Fibreoptical spatial filter velocimeter for measurement of local liquid velocity

The spatial filtering velocimetry is the basis of the new optical hydrometric measuring probe. Like a laser Doppler velocimeter, the hydrometric probe determines the velocity of tracer particles in the liquid flow. Essential parts of the hydrometric probe are a differential fibreoptical spatial filter and an illumination source. The tracer particles are imaged onto the spatial filter by a shadow projection with a parallel light beam. The hydrometric probe can be used for velocity and flow measurements in pipes and river flows. The technique permits the design of robust and low cost optical hydrometric probe which can be used for different hydrometric applications.     

Three-dimensional MHD simulations of the electromagnetic flowmeter for laminar and turbulent flows

The interaction between an electrically conducting fluid and an external magnetic field in an ideal cylindrical electromagnetic flowmeter is numerically investigated for both laminar and turbulent flows. Induced electric potential in the fluid, and the difference in potential at the measuring electrodes are directly obtained by including MHD effects in the CFD simulations. Fully developed laminar and turbulent flows are simulated. The computed electric potential difference on the electrodes agrees with analytical values for small Hartmann number cases, where the induced Lorentz force is small. Turbulent flow produces a more uniform electric potential distribution in the flow meter cross-section than laminar flow. These integrated MHD/CFD simulations couple the MHD effect with flow dynamics without deriving a weighting function with an assumed velocity profile, which will be necessary for electromagnetic flow meters when the Hartmann number is not small.     

内置扭带换热管三维流动与传热数值模拟

对自转扭带换热管内流体的运动进行了分析,根据流体在自转扭带管内的切向运动特点,提出将自转扭带等效虚拟于静止扭带的思路。建立内置螺旋扭带换热管流体流动的三维物理模型,采用大型CFD软件FLUENT6.0中的RNG k-ε模型对内置扭带换热管内的流动与传热进行了数值模拟,得到了内置扭带换热管流体流动的速度、压力、湍流强度场分布规律及传热特性。比较了静止、旋转及旋转等效虚拟静止扭带换热管的传热和阻力降特性,分析了不同螺距对强化传热和阻力降的影响。速度场的模拟值与激光测速仪试验值进行了比较,二者吻合较好。     

Velocity and turbulence in a scour hole at a vertical-wall abutment

An experiment was conducted in a laboratory flume to measure the three-dimensional turbulent flow field, using an acoustic Doppler velocimeter, in a scour hole at a vertical-wall abutment under a clear water scour condition. The distributions of time-averaged velocity components, turbulence intensity components and Reynolds stresses at different vertical sections are presented. In the upstream, a large primary vortex associated with the downflow exists inside the scour hole. On the other hand, in the downstream, the flow field is chaotic due to the vortex shedding. The data presented in this paper would be useful for the development and validation of a flow field model, which can be used for the estimation of scour depth at bridge abutments.     

Unsteady magnetohydrodynamic couette flow

The effect of a uniform transverse magnetic field on the Couette flow of an electrically conducting fluid between two parallel plates for impulsive and uniformly accelerated motion of one of the plates is discussed. The magnetic lines of force are assumed to be fixed relative to the moving plate. The Laplace transform technique has been used to obtain the expressions for the velocity field and skin friction. The effect of the magnetic field is to increase the velocity field in both cases.     

Demonstration of a laser vorticity probe in turbulent boundary layers

A laser-based probe for the nonintrusive measurement of velocity gradient and vorticity was demonstrated in turbulent boundary layers. Unlike most other optical methods, the current technique provides an estimate of the velocity gradient, without having to first measure velocity at multiple points. The measurement principle is based on the heterodyne of coherent light scattered from two adjacent particles. The beat frequency of the heterodyne is directly proportional to the velocity gradient. The probe is assembled from commercially available, inexpensive optical components. A laser Doppler velocimeter (LDV) processor is used to analyze the heterodyne signal. A component of vorticity is obtained by using two appropriately aligned velocity gradient probes. The optical probes developed were used in turbulent boundary layers to measure local, time-frozen velocity gradients partial differential u / partial differential y, partial differential v / partial differential x, and partial differential v / partial differential y, as well as the spanwise vorticity. The measurements were compared to those inferred from LDV measurements in the same facility and to data available in the literature.     

Unsteady velocity profiling in bores and positive surges

In an open channel, steady flow conditions may be achieved when the discharge and boundary conditions remain constant for a reasonable period of time. The operation of any regulation device (e.g. gate) is associated with some unsteady surge motion. In the present study, new velocity profiling measurements were performed systematically under controlled flow conditions. Both steady and unsteady measurements were conducted in a relatively large laboratory facility. An ensemble-averaged technique was applied in unsteady flows to investigate positive surges. The experiments were repeated 25 (or 50) times for each controlled flow condition and the results were ensemble-averaged. The quality and accuracy of the Profiler data set were validated against data collected with an acoustic Doppler velocimeter, in both steady and unsteady rapidly-varied flows. A careful sensitivity analysis was conducted to test the appropriate number of runs. The results indicated that the selection of 25 runs was suitable for ensemble-averaging in rapidly-varied unsteady flows. Some instrumental error was observed however with the velocity profiler. Outside the boundary layer, the Profiler tended to produce errors in terms time-averaged velocity data and velocity fluctuations for a number of points in a profile. Overall, the study demonstrated that the propagation of positive surges is a highly unsteady turbulent process, and the performance of ADV Vectrino II Profiler in such an unsteady turbulent flow was satisfactory, provided that a careful validation was undertaken for all Profiler outputs.     

磁场复合电解加工流场分布特性的数值模拟

采用欧拉-欧拉多流体方法,对磁场复合电解加工的气液两相流进行了数值模拟,研究磁场作用下电解质流场流速、气泡体积分数分布等流体力学信息.仿真表明,静液状态时,在磁场的作用下,液相产生了流速,流速呈环状分布,气相呈搅拌状分布.入口添加流速后,在低流速下,液相速度仍呈环状分布,气相呈搅拌状分布;在较高流速下,液相和气相的分布与无磁场时的分布相近.     

三叶折叶浆搅拌流场三维速度分布规律的研究

用激光测速技术测定了三叶折叶浆搅拌流场的三维速度分布规律,通过流场分析发现该类搅拌浆轴向流动特性较差,为进一步研究开发新型高效轴流式搅拌器奠定了基础。     

Quantitative multiphase flow characterisation using an Earth's field NMR flow meter

We present a novel nuclear magnetic resonance (NMR) multiphase flow metering system with the ability to interpret the flow regime and quantify both the liquid volumetric flowrate and holdup for gas-liquid flows. The flow measurement apparatus consists of a pre-polarising permanent magnet upstream of an Earth's field radio frequency NMR detection coil. In this work, the system is applied to measure the free induction decay (FID) NMR signal of gas-liquid flows at a range of flow rates in both the stratified and slug flow regimes. Tikhonov regularisation is applied to fit a model equation to the acquired FID signal in order to determine the relevant liquid velocity probability distribution. Signal interpretation applied to the individual NMR scans allows monitoring of both the liquid velocity and holdup with time. The NMR estimate of the liquid holdup is comparable to video analysis of the flowing stream through a transparent pipe section. The accuracy of the NMR metering system is successfully validated against an independent in-line rotameter measurement of the liquid volumetric flowrate during multiphase flow. Finally, analysis of the temporal variation in measured liquid flowrate is shown to clearly distinguish the stratified and slug flow regimes.     

双流道泵内非定常流动数值模拟及粒子图像测速测量

为探讨双流道泵内部的非定常流动机理,采用Fluent软件,基于滑移网格技术、 湍流模型计算了一双流道泵在不同工况下的内部流动,并将计算结果与粒子图像测速仪(Particle image velocimeter, PIV)实测结果进行比较。结果表明：计算所得双流道泵内部流场符合叶轮机械内部流动的一般规律,且与PIV实测结果总体变化趋势一致;由于双流道泵结构特殊,其进口处的流动状态与普通叶轮相差较大,出口处的流动状态与普通叶轮类似;叶轮进口处,流体基本沿流道吸力面流动,流道工作面上的相对速度很小,存在严重的脱流和旋涡;叶轮出口处,压力面和吸力面的速度趋于相等,射流—尾迹现象并不明显;由于叶轮—蜗壳动静干涉,两个叶轮流道内的静压分布有所不同;同一流道内,静压随着半径的增加而逐步增大,压力面侧静压大于吸力面侧;蜗壳流道内静压随半径增大,最大静压值在隔舌处。此项研究不仅加深了人们对双流道泵内非定常流动图画的理解,从而进一步完善双流道泵设计方法,同时也可为其他类型泵的内流研究提供借鉴。     

Comparative study of a turbulent wall-attaching offset jet and a plane wall jet

The flow field characteristics of a two-dimensional wall-attaching offset jet (WAOJ) are experimentally investigated by comparing with those of a turbulent plane wall jet(PWJ). The mean velocity, the turbulent stresses and triple velocity correlations are measured with a split film probe and anX wire probe. Even with the strong influence of the suction pressure field in the recirculation bubble at the lower corner, it is found that the WAOJ in the wall jet region has a close similarity with the PWJ. Especially, the decay of maximum velocity and the upper jet spread along the maximum velocity line of the WAOJ are virtually the same as those of the PWJ. The mean velocity profile of the WAOJ attains similarity after the jet impingement onto the lower plate. However the profiles of second and third-order moments of fluctuating velocities vary rapidly before the impingement and then relax very slowly to the similarity profiles of the PWJ.     

基于铁磁流体的液压驱动方法研究

在机器人液压驱动控制中,传统机械阀结构复杂,体积较大,故基于铁磁流体的超顺磁性质,在机器人液压驱动的基础上提出了新型液压驱动控制方法,即使用铁磁流体对传统液压系统中的液压流质进行改良,外加磁场直接控制高压液流的流向,实现系统压力的换向。在铁磁流体动力学(Ferro Hydro Dynamics, FHD)的基础上建立了喷射在空气介质中的铁磁流体在外磁场作用下流动的数学模型,并利用二次开发的Fluent软件建立了气液两相的欧拉(Eulerian)模型,模拟了在不同磁场强度下铁磁流体液流的偏移及分散等动力学行为。结果表明,在0.08 T磁场作用下,当流出高度为30 mm时,液流的偏移量为1.1 mm,并通过实验验证了模型的正确性。在新型铁磁流体液压驱动系统中,高压液流的激励由磁场提供,不需运动部件。该方案具有结构紧凑简单、施加激励参数易调节、响应迅速等优点。     

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.     

Relative errors in evaluating the electromagnetic flowmeter signal using the weight function method and the finite volume method

In the signal analysis of an electromagnetic flowmeter, the weight function method (WFM) involves a numerical integration of products between the magnetic flux density vector, the weight function and the velocity profile function in two-dimensional analysis. The electromagnetic flowmeter signal can also be predicted by solving the governing flowmeter equation by the finite volume method (FVM). The present study is aimed at comparing the calculation accuracies of WFM and FVM. The test flows are fully developed laminar and turbulent flows in a straight pipe under uniformly distributed magnetic field. The calculation accuracies are also compared between different O-type grid systems. It was found that the accuracy of both methods depended strongly on the grid system. In particular, for turbulent flow whose axial velocity component changes very rapidly near the wall, the accuracy of WFM was found to depend strongly on the cell configuration near the electrodes.     

Laboratory measurement and numerical simulation of flow and turbulence in a meandering-like flume with spurs

The paper reported herein presents the laboratory measurement of near-bed flow and turbulence induced by non-submerged spurs protruding from the bank of a meandering-like laboratory flume with smooth rigid bed. The flow property was measured using a 3D acoustic doppler velocimeter for various combinations and locations of spurs in order to assess their effect on mean flow field. Likewise, turbulent characteristics were computed from the measured data for one of the experimental cases. Furthermore, a 2D numerical model was developed for the simulation of mean flow property, turbulent intensities as well as vorticity field using cubic-interpolated pseudoparticle (CIP) numerical technique. The simulated down-stream and cross-stream mean flow property as well as turbulent intensities in shear layer was found to be in good agreement with the experimental results. The numerical simulation of vortices, generated from the tip of the spur, was seen to be reliable. In addition, the migration of small vortices was visualized in the experimental flume using a simple technique.     

Two-component laser Doppler anemometer for measurement of velocity and turbulent shear stress near prosthetic heart valves

The velocity and turbulent shear stress measured in the immediate vicinity of prosthetic heart valves play a vital role in the design and evaluation of these devices. In the past hot wire/film and one-component laser Doppler anemometer (LDA) systems were used extensively to obtain these measurements. Hot wire/film anemometers, however, have some serious disadvantages, including the inability to measure the direction of the flow, the disturbance of the flow field caused by the probe, and the need for frequent calibration. One-component LDA systems do not have these problems, but they cannot measure turbulent shear stresses directly. Since these measurements are essential and are not available in the open literature, a two-component LDA system for measuring velocity and turbulent shear stress fields under pulsatile flow conditions was assembled under an FDA contract. The experimental methods used to create an in vitro data base of velocity and turbulent shear stress fields in the immediate vicinity of prosthetic heart valves of various designs in current clinical use are also discussed.     

Spatial filtering velocimeter using frequency shifting by the method of rotating kernel

A new approach of frequency shifting by rotating kernel is proposed to improve the performance of a spatial filtering velocimeter, used to provide accurate velocity information for a vehicle self-contained navigation system. A linear CMOS image sensor was employed both as a spatiotemporal differential spatial filter and as a photodetector. The filtering operation was fully performed in FPGA and is realized by applying a rotating kernel to the pixel values of the image. Theoretical analysis showed this method could double the maximum measurable velocity. The power spectrum of the output signal was obtained by fast Fourier transform (FFT), and was corrected by a frequency spectrum correction algorithm, named energy centrobaric correction. This velocimeter was used to measure the moving velocities of a conveyor belt. Experimental results verified the method’s ability of reducing the output signal frequency and standard uncertainty of velocity measurement. What is more, the undesired output introduced by frequency shifting to the power spectrum of the output signal was deeply investigated and a new method was proposed to eliminate the undesired component in output signals. This velocimeter aims at providing accurate velocity information for vehicle autonomous navigation system.     

Validation of magnetic resonance velocimetry for mean velocity measurements of turbulent flows in a circular pipe

Magnetic resonance velocimetry (MRV) is a versatile flow visualization technique that has been utilized for medical applications. Recently, MRV has been used to visualize engineering flows, but most engineers are still unfamiliar with the technique. In this paper, we introduce the basic principles and experimental configurations of MRV in detail and evaluate the accuracy of MRV applied to measure the mean velocity fields of turbulent flows in a circular pipe. A Philips Achieva 3.0 T Tx MRI scanner is used to provide a magnetic field and acquire resonance signals for flow visualization. Fully developed turbulent flows with Reynolds numbers of 6800, 9900 and 19400 were measured, and the axial mean velocity vectors were obtained with a spatial resolution of 0.5 mm for the three directions. Results show that the mean velocity profiles are in good agreement with reference data sets when properly scaled in both the inner and outer layers.

     

Spectral Method of Detection of Laser Doppler Velocimeter Signals in Turbulent Flows

A spectral method of detection of laser Doppler velocimeter signals in turbulent flows is proposed. The method is based on estimating the signal/noise ratio adapted to the width of the power spectral density of the signal and comparing its value with the recognition threshold. Numerical simulations show that the error of the signal/noise ratio estimates is 1% for different velocities of the turbulent flow. Physical experiments aimed at measuring turbulent aerodynamic processes show that the proposed method offers a possibility of eliminating the errors of calculating the mean value and the velocity deviation equal to 15 and 78% by means of eliminating the signals with low signal/noise ratios from the processing system. Application of the proposed method also ensures lower nonuniformity of the signal/noise ratio estimates than that ensured by the famous method developed by Tropea for a wide range of turbulent velocities.