraul Eng 110(11):1613–1641, 1984b] formulation. Both a qualitative (i.e. visual comparison) and a quantitative tool [Brier Skill Score (BSS); described in Sutherland et al. in Coast Eng 51:917–939, 2004b] are applied to assess the similarity of simulations when compared to model predictions and observations. Tests confirmed the reliability and time efficiency of the ensemble technique, since it reproduced very well the results of a reference run, a computation based on the observed boundary conditions. For the spring-neap cycle modelled, the BSS was of 0.91 (a perfect modelling would have a BSS of 1), with a reduction in the simulation time on the order of 80%. For the 6-month-period simulation, results were also excellent: BSS=0.92 and a computer time reduction of 85%. In principle, this method has the advantage of being applied to any process-based numerical model. 相似文献
11.
Initial ensemble generation and validation for ocean data assimilation using HYCOM in the Pacific 总被引：2，自引：0，他引：2
A method to initialize an ensemble, introduced by Evensen (Physica, D 77:108–129, 1994a; J Geophys Res 99(C5):10143–10162, 1994b; Ocean Dynamics 53:343–367, 2003), was applied to the Ocean General Circulation Model (OGCM) HYbrid Coordinate Ocean Model (HYCOM) for the Pacific Ocean.
Taking advantage of the hybrid coordinates, an initial ensemble is created by first perturbing the layer interfaces and then
running the model for a spin-up period of 1 month forced by randomly perturbed atmospheric forcing fields. In addition to
the perturbations of layer interfaces, we implemented perturbations of the mixed layer temperatures. In this paper, we investigate
the quality of the initial ensemble generated by this scheme and the influence of the horizontal decorrelation scale and vertical
correlation on the statistics of the resulting ensemble. We performed six ensemble generation experiments with different combinations
of horizontal decorrelation scales and with/without perturbations in the mixed layer. The resulting six sets of initial ensembles
are then analyzed in terms of sustainability of the ensemble spread and realism of the correlation patterns. The ensemble
spreads are validated against the difference between model and observations after 20 years of free run. The correlation patterns
of six sets of ensemble are compared to each other. This study shows that the ensemble generation scheme can effectively generate
an initial ensemble whose spread is consistent with the observed errors. The correlation pattern of the ensemble also exhibits
realistic features. The addition of mixed layer perturbations improves both the spread and correlation. Some limitations of
the ensemble generation scheme are also discussed. We found that the vertical shift of isopycnal coordinates provokes unrealistically
large deviations in shallow layers near the islands of the West Pacific. A simple correction circumvents the problem.
相似文献
| Liying WanEmail: |
12.
Although large-scale tidal and inertial motions dominate the kinetic energy and vertical current shear in shelf seas and ocean, short-scale internal waves at higher frequencies close to the local buoyancy frequency are of some interest for studying internal wave breaking and associated diapycnal mixing. Such waves near the upper limit of the inertio-gravity wave band are thought to have relatively short O (102–103 m) horizontal scales and to show mainly up- and downward motions, which contrasts with generally low aspect ratio large-scale ocean currents. Here, short-term vertical current (w) observations using moored acoustic Doppler current profiler (ADCP) are presented from a shelf sea, above a continental slope and from the open ocean. The observed w, with amplitudes between 0.015 and 0.05 m s−1, all span a considerable part of the water column, which is not a small vertical scale O(water depth) or O (100–500 m, the maximum range of observations), with either 0 or π phase change. This implies that they actually represent internal waves of low vertical modes 1 or 2. Maximum amplitudes are found in layers of largest stratification, some in the main pycnocline bordering the frictional bottom boundary layer, suggesting a tidal source. These ‘pycnocline-w’ compose a regular train of (solitary) internal waves and linearly decrease to small values near surface and bottom. 相似文献
13.
采用高精度的拟谱方法,数值模拟了层结海洋中小振幅内行进波的演变和破碎过程.在演变过程中,导致内波破碎的PSI不稳定机制在共振相互作用中逐渐占据主导地位,能量从初级波向低频、高波数运动缓慢传递并形成一次级波包,随即破碎发生.破碎后产生的层化湍流引起的强烈混合以及湍流间歇性可从总能量和涡度峰度随时间的变化趋势看出.我们分析了层化湍流的一些统计特性,包括动能和有效位能沿垂向波数ky的功率谱.结果表明,动能和有效位能谱都存在一个谱段满足k-3y律,且分别可表示为01N4k-3y和02N4k-3y(N为Brunt Visl频率),通常称其为浮力子区.另外,我们分析了Cox数(湍流扩散系数与分子扩散系数之比),在层化湍流维持在一定强度时,计算结果和由海洋内区观测(远离内波强生成源和复杂地形)所推测的结论较为吻合. 相似文献
14.
Hans van Haren 《Ocean Dynamics》2012,62(8):1123-1137
During a period of 3?days, an accurate bottom-pressure sensor and a four-beam acoustic Doppler current profiler (ADCP) were mounted in a bottom frame at 23?m in a narrow sea strait with dominant near-rectilinear tidal currents exceeding 1?m?s?1 in magnitude. The pressure record distinguishes small and short surface waves, wind- and ferry-induced near-surface turbulence and waves, large turbulent overturns and high-frequency internal waves. Typical low-frequency turbulent motions have amplitudes of 50?N?m?2 and periods of about 50?s. Such amplitudes are also found in independent estimates of non-hydrostatic pressure using ADCP data, but phase relationships between these data sets are ambiguous probably due to the averaging over the spread of the slanted acoustic beams. ADCP's echo amplitudes that are observed in individual beams show much better phase correspondence with near-bottom pressure, whether they are generated near the surface (mainly air bubbles) or near the bottom (mainly suspended sediment). These 50-s motions are a mix of turbulence and internal waves, but they are not due to surface wave interactions, and they are not directly related to the main tidal flow. Internal waves are supported by stratification varying between extremely strong thin layer and very weak near-homogeneous stratification. They are driven by the main flow over 2-m amplitude sand-wave topography, with typical wavelengths of 150?m. 相似文献
15.
Guillaume Martinat Ying Xu Chester E. Grosch Andrés E. Tejada-Martínez 《Ocean Dynamics》2011,61(9):1369-1390
Turbulent shear flows on shallow continental shelves (here shallow means that the interaction with the solid, no-slip bottom
is important) are of great importance because tide- and wind-driven flows on the shelf are drivers of the transfer of momentum,
heat, and mass (gas) across the air–sea interface. These turbulent flows play an important role because vertical mixing and
current are vectors for the transport of sediment and bioactive material on continental shelves. Understanding the dynamics
of this class of flows presents complications because of the presence of a free surface and also because the flow can be driven
by a pressure gradient (a tidal current), a stress at the free surface (a wind-driven current), or a combination of both.
In addition, the flow can be modified by the presence of a wave field that can induce Langmuir circulation (Langmuir, Science
87:119–123, 1938). Large eddy simulation is used to quantify the effects of pressure gradient and wind shear on the distinctive structures
of the turbulent flow. From these computations, an understanding of the physics governing the turbulence of pressure-driven
and wind-driven flows, how they can interact in a normal or a tangential direction, and the effect of wave forcing on these
flows is obtained. 相似文献
16.
The current study deals with a parameterization of diapycnal diffusivity in an ocean model. The parameterization estimates the diapycnal diffusivity depending on the location of tidal-related energy dissipation over rough topography. The scheme requires a bottom roughness map that can be chosen depending on the scales of topographic features. Here, we implement the parameterization on an ocean general circulation model, and we examine the sensitivity of the modeled circulations to different spatial scales of the modeled bottom roughness. We compare three simulations that include the tidal mixing scheme using bottom roughness calculated at three different ranges of spatial scales, with the largest scale varying up to 200 km. Three main results are discussed. First, the dependence of the topographic spectra with depth, characterized by an increase in spectral energy over short length scales in the deep ocean, influences the vertical profile of the diffusivity. Second, the changes in diffusivities lead to different equilibrium solutions in the Atlantic meridional overturning circulation and bottom circulation. In particular, the lower cell of the Atlantic overturning and the bottom water transport in the Pacific Ocean are stronger for stronger diffusivities at the corresponding basins and depths, and the strongest when using the small-scale roughness map. Third, a comparison of the density fields of the three simulations with the density field of World Ocean Atlas dataset, from which the models are initialized, shows that among the simulations with three different roughness maps, the one using small-scale bottom roughness map has the smallest density bias.
相似文献17.
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
18.
混合是海洋中普遍存在的一种海水运动形式,对多个海洋学分支的研究具有重要的影响.随着物理海洋学的研究重心从大尺度向中小尺度现象过渡,近年来混合问题的研究重心也逐渐转向了中小尺度现象.内波与中尺度涡都是非常重要的中小尺度物理海洋学现象,对海洋能量在不同尺度中的级联发挥着重要的作用.本文基于地震海洋学研究了海洋混合参数的提取方法,并以南海内波和地中海涡旋为例进行了计算和分析.结果显示,南海内波在200~600m深度范围内所引起的混合可达10-2.79 m2·s-1左右,比大洋的统计结果10-5 m2·s-1高出两个数量级以上.而地中海涡旋所引起的湍流混合率可达10-3.44 m2·s-1左右,与大洋统计结果相比高出1.5个数量级左右,并且地中海涡旋下边界的混合要强于上边界,这一特征与前人的研究一致,另外涡旋上边界之上以及侧边界的外侧也具有非常高的混合率. 相似文献
19.
Eleftheria Exarchou Jin Song von Storch Johann H. Jungclaus 《Ocean Dynamics》2012,62(10-12):1545-1563
The current study deals with a parameterization of diapycnal diffusivity in an ocean model. The parameterization estimates the diapycnal diffusivity depending on the location of tidal-related energy dissipation over rough topography. The scheme requires a bottom roughness map that can be chosen depending on the scales of topographic features. Here, we implement the parameterization on an ocean general circulation model, and we examine the sensitivity of the modeled circulations to different spatial scales of the modeled bottom roughness. We compare three simulations that include the tidal mixing scheme using bottom roughness calculated at three different ranges of spatial scales, with the largest scale varying up to 200?km. Three main results are discussed. First, the dependence of the topographic spectra with depth, characterized by an increase in spectral energy over short length scales in the deep ocean, influences the vertical profile of the diffusivity. Second, the changes in diffusivities lead to different equilibrium solutions in the Atlantic meridional overturning circulation and bottom circulation. In particular, the lower cell of the Atlantic overturning and the bottom water transport in the Pacific Ocean are stronger for stronger diffusivities at the corresponding basins and depths, and the strongest when using the small-scale roughness map. Third, a comparison of the density fields of the three simulations with the density field of World Ocean Atlas dataset, from which the models are initialized, shows that among the simulations with three different roughness maps, the one using small-scale bottom roughness map has the smallest density bias. 相似文献