桥墩-冰塞-局部冲刷相关问题研究进展

桥梁建设改变了河流的边界条件、水流条件和河床泥沙的运动状态,冬季河流中有冰塞出现时则产生相互影响作用.近年来,基于冰塞稳定性力学分析,开展了桥墩影响下冰塞稳定性研究,所得桥墩影响下冰塞稳定性判别公式计算结果和实测资料能够较好得吻合;研究了桥墩对河道卡封以及临界流凌密度的影响,计算得到的临界流凌密度与实验值较为接近;介绍...     

Flow Field Around Single and Tandem Piers

The present study provides a comparison between the flow pattern around two circular piers in tandem and a single pier set up on a moderately rough flat bed in a laboratory flume. Velocities are measured by an Acoustic Doppler Velocimeter (ADV). The contours of the time-averaged velocity components, Reynolds shear stresses, turbulence intensities and turbulence kinetic energy at different planes are presented. Streamlines and vectors are used to study the flow features. The analysis of power spectra around the piers is also presented. The results show that the presence of downstream pier changes the flow structure to a great extent, particularly in the near-wake region. Within the gap between the two piers, a stronger and substantial upflow is shaped. However, a weaker transverse-deflection is formed in comparison with that in the single pier. Near the bed, the velocity of flow approaching the downstream pier decreases to 0.2–0.3 times of the approach mean velocity (U ₀) due to the sheltering effect of the upstream pier. In the wake of downstream pier, the flow structure is completely different from the one in the wake of single pier. In comparison with the single pier, the values of turbulence kinetic energy and turbulence intensities show a considerable decrease around the tandem piers. In tandem piers, the high values of turbulence characteristics are found near the downstream pier. There is a recirculation zones just upstream of the sheltered pier close to the bed and another behind that pier near the free surface. The results show a decrease in the strength of vortical structure in the wake of tandem piers in comparison with single pier. It is shown that the formation of flow with different Reynolds number along the flow depth due to the effect of bed roughness, as well as pier spacing, can influence the type of flow regime of tandem case. In addition to enhancing the flow structure indulgence, the present detailed measurements can also be used for verification of numerical models.     

The turbulent flow field around a circular cylinder

The flow field around a circular cylinder mounted vertically on a flat bottom has been investigated experimentally. This type of flow occurs in several technical applications, e.g. local scouring around bridge piers. Hydrogen bubble flow visualization was carried out for Reynolds numbers ranging from 6,600 to 65,000. The main flow characteristic upstream of the cylinder is a system of horse-shoe vortices which are shed quasi-periodically. The number of vortices depends on Reynolds number. The vortex system was found to be independent of the vortices that are shed in the wake of the cylinder. The topology of the separated flow contains several separation and attachment lines which are Reynolds number dependent. In the wake region different flow patterns exist for each constant Reynolds number.     

Improvements on bed‐shear stress formulation for pier scour computation

This paper introduces an improved formula for the bed‐shear stress by applying the vorticity effect and its application in a 3D flow and sediment model to estimate scouring around bridge piers. Up to now, the sediment transport formulae used for computing pier scour were developed based on the general scouring in unobstructed flow. The capability for numerical models to predict local scour around bridge piers was severely restricted by the sediment transport formulae. The new formula introduced in this paper can take into account vortices that affect the local scour process by adding some terms into the classic bed‐shear stress equation. The 3D numerical model system used in this study consists of three modules: (a) an unsteady hydrodynamic module; (b) a sediment transport module; and (c) a Fation module. The hydrodynamic module is based on the 3D RANS equations. The sediment transport module is comprised of semi empirical models of suspended load and non‐equilibrium bed load. The bed‐deformation module is based on the mass balance for sediment. The model was used to simulate pier scour in tree different test cases: (1) a circular pier; (2) a square pier; and (3) a rectangular pier, by applying the ordinary sediment equation and the newly introduced sediment equation. Results of both numerical simulations were compared against laboratory measured data and also in case 1 with result of Olsen and Melaaen (J. Hydraul. Eng. 1993; 119 (9):1048–1054). Comparisons show that the new sediment formula could predict the scour more accurately than the ordinary one. Copyright © 2010 John Wiley & Sons, Ltd.     

Computation of three‐dimensional flow around square and circular piers

The aim of the present study is to investigate, by numerical simulation, the three‐dimensional turbulent flow field around square and circular piers. The numerical model employs a finite volume method based on MacCormack's explicit predictor–corrector scheme to solve weakly compressible hydrodynamic equations for turbulent flow. Computed results are compared with Dargahi's experimental measurements to assess the validity of the proposed model. Very good agreements are obtained. The results of flow simulation indicate that near the upstream face of the pier there exists a downflow, which joins the separated flow to form the horseshoe vortex stretched around the pier. This horseshoe vortex interacts with the wake vortex to create the upflow behind the pier. These phenomena appear to be very important to the mechanism of scouring around the pier. In general, the flow patterns for the square and circular piers are similar. However, the strengths of the downflow and horseshoe vortex are greater in the case of the square pier. The position of the horseshoe vortex around the circular pier is closer to the front face than that around the square pier. In the meantime, the domain of the wake flow in the case of the square pier is greater than that in the case of the circular one. Copyright © 2000 John Wiley & Sons, Ltd.     

钢筋混凝土桥墩弯剪数值分析模型

基于修正的压力场理论MCFT(The Modified Compression Field Theory)和纤维单元模型建立了钢筋混凝土桥墩的弯剪数值分析模型,以MCFT理论确定桥墩的剪切力-剪切位移关系,并与考虑桥墩弯曲变形的纤维单元模型组合,共同考虑桥墩的弯-剪-轴力耦合作用.通过与六个弯剪破坏控制的圆形截面钢筋混凝土桥墩拟静力试验结果的对比,对分析模型进行了验证.主要认识结论为基于MCFT理论可准确地计算弯剪破坏桥墩的屈服荷载、极限荷载和弹性阶段剪切刚度,剪切开裂是引起钢筋混凝土构件剪切力-剪切位移关系刚度突变的主要因素,而弯曲开裂与纵筋屈服对刚度的影响较小;分析模型对弯剪破坏桥墩的滞回曲线、弯曲与剪切变形成分均进行了较为准确的模拟分析.     

Numerical simulation of Reynolds number effects on velocity shear flow around a circular cylinder

Three-dimensional Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) are performed to investigate the shear effects on flow around a circular cylinder at Reynolds numbers of Re=60–1000. The shear parameter, β, which is based on the velocity gradient, cylinder diameter and upstream mean velocity at the center plane of the cylinder, varies from 0 to 0.30. Variations of Strouhal number, drag and lift coefficients, and unsteady wake structures with shear parameter are studied, along with their dependence on Reynolds number. The presented simulation provides detailed information for the flow field around a circular cylinder in shear flow. This study shows that the Strouhal number exhibits no significant variation with shear parameter. The stagnation point moves to the high-velocity side almost linearly with shear parameter, and this result mainly influences the aerodynamic forces acting on a circular cylinder in shear flow. Both the Reynolds number and shear parameter influence the movement of the stagnation point and separation point. Mode A wake instability is suppressed into parallel vortex shedding mode at a certain shear parameter. The lift force increases with increasing shear parameter and acts from the high-velocity side to the low-velocity side. In addition, a simple method to estimate the lift force coefficient in shear flow is provided.     

An application of the finite difference‐based lattice Boltzmann model to simulating flow‐induced noise

This paper presents a novel approach to simulate aerodynamically generated sounds by modifying the finite difference‐based lattice BGK compressible fluid model for the purpose of speeding up the calculation and also stabilizing the numerical scheme. With the model, aerodynamic sounds generated by a uniform flow around a two‐dimensional circular cylinder at Re = 150 are simulated. The third‐order‐accurate up‐wind scheme is used for the spatial derivatives, and the second‐order‐accurate Runge–Kutta method is applied for the time marching. The results show that we successively capture very small acoustic pressure fluctuations, with the same frequency of the Karman vortex street, much smaller than the whole pressure fluctuation around a circular cylinder. The propagation velocity of the acoustic waves shows that the points of peak pressure are biased upstream owing to the Doppler effect in the uniform flow. For the downstream, on the other hand, it is faster. It is also apparent that the amplitude of sound pressure is proportional to r^?1/2, r being the distance from the centre of the circular cylinder. Moreover, the edgetone generated by a two‐dimensional jet impinging on a wedge to predict the frequency characteristics of the discrete oscillations of a jet‐edge feedback cycle is investigated. The jet is chosen long enough to guarantee the parabolic velocity profile of the jet at the outlet, and the edge is of an angle of α = 23°. At a stand‐off distance w, the edge is inserted along the centreline of the jet, and a sinuous instability wave with real frequency is assumed to be created in the vicinity of the nozzle exit and to propagate towards the downstream. We have succeeded in capturing small pressure fluctuations resulting from periodic oscillation of jet around the edge. Copyright © 2006 John Wiley & Sons, Ltd.     

MHD free convection flow of visco-elastic fluid past an infinite vertical porous plate

 This work provides a comprehensive theoretical analysis of a two-dimensional unsteady free convection flow of an incompressible, visco-elastic fluid past an infinite vertical porous plate. Solutions for the zero order perturbation velocity profile, the first order perturbation velocity and temperature profiles in closed form are obtained with the help of Laplace transform technique. The numerical solutions are carried out for the Prandtl number 0.1, 0.72, 1.0, 1.5 and 2.0 which are appropriate for different types of liquid metals and for different values of magnetic field parameter, M. Received on 1 September 1999     

GROUND EFFECT OF FLOW AROUND AN ELLIPTIC CYLINDER IN A TURBULENT BOUNDARY LAYER

The flow characteristics around an elliptic cylinder with an axis ratio of AR=2 located near a flat plate were investigated experimentally. The elliptic cylinder was embedded in a turbulent boundary layer whose thickness is larger than the cylinder height. For comparison, the same experiment was carried out for a circular cylinder having the same vertical height. The Reynolds number based on the height of the cylinder cross-section was 14000. The pressure distributions on the cylinder surface and on the flat plate were measured for various gap distances between the cylinder and the plate. The wake velocity profiles behind the cylinder were measured using hot-wire anemometry. In the near-wake region, the vortices are shed regularly only when the gap ratio is greater than the critical value of G/B=0·4. The critical gap ratio is larger than that of a circular cylinder. The variation of surface pressure distributions on the elliptic cylinder with respect to the gap ratio is much smaller than that on the circular cylinder. This trend is more evident on the upper surface than the lower one. The surface pressures on the flat plate recover faster than those for the case of the circular cylinder at downstream locations. As the gap ratio increases, the drag coefficient of the cylinder itself increases, but the lift coefficient decreases. For all gap ratios tested in this study, the drag coefficient of the elliptic cylinder is about half that of the circular cylinder. The ground effect of the cylinder at small gap ratio constrains the flow passing through the gap, and restricts the vortex shedding from the cylinder, especially in the lower side of the cylinder wake. This constraint effect is more severe for the elliptic cylinder, compared to the circular cylinder. The wake region behind the elliptic cylinder is relatively small and the velocity profiles tend to approach rapidly to those of a flat plate boundary layer     

Flow around submerged structures subjected to shallow submergence over plane bed

The results of an experimental investigation on the flow field around submerged structures on horizontal plane beds, measured by an acoustic Doppler velocimeter (ADV), are presented. Experiments were conducted for various conditions of submergence, having submergence factors ranging from 1.0 to 2.0 and average flow velocity ranging from 0.25 to 0.51 m/s. The Froude number and the Reynolds number of the approaching flow for different runs are in the range of 0.18–0.42 and 50 000–76 500, respectively. The vertical distributions of time-averaged three dimensional velocity components and turbulence intensity components at different radial distances from the submerged structures are plotted. Deceleration and acceleration of the approaching flow around the submerged body are evident from the vertical distributions of the horizontal velocity component, whereas the lifting and diving nature of the flow are indicated by the vertical velocity component distributions. The vertical distributions of the horizontal velocity component indicate reduction of 30% of the non-dimensional time-averaged horizontal velocity component magnitude for the cylinder of diameter 11.5 cm in comparison to the cylinder of diameter 10 cm. Also, there is an increase of 10–25% in the horizontal velocity component at different radial sections. The flow is three dimensional in the downstream of the submerged structure. The velocity and the turbulent intensity components are also well predicted by FLUENT. The flow characteristics in the wake and the induced bed shear stress are also analyzed with FLUENT.The profiles of non-dimensional shear velocity deviate from the log law in the wake and the far downstream directions. The scour prone regions may be identified from the profiles of the induced bed shear stress around the submerged structure.     

Interaction between the near wake and the cross-section variation of a circular cylinder in uniform flow

The flow around a circular cylinder with a cross-section variation is experimentally investigated. Particle Image Velocimetry (PIV) is used to scrutinize the interaction of the cylinder’s wall with its near wake. The Reynolds number based on the cylinder’s diameter and freestream velocity is 80 × 10³, corresponding to the upper subcritical flow regime. At a forcing Strouhal number of St _f = 0.02, the maximum vorticity level around the cylinder is reduced by more than 50% as compared to its uncontrolled value. The topology of the bulk flow confined between the primary vortical structure and the cylinder surface is modified resulting in substantial drag reduction.     

Numerical solution for the flow of viscoelastic fluids around an inclined circular cylinder.

Finite difference solutions have been obtained by the perturbation method to investigate the influence of shear thinning and elasticity on the flow around an inclined circular cylinder of finite length in a uniform flow. In this numerical analysis a generalized upper-convected Maxwell model, in which the viscosity changes according to the Cross model, has been used.The local flow over the cylinder is only slightly deflected. However, in the wake flow behind the cylinder the particle path is remarkably influenced by the axial flow and rapidly flows up parallel to the cylinder's axis. Then it gradually rejoins direction of the incoming flow. It is found that viscoelastic fluids are prone to flow axially in the vicinity of the cylinder. The numerical predictions generally agree with the flow visualization results.The numerical solutions also demonstrate that elasticity has a strong effect on the velocity profile especially around both ends of the cylinder; elasticity increases the asymmetric profiles of both circumferential velocity and axial velocity with respect to equal to 90° and decreases a difference in the circumferential velocity between the windward end and the leeward end.For non-Newtonian fluids, the length of the wake flow is influenced by not only the Reynolds number but also the cylinder diameter and it is larger for the cylinder with the smaller diameter at the same Reynolds number.Partly presented at the 9th Australasian Fluid Mechanics Conference, University of Auckland, New Zealand, 8–12 December, 1986     

A semi-implicit lagrangian scheme for 3D shallow water flow with a two-layer turbulence model

A semi-implicit Lagrangian finite difference scheme for 3D shallow water flow has been developed to include an eddy viscosity model for turbulent mixing in the vertical direction. The α-co-ordinate system for the vertical direction has been introduced to give accurate definition of bed and surface boundary conditions. The simple two-layer mixing length model for rough surfaces is used with the standard assumption that the shear stress across the wall region at a given horizontal location is constant. The bed condition is thus defined only by its roughness height (avoiding the need for a friction formula relating to depth-averaged flow, e.g. Chezy, used previously). The method is shown to be efficient and stable with an explicit Lagrangian formulation for convective terms and terms for surface elevation and vertical mixing handled implicitly. The method is applied to current flow around a circular island with gently sloping sides which produce periodic recirculation zones (vortex shedding). Comparisons are made with experimental measurements of velocity using laser Doppler anemometry (time histories at specific points) and surface particle-tracking velocimetry.     

Scour Hole Influence on Turbulent Flow Field around Complex Bridge Piers

Experimental results of detailed flow measurements using an Acoustic-Doppler Velocimeter (ADV) around a complex bridge pier (CBP) are presented. The pier consists of a column, a pile cap (PC) and a 2×4 pile group. The time-averaged velocities, turbulence intensities, and Reynolds stresses are studied and presented at different horizontal and vertical planes. Streamlines obtained from the velocity fields are used to show the complexity of the flow around the pier. It is shown that the main feature of the flow responsible for the entrainment of the bed sediments is a contracted (pressurized) flow below the PC toward the piles. A deflected flow around the PC and a strong down-flow along its sides are observed and have been measured. It is shown that these flow patterns also cause sediment entrainment. Vortex flow behind the PC and amplification of turbulence intensity along its sides near the downstream region can be other reasons for the scour hole (SH) development. Turbulence intensities and Reynolds shear stresses are presented and discussed. A comparison is made between the flow field measured with the equilibrium SH and that measured on the fixed flat-bed. The results show that the flow field around the PC is considerably influenced by the development of the SH. The extent of the wake region at the rear of the PC is about 1.4 times larger for the fixed bed (FB) than for the scoured bed (SB). Moreover, the size of the core of high turbulent kinetic energy K, as well as the maximum values of K behind the column for the FB case is larger than that of the SB case. When a scour hole develops, the flow below the PC around the piles is considered to be the main cause of the scour. This is the first time that these observations about the flow and turbulence field around a complex bridge pier are reported and analyzed. In addition to improving the understanding of the flow structure, the present detailed measurements can also be used for benchmarking and verification of numerical models.     

Turbulent shear stress profiles in a bubbly channel flow assessed by particle tracking velocimetry

Particle tracking velocimetry (PTV) is applied to a bubbly two-phase turbulent flow in a horizontal channel at Re = 2 × 10⁴ to investigate the turbulent shear stress profile which had been altered by the presence of bubbles. Streamwise and vertical velocity components of liquid phase are obtained using a shallow focus imaging method under backlight photography. The size of bubbles injected through a porous plate in the channel ranged from 0.3 to 1.5 mm diameter, and the bubbles show a significant backward slip velocity relative to liquid flow. After bubbles and tracer particles are identified by binarizing the image, velocity of each phase and void fraction are profiled in a downstream region. The turbulent shear stress, which consists of three components in the bubbly two-phase flow, is computed by analysis of PTV data. The result shows that the fluctuation correlation between local void fraction and vertical liquid velocity provides a negative shear stress component which promotes frictional drag reduction in the bubbly two-phase layer. The paper also deals with the source of the negative shear stress considering bubble’s relative motion to liquid.     

Horizontal bridge-soil interaction assessment by dynamic testing

This paper presents a methodology to assess the real longitudinal stiffness of a bridge including the foundation-soil interaction. The method is based on a dynamic test of the bridge complemented by structural identification. The experimentally obtained natural frequencies of the bridge in the horizontal and vertical planes are used to derive, via a parameter identification procedure and a dynamic model of the bridge, the most approximate values of the Winkler coefficients simulating the interaction of foundation with ground. The theoretical background of the procedure is first presented and then demonstrated on a prestressed concrete highway bridge with a total length of 2040 m (normal span length of 40 m). The bridge is located near the sea coast, and large variations in the soil properties were observed between the piers. As a consequence, it was not possible to assess in a reliable way the actual global stiffness of this bridge (due to the interaction of deck, piers, abutments and foundations) in front of horizontal forces (vehicle braking, earthquake, wind and so on) based on the results of the geotechnical survey. The dynamic excitation in the longitudinal direction was easily achieved by means of controlled vehicle braking at different points on the bridge. The results show the feasibility of using dynamic testing with vehicle braking as excitation to deduce the correct behavior of the bridge under the effect of horizontal loads.     

Extended Proper Orthogonal Decomposition: Application to Jet/Vortex Interaction

The Proper Orthogonal Decomposition (POD) is used in the present work to study the interactions between different regions of a flow. The standard analysis would select structures that are best correlated with the entire fluctuating velocity field. It is therefore not helpful if one flow region S of interest contains only a small percentage of the total kinetic energy. Using POD modes computed in the sub-domain S only, extended modes are introduced using the method of snapshots. We demonstrate that they provide a decomposition of the velocity field in the whole domain and that the extended mode number p provides the only local contribution to the velocity field correlated with the projection of the velocity field on POD mode p in S. This method is general and can be applied to either experimental or numerical velocity fields. As an example, it is applied to the analysis of an internal turbulent flow in a model engine cylinder with tumble. Data are obtained at a given phase with Particle Image Velocimetry. We focus our analysis on the middle of the intake stroke when the energy containing intake jet rolls up to feed a large vortex structure. Preferred directions of the jet/vortex interaction are clearly identified. This revised version was published online in July 2006 with corrections to the Cover Date.     

ONSET OF ASYMMETRY: FLOW PAST CIRCULAR AND ELLIPTIC CYLINDERS

A computational study of the development of two- dimensional unsteady viscous incompressible flow around a circular cylinder and elliptic cylinders is undertaken at a Reynolds number of 10,000. A higher- order upwind scheme is used to solve the Navier–Stokes equations by the finite difference method in order to study the onset of computed asymmetry around bluff bodies. For the computed cases the ellipses develop asymmetry much earlier than the circular cylinder. The receptivity of the computed flows in the presence of discrete roughness and surface vibration is studied. Finally, the role of discrete roughness in triggering asymmetry for flow past a circular cylinder is studied and compared with flow visualization experiments at Re=10,000     

The interactions between wave-currents and offshore structures with consideration of fluid viscosity

Study of the flow field around the large scale offshore structures under the action of waves and viscous currents is of primary importance for the scouring estimation and protection in the vicinity of the structures. But very little has been known in its mechanism when the viscous effects is taken into consideration. As a part of the efforts to tackle the problem, a numerical model is presented for the simulation of the flow field around a fixed vertical truncated circular cylinder subjected to waves and viscous currents based on the depth-averaged Reynolds equations and depth-averagedk-ɛ turbulence model. Finite difference method with a suitable iteration defect correct method and an artificial open boundary condition are adopted in the numerical process. Numerical results presented relate to the interactions of a pure incident viscous current with Reynolds numberRe=10⁵, a pure incident regular sinusoidal wave, and the coexisting of viscous current and wave with a circular cylinder, respectively. Flow fields associated with the hydrodynamic coefficients of the fixed cylinder, as well as corresponding free surface profiles and wave amplitudes, are discussed. The present method is found to be relatively straightforward, computationally effective and numerically stable for treating the problem of interactions among waves, viscous currents and bodies. The project supported by the National Natural Science Foundation of China and Foundation of State Key Laboratory of Ocean Engineering at Shanghai Jiao Tong University.