On diffusion velocity and the mass conservation equation for components

The mass migration velocity(absolute velocity)of component i in a multicomponent flow is equal to the convection velocity(frame velocity)plus the diffusion velocity(relativevelocity).The diffusion velocity as well as the corresponding diffusion coefficient depends on how the convection velocity is adopted.In turbulent flow,the mass migration velocity of component i is(?)(mass-weighted time average velocity).The diffusion velocity(-a)consists of turbulent diffusionvelocity(?)and molecular diffusion velocity(?)(?is the simple time average velocity of component i and a is a certain convection velocity).So,the part of turbulent diffusion velocity is independent of what convection velocity is taken.In the mass conservation equation for component i,the expression for the diffusion term on its right-hand side will change when the convection velocity on its left-hand side changes.In turbulent flow,there could be no diffusion terms,or a turbulent diffusion term only,or both the turbulent and molecular diffusion     

深空天文测速自主导航速度矢量合成误差传递分析

针对深空天文测速自主导航系统速度矢量合成过程中的误差传递问题,推导了视向速度测量误差与定速误差统计特性之间的映射关系,获得了当测量误差满足零均值高斯分布时的定速误差概率密度函数,给出了在特定条件下定速误差均值与方差的解析表达式。理论与仿真分析均表明,当三个恒星视线方向两两正交时,测量误差对定速误差的影响达到最小,仿真结果还给出了目标恒星夹角对误差传递的影响,这些研究结果可为测速导航系统的目标选取提供理论依据。     

Effect of gas expansion on the velocity of a Taylor bubble: PIV measurements

The effect of gas expansion on the velocity of a Taylor bubble was studied experimentally. The velocity field in the liquid ahead of a Taylor bubble was measured by particle image velocimetry (PIV), and the bubble velocity was measured with two pairs of laser diodes and photocells. The experiments were done in a 7.0 m long vertical tube with a 32 mm internal diameter. Solutions of carboxymethylcellulose (CMC) polymer with weight percentages between 0.01% and 0.1% were used. The expansion of slug gas induces an increase in the bubble velocity and a corresponding displacement of the liquid ahead of the bubble. The velocity of the bubble increases by an amount equal to the maximum velocity in the liquid displaced. For the solutions studied, the induced velocity profile was parabolic and the bubble velocity increase was equal to the liquid velocity at the tube axis, i.e., twice the mean velocity in the liquid displaced. The corrected velocity obtained by subtracting the velocity increase from the value of the bubble velocity is independent of the bubble length.     

Efficient approaches of determining the motion of a spherical particle in a swirling fluid flow using weighted residual methods

The motion of a spherical particle released in a swirling fluid flow is studied employing the least-squares method and method of moments. The governing equations are obtained and solved employing the two methods. The accuracy of the results is examined against the results of a fourth-order Runge–Kutta numerical method. The effects of various parameters, namely the initial radius, initial radial velocity, initial angular velocity, and drag-to-inertia ratio, on the non-dimensional velocity profiles and particle position distribution are considered. The results show that the radial velocity increases over time while the angular velocity decreases, and that an increase in the initial radial velocity increases the particle radial distance and angular velocity but decreases the radial velocity profile.     

Analytical solution to stagnation-point flow and heat transfer over a stretching sheet based on homotopy analysis

This paper is concerned with two-dimensional stagnation-point steady flow of an incompressible viscous fluid towards a stretching sheet whose velocity is proportional to the distance from the slit. The governing system of partial differential equations is first transformed into a system of dimensionless ordinary differential equations. Analytical solutions of the velocity distribution and dimensionless temperature profiles are obtained for different ratios of free stream velocity and stretching velocity, Prandtl number, Eckert number and dimensionality index in series forms using homotopy analysis method（HAM）. It is shown that a boundary layer is formed when the free stream velocity exceeds the stretching velocity, and an inverted boundary layer is formed when the free stream velocity is less than the stretching velocity. Graphs are presented to show the effects of different parameters.     

用管轴流速确定动脉中血液振荡流的速度剖面

本文通过求解圆管内血液振荡流的基本方程，求得圆管内血液流的速度与压力梯度之间的关系式，文章提出一种利用管轴外流速计算管内压力梯度，进而确定血液振荡流动速度分布的方法，该方法用于检测活体血管内血液振荡流的速度剖面，具有操作简单，精度较高的优点，最后，以人体颈动脉为例，讨论血液周期振荡流的速度分布特征，发现在任意时刻，除了邻近管壁速度迅速降为零之外，沿管截面速度分布相当均匀，呈现出与定常流不同的速度分布特征。     

Slip velocity measurements in an upward bubbly flow by combined LDA and electrodiffusional techniques

 An experimental technique for the measurement of the local slip velocity of spherical bubbles is reported. It is based on the measurement of the local liquid velocity by an electrodiffusional method, and the bubble velocity by a specially adapted LDA (Laser Doppler anemometer) with a short measuring volume. The bubble velocity is measured taking into account the shift between the bubble centre and the centre of the LDA measuring volume. The slip velocity is obtained by subtracting the liquid velocity from the bubble velocity at the point corresponding to the bubble centre. The technique is applicable for flows with high velocity gradients. Results of the slip velocity measurements in an upward bubbly flow at laminar pipe Reynolds numbers are presented. Received: 25 July 1996/Accepted: 13 April 1998     

低速水流下不同截面形状质量块压电能量收集器的实验研究

本文设计了一种适用于低流速水流中的悬臂梁式压电能量收集器，利用明渠流弯道水槽对五种质量相同、截面形状不同质量块的压电能量收集器进行实验研究，通过改变水的流速，得到不同质量块能量收集器收集功率和频率随流速的变化规律并进行分析。结果表明：在实验流速范围内，各质量块均存在起振流速，当低于起振流速时，收集的均方根功率（RMS Power）约为零，超过起振流速后，收集功率随流速增加而改变；形状相似的质量块，起振流速较为接近，四棱柱的起振流速最低，圆柱次之，三棱柱的最大；质量块截面形状不同，能量收集器性能不同，其中以三棱柱（70°）的输出性能最好，在流速为0.54 m/s时收集到的最大功率为2.02mW，分别是圆柱、四棱柱（50）和三棱柱（60°）的1.06、2.58、1.36倍。本文的研究可以为类似压电能量收集器的设计提供借鉴。     

On tortuosity and areosity tensors for porous media

An average streamwise channel velocity is proposed as a more accurate representation of the actual intrapore velocity than the intrinsic phase average velocity. A relationship is derived between the average streamwise channel velocity and the interstitial velocity and superficial velocity. New definitions of tortuosity and areosity as second-order tensors are proposed for porous media in general. Novel names, semantically in line with the respective physical meanings, are proposed for these quantities. The definitions produce results which conform with several other published results and are applicable to anisotropic media in general.     

Influence of the inlet velocity profiles on the prediction of velocity distribution inside an electrostatic precipitator

The influence of the velocity profile at the inlet boundary on the simulation of air velocity distribution inside an electrostatic precipitator is presented in this study. Measurements and simulations were performed in a duct and an electrostatic precipitator (ESP). A four-hole cobra probe was used for the measurement of velocity distribution. The flow simulation was performed by using the computational fluid dynamics (CFD) code FLUENT. Numerical calculations for the air flow were carried out by solving the Reynolds-averaged Navier–Stokes equations coupled with the realizable k-ε turbulence model equations. Simulations were performed with two different velocity profiles at the inlet boundary – one with a uniform (ideal) velocity profile and the other with a non-uniform (real) velocity profile to demonstrate the effect of velocity inlet boundary condition on the flow simulation results inside an ESP. The real velocity profile was obtained from the velocity measured at different points of the inlet boundary whereas the ideal velocity profile was obtained by calculating the mean value of the measured data. Simulation with the real velocity profile at the inlet boundary was found to predict better the velocity distribution inside the ESP suggesting that an experimentally measured velocity profile could be used as velocity inlet boundary condition for an accurate numerical simulation of the ESP.     

基于碰撞模型的斜坡滚石颗粒速度预测

滑坡滚石灾害是西部山区常见的地质灾害类型,具有突发性和随机性强的特点,是山区地质灾害预测和防治工作的重点和难点. 本文基于颗粒接触理论,考虑影响斜坡滚石碰撞过程中的随机因素,建立了用于预测斜坡滚石颗粒碰撞后速度的理论模型. 根据冲量及冲量矩定理建立滚石颗粒碰撞基本方程,得到斜坡滚石颗粒碰撞后反弹速度的解析解. 结果表明:斜坡滚石碰撞后反弹速度的解析解包含了坡角、坡体上被碰颗粒速度以及角度、入射速度和角度以及撞击角度等随机因素;当考虑入射滚石颗粒与坡体上被碰颗粒的撞击角度变化时,模型预测结果与试验结果吻合较好;本文进一步预测了滚石颗粒碰撞后颗粒反弹线速度和角度以及反弹旋转角速度的概率分布情况. 结果显示,反弹颗粒速度和角度以及反弹旋转角速度的概率分布均服从高斯分布;当坡体上被碰颗粒速度和坡角发生变化时,其对反弹颗粒速度和角度以及反弹旋转角速度概率分布定性上没有影响,但是对概率分布的中心参数有显著影响.      

叠氮化铜微装药爆轰驱动飞片的数值模拟

为了优化叠氮化铜微装药器件的设计,探究叠氮化铜爆轰驱动飞片的作用原理,根据微装药器件的实际设计和相关实验,采用ANSYS/LS-DYNA流固耦合算法对叠氮化铜爆轰驱动飞片的作用过程作了数值模拟。具体研究了加速膛长度对飞片的平整性和完整性的影响,分析了微装药的尺寸与飞片速度之间的关系。研究结果表明:加速膛的长度对飞片的完整性、平整性和速度具有重要影响,在过长的加速膛中飞片飞行时易发生破碎,加速膛过短飞片的驱动速度不能达到最佳。装药尺寸与飞片速度之间关系密切,装药直径对飞片速度的前期成长影响不大,但对飞片获得的最大速度却有较为明显的影响;装药的直径大于0.8 mm时,增加装药直径并不能使飞片的最大速度明显增加。     

Critical velocity in a contact elasticity problem for the case of transonic punch velocity

The paper deals with a dynamic contact problem in the presence of friction forces in the transonic range of punch velocities, where the punch velocity exceeds the transverse wave velocity but is still less than the longitudinal wave velocity. It is shown that there exists a critical velocity at which the solution structure and the character of its behavior on the boundary of the contact region change. This velocity is $\sqrt 2 $ times the transverse wave velocity. The existence of this velocity is possibly related to the surface wave velocity under restricted deformation conditions.     

近壁泊肃叶流中活性粒子迁移规律研究

研究活性粒子在剪切流中的迁移规律对实现颗粒分离和过程强化均具有重要意义.基于耗散粒子动力学理论,建立了描述微通道内近壁泊肃叶流中活性粒子迁移运动的数学模型,考察了活性粒子圆周运动角速度、手性诱导角速度、直行运动速度和转向扩散系数对大肠杆菌和常规活性粒子横向迁移速度和受迫转向频率的影响规律,并确定近壁剪切流中活性粒子横向迁移的形成机制.结果表明,近壁剪切流场中大肠杆菌的横向迁移速度随剪切速率增大先快速增加继而趋于稳定;大肠杆菌横向迁移速度随圆周运动角速度增大而减小,随手性诱导角速度、直行运动速度和转向扩散系数的增大而增大;大肠杆菌的受迫转向频率受圆周运动角速度、直行运动速度和转向扩散系数的影响小,而随手性诱导角速度的增大而加快;相比大肠杆菌,常规活性粒子横向迁移速度显著减小、受迫转向频率明显变慢,二者受直行运动速度和转向扩散系数的影响规律与大肠杆菌类似.直行运动是活性粒子形成横向迁移运动的前提,其他运动参数和结构参数均可一定程度促进或抑制活性粒子在近壁剪切流场中的横向迁移.     

Velocity measurements in a two-phase turbulent jet

The motion of oil droplets in a round turbulent air jet is investigated experimentally. Direct information on the droplets' average velocity is obtained by means of a Laser Doppler velocimeter. Average velocity profiles of the droplets are measured along the axis of the jet and transverse to it. The results are compared to the free jet expansion.The jet Reynolds number is in the range 10⁴–10⁵, the droplets' diameters are 50 μm and below and the volume concentration of the oil in the air is 10^?6.At the jet exit, the air velocity is higher than the droplets' velocity, at the developed region of the jet the droplets' velocity is found to be higher than the free air jet velocity at the same location. In the radial direction, the velocity profiles of the droplets are self similar and the droplets' velocity is lower than the free air jet velocity at the same location. The droplets' velocity decay along the axis of the jet is slower than the air velocity in the free jet and the two-phase jet is narrower than the submerged free air jet at the same exit velocity.     

Flow analysis for wormlike micellar solutions in an axisymmetric capillary channel

Flows of wormlike micellar solutions in an axisymmetric capillary channel were studied both numerically and experimentally. In the experiments, an aqueous solution of cetyltrimethylammonium bromide (CTAB) with sodium salicylate (NaSal) was used as a test fluid. The mole concentration of CTAB is 0.03 mol/l, and that of NaSal is 0.06 mol/l. The velocity distribution was measured with a particle tracking velocimetry and flow visualization experiments were performed. The velocity profile showed a plug-like shape and had inflection points where the velocity gradient rapidly changed. High-shear-rate regions near the channel wall spread with increasing the average velocity. Moreover, the flow turned out to be unstable at high average velocities, and when the flow was unstable, white turbidity was observed near the capillary wall. Shear rates showing a white turbidity were included in the range of shear rate where a shear-rate jump in a flow curve occurred. These results suggest that both the characteristic velocity profile and the emergence of white turbidity relate the shear-rate-jump property of wormlike micellar solution. In the numerical analysis, startup flows were considered. A modified Bautista–Manero model was employed as a constitutive equation, and startup flows at a constant average velocity were numerically simulated. The velocity profile at steady state predicted by the numerical simulation adequately agreed with corresponding experimental data. The velocity profile changes from Newton-like to plug-like with time. Inflection points in velocity profile appeared and moved towards the center-side with time. Temporal changes in both velocity gradient and fluidity indicated that the behavior in velocity depended on the shear-rate-jump property of wormlike micellar solution. The velocity gradient rapidly changed around the inflection point and the range of velocity gradient corresponds to that where a white turbidity was observed in the experiments. An erratum to this article can be found at     

Local solids flow structure in a countercurrent liquid-upward and solids-downward fluidized bed (CCLSFB) system

The local solids holdup and local particle velocity in a Countercurrent Liquid-upward and Solids-downward Fluidized Bed (CCLSFB) were investigated in details using optical fiber probes with two different models in a Plexiglas column of 1.5 m in height and 7.0 cm in inner diameter. A new flow regime map including fluidized bed, transition, and flooding regimes was established. The axial solids holdup distribution is almost uniform at low liquid velocity and/or solids flowrate and becomes less uniform with higher solids holdup at the top of the column after the operating liquid velocity is reaching the flooding velocity. The radial solids holdup profile is also nearly flat with a slightly lower solids holdup in the near-wall region at low liquid velocity and solids flowrate but becomes nonuniform as the operating liquid velocity approaches the flooding velocity. Two equations were also proposed to correlate radial local solids holdups. The descending particle velocity in CCLSFB increases with the decrease of the liquid velocity and the increase of the solids flowrate. A generally uniform particle velocity distribution was found in the axial direction, as well as in the radial direction except for a small decrease near the wall. These results on the local solids flow structure would provide basic information and theoretical supports for the design and industrial application of CCLSFB.     

磁探针速度传感器研究

应用Hopkinson杆装置,对磁探针速度计进行了实验测定。发现对待测自由面速度阶跃,磁探针有两种类型稳定的输出信号,分别对应于不同的速度范围。对某个特定磁探针来说,其初始信号峰值与速度成正比的条件是速度大于某个阈值。实验还测定了间距大小对磁探针输出信号的影响,给出了磁探针对阶跃实验的响应波形。基于实验条件,本文探讨了磁探针原理、测量电路特性以及靶板涡流的穿透深度等理论问题,得到的结果可以定性解释磁探针的两种特征信号以及它们间的转化机理。本文工作表明,磁探针可以用来定量测量自由面速度的初始阶跃幅度,准确测定全时域的速度阶跃时刻,但不足以确定整个速度历史。为使这种传感器更加成熟,其结构和线路有待改进。     

A STUDY ON DROPLET VELOCITY OF ETHANOL DURING ELECTROSPRAYING PROCESS AT CONE-JET MODE

研究液滴在静电喷雾下的速度特性是理解喷雾形态的形成及演化的关键.结合锥射流模式下乙醇静电喷雾实验数据,建立了静电喷雾二维轴对称模型.基于离散相液滴运动方程、连续相空气运动方程、电场方程以及用户自定义函数,进行了数值求解,获得了锥射流模式下的乙醇静电喷雾形态、空间电场分布以及液滴速度场分布.考虑了不同空气入口流速的影响,得到了乙醇/空气同轴射流静电喷雾形态的变化规律.结果表明,喷雾外围液滴与空气流场有较强的相互作用,导致喷雾中轴线附近的液滴速度分布变化较小,而在喷雾外围处的液滴速度分布沿径向剧烈变化;随着空气入口速度的增大,乙醇/空气同轴射流静电喷雾形态先趋于发散,当空气入口速度大于喷雾外围液滴轴向速度时,喷雾形态则趋于聚拢.因此,除改变施加电压、液体流量和电极结构外,通过控制空气入口速度来影响喷雾液滴速度场,也可获得不同的静电喷雾效果.     

A hybrid RANS/LES simulation of high-Reynolds-number channel flow using additional filtering at the interface

A hybrid method combining large eddy simulation (LES) with the Reynolds-averaged Navier-Stokes (RANS) equation is used to simulate a turbulent channel flow at high Reynolds number. It is known that the mean velocity profile has a mismatch between the RANS and LES regions in hybrid simulations of a channel flow. The velocity mismatch is reproduced and its dependence on the location of the RANS/LES interface and on the type of RANS model is examined in order to better understand its properties. To remove the mismatch and to obtain better velocity profiles, additional filtering is applied to the velocity components in the wall-parallel planes near the interface. The additional filtering was previously introduced to simulate a channel flow at low Reynolds number. It is shown that the filtering is effective in reducing the mismatch even at high Reynolds number. Profiles of the velocity fluctuations of runs with and without the additional filtering are examined to help understand the reason for the mismatch. Due to the additional filtering, the wall-normal velocity fluctuation increases at the bottom of the LES region. The resulting velocity field creates the grid-scale shear stress more efficiently, and an overestimate of the velocity gradient is removed. The dependence of the velocity profile on the grid point number is also investigated. It is found that the velocity gradient in the core region is underestimated in the case of a coarse grid. Attention should be paid not only to the velocity mismatch near the interface but also to the velocity profile in the core region in hybrid simulations of a channel flow at high Reynolds number. PACS47.27.Eq; 47.27.Nz; 47.60.+i