Numerical Investigation of Plunging Density Current

When a buoyant inflow of higher density enters a reservoir, it sinks below the ambient water and forms an underflow. Downstream of the plunge point, the flow becomes progressively diluted due to the fluid entrainment. The entrainment rate is strongly dependent on the Richardson number and reaches a constant value well downstream of the plunge point. This study is concerned with the analysis of the plunging phenomenon and the determination of the entrainment. A k-ε model including buoyancy effects, both in a sloping and a diverging channel, is used to reproduce the main flow characteristics. A relation between the depth at the plunge point in a channel of constant width and in a diverging channel is established, and theoretical results for the calculation of the dense layer thickness are provided. The latter indicates that the spreading rate of the dense layer in a diverging channel is a function of both the entrainment rate and the channel width. The predictions of the plunge line location are in agreement with most semiempirical equations.     

Energy Dissipative Plunging Flows

The analysis of plunging flows using energy dissipation hypothesis is presented in this paper. The classic two-control-volume theory has recently been found questionable and needs to be corrected. In this study, we avoid the questionable part of that framework and use a single control volume for the analysis of plunging flows. Furthermore, we hypothesize that the increase in kinetic energy in the underflow and entrained flow should be no greater than the loss in potential energy as the inflow plunges. In the current framework, the entrained flow rate does not exceed approximately 40% of the total inflow rate. The densimetric Froude number at the plunge point, Fdp = q/, where q = inflow rate per unit width; g′ = reduced gravity of inflow; and Hp = flow depth at the plunge point, is typically in the range of 0.3相似文献   

Three-Dimensional Modeling of Negatively Buoyant Flow in Diverging Channels

Diverging channels, also known as diffusers, represent common natural and industrial outlets to lakes, reservoirs, and rivers. If the outflow in a diffuser has a larger density than the ambient water, the inflow may plunge and form a density underflow. In this paper, a three-dimensional numerical study was conducted to gain insight into the mechanism of negatively buoyant flows in diffusers with a sloping bottom. Of particular interest is the formation of separated flows such as wall-jet and free-jet flows. Various cases of plunging and the associated density current in a diffuser with different divergence angles and inflow densimetric Froude numbers are considered. The model successfully simulates the formation of attached flow, wall jets, and free jets in a negatively buoyant environment. The onset, evolution, and stabilization of a stall and the subsequent development of a wall jet in a negatively buoyant flow are investigated in detail. Computed results also show favorable agreement with some published experimental data on density current generated by the plunging of cold water in ambient warm water in a diverging channel.     

Time-Varying Underflow into a Continuous Stratification with Bottom Slope

Results are presented from a laboratory investigation of a continuous discharge gravity current moving down an inclined plane into a linearly stratified fluid; the density of the inflow decreasing linearly with time, initially larger and finally smaller than the bottom ambient density. The inflowing water was observed to follow both underflowing and intrusive flow regimes. Hence, during the time in which the inflow was denser than the water in the stratified reservoir, an underflow was observed to descend down the sloping bottom with a speed that was consistent with that given by the theory for a buoyancy-conserving gravity current on gentle slopes. However, the continuous decrease of the density at the source shortly lead to an unstable density distribution within the initial underflow, which then collapsed into an intrusion that traveled as a horizontal gravity intrusion. Scaling arguments were used to identify both the position and time to the breakup of the underflow. To the end of the experiment, multiple intrusions were established successively at different depths in between the initial underflow and the surface buoyant plume.     

Cryopreservation of unfertilized mouse oocytes: the effect of replacing sodium with choline in the freezing medium

Although embryo cryopreservation has become commonplace in many species, effective methods are not available for routine freezing of unfertilized eggs. Cryopreservation-induced damage may be caused by the high concentration of sodium ions in conventional freezing media. This study investigates the effect of a newly developed low-sodium choline-based medium (CJ2) on the ability of unfertilized, metaphase II mouse eggs to survive cryopreservation and develop to the blastocyst stage in vitro. Specifically, the effects of cooling to subzero temperatures, thawing rate, LN2 plunge temperature, and equilibration with a low-sodium medium prior to freezing are examined. In contrast to cooling to 23, 0, or -7.0 degreesC in a sodium-based freezing medium (ETFM), cooling in CJ2 had no significant negative effect on oocyte survival or development. Oocytes frozen in CJ2 survived plunging into LN2 from -10, -20, or -33 degreesC at significantly higher rates than oocytes frozen in ETFM. With the protocol used (1.5 M PrOH, 0.1 M sucrose, -0.3 C/min, plunging at -33 degreesC) rapid thawing by direct submersion in 30 degreesC water was more detrimental to oocyte survival than holding in air for 30 or 120 s prior to transfer to water. Equilibration of unfertilized oocytes with a low-sodium medium prior to cryopreservation in CJ2 significantly increased survival and blastocyst development. These results demonstrate that the high concentration of sodium in conventional freezing media is detrimental to oocyte cryopreservation and show that choline is a promising replacement. Reducing the sodium content of the freezing medium to a very low level or eliminating sodium altogether may allow oocytes and other cells to be frozen more effectively.     

Coupling an Underflow Model to a Three-Dimensional Hydrodynamic Model

A separate underflow model is coupled to the three-dimensional (3D) estuary and lake computer model. The underflow equations are solved on a two-dimensional (2D) grid underlying the 3D model grid. The underflow model entrains ambient water whose properties are given by the fluid properties of the bottom boundary cells in the 3D model. This new approach allows improved representation of underflow effects in z-coordinate models by reducing numerical convective entrainment. An idealized case is used to illustrate the benefits of the underflow model. Comparisons of model results and field data for a saline underflow event in Lake Ogawara and a cold-water underflow in Lake Kinneret demonstrate improved model capability in representing underflow events that are thin compared to the vertical grid scale.     

Entrainment and Turbulence in Saline Underflow in Lake Ogawara

Under certain tidal conditions, a saline underflow originating in the Pacific Ocean moves into Lake Ogawara, Japan. The underflow consists of a uniform saline bottom layer that is slightly warmer than the ambient and an interfacial shear layer in which the velocity and density are decreased. Within the experimental area the underflow is confined to a channel approximately 1 km wide and is essentially two-dimensional. The underflow had a bulk Richardson number, defined in terms of the mean properties, between 1 and 2. The rate of entrainment into the bottom layer was calculated using two distinct methods. The first method used the change in the maximum salinity of the underflow measured at two stations along the path of the underflow to infer the amount of ambient water entrained. The second method made direct measurements of vertical mass fluxes with a profiler. The agreement between the two methods was excellent. The measured entrainment coefficients were consistent with the derived entrainment law. The turbulent structure of the flow was mapped for a 3 h quasisteady period of the flow. Turbulence is predominantly generated on the bottom boundary and is transported vertically to the density interface, where it leads to mixing.     

Physical Modeling of Hydrodynamic Separators Operating with Underflow

Model studies using geometrically similar 300-mm-diameter and 1,600-mm-diameter hydrodynamic separators operating with an underflow have confirmed earlier results indicating that a combination of Froude and Hazen scale laws should be used when predicting prototype separator performance. A hybrid equation is proposed to scale the model inflow rates. This equation provides a good fit to the observed prototype flow versus efficiency data. The need to scale particles using a Hazen scale for settling velocity and a Froude scale for flows is discussed. Treatment factors are calculated for the different particle sizes used, and these confirm the suitability of the device to remove particles with higher settling velocities while passing organic particles with low settling velocities forward to treatment when the device is configured as a grit separator.     

基于循环系统的膏体浓密机底流调控及其数学模型

为了保证浓密机在高料位下不压耙,一般通过增设循环系统使料浆始终处于活化状态,降低耙架运行阻力.然而,目前循环参数对底流的影响规律不明确,造成系统的设计及应用缺乏科学依据,为此开展了循环参数对底流的调控研究.分析循环系统的作用原理,将循环系统作用范围划分为两大区域,揭示循环参数对底流的调控机制,运用微积分原理对区域内的底流体积分数变化进行求解,最终建立浓密机底流调控数学模型.最后,利用该模型对底流循环实验参数进行验证.研究结果表明:开启底流循环后,底流体积分数开始降低并最终趋于稳定,底流体积分数差随着循环流量及循环高度增大而增大,体积分数变化幅度为0.7%~2.2%,稳定所需时间随流量及高度增加而减小.该理论模型完全吻合验证结果函数,为循环系统的设计及运行提供理论依据.     

Modeling Total Dissolved Gas Concentration Downstream of Spillways

Dams are often operated to facilitate downstream juvenile anadromous fish migration over the spillways, but such operation can cause high dissolved concentrations of oxygen and nitrogen that can be harmful to fish. The concentration of total dissolved gas (TDG) in the flow changes with distance downstream of the spillway crest and depends on the geometric configuration of the spillway and on hydraulic and operating conditions. A model is presented that simulates the physical processes of gas transfer with the goal of having an accurate and more widely applicable TDG model for plunging spillway discharges. Bubble transfer is dominant in the stilling basin, while water surface transfer is dominant downstream. Sensitivity analyses suggest which physical processes are important for accurate total dissolved gas predictions. Instantaneous bubble coalescence and breakup based upon local turbulence conditions is an appropriate assumption. Vertical bubble profiles do not need to be simulated in this type of model. Water surface roughness provides a significant increase to surface transfer. Tailwater depth is important to downstream TDG concentrations. Finally, a 10% difference in air entrained at the plunge point causes relatively minor differences in TDG of 1.4 and 3.1% at low and high discharges, respectively.     

深锥浓密机底流质量分数与停留时间关系研究

料浆停留时间是保证深锥浓密机底流质量分数的必要条件,为了探讨底流质量分数与停留时间的关系,推导了深锥浓密机底流质量分数与停留时间之间的数学模型.将深锥浓密机外形结构及尾砂物理特性参数代入数学模型,即可得到该浓密机达到所需底流质量分数的停留时间.采用某矿山深锥浓密机对数学模型进行工程应用举例,在底流质量分数为55.82％...     

深锥浓密机底流浓度模型及动态压密机理分析

泥层高度和底流浓度是深锥浓密机最为重要的两个参数,因此有必要研究底流浓度随泥层高度的变化规律.采用自制小型深锥浓密机,对尾矿非连续/连续动态压密过程进行了物理实验;借助于有效孔隙比与泥层压强间遵循的Power函数关系,结合对尾矿颗粒的受力分析,推导出了底流浓度与泥层高度的数学模型,揭示了浓密机底流浓度与泥层高度的内在关系,并从尾矿颗粒空间结构的角度解释了该模型的变化规律;结合矿山生产对于底流浓度的要求,应用该数学模型,为其推荐了泥层高度的合理范围,验证了底流浓度数学模型的可靠性.该模型为深锥浓密机的设计和运行提供了理论依据.     

Soil Moisture Flow Modeling with Water Uptake by Plants (Wheat) under Varying Soil and Moisture Conditions

A variably saturated soil moisture flow model is developed for planted soils with depth varying properties by incorporating a nonuniform macroscopic root water uptake function. The model includes spatial and temporal variation of the root density with dynamic root growth for simulating water uptake by plants along with the impact of soil moisture availability. The governing partial differential moisture flow equation integrated over the depth with a plant water uptake term is solved numerically by the implicit finite difference method using an iterative scheme. The model is first tested for barren soils for two profiles considering constant and depth varying soil characteristics under constant inflow condition. The results obtained are later tested with experimental data available in the literature. A nonuniform plant water uptake term is subsequently incorporated in the model and water uptake by wheat plants under different soil moisture availability conditions is studied. Finally, the moisture flow model is validated with field data of rain fed wheat (Triticum aestivum) using a dynamic root growth model for a layered root zone soil profile. The simulated soil moisture regime of the layered root zone shows a reasonably good agreement with the observed data.     

Simulation of Scour Process in Plunging Pool of Loose Bed-Material

The scouring process in a plunge pool of loose bed with uniform bed-materials due to a two-dimensional plane impinging jet was simulated computationally. The finite-element-based unsteady three-dimensional model, CCHE3D, with k-ε turbulence closure was employed to solve the flow field. It has long been recognized that the unsteady behavior of the turbulent jet fluctuation plays an important role in scouring and transporting sediment in the plunge pool. In order to model this phenomenon realistically, one has to consider the effects of both shear stress and the life force on sediment particles due to pressure fluctuation. The latter has been taken into account by using empirical relationships of flume data. Both of these effects have been incorporated in the nonequilibrium sediment transport model consisting of sediment pickup rate and step length adopted for the jet scour problem. The model constant relating to the fluctuating lift force was calibrated using an empirical equation to predict the quasi-equilibrium scour depth. The results simulated by the model proposed here agree reasonably well with experimental data.     

Bubble Entrainment and Distribution in a Model Spillway with Application to Total Dissolved Gas Minimization

This paper focuses on an experimental study of the two-phase flow downstream of a laboratory model fish bypass. Experiments were performed on a 1:24 scale laboratory model of a fish bypass under consideration for construction at Wanapum Dam, on the Columbia River in Washington. The model was operated at the design condition of skimming flow regime and at the possible off-design plunging and surface jump regimes. Void fraction data were collected using an optical phase detection probe on a three-dimensional grid, and the phase indicator function was recorded at selected locations. It was found that in the laboratory model, the skimming flow regime effectively prevents bubbles from reaching deep into the tailrace, resulting in a considerably lower void fraction than plunging and surface jump regimes. For this geometry, the surface jump regime entrains air deeper than the plunging regime. To observe trends, the instantaneous source of total dissolved gas was estimated for the three regimes using the model data and several simplifying assumptions. Time distributions of the indicator function are also reported.     

膏体浓密机扭矩计算模型及其影响因素

基于泥床料浆的非牛顿体特征,以及料浆初始剪切应力与浓度的变化规律,本文提出了耙架扭矩计算模型.模型预测结果显示,耙架扭矩随底流浓度升高而增大,底流浓度对浓密机运行时的扭矩百分比的影响幅度可达4.67%,两者呈现为多项式函数关系.与此同时,泥床高度对耙架扭矩影响较小,影响幅度仅为0.27%.通过某铅锌矿膏体浓密机64 h运行过程实时监测数据对比,验证了模型关于底流浓度和泥床高度对扭矩的影响规律.     

Friction Factors for Spatially Varied Flow with Increasing Discharge

This paper describes an experimental investigation of how friction factors change for spatially varied flow in sloping channels receiving lateral inflow. The results are compared with those of Beij in 1934, and it is concluded that uniform flow resistance coefficients are not always appropriate for spatially varied flow. Moreover, the common technique of assuming a constant friction factor over the entire length of the channel has been found to have little theoretical justification. The method of Keulegan in 1952 for calculating friction factors in spatially varied flow gives a better estimate, but does not explicitly take account of the lateral inflow rate or velocity. Beij’s 1934 experimental data, which was used by Keulegan does not show a systematic variation of friction factor with lateral inflow rate for a constant Reynolds number although this may be due to the low flowrates used. The results of the present study indicate that the friction factor increases with lateral inflow rate for a constant Reynolds number in the experiments that included subcritical and supercritical flow conditions. A method for calculating friction factors which allows for lateral inflow is presented as a precursor to the development of a general method of evaluating friction factors for spatially varied flow with increasing discharge.     

底流泵变频器稳定运行的技术探讨

文章分析底流泵变频器的常见故障及原因。通过更换变频器及优化参数,成功解决了底流泵变频器运行中的各类故障,降低了底流泵变频器的故障率,为底流泵变频器及整个尾矿区域的稳定运行打下良好的基础。     

A model of blood interaction with optical-fluid guide for laser angioplasty

A mathematical model is developed to describe the flow and mixing of blood and optical fluid used in liquid-guided light for laser angioplasty. The model is based on a two-fluid formulation in which separate transport equations are solved for the blood and the optical fluid. Empirical relations, established in prior work, are used to represent interfluid transport of momentum. Both steady and phasic inflow conditions are considered. Parametric calculations are performed showing effect of relative flow rates of blood and optical fluid, on the mixing phenomena. The relative velocity considered (based on average blood velocity) ranged from 0.08 to 0.28 m/sec. No allowance has been made for ablation of the plaque. The predicted results include spatial distribution of the velocity field and the existence probabilities (volume fractions) that provide a measure of the extent of mixing between the fluids. It is found that the degree of mixing is adversely affected by the relative inflow velocity between blood and optical fluid and the pulsatility of blood inflow. Deep penetration of the optical fluid is predicted at high relative velocity and at the end of diastolic and early systolic stages of the cardiac cycle.     

Optimal Furrow Design.?II: Explicit Calculation of Design Variables

The furrow irrigation system design problem (at minimum cost) is significantly simplified by analytically solving it. For a specified furrow length, a simple algebraic equation is derived to directly calculate the appropriate inflow rate (and cutoff time) so that the minimum cost of the furrow system is obtained. The proposed equation is independent of the water and labor cost coefficients. Comparison tests indicated that the optimum inflow rate values obtained analytically were in close agreement to the optimum values obtained using the outcome of the zero-inertia numerical model. The method is extended for furrow design considering the furrow length also as a design variable. The optimum number of distribution lines and widthwise furrow sets are easily determined by a simple calculation procedure.