Numerical investigation of interaction between rising bubbles in a viscous liquid

The rising behavior of bubbles undergoing bubble-bubble interaction in a viscous liquid is studied using a two-dimensional direct numerical simulation. Level contour reconstruction method (LCRM), one of the connectivity-free front tracking methods, is applied to describe a moving interface accurately under highly deformable conditions. This work focuses on the effects of bubble size on the interaction of two bubbles rising side-by-side in a stagnant liquid. Several characteristics of bubble-bubble interaction are analyzed quantitatively as supported by energy analysis. The results showed clear differences between small and large bubbles with respect to their interaction behavior in terms of lateral movement, vortex intensity, suppression of surface deformation, and viscous dissipation rate. Distributions of vorticity and viscous dissipation rate near the bubble interfaces also differed depending on the size of the bubbles. Strong vortices from large bubbles triggered oscillation in bubble-bubble interaction and played a dominant role in the interaction process as the size of bubbles increases.

     

螺旋微流道中液滴离心驱动

为研究离心驱动下液滴在螺旋微流道中的流动特性,建立了液滴流动物理模型;提出表面耗散的表达式并结合离心实验对其进行修正;利用仿真软件COMSOL计算黏性耗散公式中的系数;根据驱动功与耗散能守恒进而得到流速的理论公式;展开多因素多水平实验测量液滴的实际流速。结果表明:实际流速略小于理论预测值,但各因素对实际流速的影响与预测公式相符;液滴流动的特征表现为,毛细数和离心-邦德数呈正相关的线性关系,且较高的离心-邦德数对应的线性度更强。为其他方式驱动的液滴研究和相关应用的开发提供了理论借鉴和依据。     

Numerical analysis of turbulent flow and heat transfer in a square sectioned U-bend duct by elliptic-blending second moment closure

A second moment turbulence closure using the elliptic-blending equation is introduced to analyze the turbulence and heat transfer in a square sectioned U-bend duct flow. The turbulent heat flux model based on the elliptic concept satisfies the near-wall balance between viscous diffusion, viscous dissipation and temperature-pressure gradient correlation, and also has the characteristics of approaching its respective conventional high Reynolds number model far away from the wall. Also, the traditional GGDH heat flux model is compared with the present elliptic concept-based heat flux model. The turbulent heat flux models are closely linked to the ellipticblending second moment closure which is used for the prediction of Reynolds stresses. The predicted results show their reasonable agreement with experimental data for a square sectioned U-bend duct flow field adopted in the present study.     

盖驱动腔的动理学理论数值分析

为了探讨盖驱动腔流场的影响因素和分布规律,本文提出采用气体动理学格式对其流场开展研究.论文结合盖驱动腔的物理模型,采用有限体积法对Boltzmann方程开展了数值过程推导和数值计算.给出了数值计算过程中以虚网格技术实现的绝热无滑移边界条件,计算了不同雷诺数下盖驱动方腔的流场和流线特征,提取了两条正交中心线上的速度分布并...     

A low Reynolds number dissipation rate equation model using the dissipation rate tensor equation and elliptic-blending equation

The purpose of this study is to theoretically supplement the dissipation rate equation model that has been adopted and used for the elliptic-blending second-moment closure widely and currently used for analysis of turbulent flow, and eventually to enhance the theoretical validity of the model. The new dissipation rate equation model was derived by using the dissipation rate equation in terms of length scale that can be applied both in the near-wall region and to homogeneous flow and by using the dissipation rate tensor equation model. The newly derived dissipation rate equation model is applied in the existing elliptic-blending model as it is. To test the model equation we conducted a numerical analysis of non-rotating and rotating channel flows, channel flow with uniform transpiration, square duct flow, and 3-dimensional curved duct flow before comparing the analysis results with DNS data and the measurements. In regard to all flow fields adopted in this study, the expected results showed a high satisfaction in comparison with DNS data and measurements, thereby proving the theoretical validity of the new model.     

Turbulent structures,integral length scale and turbulent kinetic energy (TKE) dissipation rate in compound channel flow

In the present study, high data rate measurements were obtained for the streamwise and vertical velocity components using 2D Laser Doppler Velocimeter. The turbulent field in a straight compound-channel flow was characterized for three different uniform flow water depths, corresponding to “deep flows”, “intermediate flows” and “shallow flows” conditions. Several methodologies were studied to process the data and to obtain autocorrelation functions, integral length scale and turbulence kinetic energy (TKE) dissipation rate. The Sample and Hold method was adopted to interpolate the unevenly spaced record and calculate the autocorrelation function; the integral-stop-value 1/e was used to estimate the integral length scale; and the TKE dissipation rate was estimated through the velocity energy spectrum. A double shear layer composed of two counter-rotating vertical oriented vortices, interacting with the secondary currents, is observed in the interface region for deep flow conditions. By decreasing the water depth, the interface region becomes dominated by a strong mixing layer of vertical oriented vortices with high TKE dissipation rate and large integral length scale, acting as a vertical wall to the weak secondary currents that develop at the main channel. The determination of the integral length scale permits to confirm the existence and the strength of these turbulence structures, unveiling the strong mixing layer as the origin of the largest integral length scales, even larger than the flow depth, and as the most efficient mechanism to redistribute turbulence generated at the bottom towards upper flow regions. Despite the high complexity of turbulence structures present in the flow, for all water depths, a linear dependence is depicted between integral length scale, TKE dissipation rate, and streamwise turbulence intensity.     

超声外场对微通道中熔体黏性耗散效应的影响

针对聚合物熔体在微尺度通道中流动时的黏性耗散效应对其流动行为的影响,通过自行构建的带有温度传感器和超声振子的微注塑成型试验系统,采用单因素成型试验方法,对聚丙烯(Polypropylene, PP)和高密度聚乙烯(High-density polyethylene, HDPE)两种聚合物材料在不同工艺参数和超声外场作用下,流经矩形截面微通道时由黏性耗散效应引起的通道出口熔体温升进行试验测量。结果表明,微通道中熔体的黏性耗散效应随注射速度的增加而增强,随入口熔体温度和模具温度的升高而减弱;与不加超声振动相比,施加超声振动使两种材料的微通道出口熔体温升值明显升高;但材料自身的微观分子结构及其热物理性能不同,其温升增幅差别较大。试验注射速度下,施加超声振动比不加超声振动时的PP熔体温升增幅高出34.7%,而HDPE熔体的温升增幅则高达71.7%。当超声频率和工艺参数一定时,增大超声功率使PP熔体的微通道出口温升增加了24.8%,HDPE熔体的温升增加了83.6%。可见施加超声外场作用能使微通道中聚合物熔体的黏性耗散效应明显增强。     

The Shear Properties of Poly (N-Alkyl Methacrylates) in Concentrated Contacts

The paper describes experiments where thin films of polymers are deposited on smooth rigid substrates and rigid smooth sliders are slid over the organic films. The influence of temperature and pressure on the interfacial shear properties of a range of poly n-alkyl methacrylates is presented. The behavior of these materials is compared with that of polythenes. The effect of progressively increasing the length of the hydrocarbon side chain moiety in the methacrylates is investigated; the behavior of the polythenes is regarded as the limiting case for infinitely long hydrocarbon side chains.

The data are treated using a very simple plastic flow model based on viscous energy dissipation. The parameters obtained for this model are then considered in terms of the likely molecular processes occurring during sliding.     

Experiment and simulation study on cavitation flow in pressure relief valve at different hydraulic oil temperatures

Hydraulic oil is the “blood” of hydraulic system, its high temperature in low-pressure hydraulic system would promote the development of cavitation and cause severe erosion of pressure relief valve. The influence of high oil temperature on the distribution of pressure field, velocity field and vapor volume fraction are discussed experimentally and numerically. The results show that with the increasing oil temperature, the viscosity of the oil decreases, and the flow rate increases, resulting the decreasing pressure at the orifice. Higher oil temperature promotes the occurrence of cavitation in the pressure relief valve, wider low-pressure zone could be found and cavitation bubble developed more fully and towards the valve core head. When the oil temperature increases from 303 K to 353 K, the cavitation intensity rises more sharply, but the growth rate of cavitation intensity increases firstly and then decreases with the increasing input pressure. Furthermore, based on the field synergy theory, the flow resistance and energy dissipation under different oil temperatures are evaluated. Both of large viscous dissipation and effective viscosity coefficient are mainly concentrated at the orifice, which are all effected by the oil temperature, so as to the characteristics of cavitation flow. The average field synergy cosine angle and the average viscosity coefficient decreases gradually with the increasing oil temperature, while the average vapor volume fraction increases. The energy dissipation is reduced by 3.3 × 10⁷ (W m⁻³) while the hydraulic oil temperature increases from 303 K to 353 K. Appropriate hydraulic oil temperature could provide favourable working conditions for the pressure relief valve which is beneficial for extending the hydraulic system's service life.     

Viscous dissipation in micro-channels

Viscous dissipation effects in circular micro-tubes and rectangular micro-channels were investigated theoretically in a fully developed laminar flow region. From the energy conservation law, simple theoretical equations were derived for the evaluation of the viscous dissipation effect. The proposed equations were verified by comparing to other researchers’ experimental measurement and numerical results. Wall heat flux, fluid velocity, operating temperature, tube diameter, and working fluids’ effects on viscous dissipation were evaluated. It was found that viscous heating increases with decreasing aspect ratio in rectangular channels. The classical macro methods for heat transfer and pressure drop predictions agree well with the measurement and numerical predictions in micro-channels. Finally, a criterion for the evaluation of viscous dissipation was provided with the Brinkman number and heat balance concept.     

两栖车两相绕流场的模拟与水上快速性分析

两栖车形体复杂,其水上绕流场和水上性能难于用传统手段进行预报,计算流体力学为解决这一问题开辟了新途径。以计算流体力学中广泛使用的雷诺时均纳维—斯托克斯方程为基本控制方程,采用切应力输运型 湍流模型结合流体体积分数法,进行两栖车水上两相绕流场的数值模拟。控制方程由有限体积法离散,压力速度耦合采用SIMPLE策略处理,代数方程由高斯—赛德尔法求解。阻力是快速性的主要指标,以其为判据对比试验与模拟得到的阻力结果,相对误差在5%左右。构建阻力—速度模型,模型的系统参数由最小二乘法进行辨识后得到结论,兴波阻力与航速的1.88次方成正比,摩擦阻力与航速的1.76次方成正比,粘压阻力与航速的3.54次方成正比。得到的系统模型为定性分析两栖车水上快速性提供了依据。     

Drag reduction for flow past a square cylinder through corner chamfering

This study investigates the unsteady incompressible flow around a square cylinder with different chamfer ratios (CRs) using a commercial finite volume code, ANSYS Fluent. CR ranges from 0.0 (sharp square cylinder) to 0.5 (diamond cylinder) with variable increments. Detailed analysis of flow characteristics is conducted at Reynolds number (Re) = 2100. Additionally, simulation is extended to cover Re, i.e., Re = 100, 500, and 10000. The simulation results show that cylinder with CR = 0.1 outperforms all other cases by enabling a drag reduction of about 60 % at Re = 10⁴. Drag has an inverse relationship with the wake closure length. Time-averaged coefficient of pressure, streamlines, and vorticity contours are also discussed to better understand near-wake features and the physics of drag reduction.

     

Influence of the flow velocity on a Kovasznay type vorticity probe

The output signal of a Kovasznay type vorticity probe is in first-order approximation proportional to the longitudinal component of the vorticity omegax= partial differentialw/ partial differentialy- partial differentialv/ partial differentialz and does not depend on the two transverse components of the flow velocity v, w. An experimental investigation of the influence of all three fluctuating velocity components on the longitudinal vorticity signal showed that their influence may not be neglected. The error in the probe response caused by the longitudinal component of the flow velocity was easily corrected using the instantaneous longitudinal velocity component and digital measuring techniques. On the other hand, the error caused by the two transverse velocity components could not be corrected. The contamination of the vorticity signal produced by the two transverse velocity components has been calculated by considering first- and second-order terms. The agreement between the calculations and experimental measurements is good. It is concluded that in a turbulent flow field this probe cannot be used without the simultaneous knowledge of the instantaneous transverse velocity components.     

不同微结构静压轴承油膜流动特性研究

在剪切力和压缩力共同作用下,液体静压轴承黏性油膜的液阻和流速会发生变化,导致油膜的散热能力不稳定,而增加油膜流动阻力,减小流动速度可以有效提高油膜的散热能力。为增加流体的扰动进而增强换热,在静压轴承工作面上加工不同的微结构(矩形、三角形、椭圆形),通过数值仿真方法研究微结构在不同跨度、不同深度、不同间距下对轴承工作面油膜流动速度的影响,得到黏性油膜增阻减速的有效范围。结果表明：综合微结构深度、跨度、间距变化对油膜液阻的影响,矩形微结构增阻效果最明显,椭圆形微结构次之,三角形微结构最差;当微结构间距单一变化时,只有矩形微结构可起到降低流场平均速度的作用。因此,矩形微结构可起到增阻减速的作用,且增阻减速的最佳间距范围为0.01～0.04 mm。     

Flow structure in the downstream of square and circular cylinders

In this experimental work, a technique of digital particle image velocimetry (DPIV) is employed to characterize instantaneous vorticity and time-averaged velocity, vorticity, root mean square (rms) velocities, Reynolds stress correlations and phase-averaged contours in the downstream of circular, sharp-edged square and 45^∘ orientated square cylinders in a uniform flow. Strouhal numbers for 550≤Re≤3400 are calculated from wake flow patterns. Shear layers surrounding the recirculation bubble region behind the cylinder are discussed in terms of flow physics and vortex formation lengths of large-scale Kármán vortices. Enhancement levels of Reynolds stress correlations associated with cross-stream velocity are clarified. Finally, flow structures depending on the cylinder geometry and Reynolds number are interpreted with quantitative representations.     

光学镜面离子束加工材料去除效率

分析了离子束加工材料去除效率、不同材料之间的相对去除效率与工艺参数的关系。基于Sigmund溅射理论,建立表征去除效率的指标－法向去除率、体积去除率和溅射产额与束能、束流以及入射角度之间的关系模型。以石英、微晶和K4等为样件,实验分析了去除效率与工艺参数的关系,验证了模型的正确性。分析结果表明材料去除效率随束流线性增大;约与束能平方根呈线性关系;随着入射角度先缓慢增大,约在60~80°达到最大值,尔后迅速降为零。不同材料之间的相对去除效率与束流无关;与束能的关系较弱,可以忽略;随角度变化较为明显。     

Reverse flow in a square duct with an obstruction at the entry

In this paper, the reverse flow in a square duct with an obstruction at the front (which is a square plate), is investigated using particle image velocimetry (PIV). The gap g between the obstruction and the entry to the duct was systematically varied, and it was found that maximum reverse flow occurs around a g/w value of 0.75. The velocity vectors, vorticity plots, and other details described indicate that the flow field is different compared with the two-dimensional channel case.     

Viscous dissipation influencing viscosity of polymer melt in micro channels

Determination of melt rheological behavior within micro-structured geometry is very important for the accurate simulation modeling of micro-molding. Yet studies on the rheological behavior of polymer melts, flowing through micro channels, are complicated due to a large number of factors affecting the melt viscosity. One factor, viscous dissipation, is investigated in the current work through a novel experimental technique to determine the viscous dissipation of a polymer melt flowing through several micro channels with identical aspect ratio. Relative tests are conducted with the melt of high density polyethylene (HDPE) at different temperatures being extruded through the capillary dies with diameters 1000μm, 500μm and 350μm, respectively. It was found that the temperature rise due to viscous dissipation decreases significantly with the reduction of the characteristic size of micro channel at the same shear rate. In addition, based on the suggested model of radial temperature distribution, the influence of viscous heating on the melt viscosity is investigated. The results indicate that viscous dissipation does not play a significant role.     

Estimation of velocity profile effects in the shell-type Coriolis flowmeter using CFD simulations

This paper presents a study of the axisymmetric velocity profile effects in the shell-type Coriolis flowmeter with a second circumferential mode, which exploits the results of computational fluid dynamics simulations. Simulations were carried out for viscous fluid flow in the vibrating measuring tube, whose mode shape remained fixed during the transient simulation process. We observed time responses of the integral anti-symmetric fluid forces acting on the inner wall of the measuring tube. Their magnitudes and their relative variations with the mass flow rate of the fluid were used for the integral estimation of the velocity profile effect. Simulation results are presented for different fluid velocities through the measuring tube and show considerable loss of flowmeter’s sensitivity in the range of lower Reynolds numbers. The results are also compared with the weight vector estimations of the velocity profile effect and are evaluated for two different turbulent models.     

高速离心叶轮内部三维粘性流场的数值模拟

采用一种快速求解三维黏性流场的计算方法求解某离心压气机内部复杂的流场，该方法利用Denton J．D．教授的粘性体积力法模拟粘性对叶轮机械内部流动的影响，采用时间推进法和有限体积差分格式对叶轮机械内部的流动进行求解。为加速收敛，使用了多重网格法，当地时间步长和局部残差光顺技术。对某离心压气机内部流动进行了详细的数值模拟，得出了压气机转子性能、叶顶周向平均静压以及准正交面上子午速度分布图，计算结果与试验结果吻合良好。采用该方法详细分析了不同工况下离心压气机内部流场。