Application of LSPIV to measure supercritical flow in steep channels with low relative submergence

Measuring flow discharge has always been one of the most important concerns of water experts. To measure discharge in streams using velocity-area method it is necessary to quantify average velocity of the flow. It is not feasible to measure velocity by contact approaches like current meters under certain conditions such as in flood periods or for very shallow flows. Flow surface image velocimetry methods as non-intrusive solutions have recently been widely utilized to measure discharge in open channels. One of these methods is a variety of PIV method named LSPIV which has been very popular due to the elimination of laser application. In this study, LSPIV was used to measure 2D velocity field over the surface of steep supercritical flow. The obtained surface velocity data were used to calculate Velocity Index (VI) which is multiplied by surface velocity to convert it to mean velocity and subsequently flow discharge. Also, a few relations were proposed to calculate the VI according to the slope and relative submergence. Since, Velocity Index has been so far mostly studied for subcritical conditions, results of this study may be applied for measuring supercritical flows. Eventually, the proposed method was verified to be used for discharge measurement and was proven quite precise in this regard.     

Application of large scale PIV in river surface turbulence measurements and water depth estimation

Large-Scale Particle Image Velocimetry (LSPIV) has emerged as a reliable technology to measure river surface flow velocity distribution and can be applied to estimate river discharge. Fewer studies have explored the capability of surface turbulence measurements using LSPIV. In this paper, LSPIV is applied to evaluate statistics of surface turbulence of a natural river. Turbulence measurements including velocity fluctuation, velocity spectra and the dissipation rate of turbulent kinetic energy (TKE) are validated by comparing with those measured by an Acoustic Doppler Velocimeter (ADV). Traditionally, estimation of stream discharge through LSPIV needs a secondary measurement to determine river bathymetry and water depth. A new method is presented here to demonstrate that for a fully developed and channel-controlled flow, the cross section geometry can be estimated from the combined measurements of surface mean velocity and the dissipation rate, following the Manning-Strickler formula. Therefore, river discharge can be estimated with LSPIV along with a calibrated Manning's roughness, without additional bathymetry survey. The proposed new method is applied to measure discharge in Milwaukee River (Milwaukee, Wisconsin, U.S.A.), which agreed well with data obtained from a nearby streamgage station.     

The effect of air velocity on slugs in a confined channel

The present work focuses on the study of slugs occurring in a two-phase flow of a confined rectangular channel: conditions of appearance and effect on the flow behavior. Three-dimensional numerical simulations have been carried out to examine the effect of superficial air velocity on flow behavior. The Volume Of Fluid model (VOF) is used to track the air-water interface. Validation of the numerical model is obtained by comparing the results of the simulated axial velocity with experimental data determined using the Laser Doppler Anemometry (LDA) technique. The numerical results revealed that for a fixed water level and superficial water velocity, higher superficial air velocities generate a slug flow that causes channel blockage. The position of these slugs and the timing of their occurrence were correlated in terms of air and water superficial Reynolds numbers.     

Validating dilute settling suspensions numerical data through MRI,UVP and EIT measurements

The measurement of fluid dynamic quantities are of great interest both for extending the range of validity of current correlations to be used in equipment design and for verification of fundamental hydrodynamic models. Studies where comparisons are made between imaging techniques serve to provide confidence on the validity of each technique for the study of multiphase flow systems. The advantage of cross-validation is that it can help establish the limitations of each technique and the necessary steps towards improvement. A small amount of comparative studies are found in the literature and none of them reports the study of settling particles suspension flow using simultaneously Ultrasonic Velocity Profiling (UVP), Magnetic Resonance Imaging (MRI) and Electrical Impedance Tomography (EIT), at least not to the best of the authors knowledge. In the present paper the authors report efforts made on the characterization of dilute suspensions of glass particles in turbulent flow, with increasing flow velocities and particles concentrations, in a pilot rig at a laboratorial scale, using both MRI, EIT and UVP: direct comparisons of EIT, MRI and UVP measurements acquired and mixture model numerical simulations are presented and the level of agreement explored.     

Application of spherical-rod float image velocimetry for evaluating high flow rate in mountain rivers

Spherical-rod float image velocimetry (SFIV) is a convenient technique combining the positive functions of a rod float velocimetry (RFV) and large-scale particle image velocimetry (LSPIV) for measuring high flow rate in mountain rivers. The SFIV is the principle that the sphere allowing little image distortion according to the orientation is used as a floating tracer for LSPIV. The drifting distances of a spherical-rod float were calculated by geometrical interpretation of spherical images recorded in an experimental open channel and mountain rivers. The depth-reflecting velocities estimated by SFIV in the rivers as in the open channel coincided approximately with the velocities by visual observation from river bank despite of the long shooting distance, weather impact, and flow complicated by topography and bed materials. The velocity coefficients obtained from the experimental channel were used to evaluate depth-averaged velocity for river discharges. The high discharges estimated by SFIV in mountain rivers distributed mostly within the range of the rating curve established by RFV. The results show that the safe and efficient SFIV is a highly applicable technique in mountain rivers with the high flow rate and complex flow. In order to practically use SFIV in mountain rivers, additional studies are required for velocity coefficients depending on the water depth and draft.     

Evaluation of hot-film,dual optical and Pitot tube probes for liquid–liquid two-phase flow measurements

An experimental study of kerosene–water upward two-phase flow in a vertical pipe was carried out using hot-film, dual optical and Pitot tube probes to measure the water, kerosene drops and mixture velocities. Experiments were conducted in a vertical pipe of 77.8 mm inner diameter at 4.2 m from the inlet (L/D=54). The tests were carried out for constant superficial water velocities of 0.29, 0.59 and 0.77 m/s (flow rates = 83, 167 and 220 l/min) and volume fractions of 4.2%, 9.2%, 18.6% and 28.2%. The Fluent 6.3.26 was used to model the single and two-phase flow and to reproduce the results for the experimental study. Two methods were used to evaluate the accuracy of the probes for the measurement of the velocities of water, drops and mixture for two-phase flow: (i) comparison of measured local velocities with predictions from the CFD simulation; (ii) comparison between the area-averaged velocities calculated from the integration of the local measurements of water, drops and mixture velocities and velocities calculated from flow meters’ measurements.The results for single phase flow measured using Pitot tube and hot-film probe agree well with CFD predictions. In the case of two-phase flow, the water and drops velocities were measured by hot-film and dual optical probes respectively. The latter was also used to measure the volume fraction. These three measured parameters were used to calculate the mixture velocity. The Pitot tube was also used to measure the mixture velocity by applying the same principle used for single phase flow velocity. Overall the mixture local velocity measured by Pitot tube and that calculated from hot-film and dual optical probe measurements agreed well with Fluent predictions. The discrepancy between the mixture area-averaged velocity and velocity calculated from flow meters was less than 10% except for one test case. It is concluded that the combined hot-film and optical approach can be used for water and drop velocity measurements with good accuracy for the flow conditions considered in this study. The Pitot tube can also be used for the measurement of mixture velocities for conditions of mixture velocities greater than 0.4 m/s. The small discrepancy between the predictions and experimental data from the present study and literature demonstrated that both instrumentation and CFD simulations have the potential for two-phase flow investigation and industrial applications.     

Stroboscopic two-dimensional ultrasonic velocity profiling for measuring flow transition in Taylor Couette systems

Flow characterization in a Taylor Couette system was made by investigating the radial velocity component with Ultrasonic Doppler Velocimetry based flow mapping. With the technique presented in this work, it is possible to measure the radial velocity components for variable axial position in a Couette cell within Taylor vortex flow (TVF), wavy vortex flow (WVF), modulated vortex flow (MVF) as well as spiral vortex domains in a conical shaped gap. The resulting maps for the different flow states show the location of vortices in the annular gap between the inner and outer cylinder. Cylindrical and conical concentrically rotating inner bodies were applied and respective flow patterns were analyzed. The method uses a stroboscopic triggering to synchronize flow measurements and rotational motion. The oscillation frequency f of unsteady motion in WVF, MVF, and spirals can be obtained from the power spectrum of velocity. The UVP transducer was preferably positioned in radial direction, perpendicular to the surface of the inner rotating body for measuring the radial velocity component. At the same time, the transducer was moved with constant velocity vertically along the outer cylinder height.     

Pressure field estimation from ultrasound Doppler velocity profiler for vortex-shedding flows

A novel algorithm of pressure field estimation based on ultrasound velocity profiler (UVP) is developed. The method consists of UVP measurement of velocity distribution in fluid flows and numerical analysis of the measured data using fluid dynamics equations. We introduce equation of continuity, incompressible Navier-Stokes equation and proper orthogonal decomposition (POD) into the basic algorithm, so that pressure field of space-time two-dimensional unsteady fluid flow is fully reconstructed. Since UVP is based on ultrasound Doppler principle, the local instantaneous pressure distribution is obtained non-intrusively. The performance of an algorithm is evaluated for vortex shedding flow behind a circular cylinder at Re = 1000. Considering the specification of UVP, the optimal method of experimental data conversion to pressure information is proposed. We have found that the one-dimensional velocity measurement by UVP upon Taylor's frozen hypothesis is suitable for evaluation of pressure field in wake of the cylinder. The present algorithm is also demonstrated for opaque fluid flows by considering vortex flow in milk.     

河流水面成像测速研究进展

大尺度粒子图像测速(LSPIV)是一种用于测量大面积(数百平方米)水体表面流速的非接触式全场流速测量技术。不仅可用于常规条件下明渠紊动特性和时均特性的研究,更具有极端条件下河道水流监测的应用潜力。然而受复杂现场环境的影响,该技术在天然河流的应用中面临着诸多挑战。从水流示踪方式、流场图像采集、水面目标增强、运动矢量估计、时均流场重建、水面流场定标及不确定度评估七个方面综述了国内外对河流水面成像测速(RSIV)方法的研究进展。通过对现有方法的总结和讨论,提出改进技术方法的研究需求,以使其成为水动力学和水文学中有力的测量工具。     

Improved volume-of-fluid (VOF) model for predictions of velocity fields and droplet lengths in microchannels

Accurate measurement of flow in microchannels is imperative to better understand their flow behaviour, which aids in the design of microfluidic devices. In this work, we present an improved VOF model based on smoothing functions that can effectively minimise the issue of spurious velocities, which causes numerical simulations in microchannels to be less accurate. We use the smoothed VOF to simulate the velocity fields and droplet lengths in microchannels and compare the results with experimental data. The results show that the smoothed VOF is able to simulate flow in microchannels more accurately than the standard VOF model. Microchannel simulations using the standard VOF model are less accurate because the spurious velocities produces artificially higher velocity regions in the flow field results. The spurious velocities also induce a higher but non-physical shear stress during the droplet formation process, resulting in droplets forming prematurely with shorter lengths. Hence the smoothed VOF which resolves the issue of spurious velocities is shown to be a more viable tool in predicting the flow in microchannels by means of numerical simulations.     

Using µPIV to Investigate Fluid Flow in a Pocketed Thrust Bearing

This article presents the results of an experimental and analytical investigation of the fluid flow in a pocketed thrust bearing. An experimental test rig was designed, developed, and used to visualize fluid flow in pocketed thrust bearings. Microparticle image velocimetry (μPIV) was used to measure fluid flow inside the pocket of a thrust bearing. The thrust bearings were constructed by gluing precision shim stocks to a flat BK7 glass disk in contact with a polished steel disk. The precision shim stock provides the desired pocket depth for the bearing. A polished steel disk in contact with the thrust bearing was driven by a motor in order to induce fluid flow within the pockets. μPIV was then employed to measure the shear-driven cavity flow and generate the quiver plots of the flow field. Three different lubricants were used at various speeds and a constant load to measure the effects of speed and viscosity on the flow out of the pocketed thrust bearing. In order to achieve the analytical aspect of this research, a model was developed to predict the film thickness, cavitation area, and pressure distribution generated within the bearing. The cavitation areas obtained from the model were compared with the experimental results. The results corroborate well. The calculated pressure and film thickness were then used to determine the 3D velocity profiles within the pocketed thrust bearing. The measured velocities obtained from the experimental images were compared to the analytical velocity fields. Comparing the measured velocities with the analytical model, the depth of the microparticles in the bearing pocket was determined. Using this approach, the μPIV-measured 2D velocity field was converted into a 3D velocity field, which illustrates the fluid motion inside a pocketed thrust bearing at various speeds and viscosities.     

Application of ultrasonic multi-wave method for two-phase bubbly and slug flows

This paper proposes a measurement technique for two-phase bubbly and slug flows using ultrasound. In order to obtain both liquid and gas velocity distributions simultaneously, a new technique for separating liquid and gas velocity data is developed. The technique employs a unique ultrasonic transducer referred to as multi-wave transducer (TDX). The multi-wave TDX consists of two kinds of ultrasonic piezoelectric elements which have different resonant frequencies. The central element of 3 mm diameter has a basic frequency of 8 MHz and the outer element has a basic frequency of 2 MHz. The multi-wave TDX can emit the two ultrasonic frequencies independently. In our previous investigations, both elements were connected with two ultrasonic velocity profile (UVP) monitors to measure liquid and bubble velocity distributions. However, the technique was limited to the measurement of bubbly flows at low void-fraction. Furthermore, it was impossible to synchronize the instantaneous velocities of liquid and bubbles because of the facility limitation. In order to overcome these disadvantages, cross-correlation method is employed for the measurements in this study. In order to apply the technique to flow measurements, ultrasound pressure fields are measured. As a result, it is found that the TDX must be set 20 mm away from the test section. The technique is applied to measuring bubbly and slug flows. By the combination of 2 and 8 MHz ultrasonic echo signals, the echo signals are distinguished between reflected from particles and bubbles. Compared with the results of obtaining with the multi-wave method and a high-speed camera, it is confirmed that the technique can separate the information of liquid and gas phases at a sampling rate of 1000 Hz.     

3D Particle image velocimetry test of inner flow in a double blade pump impeller

The double blade pump is widely used in sewage treatment industry,however,the research on the internal flow characteristics of the double blade pump with particle image velocimetry(PIV) technology is very little at present.To reveal inner flow characteristics in double blade pump impeller under off-design and design conditions,inner flows in a double blade pump impeller,whose specific speed is 111,are measured under the five off-design conditions and design condition by using 3D PIV test technology.In order to ensure the accuracy of the 3D PIV test,the external trigger synchronization system which makes use of fiber optic and equivalent calibration method are applied.The 3D PIV relative velocity synthesis procedure is compiled by using Visual C++ 2005.Then absolute velocity distribution and relative velocity distribution in the double blade pump impeller are obtained.Test results show that vortex exists in each condition,but the location,size and velocity of vortex core are different.Average absolute velocity value of impeller outlet increases at first,then decreases,and then increases again with increase of flow rate.Again average relative velocity values under 0.4,0.8,and 1.2 design condition are higher than that under 1.0 design condition,while under 0.6 and 1.4 design condition it is lower.Under low flow rate conditions,radial vectors of absolute velocities at impeller outlet and blade inlet near the pump shaft decrease with increase of flow rate,while that of relative velocities at the suction side near the pump shaft decreases.Radial vectors of absolute velocities and relative velocities change slightly under the two large flow rate conditions.The research results can be applied to instruct the hydraulic optimization design of double blade pumps.     

Measurement and analysis of flow behaviour in complex geometries using the Ultrasonic Velocity Profiling (UVP) technique

In this project a non-Newtonian CMC model fluid was tested in two different complex geometries using Ultrasonic Velocity Profiling (UVP). Velocity profiles were measured at three different positions at the center (contraction) of a specially manufactured 50% open diaphragm valve. The complex geometry coordinates and velocity magnitudes were analysed and compared to the bulk flow rate measured using an electromagnetic flow meter. The difference between the calculated and measured flow rates varied from 15% to 25%. A complete flow map in the axial direction from developed to contracting flow was also measured by scanning the transducer along a hyperbolic contraction using a high precision robotic arm set-up. Experimental results obtained using UVP showed good agreement (10%) with theoretical predictions. Results showed that it was possible, for the first time, to measure quantitative velocity data for non-Newtonian flow in a complex geometry, such as a diaphragm valve. It was found that the most important problem in order to increase measurement accuracy is the estimation of wall interface positions, which is due to the ultrasonic transducer’s near field. This problem can be eliminated by the introduction of a next generation transducer, which is currently under development.     

2D电液比例换向阀阀芯卡紧力分析

针对2D电液比例换向阀阀芯卡滞现象,应用缝隙流动原理,对2D阀芯有无偏心情况下的径向卡紧力进行系统理论分析,得到2D阀芯液压卡紧力计算方法;运用MATLAB软件进行数值计算,得出2D阀芯径向卡紧力与偏心量和高低压孔夹角间的关系;根据2D阀特性,提出2D电液比例换向阀阀芯改进措施,应用Fluent 软件对阀芯表面的流场进行CFD仿真分析,比较了改进前后的流速矢量和压力分布情况,验证了改进措施的正确性。改进后的2D电液比例换向阀在中高压实验中无“卡滞”现象出现,实现了高压大流量的比例控制。     

Application of video imagery techniques for low cost measurement of water surface velocity in open channels

Developments in digital video recording technology make the video imagery tools more popular for velocity measurement in water flows. This has especially been of large interest due to its inherent advantage of non-contact nature which is quite handy in extreme flow conditions. Particle Image Velocimetry (PIV), Particle Tracking Velocimetry (PTV) and Large Scale Particle Tracking Velocimetry (LSPTV) are applied to free surface channel flow for water surface velocity measurement. Experiments are conducted to measure either a single point velocity applying PTV or velocity profiles across the channel width applying PIV on the water surface in a rectang typical velocities of nearly 1 andular tilting flume for various flow conditions. Technical issues regarding tracer particle size and type, travel distance, lighting, recording speed, camera position, image distortion and state of flow are discussed. Measured data is compared to computational results obtained from a numerical model involving a non-linear turbulence model capable of predicting turbulence driven secondary flows. Confirmation of reasonable match between computational and experimental results whereby applying mutual collaboration of them for discharge measurement has been attested. In addition to discharge, boundary roughness has also been predicted as an outcome of the numerical solution.     

An experimental device for measuring radial flow velocity profiles of yield stress fluids

Measuring the radial flow velocity field of yield stress fluids (YSFs) between two parallel disks provides crucial data to understand the underlying flow phenomena. However, direct velocimetry of YSFs in the radial flow configuration remains a challenge, due to the complex fluid rheology and geometry constraints. In this paper, we present an experimental device for measuring YSF radial flow velocity profiles. Ultrasound Velocity Profiling (UVP) is used to non-intrusively measure the velocity profiles. The Tikhonov regularization method is implemented to obtain smooth velocity profiles, which are used to calculate the plug-flow region. Compared to our previous work on radial flow, the current contributions include: (i) additional structural frame members to maintain a constant aperture, (ii) wall slip reduction, and (iii) an improved velocity profile plug-detection algorithm. The results show that the experimental device and the measurement method are effective for further studying radial flow behavior of YSFs for industrial applications.     

Smokestack gas velocity measurements using 3D pitot tubes in a coal-fired power plant

On the path to carbon neutrality to reduce greenhouse gas (GHG) emissions, the Korean government has mandated legislation for controlling and monitoring GHG emissions emitted from smokestacks. A continuous emission measurement (CEM) method is considered to be the most reliable for determining CO2 emissions from stationary sources. In Korea, an S-type Pitot tube is the most popular technique to measure the gas velocity in a smokestack, but it will result in a certain error when the non-axial velocity components exist. To vanquish this limitation, Korea Research Institute of Standards and Science (KRISS) developed a nulling smokestack flow measurement (NSFM) instrument equipped with 3D Pitot tubes for taking on-site stack gas velocity measurements. 3D Pitot tubes used in this research, such as prism Pitot tube and sphere Pitot tube, are calibrated in the KRISS airspeed system. The instrument using 3D Pitot tubes with the nulling technique is expected to diminish the restriction on S-type Pitot tubes, and to enhance the quality of the GHG emission measurements in the smokestack. The 3D Pitot tubes can measure both axial and non-axial velocity components of a flow, whereas the S-type Pitot tubes can measure only the axial velocity component. The averaged axial velocity of the stack gas as measured by this instrument has expanded uncertainty of 3.3% (P = 95%, k = 2) for both prism and sphere Pitot tubes.     

基本式和差动式旋转执行器研究 

基于磁控形状记忆合金（MSMA）直线执行器的研究,提出了基本式和差动式两种类型的MSMA旋转执行器。分别介绍了两种执行器的工作原理、结构和控制策略,推导了执行器的位移和转速公式,绘制出了MSMA元件的相对磁导率与磁通密度的关系曲线。在磁场有限元分析的基础上设计并研制了旋转执行器样机,比较了不同结构和不同控制策略下样机的实验速度曲线。样机的实验结果验证了MSMA旋转执行器工作机理的正确性和设计方法的可行性。