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1.
John A. T. Bye 《地球物理与天体物理流体动力学》2013,107(1-4):135-166
Abstract A unified analysis is given of the critical conditions for the onset of stratification due to either a vertical or a horizontal buoyancy flux, with tidal or wind stirring. The critical conditions for the onset of stratification with a horizontal buoyancy flux are found to be of the form of ratios of the tidal slope, or wind setup, to the equivalent surface slope due to the lateral density gradient. These ratios, which are easily determined from sea data, indicate that the profiles of critical flux Richardson Number, averaged over the stirring cycle, are similar to those inferred from the laboratory experiments of Hopfinger and Linden (1982) in which there is zero mean shear turbulence with a stabilising buoyancy flux, and also that the efficiency for the conversion of kinetic energy to potential energy for tidal stirring is similar to that for wind stirring. The observed much greater efficiency for wind stirring, compared with tidal stirring with a vertical buoyancy flux, is also consistent with the existence of flux Richardson Number profiles in the sea similar to those occurring in the corresponding laboratory experiments. Using the solution of the turbulent kinetic energy equation for the water column, the relative importance of the production of turbulent kinetic energy, and its diffusion by turbulence are assessed, and the critical conditions for the onset of stratification with a vertical buoyancy flux are shown to reduce the classical Simpson—Hunter form. 相似文献
2.
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
3.
The dispersion of floaters, small organic particles lighter than water, on the free surface of an open turbulent channel flow subject to thermal stratification is studied by Direct Numerical Simulation (DNS) of turbulence and Lagrangian Particle Tracking (LPT). Constant heat flux is maintained at the free surface of the channel, the bottom wall is adiabatic and the turbulent flow is driven by a pressure gradient. This archetypal flow setup mimics an environmentally plausible situation which can be found in terrestrial water bodies. The free surface turbulence characteristic of such flows has a strong influence on the distribution of the floaters: the objective of this work is to study the effect of different regimes of stable stratification on the surface distribution of floaters. The distribution of the floaters can possibly influence the transfer of chemical species across the water/atmosphere interface. Our results show that the modification of turbulence due to the thermal stratification strongly influences the settling velocity of floaters in the bulk of the flow. At the surface, stratification effects are also observed on the clustering of the floaters: the filamentary patterns of floaters observed in unstratified turbulence are progressively lost as thermal stratification increases, and the distribution of the floaters remains roughly two-dimensional. 相似文献
4.
The physical nature of motions with scales intermediate between approximately isotropic turbulence and quasi-linear internal gravity waves is not understood at the present time. Such motions play an important role in the energetics of small scales processes, both in the ocean and in the atmosphere, and in vertical transport of heat and constituents. This scale range is currently interpreted either as a saturated gravity waves field or as a buoyancy range of turbulence.We first discuss some distinctive predictions of the classical (Lumley, Phillips) buoyancy range theory, recently improved (Weinstock, Dalaudier and Sidi) to describe potential energy associated with temperature fluctuations. This theory predicts the existence of a spectral gap in the temperature spectra and of an upward mass flux (downward buoyancy and heat fluxes), strongly increasing towards large scales. These predictions are contrasted with an alternate theory, assuming energetically insignificant buoyancy flux, proposed by Holloway.Then we present experimental evidences of such characteristic features obtained in the lower stratosphere with an instrumented balloon. Spectra of temperature, vertical velocity, and cospectra of both, obtained in homogeneous, weakly turbulent regions, are compared with theoretical predictions. These results are strongly consistent with the improved classical buoyancy range theory and support the existence of a significant downward heat flux in the buoyancy range.The theoretical implications of the understanding of this scale range are discussed. Many experimental evidences consistently show the need for an anisotropic theory of the buoyancy range of turbulence. 相似文献
5.
An explanation for salinity- and SPM-induced vertical countergradient buoyancy fluxes 总被引：1，自引：1，他引：0
Measurements of turbulent fluctuations of velocity, salinity, and suspended particulate matter (SPM) are presented. The data
show persistent countergradient buoyancy fluxes. These countergradient fluxes are controlled by the ratio of vertical turbulent
kinetic energy (VKE) and available potential energy (APE) terms in the buoyancy flux equation. The onset of countergradient
fluxes is found to approximately coincide with larger APE than VKE. It is shown here that the ratio of VKE to APE can be written
as the square of a vertical Froude number. This number signifies the onset of the dynamical significance of buoyancy in the
transport of mass. That is when motions driven by buoyancy begin to actively determine the vertical turbulent transport of
mass. Spectral and quadrant analyses show that the occurrence of countergradient fluxes coincides with a change in the relative
importance of turbulent energetic structures and buoyancy-driven motions in the transport of mass. Furthermore, these analyses
show that with increasing salinity-induced Richardson number (Ri), countergradient contributions expand to the larger scales of motions and the relative importance of outward and inward
interactions increases. At the smaller scales, at moderate Ri, the countergradient buoyancy fluxes are physically associated with an asymmetry in transport of fluid parcels by energetic
turbulent motions. At the large scales, at large Ri, the countergradient buoyancy fluxes are physically associated with convective motions induced by buoyancy of incompletely
dispersed fluid parcels which have been transported by energetic motions in the past. Moreover, these convective motions induce
restratification and enhanced settling of SPM. The latter is generally the result of salinity-induced convective motions,
but SPM-induced buoyancy is also found to play a role. 相似文献
6.
Northern rivers experience freeze‐up over the winter, creating asymmetric under‐ice flows. Field and laboratory measurements of under‐ice flows typically exhibit flow asymmetry and its characteristics depend on the presence of roughness elements on the ice cover underside. In this study, flume experiments of flows under a simulated ice cover are presented. Open water conditions and simulated rough ice‐covered flows are discussed. Mean flow and turbulent flow statistics were obtained from an Acoustic Doppler Velocimeter (ADV) above a gravel‐bed surface. A central region of faster flow develops in the middle portion of the flow with the addition of a rough cover. The turbulent flow characteristics are unambiguously different when simulated ice covered conditions are used. Two distinct boundary layers (near the bed and in the vicinity of the ice cover, near the water surface) are clearly identified, each being characterized by high turbulent intensity levels. Detailed profile measurements of Reynolds stresses and turbulent kinetic energy indicate that the turbulence structure is strongly influenced by the presence of an ice cover and its roughness characteristics. In general, for y/d > 0·4 (where y is height above bed and d is local flow depth), the addition of cover and its roughening tends to generate higher turbulent kinetic energy values in comparison to open water flows and Reynolds stresses become increasingly negative due to increased turbulence levels in the vicinity of the rough ice cover. The high negative Reynolds stresses not only indicate high turbulence levels created by the rough ice cover but also coherent flow structures where quadrants one and three dominate. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
7.
We determine the nonlinear drift velocities of the mean magnetic field and nonlinear turbulent magnetic diffusion in a turbulent convection. We show that the nonlinear drift velocities are caused by three kinds of the inhomogeneities; i.e., inhomogeneous turbulence, the nonuniform fluid density and the nonuniform turbulent heat flux. The inhomogeneous turbulence results in the well-known turbulent diamagnetic and paramagnetic velocities. The nonlinear drift velocities of the mean magnetic field cause the small-scale magnetic buoyancy and magnetic pumping effects in the turbulent convection. These phenomena are different from the large-scale magnetic buoyancy and magnetic pumping effects which are due to the effect of the mean magnetic field on the large-scale density stratified fluid flow. The small-scale magnetic buoyancy and magnetic pumping can be stronger than these large-scale effects when the mean magnetic field is smaller than the equipartition field. We discuss the small-scale magnetic buoyancy and magnetic pumping effects in the context of the solar and stellar turbulent convection. We demonstrate also that the nonlinear turbulent magnetic diffusion in the turbulent convection is anisotropic even for a weak mean magnetic field. In particular, it is enhanced in the radial direction. The magnetic fluctuations due to the small-scale dynamo increase the turbulent magnetic diffusion of the toroidal component of the mean magnetic field, while they do not affect the turbulent magnetic diffusion of the poloidal field. 相似文献
8.
This numerical investigation was carried out to advance mechanistic understanding of sediment transport under sheet flow conditions. An Euler–Euler coupled two-phase flow model was developed to simulate fluid–sediment oscillatory sheet flow. Since the concentration of sediment particles is high in such flows, the kinematics of the fluid and sediment phases are strongly coupled. This model includes interaction forces, intergranular stresses and turbulent stress closure. Each phase was modeled via the Reynolds-Averaged Navier–Stokes equations, with interphase momentum conservation accounting for the interaction between the phases. The generation and transformation of turbulence was modeled using the two-equation k–ε turbulence model. Concentration and sediment flux profiles were compared with experimental data for sheet flow conditions considering both symmetric and asymmetric oscillatory flows. Sediment and fluid velocity variations, concentration profiles, sediment flux and turbulence parameters of wave-generated sheet flow were studied numerically with a focus on sediment transport characteristics. In all applications, the model predictions compared well with the experimental data. Unlike previous investigations in which the flow is driven by a horizontal pressure gradient, the present model solves the Navier–Stokes equations under propagating waves. The model’s ability to predict sediment transport under oscillatory sheet flow conditions underscores its potential for understanding the evolution of beach morphology. 相似文献
9.
《Journal of Volcanology and Geothermal Research》2005,139(1-2):33-57
The dynamics of pyroclastic surges accompanied by co-ignimbrite plumes is investigated numerically. The numerical simulations are performed with a newly developed numerical model, which is based on the Navier–Stokes equations for time-dependent flows of a compressible fluid in two-dimensional Cartesian coordinates. We regard pyroclastic surges as dilute turbulent suspensions in which hot gases and fine solid particles are homogeneously mixed owing to vigorous turbulence. In other words, the gas–particle mixture is treated as a single-phase fluid whose bulk density is represented by averaging the density of each component in the numerical model. We focus on the effect of buoyancy forces generated by the thermal expansion of the air mixed into pyroclastic surges from the calm surroundings. For our purpose, the numerical model is designed to simulate relatively simple flows spreading over a horizontal flat surface. Topographic irregularity and the sedimentation process of solid particles are neglected in the present simulations. The motion of pyroclastic surges is generated by the instantaneous release of a gas–particle mixture whose density is initially larger than the ambient air density and changes nonlinearly with the temperature and concentration of suspended solid particles. Turbulent mixing is evaluated by adopting the Smagorinsky model. By employing cubic interpolated pseudo-particle (CIP) method and C-CUP method, we obtain the fine structure of flows. The behavior of calculated flows agrees fairly well with observed pyroclastic surges in nature. The current head, which remains hot and dense, keeps spreading over a horizontal surface at a speed of about 20 m s−1. The spreading speed is of the order of the speed of a gravity current that excludes the influence of thermal expansion. Besides, turbulent mixing between the basal dense layer and the ambient air is enhanced by the successive development of an interfacial less-dense layer. This results in the formation of a number of buoyant plumes rising above a horizontally spreading current. Consequently, the tails of the current thickens as time progresses. A parametric study shows that the initial temperature of a gas–particle mixture should be higher than about 600 K when buoyant plumes occur owing to the thermal expansion of mixed air. The result is quantitatively interpreted by introducing a diagram that describes the relationship among the bulk density, temperature and concentration of solid particles suspended in pyroclastic surges. 相似文献
10.
Various circulatory mechanisms within the mesoscale range of tidal flows in Eastern Long Island Sound (LIS) are investigated. Two 13-day time series of current measurements were obtained using three moorings arranged in the form of a triangle. Each mooring contains a single Aanderaa current meter located at a depth of 18 to 20 m.Statistical analysis provides evidence of residual circulation effects of topographic origin in the average flow field. The relative importance and periods of orderly residual patterns and turbulent motions are examined using kinetic energy spectra for the longitudinal and transverse components of the flow. The analyses reveal residual oscillations having nearly the same time scales for all moorings and horizontal turbulent eddies which display a diversity of temporal scales ranging from 0.6 to 2.5 h. Within the high-frequency band of temporal scales the computed spectral curves for longitudinal components generally follow Kolmogorov's ‘ ?5/3 law’, while the spectra of transverse components have a slope of approximately ?1.A computational technique is described to calculate horizontal eddy coefficients in various directions using the mixing length hypothesis. The coefficients developed using this technique are used to investigate various oscillatory mechanisms in the tidal flows. 相似文献
11.
In this study,the characteristics of turbulence transport and intermittency and the evolutionary mechanisms were studied in different pollution stages of heavy haze weather from December 2016 to January 2017 in the Beijing area using the method developed by Ren et al.(2019) as the automatic identification of atmospheric spectral gaps and the reconstruction of atmospheric turbulence sequences.The results reveal that turbulence intermittency is the strongest in the cumulative stage(CS)of heavy haze weather,followed by in the transport stage(TS),and it is the weakest in the dissipation stage(DS).During the development and accumulation of haze pollution,buoyancy contributes negatively to turbulent kinetic energy(TKE),and horizontal wind speed is low.The classical turbulent motion is often affected by submesoscale motion.As a result,the calculation results of turbulence parameters are affected by submesoscale motion,which causes intensified turbulence intermittency.During the dissipation of pollution,the downward momentum transfer induced by low-level jets provides kinetic energy for turbulent motion in the near surface layer.The turbulent mixing effect is enhanced,and intermittency is weakened.Due to the intermittency of atmospheric turbulence,turbulence parameters calculated from the original fluctuation of meteorological elements may be overestimated.The overestimation of turbulence parameters in the CS is the strongest,followed by the TS,and the DS is the weakest.The overestimation of turbulent fluxes results in an overestimation of atmospheric dissipation capability that may cause an underestimation of pollutant concentrations in the numerical simulations of air quality. 相似文献
12.
A hydrodynamic model is employed to derive the magnitude of on-shelf fluxes through a shelf-break canyon for a wide range of canyon sizes and ambient oceanic conditions. Predicted canyon-upwelling fluxes are of the order of 0.05–0.1 Sv (1 Sv=1 million m3/s), being several orders of magnitude greater than upslope fluxes in the bottom Ekman layer on the ambient continental slope. On the basis of ∼150 simulations conducted, a bulk formula of upwelling flux in a submarine canyon is derived. For typical conditions, the upwelling flux varies quadratically with forcing strength (speed of incident flow), linearly with canyon depth, and is inversely proportional to the buoyancy frequency of the density stratification inside the canyon. Other parameters such as density stratification above shelf-break depth and bottom friction are found to have minor influences on the resultant canyon-upwelling flux. 相似文献
13.
Numerical experiments were conducted to investigate the ocean's response to the precipitation. A squall line observed in TOGA COARE was simulated. The simulation reproduced some of the observed ocean responses to the precipitation, such as the formation of a fresh water layer, surface cooling and the variation of upper layer turbulent mixing. The precipitation-induced fresh layer can cause the vertical turbulent diffusivities to decrease from the surface to a depth of about 11–13 meters within a few hours. After the rainfall, the turbulence increases near the surface of the ocean due to the combined effect of increased shear and wind forcing, but decreases with depth due to the development of a stable layer. The main reason for the turbulence variation is the decrease in the vertical turbulence flux below the surface fresh layer because of increased static stability. Sensitivity experiments reveal that the sea-surface temperature increases faster after rainfall due to the formation of a shallow fresh water layer near the surface. 相似文献
14.
We present analyses of plume rise into a linearly stratified environment, either with or without a uniform horizontal flow. In the case of a still ambient, we collate results on plume spreading height and volume flux, enabling the speed of the spreading intrusion in the buoyancy-inertia regime to be expressed in terms of the fundamental parameters of plume buoyancy flux and ambient buoyancy frequency. A theoretical expression for the final volume flux emanating from the plume-rise region, in terms of maximum rise height, is also derived. Hence it is shown that the ratio of the intrusion radius to the maximum rise height is a simple function of ambient buoyancy frequency and time. In the case of a wind, we analyse the theoretical model for a rising plume to obtain predictions for the downwind volume flux, and subsequent lateral spread, in the limits of strong and weak wind. We identify a regime of very weak wind, which may be modelled as a passive advection of plume flow in the still case as a first approximation. Numerical solutions of a general model are presented which show that it predicts a peak in entrainment, and hence volume-flux growth, in the case of intermediate wind strength. We verify the crosswind model predictions of lateral spread, upwind penetration and entrainment by comparison with large-eddy simulations. 相似文献
15.
16.
《国际泥沙研究》2019,34(6):550-563
The effects of turbulence on water-sediment mixtures is a critical issue in studying sediment-laden flows. The sediment concentrations and particle inertia play a significant role in the effects of turbulence on mixtures. A two-phase mixture turbulence model was applied to investigate the turbulence mechanisms affecting sediment-laden flows. The two-phase mixture turbulence model takes into account the complicated mechanisms arising from interphase transfer of turbulent kinetic energy, particle collisions, and stratification. The turbulence in sediment-laden flows is the result of the interaction of four factors, i.e. the production, dissipation, diffusion, and inter-phase transfer of turbulent kinetic energy of mixtures. The turbulence production and dissipation are two dominant processes which balance the turbulent kinetic energy of mixtures. The turbulence production represents turbulence intensity, while the inter-phase transfer of turbulent kinetic energy denotes the effect of particles on the turbulence of sediment-laden flows. Although, the magnitude of the inter-phase interaction term is much less than that of the turbulence production and dissipation terms, due to an approximate local balance between production and dissipation of the turbulent kinetic energy, even the small order of the inter-phase interaction has a significant impact on the turbulent balance of sediment-laden flows. The presence of particles plays a duel role in the turbulence dissipation of mixtures: both promotion and suppression. An important parameter used to determine the turbulent viscosity of mixtures, which is constant in clear water, is the function of the sediment concentration and particle inertia in sediment-laden flows. 相似文献
17.
18.
Steady convective exchange flows down slopes 总被引：2，自引：0，他引：2
Jeff J. Sturman Carolyn E. Oldham Greg N. Ivey 《Aquatic Sciences - Research Across Boundaries》1999,61(3):260-278
19.
The flow caused by the discharge of freshwater underneath a glacier into an idealized fjord is simulated with a 2D non-hydrostatic model. As the freshwater leaves horizontally the subglacial opening into a fjord of uniformly denser water it spreads along the bottom as a jet, until buoyancy forces it to rise. During the initial rising phase, the plume meanders into complex flow patterns while mixing with the surrounding fluid until it reaches the surface and then spreads horizontally as a surface seaward flowing plume of brackish water. The process induces an estuarine-like circulation. Once steady-state is reached, the flow consists of an almost undiluted buoyant plume rising straight along the face of the glacier that turns into a horizontal surface layer thickening as it flows seaward. Over the range of parameters examined, the estuarine circulation is dynamically unstable with gradient Richardson number at the sheared interface having values of <1/4. The surface velocity and dilution factors are strongly and non-linearly related to the Froude number. It is the buoyancy flux that primarily controls the resulting circulation with the momentum flux playing a secondary role. 相似文献
20.
Modeling of suspended sediment particle movement in surface water can be achieved by stochastic particle tracking model approaches.In this paper,different mathematical forms of particle tracking models are introduced to describe particle movement under various flow conditions,i.e.,the stochastic diffusion process,stochastic jump process,and stochastic jump diffusion process.While the stochastic diffusion process can be used to represent the stochastic movement of suspended particles in turbulent flows,the stochastic jump and the stochastic jump diffusion processes can be used to describe suspended particle movement in the occurrences of a sequence of extreme flows.An extreme flow herein is defined as a hydrologic flow event or a hydrodynamic flow phenomenon with a low probability of occurrence and a high impact on its ambient flow environment.In this paper,the suspended sediment particle is assumed to immediately follow the extreme flows in the jump process（i.e.the time lag between the flow particle and the sediment particle in extreme flows is considered negligible）.In the proposed particle tracking models,a random term mainly caused by fluid eddy motions is modeled as a Wiener process,while the random occurrences of a sequence of extreme flows can be modeled as a Poisson process.The frequency of occurrence of the extreme flows in the proposed particle tracking model can be explicitly accounted for by the Poisson process when evaluating particle movement.The ensemble mean and variance of particle trajectory can be obtained from the proposed stochastic models via simulations.The ensemble mean and variance of particle velocity are verified with available data.Applicability of the proposed stochastic particle tracking models for sediment transport modeling is also discussed. 相似文献