Hydraulic experiments on spillway weir profiles

Spillway weirs are the system for regulating the flow in open channel flow passages. Weirs are commonly used to alter the flow of rivers to prevent floods, measure discharge and also in nuclear reactors for uniform cooling purposes. This paper describes the hydraulic experiments conducted for finding the air entrainment on water under laminar flow conditions in open channel flow passage with various profiles of spillway at flow rates in the range of 100–650?lpm. The concept of the spillway system allows stabilisation of the free level of water and avoids variation in the free level of water along the flow passage as a function of flow rate. The main problem in the spillway is the profiling of weir crest. The criteria that need to be satisfied are: (i) there should be no flow separation from the crest and (ii) there should be a uniform circumferential flow to avoid flow asymmetry in the passage. Separation of flow leads to large impact velocity of the falling water, which would lead to large-scale entrainment of air. When flow adheres to the weir crest and upstream, the impact velocity of water with the free surface would be less and hence entrainment would also be less.     

Effect of stream equalizer plate on flow distribution in open channel flow passages

Stream equalizer plates or flow distribution devices are the means for regulating a uniform flow in open channel flow passages. This paper describes the hydraulic experiment conducted on water for achieving a uniform flow upstream of the open channel flow passage under laminar flow conditions with various stream equalizer plate lengths at flow rates in the range of 100–500 liters per minute (LPM). The concept of achieving uniform flow conditions is used in many industrial applications including cooling of the nuclear reactor. The length of a stream equalizer plate, its location and its porosity are the operating parameters.     

Approaches for improving airflow uniformity in unidirectional flow cleanrooms

Airflow behavior inside a cleanroom with vertical unidirectional flow has been investigated. The design parameters, such as porosity and height of raised floor, width of cleanroom and inlet velocity profile, which affect the uniformity of air velocity distribution inside the cleanroom have been studied computationally. The Reynolds-averaged Navier–Stokes equations governing the flow are solved using a finite-volume code STAR-CD. The standard k-ε turbulence model has been used. Approaches for improving airflow uniformity have been proposed and quantitatively examined based on intensive case studies. The present results show that the uniformity increases along with the height of raised floor. Alternatively, better airflow uniformity can also be achieved through a proper allocation of floor porosity or by controlling the distribution of inlet velocity profile. Suggestions on how to design unidirectional cleanrooms with desired airflow uniformity under practical constraints have been given.     

椭圆与圆截面消防水炮炮座流动对比及试验

在内径66 mm 大回转结构形式水炮炮座的缩小试验模型上,提出含有椭圆截面形式的新型流道,采用数值模拟方法对比研究流道截面改变带来的差异,采用粒子图像测试技术PIV试验验证数值模拟结果。对比的结论为：当炮座的一部分圆截面管竖向压扁为相同过流面积的椭圆截面后,流道总流动损失增加且增量非常小;同样,反映出口紊流程度的湍动能变化量也很小。炮座出口流线发生变化,涡通量减小,出口平均速度偏差减小,径向线速度分布曲线上具有峰值减小,低谷值增大的特点,这些都说明流动均匀性获得改善。试验与数值模拟对比,管轴线速度偏差在2.3%以内,径向速度偏差约在8%以内,湍动能对比获得一致的数量量级。PIV 试验结果在流速分布上与数值模拟结果一致,但在区分两种流道的差异上需要提高试验精度。     

Characterization of two-phase flow in a single rock joint

An experimental study using a triaxial apparatus was used to analyze the two-phase flow patterns in jointed rock specimens. Rock specimens having a single natural fracture were tested for two-phase flow of water and air. Triaxial tests were conducted to characterize the two-phase flow through fractured granite specimens at low confining pressures. It was found that for a relatively smooth joint (JRC<6), bubble flow pattern occurred within the rock joint when the gas velocity is below 15 m/s. The average velocity of water usually varied between 0.1 and 0.5 m/s for bubble flow patterns. In this velocity range, air bubbles were able to form along the joint walls or to be randomly displaced within the water phase. When the gas velocity inside the rock joint exceeded 22 m/s, the flow patterns took annular form for non-zero capillary pressures (i.e., injected gas pressure is not equal to injected water pressure). At elevated (>0.25 MPa) gas injection pressures, the gas occupied the main part of the fracture and the liquid was able to flow as an unstable film forming an annular flow along the joint. When the annular flow developed, the mixture flow pattern was independent of the air flow velocity. This was due to the fact that once the injected air velocity reached a critical value (i.e., 20 m/s), water velocity inside the joint was negligible for a given confining pressure and injected water pressure. Further increase in inlet air pressures developed a single-phase air flow with no water flow.     

旋转流风量分配器空气动力性能研究

研究了用于等截面圆形直管均匀送风的旋流风量分配器，及其沿程阻力和局部阻力问题。通过测试优化出一种局部阻力系数显著降低的旋流风量分配器，并针对其特点给出了不同流量下的单位摩擦阻力变化曲线，为工程应用提供了参考依据。     

Sedimentary microbial oxygen demand for laminar flow over a sediment bed of finite length

Dead organic material accumulated on the bed of a lake, reservoir or wetland often provides the substrate for substantial microbial activity as well as chemical processes that withdraw dissolved oxygen (DO) from the water column. A model to estimate the actual DO profile and the "sedimentary oxygen demand (SOD)" must specify the rate of microbial or chemical activity in the sediment as well as the diffusive supply of DO from the water column through the diffusive boundary layer into the sediment. Most previous experimental and field studies have considered this problem with the assumptions that the diffusive boundary layer is (a) turbulent and (b) fully developed. These assumptions require that (a) the flow velocity above the sediment bed is fast enough to produce turbulent mixing in the boundary layer, and (b) the sediment bed is long. In this paper a model for laminar flow and SOD over a sediment bed of finite length is presented and the results are compared with those for turbulent flow. Laminar flow near a sediment bed is encountered in quiescent water bodies such as lakes, reservoirs, river backwaters, wetlands and ponds under calm wind conditions. The diffusive oxygen transfer through the laminar diffusive boundary layer above the sediment surface can restrict the microbial or chemical oxygen uptake inside the sediment significantly. The developing laminar diffusive boundary layer above the sediment/water interface is modeled based on the analogy with heat transfer, and DO uptake inside the sediment is modeled by Michaelis-Menten microbial growth kinetics. The model predicts that the rate of SOD at the beginning of the reactive sediment bed is solely dependent on microbial density in the sediment regardless of flow velocity and type. The rate of SOD, and the DO penetration depth into the sediment decrease in stream-wise direction over the length of the sediment bed, as the diffusive boundary layer above the sediment/water interface thickens. With increasing length of the sediment bed both SOD rate and DO penetration depth into the sediment tend towards zero if the flow is laminar, but tend towards a finite value if the flow is turbulent. That value can be determined as a function of both flow velocity and microbial density. The effect of the developing laminar boundary layer on SOD is strongest at the very lowest flow velocity and/or highest microbial density inside the sediment. Under quiescent conditions, the effective SOD exerted by a reactive sediment bed of a lake or wetland approaches zero, i.e. no or very little oxygen demand is exerted on the overlying water column, except at the leading edge.     

Indicators for the evaluation of wind tunnel test section flow quality and application to a numerical closed-circuit wind tunnel

The flow in a wind tunnel test section must meet high standards to obtain accurate and reliable measurement data. Good flow quality demands a certain degree of spatial uniformity and temporal steadiness of velocity and pressure. In this paper, a set of six new indices is developed and presented that relate spatial aspects of the mean velocity field to flow quality. One index quantifies the degree of uniformity of the velocity field and can be used directly as a flow quality indicator. The five other indices are related to different types of deviations from spatially uniform flow; skewed flow and angularity (up-flow and down-flow, swirl, cross-flow, diverging and converging flow). The indices can be used to evaluate the flow quality in existing tunnels, to assess the impact of design modifications, as well as to carry out CFD-based design of new wind tunnels. As an example, the indices are applied to assess the impact of guide vanes and screens on the test section flow quality in a numerical closed-circuit wind tunnel.     

基于冷水量变化的冷水机组性能测试与故障诊断

研究了定流量/变温差和变流量/定温差条件下冷水量变化对机组性能的影响。结果表明:大型冷水机组蒸发器侧冷水大范围变流量不会影响系统的稳定性;在一定范围内冷水机组制冷量与冷水流量呈线性变化,系统的性能系数COP基本保持稳定,这为冷水泵的节能运行提供了良好的依据;蒸发器盘管中冷水流量较大时,允许的冷水流速变化范围可以适当增加;冷水量调节的极限速度为10.1%/min,否则会造成冷水机组运行不安全;蒸发器冷水流量变化引起的冷水机组性能变化可作为故障诊断的判断依据。     

地铁站台疏散通道断面风场特性研究

针对规范中对地铁站台疏散通道断面（楼扶梯口处）仅给出风速限制的现状,采用计算机模拟和现场测试相结合的方法,对断面风场特性进行研究。站台公共区火灾排烟工况下,该断面风速呈现出沿高度方向的典型分层现象,自下而上分别为边界区、主流区、衰减区、回流区。人员活动大部分位于主流区,其内部风速为 3~6 m/s,有利于抑制烟气蔓延、补充新鲜空气。研究发现由于向下气流的渐扩效应在该断面顶部产生小面积回流,其对火灾烟气的潜在卷吸风险值得注意,并应在风速测试中判别气流方向。     

Impact of flow path length and flow rate on phosphorus loss in simulated overland flow from a humic gleysol grassland soil

In this overland flow simulation experiment, the relationships between flow path length, flow rate and the concentration of different P fractions were investigated. Overland flow was simulated using a 3 mx0.12 m laboratory flume. To remove the impact of rainfall on P lost in overland flow, simulated rainfall was not used during these experiments. Instead overland flow was generated by pumping water into the flume at the surface of the grass sod. The experimental setup allowed for the variation in flow path length and flow rate between and during experimental runs. The results demonstrated that an increase in flow path length caused an increase in Total Dissolved P (TDP), Dissolved Reactive P (DRP) and Total Reactive P (TRP) concentration in overland flow (p<0.01) while an increase in flow rate resulted in a decrease in the concentration of these P fractions in overland flow due to dilution (p<0.01). Total P (TP), Particulate P (PP) and Dissolved Organic P were not affected by the variables tested during this study. When flow path length was increased in conjunction with flow rate, there was an increase in TDP, DRP, and TRP concentrations despite the impact of greater dilution. The results indicate that variations in flow path length during a rainfall event may play a role in determining the concentration of dissolved P fractions in overland flow at field scale.     

基于Hansbo渗流的理想砂井地基固结分析

室内试验和现场观测表明,许多饱和黏性土中的渗流在小水力梯度时不能用Darcy定律描述,这应该是导致基于Darcy渗流的传统砂井固结理论有时不能很好地解释某些砂井地基固结特性的原因。引入同时考虑低速渗流幂函数曲线段和较高速渗流直线段的Hansbo渗流方程描述非Darcy渗流,在自由竖向应变假定下修正了Barron的理想砂井地基固结方程,并给出了有限差分法数值求解格式。据此探讨了Hansbo渗流参数、地基厚度等对砂井地基固结进程的影响。计算结果表明:和Darcy渗流相比,Hansbo渗流延缓了砂井地基内的孔压消散速度,导致固结速度变慢,并且径向排水对孔压消散的作用会更显著。如将Hansbo渗流方程简化为幂函数形式,则往往会高估砂井地基的固结程度,特别是在固结的初期。最后对比了自由竖向应变假定和等竖向应变假定对计算结果的影响。     

Experimental and numerical analyses of train-induced unsteady tunnel flow in subway

To analyze the unsteady three-dimensional flow in the subway tunnel caused by the passage of a train, both experimental and computational studies have been conducted. The experimental analysis of train-induced unsteady flow is conducted on a 1/20 scale model tunnel and the pressure and air velocity variations with time are presented. The three-dimensional unsteady numerical analysis using the sharp interface method for the moving boundary of an immersed solid was carried out for the same geometric configurations as the experimental analysis. The predicted numerical model results show good agreement with the experimental data.     

基于透明土技术的多孔介质孔隙流动特性研究

土体作为一种特殊多孔介质,内部孔隙通道尺度与形态随机性强,导致土体渗流场中孔隙流速分布不均,即存在优势流现象。优势流是影响污染物运移、导致土体渗透变形的重要因素。基于透明土原理,利用聚丙烯酸钠交联聚合物颗粒和蒸馏水,配制成饱和透明多孔介质,并利用一种新的研究透明多孔介质内部流场的装置及方法,将绿色光源激光器、单反相机、十字滑台等组合成简易粒子图像测速(PIV)系统,采集不同水力梯度下透明多孔介质内部流场图像,结合粒子图像测速技术,将得到的流速数据进行统计分析,揭示孔隙液体的流动特性。研究表明,利用自制简易PIV系统进行流场测量,实测孔隙流动结果与宏观流速吻合程度高,能够实现对流场的多点、无扰、高精度测量。研究发现多孔介质内部纵断面上的孔隙面积与纵断面所在位置有关,而孔隙面积越大,断面上的孔隙流速也越大。多孔介质内部孔隙流速分布规律大致相同,优势流速随着断面流速的减小而减小,优势流速越小,其概率密度越高,优势流动现象越显著。     

Laboratory measurements of velocity and turbulence field behind porous fences

Flow characteristics of turbulent wake behind porous fences have been investigated experimentally. The velocity fields were measured using the two-frame PTV method in a circulating water channel. The fence models used in this study have geometric porosity () of 0%, 20%, 40% and 65%, respectively. Each fence model was located in uniform flow whose boundary layer thickness (δ) at the fence location was about 0.1 of the fence height (H). Among the porous fences used in this study, the porous fence with porosity =20% shows the maximum reduction of mean streamwise velocity, but it has the highest vertical mean velocity at about x/H=1 location and large turbulence intensity in the near wake region. However, the porous fence with =40% has good flow characteristics for abating wind erosion with small turbulent fluctuations and a relatively large reduction in mean velocity. Except for the solid fence (=0%), two shear layers develop from the porous fences. As the fence porosity () increases, the height of the shear layer and the streamline curvature decrease. When the porosity () is greater than 40%, there is no re-circulation flow behind the fence due to the strong bleed flow, the Reynolds shear stress is nearly negligible in the entire near-wake region and relatively small turbulent kinetic energies are concentrated in the region just behind the fence (x/H<0.5). When the fence porosity is less than 20%, the Reynolds shear stress and turbulent kinetic energy are strong over the fence and in the shear layer near the reattachment region.     

循环流水力特性及模型研究

通过清水实验对气提升交替循环流复合滤料滤池进行了循环流水力特性及模型的研究。结果表明,空塔气速决定了气提升交替循环流复合滤料滤池内气含率的高低;气含率与循环液速、循环驱动力、氧传质系数成正比,与液体循环时间成反比。根据实验数据分析得出气含率-空塔气速模型和降流区液体流速-空塔气速模型,根据这两个模型选择适当的控制条件,能够为气提升交替循环流工艺应用于不同领域提供一定的理论依据。     

实验室排风柜面风速要求与实测分析

在总结各国对排风柜面风速要求的基础上,采用实验方法研究了两种拉门开度(拉门高度500mm和拉门全开)、不同设定面风速情况下,排风柜面风速的实际分布情况;结合流动显示实验,评价了排风柜的运行性能。得出结论:从面风速分布均匀性和对污染气流控制力的角度考虑,排风柜左右两侧气流通道的阻力应尽量平衡;当拉门高度为500mm时,面风速应控制在0.4～0.6m/s,拉门全开时,面风速不应小于0.4m/s。     

Studies on buoyancy driven two-directional smoke flow layering length with combination of point extraction and longitudinal ventilation in tunnel fires

This paper investigates the buoyancy-driven smoke flow layering length (both upstream and downstream) beneath the ceiling with combination of point extraction and longitudinal ventilation in tunnel fires. A theoretical model is developed based on previous back-laying model with only longitudinal ventilation, with modified actual heat release rate, as well as modified upstream and downstream opposing longitudinal air flow velocities by the induced flow velocity due to point extraction. Experiments are carried out in a reduced scale model tunnel with dimensionless of 72 m×1.5 m×1.3 m. A LPG porous gas burner is used as fire source. The smoke flow layering length both upstream and downstream are identified based on temperature profiles measured along the ceiling, for different experiment conditions. CFD simulations with FDS are also performed for the same scenarios. Results show that with combination of point extraction and longitudinal ventilation, the smoke flow layering length is not symmetric where it is longer downstream than that upstream. The upstream smoke layering length decreases, while the downstream layering length increases with increase in longitudinal ventilation velocity; and they both decrease with increase in point extraction velocity. The predictions by the proposed theoretical model agree well with the measurements and simulation results.     

基于DTM阈值分割法的孔裂隙煤岩体瓦斯渗流数值模拟

 在岩溶发育地区开展大型工程多采用钻孔灌注桩基础,而桩底溶洞的存在可能造成塌孔、埋钻、漏浆等事故,给在建工程和周围建筑造成重大影响。由于钻孔灌注桩需要泥浆护壁,现行的物探方法很难在钻孔灌注桩的泥浆内对桩底溶洞进行有效勘查。本研究结合声呐应力波传播特点,提出了一种钻孔灌注桩桩底溶洞声呐探测方法,并研发了桩底溶洞声呐探测仪(JL-SONAR)和信号分析软件(PBCA)。JL-SONAR探测仪实现了现场声呐信号的发射与采集,PBCA软件完成了探测数据的分析与整理。该技术充分利用了钻孔灌注桩的泥浆条件,可以在成孔过程中跟踪探测桩底10 m内溶洞发育情况,具有成本低、速度快、精度高等优点。并通过两个具体工程案例验证了声呐探测技术在实际工程中的应用。本文的研究对岩溶地区尤其是液态环境下的溶洞勘查提出了一种新的解决方法。     

About the penetration of a horizontal axis cylindrical vortex into the nearby downwind region of a vertical porous fence

Wind tunnel experiments have been conducted on a cylindrical vortex embedded in a low turbulence stationary horizontal stream, running through a two-dimensional narrow vertical woven fence located on the wind tunnel floor.The vortex was continuously generated upwind of the fence by means of a vortex tube located well below the fence top level, with its axis aligned with the mean velocity of the external stream. The fence installed along the entire width of the tunnel had a porosity of 70%. Visualization experiments showed that approaching the fence the vortex moves away from the mean wind direction of the adjacent stream along a rising curved trajectory while the direction of the surrounding mean flow remained nearly horizontal. The results suggest that this deviation could be promoted by the vortex slanting velocity field relative to the fence, which “sees” a fence with much lower optical porosity than the fence perpendicular velocity of the nearby mean flow.The fence top shear layer flow, which dominates the downwind evolution of the mixing layer, appears to be highly sensitive to the presence of this type of vortex. The most energetic changes in the flow due to the presence of the vortex occurred in the mixing layer region. Windbreaks are usually designed in terms of mean velocity, turbulence intensity, geometric dimensions, and porosity. The results presented in this paper suggest that the sheltering ability of a porous fence depend also on the particular flow pattern of the oncoming turbulent structures embedded in the incident wind. The results show the importance for a particular wind sheltering application in knowing a priori at least some aspects of the flow pattern of the most representative turbulent structures of the local wind.