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.     

圆柱绕流局部冲刷机制的实验研究

采用粒子图像测速技术和粒子图像识别技术研究了冲刷坑的演化过程及局部冲刷的流体动力机制。首先研究了动床条件下圆柱周围的床面演化过程，给出垂直对称面输沙率随时间变化以及圆柱下游垂直对称面的沙颗粒浓度和泥沙分布规律；其次针对典型冲刷坑形状固定底床，测量了圆柱下游的瞬时流场，并给出平均流场。结果表明尾流区平均剪应力较低，但垂向涡量和湍流强度较高。分析指出平均剪应力较低的尾流区域，局部冲刷的主要动力机制是尾涡和湍流的综合作用。     

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;介绍了明流、冰盖下桥墩周围局部冲刷等方面的相关研究工作,得出了冰盖条件下桥墩局部最大冲刷深度的回归经验公式,结果表明该公式具有很好的适用范围.对冰塞条件下桥墩附近局部冲刷问题探索性地开展了试验研究,研究表明,冰塞条件下的局部冲刷与冰盖条件下有着很大不同,不仅冲刷深度要大的多,而且桥墩周围局部冲刷过程与冰塞厚度变化相关.在冰塞条件下,桥墩处的最大冲刷深度和最大冲刷长度均随流速增大而增大,冲刷坑尾部淤积沙丘高度也受到流速的影响.就相关研究难点和今后需要进一步开展的研究工作进行了分析和展望.      

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.     

Turbulent flow around a wall-mounted cube: A direct numerical simulation

An immersed-boundary method was employed to perform a direct numerical simulation (DNS) of flow around a wall-mounted cube in a fully developed turbulent channel for a Reynolds number Re = 5610, based on the bulk velocity and the channel height. Instantaneous results of the DNS of a plain channel flow were used as a fully developed inflow condition for the main channel. The results confirm the unsteadiness of the considered flow caused by the unstable interaction of a horseshoe vortex formed in front of the cube and on both its sides with an arch-type vortex behind the cube. The time-averaged data of the turbulence mean-square intensities, Reynolds shear stresses, kinetic energy and dissipation rate are presented. The negative turbulence production is predicted in the region in front of the cube where the main horseshoe vortex originates.     

透水床面明渠湍流时空平均特征的大涡模拟与分析

基于大涡模拟数据,研究了理想粗糙透水床面明渠湍流的时空平均特性. 考虑到空间异构性,对比分析了不同位置的时空平均流速、雷诺剪应力、构造剪应力、脉动幅度的垂线分布. 结果表明:第一,顶层床面之上,空间异构性的影响较小,不同位置的双平均流速符合类似的对数分布,但由于透水床面影响,卡门常数较不透水床面小;在床面附近,空间异构性影响较大,不同位置的双平均流速分别符合线性分布与多项式分布;在透水河床内部,靠近底层球孔的双平均流速为上部球孔双平均流速的1.55 倍. 第二,床面之上,雷诺剪应力占总剪应力的95% 以上,占有主体地位;床面附近,紊动较大,构造剪应力不能忽略,其值大约占总剪应力的15%.由于流场的各向异性,纵向与垂向的脉动幅度有所差异.      

Experimental investigation on the similarity of a 3D rectangular turbulent jet

 The paper presents an experimental investigation of turbulent jets issuing from rectangular nozzles. Nozzles with aspect ratios between 3 and 10 were used. Eight different initial conditions were studied. The following jet parameters were measured and evaluated: mean velocity components, jet boundaries, jet momentum, jet entrainment, turbulence intensities and Reynolds stresses. A DISA 55M thermoanemometer and a data acquisition system BE256 were used. The influence of the initial conditions on the similarity of the flow was determined with respect to the mean axial velocity, turbulence intensity and the Reynolds stresses. A significant influence of the initial conditions on the flow structure was observed. The possibility for jet control is discussed and suggestions are given about the need to investigate different parameters. Received: 25 November 1996/Accepted: 30 October 1997     

Measurements of turbulence characteristics in an open-channel flow over a transitionally-rough bed using particle image velocimetry

The particle image velocimetry technique was used to measure characteristics of a turbulent flow over a transitionally-rough fixed bed in an open-channel flow. These conditions are typical of flows encountered in sediment transport problems. Measurements obtained with this technique were used to investigate the distributions of velocities, turbulence intensities, Reynolds stress, and third- and fourth-order moments in a region above y ⁺ = 10. The present results are in good agreement to those previously obtained on smooth walls and provide further evidence that PIV can be applied successfully to investigate turbulence in open-channel flows over a rough bed.     

The spatial inhomogeneity of turbulence above a fully rough, packed bed in open channel flow

Single point turbulence statistics measured directly above and in close proximity to the wall in a fully developed, fully rough, turbulent open channel flow are reported. In order to investigate the spatial inhomogeneity of the turbulence, the measurements were obtained over a matrix of measurement points in a plane parallel to the roughness-bed surface. The measurements were obtained with a three-component laser Doppler velocimeter (3D-LDV) system. The turbulence statistics associated with the vertical velocity component, including conditioned mean vertical velocities, rms distributions, and mean vertical momentum fluxes are emphasized. For the Reynolds and Froude numbers associated with this investigation, and with the specific roughness geometry employed in this study (a packed bed of uniform-diameter spheres), it is found that the distribution of the local mean vertical velocity, <w>, has non-zero contributions over the roughness pattern and that this contributes to a mean net vertical momentum flux into the roughness bed. However, the net vertical momentum flux due to turbulent fluctuations is positive out of the bed, consistent with smooth-wall behavior. These results are relevant to the study of sediment entrainment and suspension/deposition as well as the exchange and transport of chemical species between the channel core flow and the fluid within the roughness bed. Received: 21 July 1998/Accepted: 20 November 1999     

Comparison of methods to estimate the scour downstream of a ski jump

This study analyzes the expected depth and scour shape in the Toachi River (Ecuador) as a result of the construction of the Toachi Dam in order to avoid problems with progressive scour. The dam has a maximum height of 59 m to the foundations. The free surface weir ends in a ski jump and has been designed to operate with a peak discharge of 1213 m³/s. As each method has its limitations, the scour is studied with four complementary procedures: 1) a 1:50 Froude scale similitude laboratory model used as validation case; 2) 36 empirical formulae derived from models and prototypes; 3) a semi-empirical methodology based on pressure fluctuations-erodibility index; and 4) FLOW-3D numerical simulations. The expected scour depth is 6.65 m for the design flow. The results are close to the physical model. In the numerical analysis, turbulence is treated using the three RANS approaches. The choice of the turbulence model and the bed load coefficient in the Meyer-Peter & Müller formula are of great importance. The best results were obtained using the RNG k-ε turbulence model and a bed load coefficient below 7.     

Experimental measurement of flow past cavities of different shapes

Experiments are carried out in order to investigate the flow structure past a rectangular, triangular and semi-circular cavity of length-to-depth ratio of 2:1 using the Particle Image Velocimetry (PIV) technique. The experiments are performed in a large scale water channel with three different upstream velocities resulting in Reynolds numbers of 1230, 1460 and 1700, based on inflow momentum thickness, for each cavity type. Contours of constant averaged streamwise and transverse components of velocity, contours of constant averaged vorticity, Reynolds stress and streamline plots for each cavity type for the aforementioned three Reynolds numbers are presented. In addition, streamwise velocity, Reynolds stress and turbulence intensity values are compared for all cavity types. Effect of cavity shape on flow structure within the cavity is discussed in detail. Moreover, spectrum of instantaneous streamwise velocity fluctuations in shear layer near the downstream of the leading corner and the upstream of the trailing corner of the cavities are obtained and it was found that no organized oscillations are present in the flow; rectangular and triangular cavities have the largest amplitudes while semi-circular cavity has the smallest. Calculated turbulence intensities also reveal that the maximum turbulence intensities occur at cavity lid in the centerline section and rectangular and triangular cavities have larger turbulence intensity compared to semi-circular cavity.     

侧移土体被动桩成拱效应及主动侧土压力计算

采用三维数值模型模拟土体水平位移条件下被动桩的成拱效应。分析结果表明,被动桩的成拱效应和桩间距关系最为密切,桩间距较小时,侧移土体在桩周附近呈现出明显的土拱效应,应力等值线密集,相互连通成一整体,并向两端突出,桩间距增大时土拱效应减弱;随着作用于土体侧向荷载的增大,土拱效应增强,桩周应力区增大。在数值分析的基础上,以T.Itos土压力为依据,考虑土体的实际分层,探讨了考虑土拱效应的土压力计算方法,给出了被动桩计算模式,最后将简化方法与数值分析的桩侧土压力进行了对比。     

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.     

Unsteady RANS simulation of oscillating mould flows

Mould flow oscillations are of major importance for the performance of the continuous casting process. They are suspected to promote entrainment of slag and other unwanted secondary phases into the melt pool. These oscillating turbulent flows are investigated by means of numerical simulations. The numerical model is based on the equation of continuity and the unsteady Reynolds averaged Navier–Stokes equations. The system of flow equations is closed by a Reynolds stress turbulence model in combination with non‐equilibrium wall functions. The unsteady simulation resolves low‐frequency oscillations of the flow field. These frequencies and numerically resolved mean values are in agreement with results of corresponding model experiments. The proposed model should be advantageous in order to investigate the mechanisms of the oscillations and the process of slag entrainment in more detail. Copyright © 2006 John Wiley & Sons, Ltd.     

Anisotropic Organised Eddy Simulation for the prediction of non-equilibrium turbulent flows around bodies

The unsteady turbulent flow around bodies at high Reynolds number is predicted by an anisotropic eddy-viscosity model in the context of the Organised Eddy Simulation (OES). A tensorial eddy-viscosity concept is developed to reinforce turbulent stress anisotropy, that is a crucial characteristic of non-equilibrium turbulence in the near-region. The theoretical aspects of the modelling are investigated by means of a phase-averaged PIV in the flow around a circular cylinder at Reynolds number 1.4×10⁵. A pronounced stress–strain misalignment is quantified in the near-wake region of the detached flow, that is well captured by a tensorial eddy-viscosity concept. This is achieved by modelling the turbulence stress anisotropy tensor by its projection onto the principal matrices of the strain-rate tensor. Additional transport equations for the projection coefficients are derived from a second-order moment closure scheme. The modification of the turbulence length scale yielded by OES is used in the Detached Eddy Simulation hybrid approach. The detached turbulent flows around a NACA0012 airfoil (2-D) and a circular cylinder (3-D) are studied at Reynolds numbers 10⁵ and 1.4×10⁵, respectively. The results compared to experimental ones emphasise the predictive capabilities of the OES approach concerning the flow physics capture for turbulent unsteady flows around bodies at high Reynolds numbers.     

Finite element modelling of local scour below a pipeline under steady currents

Local scour has been identified as the main factor that causes failures of structures in offshore engineering. Numerous research efforts have been devoted to local scour around offshore pipelines in the past. In this paper, a finite element numerical model is established for simulating local scour below offshore pipelines in steady currents. The flow is simulated by solving the unsteady Reynolds-averaged Navier–Stokes equations with a standard k ? ? turbulent model closure. A sand slide scheme is proposed for the scour calculation, and bed load is considered in the proposed scour model. To account for changes in bed level, the moving mesh method is adopted to capture the water–sediment interface (bed), and the change of bed level is calculated by solving Exner–Polya equation. All the equations are discretised within the two-step Taylor–Galerkin algorithm in this paper. It is found that the sand slide model works well for the simulation of the scour, and the numerical results are shown to be in good agreement with the available experimental data.     

Moderate Reynolds number axisymmetric jet development downstream an extended conical diffuser: Influence of extension length

Low-velocity (bulk velocity of 4.4 m/s) and moderate Reynolds (7350) axisymmetrical jet development is studied by hot-film single sensor anemometry. The jet issues from a conical convergent-divergent diffuser with uniform extension (diameter 25 mm). Decreasing the length-to-diameter ratio of the extension tube from 20 down to 0.4 is shown to alter severely the mean velocity profile at the tube outlet from Blasius to top-hat whereas turbulence intensities increases from 6 up to 50%. Next, the influence of the initial velocity profile at the tube outlet on axisymmetrical jet development is assessed. The velocity development exhibits a self-similar far field characteristic for axisymmetrical jet development. Although, the jet centerline decay constant increases and the jet spreading rate decreases as length-to-diameter ratios are increased from 0.4 up to 7.2 for which the initial centerline velocity decreases. Therefore, scaling of the centerline decay constant and inverse scaling of the spreading rate with initial centerline velocity U₀ or initial velocity Reynolds number Re₀ reported for moderate Reynolds numbers and low initial turbulence levels [18,22] does not hold as the turbulence level exceeds a threshold value in the range 12–27%. In addition, the influence of initial conditions on near and far field turbulence properties is shown. A transition in near field behaviour is observed for length-to-diameters around 3.6. Flow and geometrical configurations under study are relevant to e.g. upper airway flow.     

Effect of bubbles on the turbulence near the exit of a liquid jet

The objective of this paper is to examine the effect of bubbles on the turbulence levels of a water jet. Simultaneous measurements of the axial and radial velocity components were taken in a bubbly jet with a Laser Doppler Velocimeter (LDV) and then compared to the velocities of a single phase jet at the same liquid flow rate. Mean bubble diameters ranged from 0.6 to 2 mm and the void fractions were up to about 20%. The liquid Reynolds numbers were from 5,000 to 10,000 approximately. The measurements extended to from an axial distance of 4–12 cm. It was observed that bubbles did not affect significantly the average velocity profiles in the jet. However bubbles increased the turbulence intensities in the core of the jet near the jet exit. The increase in turbulence intensities was more pronounced at lower Reynolds numbers and at higher void fractions.     

应用二维激光测速系统研究面流消能的紊动特性

本文应用二维激光测速系统对面流消能工下游典型流态的紊动特性进行了系统的试验研究,得到了不同来流条件下面流的时均流速、紊动强度及雷诺应力等沿主流与回流区的分布特征;计算并绘制了流线图,由此分析了淹没面流时底旋滚长度和佛劳德数的关系。对紊动能量的转换过程及耗散特征进行了计算分析,初步探明了消能机理。