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1.
DAN H. SHUGAR RAY KOSTASCHUK JAMES L. BEST† DANIEL R. PARSONS‡ STUART N. LANE§ OSCAR ORFEO¶ RICHARD J. HARDY§ 《Sedimentology》2010,57(1):252-272
The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate. 相似文献
2.
The dynamics of turbulent, transitional and laminar clay-laden flow over a fixed current ripple 总被引：1，自引：0，他引：1
Most aqueous sedimentary environments contain varying concentrations of fine‐grained, often clay‐rich, sediment that is transported in suspension and may modify the properties of the flow and underlying mobile bed. This paper presents results from a series of laboratory experiments examining the mean and turbulent properties of clay‐laden (kaolinite) flows, of various volumetric sediment concentrations between 0·046% and 12·7%, moving over a fixed, idealized current ripple. As the kaolinite concentration was raised, with flow velocity and depth constant, four flow types were observed to occur: (i) turbulent flow, in which flow separation is dominant in the leeside of the ripple; (ii) turbulence‐enhanced transitional flow, in which turbulence in the leeside separation zone region is enhanced; (iii) turbulence‐attenuated transitional flow, in which turbulence along the separation zone shear layer and in the free flow above it becomes damped, eventually leading to a reduction in the size of the separation zone wake region; and (iv) laminar plug flow, in which turbulence is damped and flow is almost stagnant in the lee of the ripple. Such modulation of turbulence by increasing clay concentrations suggests that many paradigms of flow and bedform dynamics, which have been based on extensive past work in clear water flows, require revision. The present results highlight a need to fully characterize the boundary conditions for turbulence modulation as a function of clay type and applied flow conditions, and the effects of such flows on fully mobile cohesionless beds. 相似文献
3.
通过水槽试验研究浅水非线性波作用下沙纹床面底层流动特性,利用CCD图像技术观测分析非对称沙纹的形成和演化规律。利用声学多普勒测速仪(ADV)测量非对称沙纹底床上的流场,得到了不同波高、周期、水深条件下的沙纹峰顶和谷底断面的瞬时速度。试验结果分析表明,浅水非线性波作用下床面上形成非对称沙纹,其近底流速具有较强紊动特性,随着距床面距离的增大紊动强度逐渐减弱。在水流方向改变时,沙纹背部具有明显漩涡运动。沙纹背后形成的漩涡能起到维持沙纹的作用。浅水非线性波作用下,沙纹的形成原因主要是床面泥沙颗粒在非对称流动和床面近壁粘性底层中漩涡结构动力作用下,作受迫摆动、推移所致。 相似文献
4.
Preliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms-1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near-bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low-relief bedwaves on upper-stage plane beds are ubiquitous and give rise to laterally extensive, mm-thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves. 相似文献
5.
N. LANCASTER 《Sedimentology》1989,36(2):273-289
Observations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the Gran Desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type. Such interactions lead to deposition of sand in the central parts of the dune, giving rise to its pyramidal shape, as well as to some extension of the linear arms. The major arms of the dune studied are oriented NE-SW, or transverse to summer SSE and winter NNW winds. An avalanche face up to 10 m high develops during the course of each season. Flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune, where it is deposited as wind ripples migrate into zones of locally reduced flow velocity. Reattachment of the separated flow occurs on the lower part of the N or S arms, parallel to the flow. Spring westerly winds move sand obliquely up the S and N arms of the dune and outwards on the E arm. Large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms. Formation of star dunes in the Gran Desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge. Convergent leeside secondary flows are developed, which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune. Examples of star dunes at different stages of their development can be documented. 相似文献
6.
Vertical profiles of suspended fine sediment concentration, tidal current velocity, and salinity were measured in May 1994 in the Changjiang Estuary. High resolution concentration profiles were obtained by using a 0.5-MHz acoustic suspended sediment, monitor. High temporal and spatial resolution acoustic profiling of fine suspension concentration provides both the instantaneous vertical profile of concentration and information on the continuous dynamic processes of fine sediment erosion, transport, and deposition. Calibrated acoustic images revealed 1) highly stratified suspensions, 2) resuspension of the cohesive mud, bed, and 3) re-entrainment of the near-bed high concentration suspensions by turbulent shear flow. Within the near-bed high concentration suspensions, two different frequencies of highly episodic resuspension processes were identified: high frequency resuspension, lasting, a few seconds low frequency resuspension, lasting a few minutes. The highest concentrations, associated with low velocity and high salinity, were found close to the cohesive mud bed. Lutoclines were persistent features during the measurements. 相似文献
7.
8.
Mean flow and turbulence structure over fixed, two-dimensional dunes: implications for sediment transport and bedform stability 总被引：4，自引：0，他引：4
Detailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed, two-dimensional dunes in a laboratory channel. Laser Doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength. The density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters, which are assessed using higher moment measures and quadrant analysis. These flow field maps illustrate that: (1) the time-averaged downstream and vertical velocities agree well with previous studies of quasi-equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest; (2) the maximum root-mean-square (RMS) of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell; (3) the maximum vertical RMS values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream, extending almost to the next crest; (4) positive downstream skewness values occur within the separation cell, whereas positive vertical skewness values are restricted to the shear layer; (5) the highest Reynolds stresses are located within the zone of flow separation and along the shear layer; (6) high-magnitude, high-frequency quadrant-2 events (‘ejections’) are concentrated along the shear layer (Kelvin-Helmholtz instabilities) and dominate the contribution to the local Reynolds stress; and (7) high-magnitude, high-frequency quadrant-4 events occur bounding the separation zone, near reattachment and close to the dune crest, and are significant contributors to the local Reynolds stress at each location. These data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation, magnitude and downstream extent of the flow separation zone and resultant shear layer. Furthermore, the origin of dune-related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting. The fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the Kelvin-Helmholtz instabilities, which are both greater for dunes than ripples. These instabilities control the local flow and turbulence structure and dictate the modes of sediment entrainment and their transport rates. 相似文献
9.
为进一步揭示沙波水流运动特性及提高沙波迎流面流速计算精度,采用两种概化模型,通过小水深沙波水槽试验,运用声学多普勒流速仪,对沙波沿程及垂线流速分布进行了测量。基于乐培九次生流理论公式,结合沙波水流特性,假定次生流在沙波迎流面上处于一个不断发展演变的过程,提出了发展函数和修正函数,得到了适用于沙波迎流面的流速垂线分布公式。研究结果表明:相对水深越小,沙波地形对迎流面水流作用越显著,使得上部流速减小、近底流速增大,且越靠近波峰这种现象越明显;建立的沙波流速公式与实测值吻合较好,能够准确地反映出迎流面流速变化规律。 相似文献
10.
Current knowledge of flow and turbulent processes acting across the sand bed continuum is still unable to unequivocally explain the mechanism(s) by which ripples become dunes. Understanding has been improved by comparative high-resolution studies undertaken over fixed bedforms at different stages in the continuum. However, these studies both ignore the role of mobile sediment and do not examine flow structure during the actual transition from ripples to dunes. The aims of the paper are: (i) to describe flow and turbulence characteristics acting above mobile bedforms at several stages across the transition; and (ii) to compare these data with those arising from experiments over fixed ripples and dunes. Laboratory experiments are presented that examine the turbulence structure across seven distinct stages of the transition from ripples to dunes. Single-point acoustic Doppler velocimeter sampling at three flow heights above a developing mobile boundary was undertaken. Time-averaged statistics and the instantaneous quadrant record reveal distinct changes in flow structure either side of the change from ripples to dunes. Initially, shear-related, high-frequency vortex shedding dominates turbulence production. This increases until two-dimensional (2D) dunes have formed. Thereafter, turbulence intensities and Reynolds stress decline and three-dimensional dunes exhibit values found over 2D ripples. This is the result of shear layer dampening which occurs when the topographically-accelerated downstream velocity increases at a faster rate than flow depth. Activity at reattachment increases due to high velocity fluid imparting high mass and momentum transfer at the bed and/or wake flapping. Suspended sediment may also play a role in turbulence dampening and bed erosion. Ejections dominate over sweeps in terms of event frequency but not magnitude. Strong relationships between inward interactions and sweeps, and ejections and outward interactions, suggest that mass and momentum exchanges are dependent upon activity in all four quadrants. The results contradict the notion present in most physical models that larger bedforms exhibit most shear layer activity. Consequently an improved model for the ripple–dune transition is proposed. 相似文献
11.
Large symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles <8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes <3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near-bed velocity and bed load transport near dune crests result in crest rounding. Long, low-angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near-bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume-based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in such flows. 相似文献
12.
Geomorphology of desert sand dunes: A review of recent progress 总被引：2，自引：0，他引：2
Through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes. Alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes. As developments with equipment allowed, field measurements became more sophisticated. However, by the mid-1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported. Most importantly, the attempt to measure the stresses imposed by the wind on the sand surface proved impossible, and the use of shear (or friction) velocity as a surrogate for shear stress also failed to deliver. At the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux. In a development paralleled in fluvial geomorphology, aeolian geomorphologists have attempted to measure and model turbulent structures over dunes. Progress has recently been made through the use of more complex numerical models based on computational fluid dynamics (CFD). Some of the modelling work has also suggested that notions of dune ‘equilibrium’ form may not be particularly helpful. This range of recent developments has not meant that field studies are now redundant. For linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground-penetrating radar images of the internal structure of the dunes.
The paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form. At least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies. More recently, attention has shifted–although not completely–to modelling approaches, and very considerable progress has been made in developing models of dune development. It is clear, however, that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology. 相似文献
13.
Theoretical and empirical analyses of flow structure, sediment transport, and sediment size characteristics at the crest of dune-like bedforms indicate that it is possible to describe, at least semi-quantitatively, the diffusion and deposition of sediment on the leeside of such structures. A numerical program based on this analysis simulates the grain-size distribution and deposition rate on the leeside of dunes for specified flow conditions and bed material. Evaluation of flow and sediment variables through the numerical simulation program shows that flow velocity, flow depth and sediment size have a strong influence on the deposition rate and texture of leeside sediment before avalanching. Sorting of the bed material, in particular, appears to exert a strong control on both the grain-size and the deposition-rate gradients. 相似文献
14.
Flows with high suspended sediment concentrations are common in many sedimentary environments, and their flow properties may show a transitional behaviour between fully turbulent and quasi‐laminar plug flows. The characteristics of these transitional flows are known to be a function of both clay concentration and type, as well as the applied fluid stress, but so far the interaction of these transitional flows with a loose sediment bed has received little attention. Information on this type of interaction is essential for the recognition and prediction of sedimentary structures formed by cohesive transitional flows in, for example, fluvial, estuarine and deep‐marine deposits. This paper investigates the behaviour of rapidly decelerated to steady flows that contain a mixture of sand, silt and clay, and explores the effect of different clay (kaolin) concentrations on the dynamics of flow over a mobile bed, and the bedforms and stratification produced. Experiments were conducted in a recirculating slurry flume capable of transporting high clay concentrations. Ultrasonic Doppler velocity profiling was used to measure the flow velocity within these concentrated suspension flows. The development of current ripples under decelerated flows of differing kaolin concentration was documented and evolution of their height, wavelength and migration rate quantified. This work confirms past work over smooth, fixed beds which showed that, as clay concentration rises, a distinct sequence of flow types is generated: turbulent flow, turbulence‐enhanced transitional flow, lower transitional plug flow, upper transitional plug flow and a quasi‐laminar plug flow. Each of these flow types produces an initial flat bed upon rapid flow deceleration, followed by reworking of these deposits through the development of current ripples during the subsequent steady flow in turbulent flow, turbulence‐enhanced transitional flow and lower transitional plug flow. The initial flat beds are structureless, but have diagnostic textural properties, caused by differential settling of sand, silt and cohesive mud, which forms characteristic bipartite beds that initially consist of sand overlain by silt or clay. As clay concentration in the formative flow increases, ripples first increase in mean height and wavelength under turbulence‐enhanced transitional flow and lower transitional plug‐flow regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater lee side erosion. As clay concentration increases further from a lower transitional plug flow, ripples cease to exist under the upper transitional plug flow and quasi‐laminar plug flow conditions investigated herein. This disappearance of ripples appears due to both turbulence suppression at higher clay concentrations, as well as the increasing shear strength of the bed sediment that becomes more difficult to erode as clay concentration increases. The stratification within the ripples formed after rapid deceleration of the transitional flows reflects the availability of sediment from the bipartite bed. The exact nature of the ripple cross‐stratification in these flows is a direct function of the duration of the formative flow and the texture of the initial flat bed, and ripples do not form in cohesive flows with a Reynolds number smaller than ca 12 000. Examples are given of how the unique properties of the current ripples and plane beds, developing below decelerated transitional flows, could aid in the interpretation of depositional processes in modern and ancient sediments. This interpretation includes a new model for hybrid beds that explains their formation in terms of a combination of vertical grain‐size segregation and longitudinal flow transformation. 相似文献
15.
Open‐framework gravel (OFG) in river deposits is important because of its exceptionally high permeability, resulting from the lack of sediment in the pore spaces between the gravel grains. Fluvial OFG occurs as planar strata and cross strata of varying scale, and is interbedded with sand and sandy gravel. The origin of OFG has been related to: (1) proportion of sand available relative to gravel; (2) separation of sand from gravel during a specific flow stage and sediment transport rate (either high, falling or low); (3) separation of sand from gravel in bedforms superimposed on the backs of larger bedforms; (4) flow separation in the lee of dunes or unit bars. Laboratory flume experiments were undertaken to test and develop these theories for the origin of OFG. Bed sediment size distribution (sandy gravel with a mean diameter of 1·5 mm) was kept constant, but flow depth, flow velocity and aggradation rate were varied. Bedforms produced under these flow conditions were bedload sheets, dunes and unit bars. The fundamental cause of OFG is the sorting of sand from gravel associated with flow separation at the crest of bedforms, and further segregation of grain sizes during avalanching on the steep lee side. Sand in transport near the bed is deposited in the trough of the bedform, whereas bed‐load gravel avalanches down the leeside and overruns the sand in the trough. The effectiveness of this sorting mechanism increases as the height of the bedform increases. Infiltration of sand into the gravel framework is of minor importance in these experiments, and occurs mainly in bedform troughs. The geometry and proportion of OFG in fluvial deposits are influenced by variation in height of bedforms as they migrate, superposition of small bedforms on the backs of larger bedforms, aggradation rate, and changes in sediment supply. If the height of a bedform increases as it migrates downstream, so does the amount of OFG. Changes in the character of OFG on the lee‐side of unit bars depend on grain‐size sorting in the superimposed bedforms (dunes and bedload sheets). Thick deposits of cross‐stratified OFG require high bedforms (dunes, unit bars) and large amounts of aggradation. These conditions might be expected to occur during high falling stages in the deeper parts of river channels adjacent to compound‐bar tails and downstream of confluence scours. Increase in the amount of sand supplied relative to gravel reduces the development of OFG. Such increases in sand supply may be related to falling flow stage and/or upstream erosion of sandy deposits. 相似文献
16.
Internal Solitary Waves (ISWs) are nonlinear, large amplitude motions of the interface between two fluids with different densities in the stratified ocean. Because of their strong vertical and horizontal current velocity, and the vortex, turbulent mixing caused by breaking, they affect marine environment, seabed sediment and man-made structures in the ocean. In the paper, we systematically analyzed and summarized the ISW-induced shoaling break mechanisms, models of suspension, and seabed dynamical response. Then, we discussed the ISW-induced sediment resuspension criteria, forming bottom and intermediate nepheloid layer and the capacity to suspend sediments in the seabed, and further put forward the unsolved problems based on the conducted work and related achievements. In shallow seas with complex terrain variations, shoaling can cause ISWs to deform, break, and split. Studies on the propagation of ISWs of depression over sloping topography have shown that an adverse pressure gradient causes the rotation of the flow separation, which produces vortices, and this results in global instability of the boundary layer and ISW burst. The separation vortices increase the bottom shear stress, vertical velocity, and near-bottom Reynolds stress, which leads to sediment resuspension and transport in the flow and vortex core. Although episodic, ISW-induced resuspension is hypothesized to be important enough to shape the topography. Shoaling ISWs may erode, resuspend and transport mud-like sediments, first towards shore by boluses, and subsequently offshore through the generation of intermediate nepheloid layers. Shoaling ISWs might be an important mechanism of muddy sediment dispersal along continental shelves. Furthermore, recent hypotheses suggest that sediment mobilization and transport caused by internal waves in general, and ISWs in particular, may be at the origin of some sedimentary structures found in the sedimentary rock record and also the hummocky-cross stratification. Observed on-shelf propagating frontal ISW most likely interacts with the sand waves, sediment waves or sand dunes. ISWs contribute to their generation, as they are trailed by considerable shear-induced turbulence and high-frequency internal waves close to the buoyancy frequency. This work is of great value for further understanding the process of ISW-induced sediment resuspension, transportation, and the capacity to suspend sediments in the seabed. It helps further study of the dynamic process of marine ecological environment dynamic process by ISW and the deep sea sedimentation process. 相似文献
17.
为了研究采样频率和样本容量对明渠紊流统计特征值的影响,利用粒子图像测速技术,对明渠恒定均匀水流的二维流速分布进行测量,获得了包含340535个样本的流场时间序列.对该大容量时间序列进行抽样,得到不同采样频率和容量条件下的一系列子序列.对比分析各子序列的时均流速、紊动强度及雷诺应力的变化特点,发现存在临界的采样频率和样本容量,紊动统计特征参数在临界值以下波动较大,而在临界值以上基本趋于稳定;要满足同一测量精度,时均流速对采样频率和样本容量的要求最低,紊动强度次之,雷诺应力最高. 相似文献
18.
The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology 总被引：2，自引：0，他引：2
The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D50 of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well‐sorted medium to coarse sand (D50 of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D50 < 0·6 mm), and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0·7–1 m and 5–10 m. Ripples and small dunes may have sinuous crestlines but primarily exhibit two‐dimensional planforms. In contrast, large dunes are primarily three‐dimensional barchanoid forms. Ripples on the backs of small dunes rarely develop to maximum steepness. Small dunes may achieve an equilibrium geometry, either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes. Secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they traverse the crestal region. However, secondary dunes more than 5 m in length are rare. The dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rarely exceed 1 m high in water depths of ≈4 m. Dune celerity increases as a function of the mean flow velocity squared, but this general relationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows downstream progression and steepens the dune form. During steady or falling stage, an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height. These diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six‐stage model of dune growth and diminution is proposed to explain variations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload‐dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation. This finding is significant, as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport. 相似文献
19.
利用声学多普勒测速仪（ADV）实测了凹口溢流堰与同侧布置竖缝组合式鱼道的三维瞬时流速,应用射流力学和紊流统计理论剖析了鱼道水池内的时均流速、紊动强度、雷诺应力、相关函数、紊动尺度等紊流结构。试验研究结果表明:溢流堰与竖缝组合式鱼道池内水流流态呈现出复杂的三维紊流结构;同竖缝相比,溢流堰对紊动强度和雷诺应力的变化有着更为显著的影响,尤其是鱼道池内表层区域;堰流区涡旋的相关性比竖缝壁面射流区好,并且堰流区存在较大尺度的涡旋结构,而竖缝壁面射流区的涡旋尺度则较小。相比单一式鱼道,组合式鱼道紊流结构更加复杂,研究结果可为优化鱼道设计、修复鱼类生境提供参考。 相似文献
20.
Yu. I. Troitskaya D. A. Sergeev O. S. Ermakova G. N. Balandina 《Doklady Earth Sciences》2010,433(1):922-926
A new method of digital optical anemometry (Particle Image Velocimetry, PIV) of turbulent flows is suggested and implemented
in the laboratory; it is based on the use of continuous laser radiation and high-speed video photography, providing continuous
statistical ensembles of flow velocity fields. Application of the method to the study of wind field over waves has allowed
us to perform, for the first time, direct measurements of velocity fields, averaged over turbulent pulsations induced by waves
in the air flow. The experiments demonstrated that the velocity fields, averaged over the turbulent pulsations, are nonseparated
even in the case of steep and breaking waves, when separation of the flow from the wave crests in the instantaneous fields
is observed. Based on comparison with the experimental data, it is shown that the average wind fields over waves are described
well quantitatively in the framework of semiempirical closure models of turbulence. 相似文献