The importance of ADCP alignment with GPS in moving-boat streamflow measurements

This paper addresses the importance of the alignment of an acoustic Doppler current profiler (ADCP) with a global positioning system (GPS) in moving-boat streamflow measurements. It presents a mathematical analysis of the discharge bias induced by a misalignment angle. A small misalignment angle may cause a significant bias in a transect discharge. The bias consists of non-directional and direction components. The directional bias is proportional to the ratio between boat velocity and water velocity. In a normal condition of ADCP streamflow measurements, however, the directional bias in transect discharges can be approximately cancelled in the average discharge of reciprocal transects, even if heading-dependent errors are involved in the misalignment. This paper also presents a trial-and-error method for estimating the misalignment angle. We analyzed the transect discharge data obtained from a field measurement on the Yangtze River at the Huangling Temple hydrology station located about 5 km downstream from the Three Gorges Dam to gain insights on the effect of misalignment. Results of this case study suggest that the data for transect discharges must be processed to remove directional bias prior to the Type A evaluation of the random uncertainty of the measured discharge. Otherwise, the estimated Type A uncertainty would be false and misleading.     

Improvements to estimate ADCP uncertainty sources for discharge measurements

The use of moving boat ADCPs (Acoustic Doppler Current Profilers) for discharge measurements requires identification of the sources and magnitude of uncertainty to ensure accurate measurements. Recently, a tool known as QUant was developed to estimate the contribution to the uncertainty estimates for each transect of moving-boat ADCP discharge measurements, by varying different sampling configurations parameters through the use of Monte Carlo simulations. QUant is not only useful for estimating ADCP discharge measurement uncertainty, but also for identifying contributions of the various sources of uncertainty.However, the software requires long computational times, and the method to estimate the uncertainty of multiple-transect measurements does not consider the correlation of the variables between transects. Therefore, improvements in QUant are needed to optimize its application for practical purposes by hydrographers immediately after discharge measurements.This work presents four approaches for optimizing the performance of QUant to estimate the contribution to the uncertainty of different selected variables on ADCP discharge measurements and describes a new method of estimating multi-transect uncertainty with the QUant model that considers the correlation of errors in selected variables between transects. The approaches for optimization and the new multi-transect uncertainty method are evaluated using a dataset of 38 field measurements from a variety of riverine settings.     

Similarity of the velocity profile in geometrically distorted flow model

A method is presented to measure the depth-averaged horizontal velocity in distorted physical models, i.e., the depth-averaged horizontal velocity can be equivalent to a point velocity measured at approximately 0.6h below the water surface (h is the water depth). Further a scale for the Chézy coefficient in distorted models is deduced, and the velocity distributions in the vertical in prototype and distorted models are analyzed. By comparing data of the vertical variation of the horizontal velocity measured in a physical distorted model (with distortion ratio of 4.0) with prototype data, inaccuracies via the one-point method, the three-point method and the five-point method are analyzed. It is concluded that it is more accurate to measure the depth-averaged velocity via the three-point method than via the one-point method in the distorted model. What's more, the point velocity at 0.6h below the water surface approximates the depth-averaged velocity in the distorted physical model and the one-point method is proposed to be used in practice.     

Application of shallow-water acoustic tomography to measure flow direction and river discharge

High-frequency Fluvial Acoustic Tomography System (FATS) was initially used to measure flow velocity and river discharge in a mountainous river. The results showed the high-frequency FATS, not only improves the velocity resolution, but also reduces the minimum operational range from 76 m to 43 m in compare with the previous type of FATS. The analysis of sound wave propagation (Ray tracing) showed the bottom topography can be the reason of multi-ray paths of sound wave in the shallow freshwater rivers. A new formula based on the continuity equation introduced to estimate the variations of the angle between the flow direction and the FATS transmission line. The flow direction was measured using two crossed FATS transmission lines of 53 kHz and 30 kHz. The results compared to the up-looking ADCP (Acoustic Doppler Current Profiler) deployed near the intersection of the two lines which measured the changes in flow direction. The results affirmed the efficiency of the proposed method. Finally, the river discharge was estimated by both FAT systems and compared to the Rating Curve method and moving-boat ADCP estimates. The relative error of the FATS discharge measurements was less than 10%.     

Analysis of acoustic Doppler current profiler mean velocity measurements in shallow flows

Acoustic Doppler current profilers (ADCPs) are commonly used instruments for measurement of natural streamflow and flow in manmade channels. Velocities measured in a profile by the instrument are used to estimate the discharge in a channel. A Teledyne RD Instruments StreamPro ADCP was used to measure the mean velocity simultaneously with a laser Doppler anemometer (LDA) in a laboratory flume. An average of 3.9% under-prediction of the mean velocity measured by the ADCP occurred when compared to the measurements of the LDA. Moreover, this study shows that the sampling duration of the measurements significantly impacts the mean point velocities measured by up to 50%.     

Effects of non-homogeneous flow on ADCP data processing in a hydroturbine forebay

Accurate modeling of the velocity field in the forebay of a hydroelectric power station is important for both power generation and fish passage, and is able to be increasingly well represented by computational fluid dynamics (CFD) simulations. Acoustic Doppler Current Profiler (ADCP) are investigated herein as a method of validating the numerical flow solutions, particularly in observed and calculated regions of non-homogeneous flow velocity. By using a numerical model of an ADCP operating in a velocity field calculated using CFD, the errors due to the spatial variation of the flow velocity are quantified. The numerical model of the ADCP is referred to herein as a Virtual ADCP (VADCP).Two applications of the VADCP are modeled in the numerical analyses presented. Firstly the virtual measurement error of the VADCP is calculated for a single instrument adjacent to the short converging intake of a powerhouse. Secondly, the flow discharge through the forebay is estimated from a transect of VADCP instruments at different distances from the powerhouse. The influence of instrument location and orientation are investigated for both cases.A velocity error of up to 94% of the reference velocity is calculated for a VADCP modeled adjacent to an operating intake and is shown to decrease with distance from the powerhouse. Qualitative agreement is observed between the calculated VADCP velocities and reference velocities by a horizontal offset distance of 18 m upstream of the powerhouse.     

A dip-corrected discharge estimation method for a river system

This work proposes a novel dip corrected velocity distribution model in combination with the entropy theory for discharge estimation in a braided river. A modified form of the dip correction factor is derived by considering the topographical complexities and applied for assessing velocity profiles in river cross-sections. The velocity profiles at different verticals are computed by employing Shanon's entropy theory. The depth-averaged velocities at different cross-sections are estimated from the computed vertical velocity profiles and substituted in the area-velocity method for the discharge calculation. The model is applied to two study areas, Majuli and Umananda of the Brahmaputra River, having both simple and braided sections. The validation of the model is performed using the observed discharge data available at the nearby gauge site for low flow condition. Results indicate that the integration of bed rugosity factor in dip corrected velocity distributions improves the accuracy of discharge estimations.     

Measurements of three-dimensional velocities of spray droplets using the holographic velocimetry system

The Holographic Particle Velocimetry system can be a promising optical tool for the measurements of three dimensional particle velocities. In this study, the holographic particle velocimetry system was used to measure the sizes and velocities of droplets produced by a commercial full cone spray nozzle. As a preliminary validation experiment, the velocities of glass beads on a rotating disk were measured with uncertainty analysis to identify the sources of all relevant errors and to evaluate their magnitude. The error of the particle velocity measured by the holographic method was 0.75 m/s, which was 4.5% of the known velocity estimated by the rotating speed of disk. The spray droplet velocities ranged from 10.3 to 13.3 m/s with average uncertainty of +-1.6 m/s, which was +-14% of the mean droplet velocity. Compared with relatively small uncertainty of velocity components in the normal direction to the optical axis, uncertainty of the optical axis component was very high. This is due to the long depth of field of droplet images in the optical axis, which is inherent feature of holographic system using forward-scattering object wave of particles.     

基于信号重采样频移处理的宽带 ADCP 分层校准方法

针对宽带声学多普勒流速剖面仪(ADCP)现有校准方法无法有效实现流速分层校准的问题,提出基于信号重采样频移 处理的声波应答校准方法,通过应答换能器组分别接收 ADCP 各波束换能器发射信号,经分段重采样、时间窗补偿等处理,产生 包含多普勒频移信息的模拟回波信号,实现对宽带 ADCP 流速分层校准。 为更符合真实水体散射情况,基于主动声呐方程建立 了回波信号幅度计算模型。 搭建了可适用于陆上和水下的宽带 ADCP 分层快速校准装置,对两个 ADCP 样机开展了校准实验, 流速范围在 0. 01~ 10 m/ s,测量不确定度优于 0. 3% v+5 mm/ s(v 为流速测量值)(k = 2),并与水槽拖车法进行了对比验证。     

Estimating the flow velocity and discharge of ADCP unmeasured area in tidal reach

Due to the ringing and side-lobe interference, acoustic Doppler current profiler (ADCP) is unable to accurately capture the complete velocity profile in open channels near the water surface and channel bottom, which are usually called unmeasured areas. At present, the flow velocities through the unmeasured areas are most commonly estimated using the power law with the power set to be the default value. However, since the flows are unsteady and nonuniform in tidal reaches, the velocity distribution model and corresponding parameters will vary with the bathymetric, tide period, etc. Therefore, the most common estimation with the power law may not be suitable in tidal reaches. In this paper, a simple determination method of the best model is proposed. Firstly, the parameters in three classical velocity distribution models, which are called power law, logarithmic law and parabolic law models, are solved by least squares based on the ADCP measured velocity cells. Then, the corresponding root-mean-square error (RMSE) of each model is used for the quantitative indicator that the model with the minimum RMSE is chosen as the best model. At last, the flow velocity and discharge of the unmeasured area are estimated by the best model. The experiments carried out in the tidal reach of Yangtze Estuary showed that vertical flow velocity distribution various with the bathymetry and tide period, and the best models averagely improved about 2.0% of the relative standard deviation (RSD) relative to the power law method in the discharge estimation, especially at some tide period the RSD of the best model was several times better than that of power law model. For Yangtze River with an annual average discharge of 3.0×10⁴ m³/s, the improvement should not be ignored. Therefore, it will be necessary to use the best model with minimum RMSE to estimate the flow velocity in tidal reach.     

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.     

Effects of sample size and concentration of seeding in LDA measurements on the velocity bias in open channel flow

A Laser Doppler Anemometer was used to measure the mean flow and turbulence in fluid experiments despite it having a problem with the mean velocity bias when a LDA is used to measure turbulent flows. It is generally considered that given a sufficiently large sample size, LDA will produce measurements free of bias, even with high turbulence intensity, but there is no relative experimental validation to demonstrate how the sample size affects the mean velocity bias. This paper first tries to find the reasonable sample size that ensures a mean velocity calculation free of bias. Furthermore, the effects of particle seeding concentration on the measurements of velocity were also considered. Throughout the experimental process the particle velocities were measured using a Dantec 2-D LDA system. To describe the effects of sample size and particle seeding concentration, this paper will also address the reasonable sample size and range of concentration in the design of water flow prior to any experimental application.     

走航式ADCP在澜沧江流量测验中的应用

通过走航式ADCP在澜沧江流量测验的实际应用,阐述了ADCP流量测量的过程及特点,明确了其适用环境以及测验使用条件.对走航式ADCP与常规缆道流速仪法测得的流量比测,验证了ADCP的测流精度,相比具有更高的测验效率,省工省时.说明ADCP测流系统技术先进,功能齐全、性能稳定、安全可靠、安装方便,数据采集自动化程度较高,在测量现场能直接地获取各类测流数据,满足澜沧江流量测验,为ADCP在云南及西南河流流量测验的推广应用抛砖引玉.     

A method for correcting discharge of boat-mounted ADCP measurements

GPS data are usually used to measure boat velocity during boat-mounted acoustic Doppler current profiler (ADCP) measurements when bottom tracking is biased by moving bed. GPS cannot provide consistently accurate boat velocity reference because of multipath errors, satellite signal reception problems, and heading errors. In addition, the computation of water velocity from an ADCP mounted onto a moving boat is a vector-algebra problem, thus the discharge calculation is subject to the compass error when GPS is used for boat velocity reference. This paper proposes a method for correcting discharge based on the idea that the discharge calculation is independent of the boat path. The processing of two sets of boat-mounted ADCP measurements integrated with differential GPS and non-differential GPS was simulated to verify the method. The results show that the proposed method performs well in both differential and non-differential GPS conditions. The relative errors range from 0.1% to 1.5% for all measurements with the mean relative errors of 0.7%. Analytical assessment of the GPS errors shows the proposed method is insensitive to the positioning accuracy of GPS, but positioning error of non-differential GPS may induce relative discharge error of more than 1% when the river or stream is narrow. On the contrary, a relatively small compass or heading error can cause a significant error in water velocity and discharge when using GPS as the boat velocity reference. Therefore, integrating a differential GPS and maintaining a slow boat speed are best practices for discharge measurement, especially for narrow streams or short boat paths.     

Experimental investigation of the 3D unsteady flow field downstream of axial fans

This experimental study, applied to axial fans with three different types of sweep (backward, radial and forward), aims at quantifying the 3D structure of the flow field from unsteady velocity measurements. The hot-wire anemometry is used to measure the velocity components in the nearfield, downstream of the fans. The data analysis leading to averaged and turbulent velocities, the components of the Reynolds’ stress tensor and the turbulent kinetic energy is presented in order to illustrate the influence of the sweep. A spectral analysis is also performed.     

Separation of the reservoir and wave pressure and velocity from measurements at an arbitrary location in arteries

Previous studies based on measurements made in the ascending aorta have demonstrated that it can be useful to separate the arterial pressure P into a reservoir pressure P* generated by the windkessel effect and a wave pressure p generated by the arterial waves: P = P*+p. The separation in these studies was relatively straightforward since the flow into the arterial system was measured. In this study the idea is extended to measurements of pressure and velocity at sites distal to the aortic root where flow into the arterial system is not known. P* is calculated from P at an arbitrary location in a large artery by fitting the pressure fall-off in diastole to an exponential function and assuming that p is proportional to the flow into the arterial system. A local reservoir velocity U* that is proportional to P* is also defined. The separation algorithm is applied to in vivo human and canine data and to numerical data generated using a one-dimensional model of pulse wave propagation in the larger conduit arteries. The results show that the proposed algorithm is reasonably robust, allowing for the separation of the measured pressure and velocity into reservoir and wave pressures and velocities. Application to data measured simultaneously in the aorta of the dog shows that the reservoir pressure is fairly uniform along the aorta, a test of self-consistency of the assumptions leading to the algorithm. Application to data generated with a validated numerical model indicates that the parameters derived by fitting the pressure data are close to the known values which were used to generate the numerical data. Finally, application to data measured in the human thoracic aorta indicates the potential usefulness of the separation.     

Point-velocity methods for flow-rate measurements in asymmetric pipe flow

Laser Doppler velocimetry (LDV) is characterized by its ability to determine local fluid velocities with high accuracy. Therefore, LDV may also be used for precise flow-rate measurements of turbulent flow in circular ducts. The uncertainty of the measurement depends mainly on the asymmetry of the axial velocity distribution and on the point-velocity method chosen to estimate the flow-rate. LDV-measurements in conjunction with velocity-area methods have been performed under different asymmetric flow conditions yielding errors in the range of one per cent. The experimental data have been transformed into analytical flow profiles, in order to investigate combinations of single-point measurements. As a result, a new multi-point method with variable centre-point factor is introduced, that reduces both the effort and uncertainty in the measurement.     

A combined pulse modulation-processing method for ADCP applications

Acoustic Doppler Current Profiler (ADCP) based on the Doppler principle is the sonar for velocity measurements, which is mainly applied in the large-scale measurements of the oceanic current velocity. In order to satisfy the increasing requirement of high measurement resolution and accuracy compared with the traditional ADCP, this paper investigates the ambiguous functions of different signals and indicates that the pulse combined by several sinusoidal signals will improve the time resolution when keeping good frequency resolution. Then the present research proposes the modulation of a combined pulse as the ADCP׳s emitted signal, as well as the corresponding signal processing method. Two examples are demonstrated to show the effectiveness of the combined pulse modulation-processing method, and it is seen that: 1. the combined pulse has the same spatial resolution as one subpulse, while achieves better frequency resolution and better signal–noise ratio than the latter; 2. although the combined pulse results in a little worse signal–noise ratio than the long sinusoidal signal, the former has much higher spatial resolution; 3. the measurement accuracy of the combined pulse is better than the broadband coded-pulse when they have the same pulse width.     

Interferometry-based Kolsky bar apparatus

A new experimental approach of the Kolsky bar system using optical interferometry is presented for determination of dynamic behavior of materials. Conventional measurements in the Kolsky bar system are based on recording the strain histories on the incident and transmitter bars with two strain gauges, and require good adhesion between the gauge and the bar. We suggest an alternative approach, based on measuring the actual velocities of the bars by using fiber-based velocity interferometry. Two fiber focusers illuminate the bars at a small angle and collect reflected Doppler-shifted light, which is interfered with a reference beam. Velocities are calculated from short-time Fourier transform and phase-based analysis, and the dynamic stress-strain curve is derived directly from the measured velocity traces. We demonstrate that the results coincide with those obtained by conventional strain gauge measurements. The new method is non-intervening and thus not affected by bar impacts, making it more robust and reliable than strain gauges.