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1.
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
2.
Flume experiments were conducted in order to monitor changes in flow turbulence intensity and suspended sediment concentration at seven stages across the ripple–dune transition and at three different positions above the bed surface. Three‐dimensional velocity measurements were obtained using an acoustic Doppler velocimeter (ADV). Suspended sediment concentration (SSC) was monitored indirectly using ADV signal amplitude. Although limited to time‐averaged parameters, the analysis reveals that SSC varies significantly with stage across the transition and with sampling height. The statistical analysis also reveals an apparent uniformity of suspended sediment concentration with height above the bed in the lower half of the flow depth at the critical stage in the transition from ripples to dunes. This is also the stage at which turbulence intensity is maximized. Statistically significant correlations were also observed between suspended sediment concentrations and root‐mean‐square values of vertical velocity fluctuations. These correlations reflect the various levels of shear‐layer activity and the distinct turbulent flow regions across the transition. Conversely, time‐averaged values of Reynolds shear stress exhibit a very weak relationship with suspended sediment concentrations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
We analyse experimental measurements of turbulent open-channel flow over hydraulically-smooth and transitionally-rough beds using the double-averaging methodology. Oil with a viscosity of 15×10?6 m2/s is used instead of water so that transitional-range roughness Reynolds numbers can be achieved with large (11.1 mm) roughness elements, allowing spatial variations in the mean velocity field to more easily be measured. Distributions of double-averaged velocities, turbulence intensities, form-induced intensities, and viscous, Reynolds, form-induced and total shear stresses are studied with comparisons made between distributions for hydraulically-smooth, transitionally-rough, and fully-rough boundaries. Measured streamwise turbulence intensities for all experiments peaked at a constant distance from the bed (z ++d + = 15) when elevation scale is adjusted using the zero-plane displacement d for the logarithmic velocity distribution. This collapse suggests that turbulence intensity distributions may be useful in assessing appropriate values of d for transitionally-rough and fully-rough boundaries. Form-induced normal and shear stresses above the roughness tops were found to collapse towards a common curve independent of roughness Reynolds number. 相似文献
4.
The acoustic Doppler velocimeter (ADV) measures three‐dimensional velocities in a small, remote sampling volume at high frequencies, however, these measurements incorporate errors that are intrinsic to the measurement technique. This paper demonstrates a new method for calculating the total measurement errors, including sampling errors, Doppler noise and errors due to velocity shear in the sampling volume associated with single‐point ADV measurements. This procedure incorporates both the effects of instrument configuration and the distribution of errors between velocity components for any probe orientation. It is shown that the ADV can characterize turbulent velocity fluctuations at frequencies up to the maximum sampling rate and that Reynolds shear stress errors are very small. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
5.
Experiments were carried out over a 2-dimentional pool with a constant length of 1.5 m and four different slopes.The distributions of velocity,Reynolds stress and turbulence intensities have been studied in this paper.Results show that as flow continues up the exit slope,the flow velocity increases near the channel bed and decreases near the water surface.The flow separation was not observed by ADV at the crest of the bed-form.In addition,the length of the separation zone increases with the increasing of entrance and exit slopes.The largest slope angle causes the maximum normalized shear stress.Based on the experiments,it is concluded that the shape of Reynolds stress distribution is generally dependent on the entrance and exit slopes of the pool.Also,the shape of Reynolds stress distribution is affected by both decelerating and accelerating flows.Additionally,with the increase in the slope angle,secondary currents are developed and become more stable.Results of the quadrant analysis show that the momentum between flow and bed-form is mostly transferred by sweep and ejection events. 相似文献
6.
Turbulent flow structures in alluvial channels with curved cross‐sections under conditions of downward seepage 
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下载免费PDF全文 Experimental investigations have been done to analyze turbulent structures in curved sand bed channels with and without seepage. Measures of turbulent statistics such as time‐averaged near‐bed velocities, Reynolds stresses, thickness of roughness sublayer and shear velocities were found to increase with application of downward seepage. Turbulent kinetic energy and Reynolds normal stresses are increased in the streamwise direction under the action of downward seepage, causing bed particles to move rapidly. Analysis of bursting events shows that the relative contributions of all events (ejections, sweeps and interactions) increase throughout the boundary layer, and the thickness of the zone of dominance of sweep events, which are responsible for the bed material movement, increases in the case of downward seepage. The increased sediment transport rate due to downward seepage deforms the cross‐sectional geometry of the channel made of erodible boundaries, which is caused by an increase in flow turbulence and an associated decrease in turbulent kinetic energy dissipation and turbulent diffusion. 相似文献
7.
Experimental results of the mean flow field and turbulence characteristics for flow in a model channel bend with a mobile sand bed are presented. Acoustic Doppler velocimeters (ADVs) were used to measure the three components of instantaneous velocities at multiple cross sections in a 135° channel bend for two separate experiments at different stages of clear water scour conditions. With measurements at multiple cross sections through the bend it was possible to map the changes in both the spatial distribution of the mean velocity field and the three Reynolds shear stresses. Turbulent stresses are known to contribute to sediment transport and the three‐dimensionality inherent to flow in open channel bends presents a useful case for determining specific relations between three‐dimensional turbulence and sediment entrainment and transport. These measurements will also provide the necessary data for validating numerical simulations of turbulent flow and sediment transport. The results show that the magnitude and distribution of three‐dimensional Reynolds stresses increase through the bend, with streamwise‐cross stream and cross stream‐vertical components exceeding the maximum principal Reynolds stress through the bend. The most intriguing observation is that near‐bed maximum positive streamwise‐cross stream Reynolds stress coincides with the leading edge of the outer bank scour hole (or thalweg), while maximum cross stream‐vertical Reynolds stress (in combination with high negative streamwise‐cross stream Reynolds stress near the bend apex) coincides with the leading edge of the inner bank bar. Maximum Reynolds stress and average turbulent kinetic energy appear to be greater and more localized over the scour hole before final equilibrium scour is reached. This suggests that the turbulent energy in the flow is higher while the channel bed is developing, and both lower turbulent energy and a broader distribution of turbulent stresses near the bed are required for cessation of particle mobilization and transport. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
8.
《国际泥沙研究》2020,35(1):42-56
Submerged vanes are hydrofoils utilized to manage the sediment transport through the river by generating the turbulence in the flow in the form of helical currents.The vanes are placed in the flow with respect to its direction at angle of 10°to 40°.In the current study,an attempt has been made to study the effect of the introduction of vanes in form of rows on parameters like turbulence intensities,Reynolds stresses,turbulent kinetic energy,anisotropy index,and the velocity profile of the flow.It is observed that the profile of variation of turbulence intensities,turbulent kinetic energy,vertical Reynolds stress and velocity over three different marked verticals on a transect are nearly identical whereas a large scatter is observed in the variation of transverse Reynolds stress over the vertical of the aforementioned vertical locations.This observation suggests that flow turbulence is homogeneous over the vertical while scattering in the variation of the transverse Reynolds stress component may be attributed to the presence of secondary currents in the flow.After introducing rows of submerged vanes,the bed turbulence is reduced,hence,helping reduce many scour related phenomenon.It is also observed that a vortex occurred at 0.85 times the height of the vane and the variation of turbulence quantities in the presence of vanes shows the existence of a peak in these quantities.It is observed that as flow moves away from the vane rows,due to the interaction of vortices and the action of vorticity,vortices dampens down and the flow regains homogeneity.After the introduction of submerged vane rows,bed shear stress reduces as fluid from the surface replaces the slow-moving fluid near the bed due to the secondary currents generated by the vanes leading to reduction in the magnitude of turbulence intensities,Reynolds stresses,and turbulent kinetic energy near the bed.The anisotropy index is observed to increase near the bed as induced secondary currents enhanced the turbulence production in the near bed region.All the profiles of parameters obtained in the current study show the existence of a peak or inflexions at a height of 0.85 H from bed(Where,H is the height of the submerged vane).Profiles of parameters obtained in the current study suggest that as the vorticity dampens the vane-generated secondary currents,the scattering in the profiles along the vertical reduces and profiles are observed to regain the variation which they had before the introduction of vane rows,suggesting that flow turbulence has regained its homogeneity. 相似文献
9.
Average velocity in streams is a key variable for the analysis and modelling of hydrological and hydraulic processes underpinning water resources science and practice. The present study evaluates the impact of the sampling duration on the quality of average velocity measurements acquired with contemporary instruments such as Acoustic Doppler Velocimeters (ADV) an Acoustic Doppler Current Profilers (ADCP). The evaluation combines considerations on turbulent flows and principles and configurations of acoustic instruments with practical experience in conducting customized analysis for uncertainty analysis purposes. The study sheds new insights on the spatial and temporal variability of the uncertainty in the measurement of average velocities due to variable sampling durations acting in isolation from other sources of uncertainties. Sampling durations of 90 and 150 s are found sufficient for ADV and ADCP, respectively, to obtain reliable average velocities in a flow affected only by natural turbulence and instrument noise. Larger sampling durations are needed for measurements in most of the natural streams exposed to additional sources of data variability. 相似文献
10.
Hossein Afzalimehr 《水文研究》2010,24(3):331-341
Non‐uniform flows encompassing both accelerating and decelerating flows over a cobble‐bed flume have been experimentally investigated in a flume at a scale of intermediate relative submergence. Measurements of mean longitudinal flow velocity u, and determinations of turbulence intensities u′, v′, w′, and Reynolds shear stress ?ufwf have been made. The longitudinal velocity distribution was divided into the inner zone close to the bed and the outer zone far from the bed. In the inner zone of the boundary layer (near the bed) the velocity profile closely followed the ‘Log Law’; however, in the outer zone the velocity distribution deviated from the Log Law consistently for both accelerating and decelerating flows and the changes in bed slopes ranging from ?2% to + 2% had no considerable effect on the outer zone. For a constant bed slope (S = ±0·015), the larger the flow rate, the smaller the turbulence intensities. However, no detectable pattern has been observed for u′, v′ and w′ distributions near the bed. Likewise, for a constant flow rate (Q = 0·040 m3/s), with variation in bed slope the longitudinal turbulent intensity profile in the longitudinal direction remained concave for both accelerating and decelerating flows; whereas vertical turbulent intensity (w′) profile presented no specific form. The results reveal that the positions of maximum values of turbulence intensities and the Reynolds shear stress depend not only on the flow structure (accelerating or decelerating) but also on the intermediate relative submergence scale. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
11.
Results are presented from a numerical simulation of three‐dimensional flow hydraulics around a mid‐channel bar carried out using the FLUENT/UNS computational fluid dynamics (CFD) software package. FLUENT/UNS solves the three‐dimensional Reynolds‐averaged form of the Navier–Stokes equations. Turbulence closure is achieved using a RNG k–ϵ model. Simulated flow velocities are compared with measured two‐dimensional velocities (downstream and cross‐stream) obtained using an electromagnetic current meter (ECM). The results of the simulation are qualitatively consistent with the flow structures observed in the field. Quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two (r=0·88) and a mean difference of 0·09 m s−1. Deviations between simulated and measured velocities may be identified that are both random and systematic. The former may reflect a number of factors including subgrid‐scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ECM orientation. Model mesh configuration, roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities. These results illustrate the potential for using CFD software to simulate flow hydraulics in natural channels with complex configurations. They also highlight the need for detailed spatially distributed datasets of three‐dimensional flow variables to establish the accuracy and applicability of CFD software. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
12.
River confluences are characterized by a complex mixing zone with three-dimensional (3D) turbulent structures which have been described as both streamwise-oriented structures and Kelvin–Helmholtz (KH) vertical-oriented structures. The latter are visible where there is a turbidity difference between the two tributaries, whereas the former are usually derived from mean velocity measurements or numerical simulations. Few field studies recorded turbulent velocity fluctuations at high frequency to investigate these structures, particularly at medium-sized confluences where logistical constraints make it difficult to use devices such as acoustic doppler velocimeter (ADV). This study uses the ice cover present at the confluence of the Mitis and Neigette Rivers in Quebec (Canada) to obtain long-duration, fixed measurements along the mixing zone. The confluence is also characterized by a marked turbidity difference which allows to investigate the mixing zone dynamics from drone imagery during ice-free conditions. The aim of the study is to characterize and compare the flow structure in the mixing zone at a medium-sized (~40 m) river confluence with and without an ice cover. Detailed 3D turbulent velocity measurements were taken under the ice along the mixing plane with an ADV through eight holes at around 20 positions on the vertical. For ice-free conditions, drone imagery results indicate that large (KH) coherent structures are present, occupying up to 50% of the width of the parent channel. During winter, the ice cover affects velocity profiles by moving the highest velocities towards the centre of the profiles. Large turbulent structures are visible in both the streamwise and lateral velocity components. The strong correlation between these velocity components indicates that KH vortices are the dominating coherent structures in the mixing zone. A spatio-temporal conceptual model is presented to illustrate the main differences on the 3D flow structure at the river confluence with and without the ice cover. © 2019 John Wiley & Sons, Ltd. 相似文献
13.
This study investigates turbulence structures in steady and non-uniform flows. Equations of Reynolds shear stress and turbulent velocity fluctuations are derived and their physical interpretations are explained. The theoretical results show that, different from previous studies, the variation of water surface can generate the wall-normal velocity, resulting in deviations of Reynolds shear stress and turbulence intensities from those in uniform flows. A self-similarity relationship is found between the Reynolds shear stress and turbulence intensities in non-uniform flows. The existence of self-similarity indicates that the effect of non-uniformity does not influence the mixing length. An empirical equation has been proposed to express the relationship based on experimental data available in the literature. Good agreement is achieved between the measured and predicted turbulence intensities by applying the self-similarity relationship. 相似文献
14.
Field measurements of settling velocities of fine sediments in Three Gorges Reservoir using ADV 总被引：1，自引：0，他引：1
《国际泥沙研究》2016,(3):237-243
The Three Gorges Reservoir (TGR) is suffering from unexpected fine sediment deposition, to better understand the fine sediment transport processes, field measurements were conducted at the Zhongxian and Fengjie reaches. A method based on the sediment diffusion equation was proposed to measure the settling velocities using the Acoustic Doppler Velocimeter (ADV). The backscatter acoustic intensities (BSI) received from the ADV were calibrated against the sediment concentrations measured via water sampling, suggesting a linear relationship in double logarithmic coordinate system. The instantaneous sediment concentration was calculated using the derived relationship, and then the settling velocity was obtained through the proposed procedure. The settling velocities of the fine particles in the TGR were found to vary with the water depth. Most of the effective settling velocities were within the range of 0.1–10 mm/s, which were larger than those of the primary particles, indicating that the flocculation was likely to occur in the TGR. Additionally, it is suggested that the turbulent motion played an important role in the flocculation in the TGR. 相似文献
15.
A field study of mean and turbulent flow characteristics upwind,over and downwind of barchan dunes 总被引：1，自引：0，他引：1
Field‐measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes. Turbulence was sampled using a high frequency sonic anemometer, deriving near‐surface Reynolds shear and normal stresses. Measurements upwind of and over a crest‐brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe. The mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress. This field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements. Downwind, the effect of different dune profiles on flow re‐attachment and recovery was apparent. With transverse incident flow, a re‐attachment length between 2·3 and 5·0h (h is dune brink height) existed for a crest‐brink separated dune and 6·5 to 8·6h for a crest‐brink coincident dune. The lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes, and a decrease in turbulence with distance characterized flow recovery. Recovery of mean velocity for the crest‐brink separated dune occurred over a distance 6·5h shorter than the crest‐brink coincident form. As the application of sonic anemometers in aeolian geomorphology is relatively new, there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle. This paper demonstrates the effect on Reynolds stresses of mathematically correcting data to the local streamline over varying dune slope. Where the streamline angle was closely related to the surface (windward slope), time‐averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines. In the close lee, however, the angle of downwardly projected (separated) flow was not aligned with the flat ground surface. Here, shear stress appeared to be underestimated by streamline correction, and corrected shear stress values were less than half of those uncorrected. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
16.
Observations of sediment resuspension and settling off the mouth of Jiaozhou Bay,Yellow Sea 总被引：2，自引：0，他引：2
We deployed bottom-mounted quadrapod equipped with acoustic Doppler current profiler (ADCP), acoustic Doppler velocimeter (ADV), and optical backscatter sensor (OBS) over two semidiurnal tidal cycles along the western coast of the Yellow Sea, China. In combination with shipboard profiling of CTD and LISST-100, we resolved the temporal and spatial distributions of tidal currents, turbulent kinetic energy (TKE), suspended sediment concentration (SSC) and particle size distributions. During the observations, tidal-induced bottom shear stress was the main stirring factor. However, weak tidal flow during the ebb phase was accompanied by two large SSC and median size events. The interactions of seiche-induced oscillations with weak ebb flow induced multiple flow reversals and provided a source of turbulence production, which stripped up the benthic fluff layers (only several millimeters) around the Jiaozhou Bay mouth. Several different methods for inferring mean suspended sediment settling velocity agreed well under peak currents, including estimates using LISST-based Stokes’ settling law, and ADCP-based Rouse profiles, ADV-based inertial-dissipation balance and Reynolds flux. Suspended particles in the study site can be roughly classified into two types according to settling behavior: a smaller, denser class consistent with silt and clay and a larger, less dense class consistent with loosely aggregated flocs. In the present work, we prove that acoustic approaches are robust in simultaneously and non-intrusively estimating hydrodynamics, SSC and settling velocities, which is especially applicable for studying sediment dynamics in tidal environments with moderate concentration levels. 相似文献
17.
Near‐bed shear stress,turbulence production and dissipation in a shallow and narrow tidal channel 下载免费PDF全文
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
18.
Pierre Weill Dominique Mouazé Bernadette Tessier Jean‐Claude Brun‐Cottan 《地球表面变化过程与地形》2010,35(14):1642-1654
Cheniers from Mont‐Saint‐Michel bay (France) are coarse shelly sand ridges migrating on the mudflat up to the salt marshes where they accumulate and merge in a littoral barrier. In this macrotidal setting and low wave forcing, the cheniers are rarely submerged. However, they are found to move up to several metres during coincidence of spring tide and wave activity. Their processes of migration, morphology and internal structure (composition of the beddings, grain size, sorting and grain arrangement) are thought to be closely related to the hydrodynamic behaviour of the coarse and shelly sediment. This paper focuses on the hydrodynamic behaviour of bioclastic sand sampled from the cheniers: settling velocities of the shell fragments were measured using a 2 m long sedimentation tube. Thresholds of motion under unidirectional current, velocity and turbulence vertical profiles were characterized in a small recirculating flume using Laser Doppler Anemometry (LDA). The flat‐shaped bioclastic particles feature low settling velocities and reveal a good resistance to the re‐suspension effect of the flow when imbricated in a sediment bed. The shear stress in the bottom boundary layer has been measured in the viscous and log sub‐layers. Nikuradse roughness heights (ks) for shell debris beds of different sizes have been quantified. It is found that ks ≈ 2·56d50. This value is close to the ones used for classic rounded sand grains despite their major differences of shape. The dual behaviour of the shell fragments (low settling velocity, good resistance to unidirectional flow) should be considered as a key to understanding how this coarse material is transported across the tidal flat, and finally accumulated as cheniers. Further flume experiments including wave activity and tidal fluctuations are necessary to better quantify these complex processes. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
19.
Douglas M. Thompson 《地球表面变化过程与地形》2004,29(11):1341-1358
The in?uence of pool length on the strength of turbulence generated by vortex shedding was investigated in a 6 m long recirculating ?ume. The experiment utilized a 38% constriction of ?ow and an average channel‐bed slope of 0·007. The base geometry for the intermediate‐length pool experiment originated from a highly simpli?ed, 0·10 scale model of a forced pool from North Saint Vrain Creek, Colorado. Discharge in the ?ume was 31·6 l/s, which corresponds to a discharge in the prototype channel of 10 m3/s. Three shorter and four longer pool lengths also were created with a ?xed bed to determine changes in turbulence intensities and energy slope with pool elongation. Three‐dimensional velocities were measured with an acoustic Doppler velocimeter at 31–40 different 0·6‐depth and near‐bed locations downstream of the rectangular constriction. The average velocity and root mean square (RMS) of the absolute magnitude of velocity at both depths are signi?cantly related to the distance from the constriction in most pool locations downstream of the constriction. In many locations, pool elongation results in a non‐linear change in turbulence intensities and average velocity. Based on the overall ?ow pattern, the strongest turbulence occurs in the center of the pool along the shear zone between the jet and recirculating eddy. The lateral location of this shear zone is sensitive to changes in pool length. Energy slope also was sensitive to pool length due to a combination of greater length of the pool and greater head loss with shorter pools. The results indicate some form of hydraulic optimization is possible with pools adjusting their length to adjust the location and strength of turbulent intensities in the center of pools, and lower their rate of energy dissipation. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
20.
Robust classification for the joint velocity‐intermittency structure of turbulent flow over fixed and mobile bedforms 下载免费PDF全文
下载免费PDF全文 Christopher J. Keylock Arvind Singh Jeremy G. Venditti Efi Foufoula‐Georgiou 《地球表面变化过程与地形》2014,39(13):1717-1728
Two datasets of turbulence velocities collected over different bedform types under contrasting experimental conditions show similarity in terms of velocity‐intermittency characteristics and suggest a universality to the velocity‐intermittency structure for flow over bedforms. One dataset was obtained by sampling flow over static bedforms in different locations, and the other was based on a static position but mobile bedforms. A flow classification based on the velocity‐intermittency behaviour is shown to reveal some differences from that based on an analysis of Reynolds stresses, boundary layer correlation and turbulent kinetic energy. This may be attributed to the intermittency variable, which captures the local effect of individual turbulent flow structures. Locations in the wake region or the outer layer of the flow are both shown to have a velocity‐intermittency behaviour that departs from that for idealized wakes or outer layer flow because of the superposition of localized flow structures generated by bedforms. The combined effect of this yields a velocity‐intermittency structure unique to bedform flow. The use of a time series of a single velocity component highlights the potential power of our approach for field, numerical and laboratory studies. The further validation of the velocity‐intermittency method for non‐idealized flows undertaken here suggests that this technique can be used for flow classification purposes in geomorphology, hydraulics, meteorology and environmental fluid mechanics. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献