Turbulent flow field over fluvial obstacle marks generated in a laboratory flume

The study is aimed at investigating the mean flow and turbulence characteristics in scour geometry developed near a circular cylinder of length 10cm placed over the sand bed transverse to the flow. The obstacle placed on a sand bed, on the way of a unidirectional flow, develops a crescent-shaped scour mark on the bed. The scour is caused by generation of vortex developed on the upstream side of the obstacle. Sand grains eroded by this vortex, are deposited on the downstream side of the obstacle as wakes. The turbulent flow field within the scour mark was measured in a laboratory flume using an Acoustic Doppler Velocimeter (ADV). The scour marks named as current crescents preserved in geological record are traditionally used as indicators of palaeocurrent direction. The distribution of mean velocity components, turbulent intensities and Reynolds stresses at different positions of the mark are presented. The experimental evidence also shows that the geometric characteristics of the scour mark (width) depend primarily on the cylinder aspect ratio, cylinder Reynolds number and sediment Froude number.     

Large Eddy Simulation for Unsteady Turbulent Field in Thin Slab Continuous Casting Mold

The unsteady turbulent flow during the continuous casting of steel is important, because it influences critical phenomena that affect steel quality. Unsteady three-dimensional flow in the mold region of the liquid pool during continuous casting of steel slabs has been computed using realistic geometries starting from the submerged inlet nozzle to the mold. The cassette filter function was used to deal with unsteady Navier-Stokes equation, and then the turbulent flow in the thin slab CCM was simulated with the large eddy simulation method combined with the Smagorinsky sub-grid scale model in this paper. And the model was verified by the Particle Image Velocimetry (PIV) experimental results which was got from a relate scientific literature. In this thesis, by means of LES, the flow characteristics in the thin slab CCM were acquired, such as the vortex distribution, the formation of the large eddy coherent structures, development, shedding and fracture process. In the same time, the turbulent asymmetric distribution was revealed even the nozzle in the centre position. Interactions between the two halves cause large velocity fluctuations near the meniscus. And the vortex is located at the low velocity side adjacent to the SEN. Along with the unsteady time development, the unsteady turbulent large vortex structures of the liquid steel in the CCM presented periodic bias flow distribution, and the period is about 20 seconds.     

Random Lock-In Intervals for Tubular Structural Elements Subject to Simulated Natural Wind

This paper reports on wind tunnel experiments with an elastically suspended circular cylinder vibrating under the excitation of natural wind of high turbulence degree. The natural wind turbulence was simulated by superposing the low frequency part of the natural wind turbulence on the background high frequency turbulence of the wind tunnel flow. This was done by controlling the propeller rotation speed according to an artificially generated low frequency speed sample function drawn from a suitable random process model. The experiment provided statistical data on the intermittent random occurrence and size of strong lock-in vibrations in resonance with the vortex shedding excitation. The purpose of the experiment was to obtain data to support the formulation of a sufficiently detailed stochastic model to allow computer simulation of reasonably realistic fatigue damage accumulation at “hot spots” of tubular structural elements subject to the natural wind. The engineering relevance of the investigation is supported by comparing it with the unrealistic highly conservative rules of wind-induced fatigue commonly given in codes of practice. The stochastic lock-in model as well as the related fatigue calculation procedure is reported in the paper.     

Flow Field at a Vertical-Wall Abutment

Experiments were conducted to measure the three-dimensional turbulent flow field, using the acoustic Doppler velocimeter, at a short vertical-wall abutment (ratio of abutment length to approach flow depth less than unity) before and after the development of a scour hole under a clear water scour condition. In the upstream, the presentation of flow field through vectors at vertical sections shows a primary vortex associated with the downflow. In the downstream, the upward flow is comprised of with irregularities owing to the vortex shedding. The flow separation near the bed and within the scour hole is evident from the turbulent kinetic energy distribution. Using Reynolds stresses, the bed shear stresses are calculated.     

入口均匀性对直方管内湍流影响的大涡模拟研究

采用大涡模拟方法对直方管内的湍流流动进行了研究，对比计算了进口处为均匀来流和非均匀来流三种工况。从瞬时速度、脉动速度和平均速度分布等方面对湍流流动进行了分析。计算结果表明：入口处均匀来流和非均匀来流在横截面都会产生二次流运动；四个角域强化了壁面流体和主流流体的动量交换；在平均雷诺数相同情况下．非均匀来流时湍流程度大，各方向湍流脉动加强，横截面二次流加强，出现多个复杂的涡流区．瞬时流场结构、涡结构要复杂得多。     

入口非均匀性对直方管内湍流影响的大涡模拟研究

采用大涡模拟方法对直方管内的湍流流动进行了研究,对比计算了进口处为均匀来流和非均匀来流三种工况。从瞬时速度、脉动速度和平均速度分布等方面对湍流流动进行了分析。计算结果表明:入口处均匀来流和非均匀来流在横截面都会产生二次流运动;四个角域强化了壁面流体和主流流体的动量交换;在平均雷诺数相同情况下,非均匀来流时湍流程度大,各方向湍流脉动加强,横截面二次流加强,出现多个复杂的涡流区,瞬时流场结构、涡结构要复杂得多。     

Numerical Computational Fluid Dynamics-Based Models of Ultraviolet Disinfection Channels

Various numerical modeling approaches, all based on computational fluid dynamics (CFD) solutions for the flow field, are studied for an ultraviolet disinfection system in which the lamps are oriented perpendicular to the flow direction. A two-dimensional model assumption is made in all simulations, for which turbulent flow solutions were obtained with commercial CFD software (FIDAP). Two modeling approaches were studied. A continuum Eulerian approach was taken in formulating an appropriate advection–diffusion equation which is solved for the viable micro-organism concentration. Alternatively, a Lagrangian approach, in which particles are numerically introduced into the flow and their trajectories through a spatially varying field of ultraviolet intensities were computed, was also investigated. The effect of modeling unsteady-flow features associated with vortex shedding and motion on the extent of disinfection was examined by comparing time-averaged results based on an unsteady-flow continuum model with the results from an analogous simulation assuming a steady flow. Under the steady-flow assumption, differences between predictions of the Eulerian continuum approach and the Lagrangian particle-trajectory approach were also considered. Both modeling approaches yielded similar predictions over a range of loadings, and tended to underestimate the extent of disinfection when compared to measurements at the pilot scale.     

Purging of a Neutrally Buoyant or a Dense Miscible Contaminant from a Rectangular Cavity. II: Case of an Incoming Fully Turbulent Overflow

Fully three-dimensional (3D) large-eddy simulation calculations of the flow past two-dimensional cavities for the case in which the incoming flow is fully turbulent are conducted to study the purging of neutrally buoyant or dense miscible contaminants introduced instantaneously inside the cavity. 3D simulations are needed because in the turbulent case (TC), as opposed to the laminar inflow case (LC) considered in the companion paper, the interactions between the coherent structures advected from the incoming channel and the eddies inside the cavity are highly 3D and have a nonnegligible effect on the mass exchange processes between the cavity and channel. Similar to the LC, it is found that the mechanism of removal of the contaminant is very different between the neutrally buoyant and buoyant cases. In the neutrally buoyant TC simulation the contaminant is ejected from the cavity due to the interactions among the large scale eddies in the separated shear layer, the coherent structures convected from the upstream channel over the cavity, and the main recirculation eddies inside the cavity. In the TC simulation with a negatively buoyant contaminant, internal wave breaking is observed to occur over the initial phases of the mixing which, along with other turbulent mixing phenomena, reduces the mean density gradient across the density interface. In the later stages, the contaminant removal and mixing processes are controlled by the interactions of the trailing edge vortex with the bottom layer containing denser contaminant beneath it and upstream of it (for the final stages when the vortex touches the cavity bottom). The oscillations in the size, position, and intensity of the trailing edge vortex are larger than the ones observed in the LC. As expected, turbulent mixing accelerates the purging process in the TC simulations.     

Influence of Inlet Shear on Structure of Wake behind a Square Cylinder

The force coefficients and the frequency of vortex shedding in the wake of a square cylinder exposed to a uniform shear flow and the flow structure around it were numerically investigated. The Reynolds number defined on the basis of cylinder width was in the range of 250–1,500. The shear parameter, namely the transverse velocity gradient, which is nondimensionalized using the obstacle width and the average incoming velocity, was varied between 0 and 0.2. Analyses were performed for a number of flow parameters using various combinations of Reynolds number and shear parameters. Results show that mean and root-mean-square values of drag coefficient initially decrease up to certain values of the shear rate and then increase with increase in shear parameter. The root-mean-square values of lift coefficient show a similar behavior. The Strouhal number decreases uniformly with increase in shear parameter. At higher shear rates, the von Kármán vortex street comprising alternating vortices breaks, and the far wake shows mainly clockwise vortices.     

RF固邦特加固材料在矿井巷道维修中的应用

随着矿井开采的不断延深和矿井开采年限的不断增加,再加上一些采动影响,使得一些主要的服务巷道离层、严重开裂、脱落,普通的锚网喷支护不能持久,使得巷修频繁。为了解决这一问题,通过对巷道现场环境的详细调查和根据巷道的实际服务要求,采用RF固邦特加固材料的粘结、加固技术对兖州煤业股份有限公司济宁三号煤矿西部回风巷进行了顶部和帮部的修复施工,使得易脱落的顶板和帮部的煤岩体牢牢粘结在了一起,矿压也均匀分布在了整个巷道断面上,从而消除了矿井巷道顶、帮压力集中造成的煤岩体垮落,可为同类矿井巷道维修提供借鉴。     

Flow around Cylinders in Open Channels

This paper presents the results of an experimental study of flow around cylindrical objects in an open channel. Cylindrical objects of equal diameter and four heights were tested under similar flow conditions producing four different levels of submergence, including a surface piercing bridge-pier-like cylinder. Different flow elements and their locations were identified using a set of flow visualization tests. Observations made from the flow visualization tests were then verified by measurements of bed-shear stress and deflected flow velocity around the cylinders. Horse-shoe vortex systems were found to appear closer to the submerged cylinders compared to a surface piercing cylinder. The increase in dimensionless bed-shear stress is found to be inversely related to the level of submergence of the cylinders. Bed-shear stress results presented in this paper will be valuable for a qualitative understanding of the scour potential of flow around submerged cylinders. Mean velocity profiles in the deflected flow region were analyzed in terms of the theories of three-dimensional turbulent boundary layer. Submergence of a cylinder has been found to suppress alternate vortex shedding and produce stronger three-dimensional flows in the downstream wake. Perry and Joubert’s model was found to be sufficiently accurate to predict the deflected velocity magnitudes around submerged cylinders. Overall, the present study will provide valuable knowledge of hydraulics of flow around submerged structures (e.g., simple fish habitat structures).     

Coherent Structures in the Flow Field around a Circular Cylinder with Scour Hole

Large-eddy simulation (LES) and laboratory-flume visualizations were used to investigate coherent structures present in the flow field around a circular cylinder located in a scour hole. The bathymetry corresponds to equilibrium scour conditions and is fixed in LES. The flow parameters in the simulation correspond to the experimental conditions in which the approach flow is fully turbulent. Detailed consideration is given to the interaction of the horseshoe vortex (HV) system within the scour hole with the detached shear layers formed from the cylinder, and the near bed turbulence. It is found that the overall structure of the HV system varies considerably in space and time, though a large, relatively stable, primary necklace vortex is present at practically all times inside the scour hole. The simulation captures the presence of bimodal chaotic oscillations inside the HV system, as well as the sharp increase in the resolved turbulent kinetic energy levels and pressure fluctuations reported in prior experimental investigations. High levels of the mean bed shear stress are observed beneath the primary necklace vortex, especially over the region where the bimodal oscillations are strong, as well as beneath the small junction vortex at the base of the cylinder. It is also found that the detachment and advection of patches of vorticity from the downstream part of the legs of the necklace vortices can induce large instantaneous bed shear stress values. When the critical bed shear stress value for sediment entrainment on a flat surface is adjusted for bed slope effects, the LES simulation correctly predicts that the distribution of the mean bed shear stress is consistent with equilibrium scour conditions.     

Numerical Simulation of Transient Flow Patterns of Upper Swirls in Continuous Slab Casting Moulds

The transient aspect of turbulent flow in a continuous slab casting mould is studied and the transient flow patterns of the upper rolls are investigated through the application of Large Eddy Simulation (LES), employing a static Smagorinsky sub‐grid scale (SGS) model. The unsteady and coherent features of the upper recirculation zones are found, and the mechanism causing vortices developing from the upper recirculation zones is described and analysed. Results show that the asymmetry of flow patterns of the upper rolls appears most of the time. The upper swirls may break into a series of relatively small scale vortices that are chaotically distributed, and large‐scale vortex shedding occurs during the switching process between flow patterns.     

Visualization of vortex shedding at the proximal side-to-end artery-graft anastomosis

The motion of guanine particles was recorded by video to visualize transitional flow phenomena in models of a proximal side-to-end anastomosis. Close examination of successive video fields revealed that above a critical Reynolds number, particles were periodically shed into the graft from a vortex situated near the anastomosis heel, and this disturbed the flow patterns in the graft causing vortex shedding to occur near to the toe of the anastomosis. The images clearly demonstrated that periodic flow structures propagated distally along the graft for over 15 tube diameters from the proximal anastomosis. The frequency of the vortex shedding was found to increase with Reynolds number. Under pulsatile conditions, the primary vortex at the heel of the anastomosis became unstable during the deceleration phase of the flow cycle and particles were shed downstream into the graft. Although it was possible briefly to observe the characteristic banded structure in the bypass graft, the flow patterns were highly three-dimensional and were quickly broken up by the accelerating flow. Dynamic flow visualization using guanine particles was found to be a complementary technique to particle tracer flow visualization and was highly effective in identifying transitional flow phenomena and the mass transport mechanisms associated with them.     

150 t钢包自由表面旋涡的数值模拟与湍流模型的选择

钢包在非稳态浇注过程中易形成旋涡,导致钢渣及空气的大量卷入,严重危害了钢的质量.为了更好的研究旋涡,采用ANSYS CFX软件,分别采用k-ε和RNG k-ε 2种湍流模型对钢包非稳态浇注过程中自由表面旋涡进行数值模拟,得到自由表面从表面旋转到旋涡贯通水口的演化过程,将两者的计算结果与相关文献以及Burgers涡模型进行相互对比,得到:两者计算的旋涡演化过程、旋涡产生临界高度无差别;两者的速度场与相关文献均一致,但RNG k-ε模型更真实的表现了旋涡的剪切流动;将两者的切向速度分布与Burgers涡对比,得到RNG k-ε湍流模型与理论模型更加吻合.综上:用RNG k-ε湍流模型计算自由表面旋涡更加正确合理.      

Fine-Scale Characterization of the Turbulent Shear Layer of an Instream Pebble Cluster

This study characterizes the shear layer and associated vortex shedding around an isolated submerged pebble cluster in a gravel-bed river. The approach combines flow visualization and high frequency three-dimensional velocity (acoustic Doppler velocimeter) measurements. Two vortex shedding modes in the wake of the cluster were identified: A small scale high frequency initial instability mode and a lower frequency mode that scales with cluster height. The lower frequency mode arose from the intermittent interaction and amalgamation of the small-scale instability vortices. Reynolds shear stresses, velocity spectra, and coherence functions indicated a dominance of longitudinal-vertical shedding vortices in the wake of the cluster. Simultaneous flow visualization was required to determine the nature and behavior of the shedding modes. Quadrant analysis revealed that Q2 and Q4 events contributed 80% of the local longitudinal-vertical component Reynolds shear stress, and demonstrated a dominance of ejection events in the wake of the cluster. Through flow visualization, the behavior of the shear layer was seen to vertically expand and contract with the passage of Q2 and Q4 events, respectively.     

钢包水口偏心率对浇注过程影响的数学模拟

 钢包的水口偏心率会显著影响浇注末期汇流旋涡的形成和发展过程。基于钢包特征设置合理的水口偏心率,有利于控制旋涡形态、抑制旋涡的危害。但是偏心率的变化如何改变流场,对浇注过程产生哪些影响,目前的研究并不充分。通过数值模拟方法,研究了水口偏心率对钢包流场和旋涡的影响规律,同时定义了“影响度”指标,定量分析了偏心率对浇注过程中表征流场和旋涡特性的14个物理量(如水口流量、旋涡角速度、湍动能、水口含气率等)的影响程度。结果表明,水口偏心率和各物理量之间存在非线性关系;不同物理量变化的临界点不同,普遍为0.5~0.7,旋涡的稳定性和容器壁面是影响临界点的主要因素。     

Bivariate Quasi-Steady Model for Prediction of Roof Corner Pressures

Extremely high-suction pressures generated beneath the conical vortex flow in the roof-corner region have a devastating effect on the building roofs in high-wind events. The application of quasi-steady theory near the roof corners of low-rise buildings deserves careful investigation for the appropriate assessment of the design wind loads. A synchronized incident wind and pressure data acquisition system was set up on the full-scale experimental building at Texas Tech Univ. Experiments were conducted systematically to simultaneously collect the incident wind and roof-corner pressure data under the influence of cornering winds. By using a conditional sampling technique, a bivariate quasi-steady model was established to incorporate the influence of both horizontal and vertical wind directional variations on the roof-corner pressures. Comparison between the measured pressures and the model-predicted pressures has shown that the quasi-steady theory in the suggested form is applicable in the roof-corner separated flow region where vortices are present. This conclusion further justifies the application and codification of quasi-steady approach for wind load assessment of low-rise buildings and other structures.     

叶片形状对涡轮桨搅拌槽内尾涡特性的影响

Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their relationship with turbulence properties in a stirred tank of 0.48 m diameter, agitated by four different disc turbines, including Rushton turbine, concaved blade disk turbine, half elliptical blade disk turbine, and parabofic blade diskturbine. Phase-averaged and phase-resolved flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail. The location, size and strength of vortices were determined by the simplified λ2-criterion and the results showed that the blade shape had great effect on the trailing vortex characteristics. The larger curvature resulted in longer residence time of the vortex at the impeller tip, bigger distance between the upper and lower vortices and longer vortex life, also leads to smaller and stronger vortices. In addition, the turbulent kinetic energy and turbulent energy dissipation in the discharge flow were determined and discussed. High turbulent kinetic energy and turbulent energy dissipation regions were located between the upper and lower vortices and moved along with them. Although restricted to single phase flow, the presented results are essential for reliable design and scale-up of stirred tank with disc turbines.