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1.
FLOW FIELD IN SCOURED ZONE OF CHANNEL CONTRACTIONS   总被引：1，自引：0，他引：1  
Experiments were conducted in a laboratory flume to measure the two-dimensional turbulent flow field in the scoured zone of channel contractions under a clear-water scour condition. The Acoustic Doppler Velocimeter (ADV) was used to detect the flow field at different vertical lines along the centerline of uncontracted (main channel) and contracted zones of the channel. The distributions of time-averaged velocity components, turbulent intensity, turbulent kinetic energy, and Reynolds stresses are presented in nondimensional graphical form. The bed shear stresses are computed from the measured Reynolds stresses being in threshold condition within the zone of contraction where bed was scoured. The data presented in this paper would be useful to the investigators for the development of kinematic flow model and morphological model of scour at a channel or river contraction.  相似文献   

2.
Flow near a model spur dike with a fixed scoured bed   总被引：1，自引：0，他引：1  
Three-dimensional flow velocities were measured using an acoustic Doppler velocimeter at a closely spaced grid over a fixed scoured bed with a submerged spur dike. Three-dimensional flow velocities were measured at 3,484 positions around the trapezoidal shaped submerged model spur dike. General velocity distributions and detailed near field flow structures were revealed by the measurement. Clear differences were revealed between flow over fixed flat and scoured beds. Strong lateral flows were the dominant cause of the observed local scour. Shear stresses were higher for the scoured bed than in the flat bed case. Decreasing rates of scour as the scour hole developed were attributed to increases in critical shear stress in the scour holes caused by the increase in the length and magnitude of adverse slopes associated with the two main scour holes.  相似文献   

3.
Experimental study has been carried out under a clear-water scour condition to explore the local scour around semi-elliptical model bridge abutments with armor-layer bed, compared with the local scour process around semi-circular abutment. Two types of model bridge abutments, namely semi-elliptical and semi-circular abutments have been used in this experimental study. The model abutments had a ratio of streamwise length of abutment to the length of abutment transverse to the flow of 2 for semi-circular abutments and 3 for semi-elliptical abutments. In total, 50 Experiments have been designed and conducted under different flow conditions such as bed shear velocities, flow depth, and dimensions of bridge abutment model, as well as grain size of the bed material. Based on these experiments, the scour process around bridge abutments has been assessed. The dependence of the equilibrium scour depth of the scour hole on hydraulic variables has been studied. Empirical equation describing the equilibrium scour depth of the scour hole around bridge abutments has been developed.  相似文献   

4.
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   

5.
Measurements of the primary and secondary velocity components were máde in two, active, braided river anabranch confluences with a simple Y-shaped plan form, in the gravelly Sunwapta River (D50 of approximately 30 mm). Flow velocity was measured at regularly-spaced intervals using a bidirectional electromagnetic current meter and the measured downstream and cross-stream velocities were converted to primary and secondary velocities to yield the secondary circulation. The primary (downstream) velocity field shows two high velocity streams from the two tributaries which merge (and, in some cases, accelerate) into a single high velocity core over the thalweg. Primary flow velocity declines as the flow expands and diverges at the downstream end of the confluence. The secondary circulation is dominated by two helical cells, back-to-back, plunging over the thalweg and diverging at the bed. This is the first confirmation of this flow structure in confluences, based on field measurements. The strength of the secondary cells declines downstream through each confluence, and laterally away from the thalweg area in cross-section. There is also a tendency for one cell, from the larger of the tributaries, to override the other. The secondary and primary flow structure and strength differs slightly between the two confluences and this is reflected in differences in scour hole form.  相似文献   

6.
The installation of free falling jet grade control structures has become a popular choice for river bed stabilization. However, the formation and development of scour downstream of the structure may lead to failure of the structure itself. The current approaches to scour depth prediction are generally based on studies conducted with the absence of upward seepage. In the present study, the effects of upward seepage on the scour depth were investigated. A total of 78 tests without and with the application of upward seepage were carried out using three different sediment sizes, three different tailwater depths, four different flow discharges, and four different upward seepage flow discharge rates. In some tests, the three-dimensional components of the flow velocity within the scour hole were measured for both the cases with and without upward seepage. The scour depth measured for the no-seepage results compared well with the most accurate relationship found in the literature. It was found that generally the upward seepage reduced the downward velocity components near the bed, which led to a decrease in the maximum scour depth. A maximum scour depth reduction of 49% was found for a minimum tailwater depth, small sediment size, and high flow discharge. A decay of the downward velocity vector within the jet impingement was found due to the upward seepage flow velocity. The well known equation of D’Agostino and Ferro was modified to account for the effect of upward seepage, which satisfactorily predicted the experimental scour depth, with a reasonable average error of 10.7%.  相似文献   

7.
Results of an experimental study on the countermeasure of scour depth at circular piers are presented. Experiments were conducted for pier scour with and without a splitter plate under a steady, uniform clear-water flow condition. The results of pier scour without splitter plate were used as a reference. Different combinations of lengths and thicknesses of splitter plates were tested attaching each of them to a pier at the upstream vertical plane of symmetry. Two different median sediment sizes (d 50 = 0.96 and 1.8 mm) were considered as bed sediment. The experimental results show that the scour depth consistently decreases with an increase in splitter plate length, while the scour depth remains independent of splitter plate thickness. In addition, temporal evolution of scour depth at piers with and without a splitter plate is observed. The best combination is found to be with a splitter plate thickness of b/5 and a length of 2b. Here, b denotes the pier diameter. An empirical formula for the estimation of equilibrium scour depth at piers with splitter plates is obtained from a multiple linear regression analysis of the experimental data. The flow fields for various combinations of circular piers with and without splitter plate including plain bed and equilibrium scour conditions were measured by using an acoustic Doppler velocimeter. The turbulent flow fields for various configurations are investigated by plotting the velocity vectors and the turbulent kinetic energy contours on vertical and horizontal planes. The splitter plate attached to the pier deflects the approach flow and thus weakens the strength of the downflow and the horseshoe vortex, being instrumental in reducing the equilibrium scour depth at piers. The proposed method of pier scour countermeasure is easy to install and cost effective as well.  相似文献   

8.
《国际泥沙研究》2022,37(5):662-674
Bridge abutment-induced sediment scour in compound river channels is time-sensitive during floods, and, thus, estimating the scour rate has significance in design. In the current study, large-scale laboratory experiments were done to examine the influence of multiple factors on scour evolution, including channel geometry, bed mobility (clear-water or live-bed), and lateral (embankment intrusion) and vertical (deck submergence) flow contractions. A new method is applied for scour measurement and determination of characteristic scour trends. Results show that the non-dimensional time scale is related to flow intensity and may peak before reaching the clear-water scour threshold; the corresponding equations also are derived. It is found that the use of scour countermeasures has a significant impact on the scour rate, and the existing equations for unprotected abutments have limitations. Under clear-water conditions, the scour rate is closely related to the overall flow contraction, which can be better reflected by the ratio of the approaching unit discharge to that at the bridge section. Specifically, the effect of vertical flow contraction on scour rate is more dominant than other factors. Two scour rate curves are derived to suit different structural and flow contraction types for clear-water scour. In contrast, the live-bed scour rate data are generally consistent and fall in a narrow band, showing less dependence on other factors. Finally, a new design approach is established to estimate the time scale and scour rate using the new equations proposed in this paper.  相似文献   

9.
1 INTRODUCTION The local scour around bridge piers and abutments has always had heightened interest to hydraulic engineers. An accurate and efficient method to estimate the local scour is greatly desired. However, the behavior of flows passing through bri…  相似文献   

10.
The effect of scour countermeasures on the mechanism of local scour around a cylinder requires clarification in order to develop design methodology for use in practice. Previous investigations on countermeasure performance, though useful, have not provided adequate measurements to support this understanding. In the present investigation, particle image velocimetry(PIV) measurements were acquired at several streamwise-vertical planes in the flow field surrounding a submerged circular cylinder wit...  相似文献   

11.
Abstract

An investigation is made of steady thermal convection of a Boussinesq fluid confined in a vertically-mounted rotating cylinder. The top and bottom endwall disks are thermal conductors at temperatures Tt and Tb with δT = Tt ? Tb >0. The vertical sidewall has a finite thermal conductance. A Newtonian heat flux condition is adopted at the sidewall. The Rayleigh number of the fluid system is large to render a boundary layer-type flow. Finite-difference numerical solutions to the full Navier-Stokes equations are obtained. The vertical motions within the buoyancy layer along the sidewall induce weak meridional flows in the interior. Because of the Coriolis acceleration, the meridional flows give rise to azimuthal flows relative to the rotating container. Strong vertical gradients of azimuthal flows exist in the regions near the endwalls. As the stratification effect increases, concentration of flow gradients in thin endwall boundary layers becomes more pronounced. The azimuthal flow field exhibits considerable horizontal gradients. The temperature field develops horizontal variations superposed on the dominant vertical distribution. As either the sidewall thermal conductance or the stratification effect decreases, the temperature distribution tends to the profile varying linearly with height. Comparisons of the sizes of the dynamic effects demonstrate that, in the bulk of flow field, the vertical shear of azimuthal velocity is supported by the horizontal temperature gradient, resulting in a thermal-wind relation.  相似文献   

12.
The turbulent flow in the local scour hole around a single non-submerged spur dyke is investigated with both experimental and numerical methods. The experiments are conducted under clear-water scour regime with an impermeable spur dyke. The scour geometry and flow velocities are measured in details with a high-resolution laser displacement meter, electro-magnetic velocimetries and PIV （Particle image velocimetry）. A 3D non-linear k-ε model is developed to simulate the complex local flow field around the scour area. The numerical model is formulated using FVM （Finite volume method） on a collocated unstructured mesh, capable of resolving complex geometries and boundaries. It is found that the simulation results are reasonably consistent with those of the experimental measurements. Based on the study results, the nature of the flow structure around a spur dyke with local scour hole is analyzed.  相似文献   

13.
The flow pattern around a cylinder, installed in a scoured channel bed, was experimentally investigated. Detailed measurements of the instantaneous 3D velocities were performed by using an Acoustic Doppler Velocity Profiler (ADVP), from which the profiles of the time-averaged velocities and turbulence stresses were obtained. It is shown that the influence of the cylinder and of the scour hole alters the approach flow; this is essentially confined to the vicinity of the cylinder and to the inside of the scour hole. The horseshoe vortex is measured as a flow reversal inside the scour hole, formed by the downward flow along the cylinder face and the reversed flow at the scour bed.  相似文献   

14.
1 INTRODUCTION'LOcal scour around a pier is a result of the interatiOn amongst the pier, the aPproach flow and theerodible bed. The Presence of a pier results in a stagnation pressure build-up in front of the pier and athree-dimensional tUrbulent flow echaracterized by the downward flow ahead of the pier and the so-called horseshoe vortex along the base of the Pier forms itself The flow modifies the erothe bed inthe vicinity of the pier when local scour takes place (Graf and AJhnakar l…  相似文献   

15.
The effect of fully submerged boulders on the flow structure in channels has been studied by some researchers. However, many natural streams have bed material with boulders that are not fully submerged under water. In many natural streams, boulders cover between 1% and 10% of the area of the stream reach. The effect of non-submerged boulders on the velocity profile and flow characteristics is very important for assessing riverbed deformation. The objectives of this paper are to find the pattern of velocity distribution around a non-submerged boulder and to compare it with the classical studies on flow resistance and Reynolds stress distribution in open channels. Also, by considering the variation in the Reynolds stress distribution at different locations around a non-submerged boulder, the effect of a non-submerged boulder on the estimation of shear velocity and resistance to flow has been investigated. Results indicates that inside the scour hole caused by a non-submerged boulder in a river velocity distributions are irregular. However, velocity distributions are regular outside the scour hole. The presence of the boulder causes a considerable deviation of the Reynolds shear stress from the classic distribution, showing a non-specific distribution with negative values. The classical methods for calculating shear velocity are not suitable because these methods do not give detailed velocity and Reynolds stress distributions in natural rivers with a lot of boulders. Thus, the effect of a non-submerged boulder on the estimation of the resistance to flow by considering the variations in velocity and Reynolds stress distributions at different locations around a non-submerged boulder is important and needs to be studied in a natural river instead of just in laboratory flumes. The negative values in Reynolds stress distribution around a boulder indicate that the classical methods are unable to predict resistance to flow, and also show strong turbulence inside the scour hole where the complex flow conditions present ambiguous Reynolds stress distributions. In the current study, to obtain a reasonable estimation of parameters in natural rivers, the classical method has been modified by considering velocity and Reynolds stress distributions through the boundary layer method.  相似文献   

16.
LOCAL SCOUR AROUND BRIDGE ABUTMENTS   总被引：2，自引：0，他引：2  
1 INTRODUCTION The protrusion of a bridge abutment or a spur dike into the main channel creates a disturbance and obstruction to the otherwise equilibrium state of sediment transportation in an alluvial river. The flow accelerates and separates at the upstream face of the abutment as it moves past the obstacle, creating a vortex trail that moves downstream in a direction approximately perpendicular to the structure. The result is that the bed around the structure is eroded locally. The l…  相似文献   

17.
Local scour may occur when a hydraulic structure is positioned in a channel with an erodible bed. Herewith investigated experimentally are the erosion and flow pattern due to a water jet passing over a channel bed at the asymptotic (final) state. The development of the scour hole, its maximum scour depth and length, are recorded and compared with available scour-depth relations. Two sets of experiments (see Table 1) were carded out. Set 1 (3 runs) was concerned with measuring the vertical instantaneous velocity distribution in the scour hole. The scour hole at the asymptotic (final) state, t=100 [h] was investigated. Set 2 (5 runs) was concerned with studying the physics of scouring. Thescour hole at about mid-state, t≈0.5 [h], was investigated; subsequently the scour-hole depth was linearly extended on the semi-log scale to 72 [h]; no velocity measurements were performed. The present data are put in context with some (popular) existing relations; recommended is a modification of some of these relations。  相似文献   

18.
REVIEW ON LOCAL SCOUR DUE TO JETS   总被引：1，自引：0，他引：1  
The safety of an apron of the energy dissipator is threatened by the large-scale scour in the downstream of the apron due to the erosive action of a horizontal jet issuing from a sluice opening. Also, large-scale deposition of the scoured sediments due to an impinging jet in a plunging pool type energy dissipator affects the passage of flow adversely in the downstream channels. Owing to the significant practical importance, the problem of local scour due to jets has been studied by many investigators, In this paper, a comprehensive review of the up-to-date investigations on local scour due to horizontal and impinging jets is presented including all possible aspects, such as scouring process, parameters affecting scour, time variation of scour velocity distribution on the apron and within the scour hole, development of boundary layer thickness, bed shear stress, scour estimation formulas and protection works.  相似文献   

19.
Abstract

A system is considered in which electrically conducting fluid is contained between two rigid horizontal planes and bounded laterally by a circular cylinder. The fluid is permeated by a strong azimuthal magnetic field. The strength of the field increases linearly with distance from the vertical axis of the cylinder, about which the entire system rotates rapidly. An unstable temperature gradient is maintained by heating the fluid from below and cooling from above. When viscosity and inertia are neglected, an arbitrary geostrophic velocity, which is aligned with the applied azimuthal magnetic field and independent of the axial coordinate, can be superimposed on the basic axisymmetric state. In this inviscid limit, the geostrophic velocity which occurs at the onset of convection is such that the net torque on geostrophic cylinders vanishes (Taylor's condition). The mathematical problem which describes the ensuing marginal convection is nonlinear, and was discussed previously for the planar case by Soward (1986). Here new features are isolated which result from the cylindrical geometry. New asymptotic solutions are derived valid when Taylor's condition is relaxed to include viscous effects.  相似文献   

20.
Information about electrical anisotropy in and around the deep borehole of the German Continental Deep Drilling Program (KTB) has been acquired using three different methods: (1) Under surface temperature and pressure conditions, the azimuthal dependence of resistivity has been measured in horizontal planes perpendicular to the axes of drill cores. (2) Simulated in-situ conditions (high temperature and pressure) have been applied to mini cores drilled at different orientations to structural features such as foliation. The resistivity was then measured in the direction of the mini cores' long axes. (3) Under quasi-in-situ (downhole) conditions, the azimuthal dependence of resistivity has been measured in horizontal planes using the Azimuthal Resistivity Imager (ARI), a down hole logging tool.These investigations have shown that electrical anisotropy is strongly related to structural features. The anisotropy of gneisses is the highest due to the strong foliation (averaged anisotropy factor 2.8). Lamprophyres are most isotropic (factor 1.2). Most rocks from KTB are amphibolites with intermediate anisotropies (factor 1.3). The anisotropies measured under surface temperature and pressure conditions and quasi-in-situ conditions on amphibolites are approximately the same, whereas anisotropies under simulated in-situ conditions are partially higher.Furthermore, the anisotropies measured on the cm to m scale using the various methods outlined above shall be compared with large scale anisotropies of the km scale measured around KTB using Magneto Tellurics.  相似文献   

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