不同曝气流速下曝气池内气液两相流混合液湍动动能的数值模拟

随着水环境综合治理范围及深度的提高,有必要优化在活性污泥工艺中耗能最多的曝气过程,即用尽可能小的能耗对已污染的水体进行净化。本文通过对一典型的推流式曝气池模型进行数值模拟研究,得出本次数值模拟的曝气池计算模型在网格数达到302400个时可以达到理想的模拟要求。通过比较不同曝气速度下曝气池特征断面的湍动动能的变化情况,得出在曝气速度达到1.463m/s~1.89m/s范围时能达到理想的曝气效果。曝气池内形成了稳定的旋流,这有利于曝气池内气体和液体的稳定混掺,从而对合理提高曝气效率从而减少能耗有一定的参考意义。

用X型热膜探针测量筛孔板上液流速度场的方法探索

Flow pattern or velocity distribution on a distillation tray is of great importance in determining tray efficiency. And design of distillation process is mainly rely on experimental data which also can be used in verification of mathematical model for describing the flow pattern on a tray.In this paper,hot-film anemometer with X-type probe is applied to the measurement of gas-liquid two-phase flow on a sieve tray.First,a calibration method that used in this paper is proposed and a computational model for data processing is presented.Then the threshold method used to separate vapor signals from those of the liquid is described.Finally,typical measuring results,which include the magnitude and direction of local velocity,and the distribution of turbulent kinetic energy,are presented.

水深对微孔曝气充氧性能的影响研究及数值模拟

以中试规模研究了不同水深下微孔曝气充氧性能的变化,并通过欧拉两相流及标准k-ε湍流模型对曝气过程进行了三维稳态流数值模拟,经获得不同水深下微孔曝气充氧性能的变化规律。结果表明,曝气水深为1~6 m时,标准氧总转移系数（KLas）随着水深的增大先减小后趋于不变,氧转移速率（SOTR）随着水深的增大而增大,氧利用率（SOTE）随着水深的增大呈现线性增大。数值模拟结果表明：水深为1 m时,湍动动能沿竖直中心线逐渐降低,水深为3~6 m时,湍动动能沿竖直中心线先增大后降低;气含率的分布随着水深的增大而逐渐均匀,曝气池中心横截面线气含率随着水深的增大而降低。

大型风力发电场对华北地区大气影响的数值模拟研究

作为一种清洁的可再生能源,风电在过去几十年中取得了快速的发展,目前中国风电装机容量居世界第一。虽然风机运行过程中不会造成污染或温室气体的直接排放,但会改变风力发电场区域地表状况,进而对大气形成潜在的扰动和影响。本研究利用大气数值模式WRF,结合两种风轮机参数化方案(地表粗糙度增加参数化方案和风机曳力参数化方案),探讨了中国北方大型风力发电场对大气要素(风速,湍流和温度)的潜在扰动及影响。通过敏感性实验研究表明,大型风力发电场会造成风电场内部和其临近区域风速的衰减,且冬季(1月)的衰减程度明显强于夏季(7月)。另外由于风轮机产生的湍动动能增加了空气的垂直混合,造成冬季(1月)风电场内部和下游地区在风轮机轮毂高度处和地表空气温度的上升,且轮毂处升温强度略高于地面。在夏季(7月),风轮机也导致在风电场临近区域地表和轮毂高度处的温度发生变化。考虑到风轮机在叶片转动区域产生大量的湍动动能,推断地面和轮毂高度温度升高主要是由于湍流混合增强,形成了湍流逆温效应。

省煤器磨损影响因素分析与数值模拟

用FLUENT软件对某电厂膜式省煤器进行了数值模拟,分析了横向间距、管径以及管子的布置方式对磨损的影响。结果表明:横向间距的增加有利于磨损的减轻;管径的增加对磨损减轻有一定的影响,而管子的布置方式对磨损的影响比较大,原因之一是顺列布置的膜式省煤器湍动能要大于错列布置的膜式省煤器的湍动能,湍动能的增加有利于磨损的减轻,这些模拟结果有利于省煤器的设计与改进。

Wave breaking on turbulent energy budget in the ocean surface mixed layer

As an important physical process at the air-sea interface, wave movement and breaking have a significant effect on the ocean surface mixed layer (OSML). When breaking waves occur at the ocean surface, turbulent kinetic energy (TKE) is input downwards, and a sublayer is formed near the surface and turbulence vertical mixing is intensively enhanced. A one-dimensional ocean model including the Mellor-Yamada level 2.5 turbulence closure equations was employed in our research on variations in turbulent energy budget within OSML. The influence of wave breaking could be introduced into the model by modifying an existing surface boundary condition of the TKE equation and specifying its input. The vertical diffusion and dissipation of TKE were effectively enhanced in the sublayer when wave breaking was considered. Turbulent energy dissipated in the sublayer was about 92.0% of the total depth-integrated dissipated TKE, which is twice higher than that of non-wave breaking. The shear production of TKE decreased by 3.5% because the mean flow fields tended to be uniform due to wave-enhanced turbulent mixing. As a result, a new local equilibrium between diffusion and dissipation of TKE was reached in the wave-enhanced layer. Below the sublayer, the local equilibrium between shear production and dissipation of TKE agreed with the conclusion drawn from the classical law-of-the-wall (Craig and Banner, 1994).

Large eddy simulation of the rotation effect on the ocean turbulence kinetic energy budget in the surface mixed layer

A non-hydrostatic, Boussinesq, and three-dimensional large eddy simulation(LES) model was used to study the impact of the Earth's rotation on turbulence and the redistribution of energy in turbulence kinetic energy(TKE) budget. A set of numerical simulations was conducted,(1) with and without rotation,(2) at different latitudes(10°N, 30°N, 45°N, 60°N, and 80°N),(3) with wave breaking and with Langmuir circulation, and(4) under different wind speeds(5, 10, 20, and 30 m/s). The results show that eddy viscosity decreases when rotation is included, indicating that rotation weakens the turbulence strength. The TKE budget become tight with rotation and the effects of rotation grow with latitude. However, rotation become less important under Langmuir circulation since the transport term is strong in the vertical direction. Finally, simulations were conducted based on field data from the Boundary Layer and Air-Sea Transfer Low Wind(CBLAST-Low) experiment. The results, although more complex, are consistent with the results obtained from earlier simulations using ideal numerical conditions.

A new approach to quantifying vehicle induced turbulence for complex traffic scenarios

Traffic-related pollutants adversely affect air quality, especially in regions near major roadways. The vehicleinduced turbulence(VIT) is a significant factor that controls the initial dilution, dispersion, and ultimately the chemical and physical fate of pollutants by altering the conditions in the microenvironment. This study used a computational fluid dynamics(CFD) software FLUENT to model the vehicle-induced turbulence(VIT) generated on roadways, with a focus on impact of vehicle-vehicle interactions, traffic density and vehicle composition on turbulent kinetic energy(TKE). We show, for the first time, that the overall TKE from multiple vehicles traveling in series can be estimated by superimposing the TKE of each vehicle, without considering the distance between them while the distance is greater than one vehicle length. This finding is particularly significant since it enables a new approach to VIT simulations where the overall TKE is calculated as a function of number of vehicles. We found that the interactions between vehicles traveling next to each other in adjacent lanes are insignificant,regardless the directions of the traffic flow. Consequently, simulations of different traffic scenarios can be substantially simplified by treating two-way traffic as one-way traffic, with less than 5% difference in the overall volume-averaged TKE. We also developed equations that allow the estimation of the overall volume-averaged TKE as a function of the number and the type of vehicles.

Test and evaluation of a moored microstructure recorder

A new moored microstructure recorder(MMR) is designed, developed, tested, and evaluated. The MMR directly measures the high-frequency shear of velocity fl uctuations, with which we can estimate the dissipation rate of turbulent kinetic energy. We summarize and discuss methods for estimating the turbulent kinetic energy dissipation rate. Instrument body vibrations contaminate the shear signal in an ocean fi eld experiment, and a compensating correction successfully removes this contamination. In both tank test and ocean fi eld experiment, the dissipation rate measured with the MMR agreed well with that measured using other instruments.

栅条式微涡除浊技术流场数值模拟

针对铁路隧道施工废水的高精度高效率除浊需求,采用在旋流式澄清池的旋流反应区中加入栅条式微涡发生器的方式来提升设备混凝除浊效率。采用数值模拟手段分析了旋流场内加入栅条前后流场内与混凝有关的特征参数变化,并采用水力实验进行效果验证。结果表明,加入栅条后,混凝流场初始段的湍动动能及湍耗散明显提升,较之空池条件下分别提升19.24%、155.59%。这种变化利于混凝反应初期,药剂的充分分散,增加颗粒的初始碰撞概率。同时,栅条引入后,增加了流场内的涡旋尺度分布范围,利于流场内不同尺度颗粒完成涡旋絮凝。在原水浊度1000 NTU的条件下,带有栅条的反应器最终上清液出水浊度达37.4 NTU,絮体分形维数可达1.71,较之空池情况下有明显提升。本研究结果可为微涡旋絮凝技术用于隧道废水处理提供参考。

Taylor-Couette流场数值模拟及絮凝效果研究

通过流体力学中的Fluent软件模拟了Taylor-Couette反应器的速度矢量分布、湍动动能分布和有效能耗分布,同时实验研究了Taylor-Couette反应器不同内筒转速下三氯化铁对高岭土悬浊液的絮凝效能.结果表明,内外筒环隙间存在亚微尺度的涡旋,当转速为30～60r.min-1时,涡旋形状闭合完整,相互分离,涡旋内部速度梯度较小,湍动动能k在0.00010～0.00023m2.s-2的范围内,有效能耗ε在0.00057～0.00189m2.s-3的范围内,絮凝效能达到最大(80%以上),湍动动能和有效能耗过大或过小均不利于形成完整闭合的涡旋,絮凝效能较差.

Structures of convection and turbulent kinetic energy in boundary layer over the southeastern edge of the Tibetan Plateau

Based on a comprehensive analysis on Sonic Anemometer and gradient data, wind profile radar(WPR) and GPS sounding data of March–August 2008 from the boundary layer(BL) tower observation system at Dali on the southeastern edge of Tibetan Plateau(TP), it is found that the strengths of turbulent kinetic energy(TKE), buoyancy term and shear term depend on vegetation cover in association with local stability and thermodynamic condition. Strong kinetic turbulence appears when near surface layer in neutral condition with the large contribution from shear term. In an unstable condition within near surface layer, the atmospheric turbulent motion is mainly thermal turbulence, as buoyancy term is obviously larger than shear term. Under a stable condition the intermittent turbulence is accompanied by weak shear and buoyancy term, and TKE is significantly less than neutral or instable condition. The study also presents that the buoyancy term contribution at Nyingchi station in the southern slopes of the TP large topography in spring is significantly larger than that at Dali over the southeastern TP edge, reflecting that the thermal turbulence makes an important contribution to convection activity in the southern slopes of TP. Dali station is located in complex terrain with mountain and valley leading to larger kinetic turbulence. From the perspective of interaction of turbulence-convection in different scales, the study revealed that the height of convective boundary layer(CBL) could reach up to 1500–2000 m. TKE, shear term, and buoyancy term in near surface layer have the notable correlations with BL height and local vertical motion. The daytime thermodynamic turbulence effect of heat flux and buoyancy term has an obvious impact on the height of CBL, whereas mechanical turbulence only exerts a less impact. Mechanical turbulence in near surface layer has a significant impact on vertical motion especially in the forenoon with impacting height of 2500–3000 m. The peaks in diurnal variations of shear term and buoyancy term correspond to the high instable periods, especially in summer forenoon. Our observation analysis characterized the convection activity triggered by TKE source and their interaction in the southeastern TP edge.

Analysis of Turbulent Heat Transfer and Fluid Flow in Channels with Various Ribbed Internal Surfaces

This paper presents results of a numerical investigation of heat transfer and flow pattern characteristics of a channel with repeated ribs on one broad wall. Numerical computations are performed for seven ribs placed on the bottom wall of a channel for Reynolds numbers ranging from 10,000 to 30,000. The newly modified ribs (the ones with convex pointing upstream/downstream rib, wedge pointing upstream/downstream rib, concave pointing upstream/downstream rib and also concave-concave rib as well as convex-concave rib), are proposed for simulation with prospect to reduce flow separation and extend reattachment area compared to the unmodified square rib. The numerical results are reported in forms of flow structure, temperature field, turbulent kinetic energy, Nusselt number, friction factor and thermal enhancement factor. The results indicate the rib with concave-concave surfaces efficiently suppresses flow separation bubble in the corner of the rib and induces large recirculation zone over those of the others, hence giving the highest Nusselt number and friction factor. On the other hand, the one with convex-concave surface provides the lowest friction factor with moderate Nusselt number. Due to the prominent effect of its low friction factor, the rib with convex-concave surface offers the highest thermal enhancement factor of 1.19.

A one-equation turbulence model for recirculating flows

A one-equation turbulence model which relies on the turbulent kinetic energy transport equation has been developed to predict the flow properties of the recirculating flows. The turbulent eddy-viscosity coefficient is computed from a recalibrated Bradshaw's assumption that the constant a1= 0.31 is recalibrated to a function based on a set of direct numerical simulation(DNS) data. The values of dissipation of turbulent kinetic energy consist of the near-wall part and isotropic part, and the isotropic part involves the von Karman length scale as the turbulent length scale. The performance of the new model is evaluated by the results from DNS for fully developed turbulence channel flow with a wide range of Reynolds numbers. However, the computed result of the recirculating flow at the separated bubble of NACA4412 demonstrates that an increase is needed on the turbulent dissipation, and this leads to an advanced tuning on the self-adjusted function. The improved model predicts better results in both the non-equilibrium and equilibrium flows, e.g. channel flows, backward-facing step flow and hump in a channel.

The impact of interphase forces on the modulation of turbulence in multiphase flows

The modulation of turbulence by particles has been rigorously investigated in the literature yielding either a reduction or an enhancement of the turbulent kinetic energy at different spatial length scales.However,a general description of the turbulence modulation in multiphase flows due to the presence of an interphase force has attracted less attention.In this paper,we investigate the turbulent modulation for interfacial and fluid-particle flows analytically and numerically,where surface tension and drag define the interphase coupling,respectively.It is shown that surface tension and drag appear as additional production/dissipation terms in the transport equations for the turbulent kinetic energies(TKE),which is of particular importance for the turbulence modelling of multiphase flows.Furthermore,we study the modulation of turbulence in decaying homogenous isotropic turbulence(HIT)for both types of multiphase flow.The results clearly unveil that in both cases the energy is reduced at large scales,while the small-scale energy is enhanced compared to single-phase flows.Particularly,at large scales surface tension works against the turbulent eddies and hinders the ejection of droplet from the corrugated interface.In contrast,at the small scales,the surface tension force and the velocity fluctuations are aligned leading to an enhancement of the energy.In the case of fluid-particle flows,particles retain their energy longer than the surrounding fluid increasing the energy at the small scales,while at the large scales the particles do not follow exactly the surrounding fluid reducing its energy.For the latter effect,a considerable dependence on the particle Stokes number is found.