Accurate Flow Measurements Research Articles

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.

The effects of probe misalignment on sap flux density measurements and in situ probe spacing correction methods

The accurate measurement of sap flow is important for estimating transpiration. This paper is focused on the errors in sap flux density (J) caused by probe misalignment and on how to reduce these errors. We found that probe misalignment caused errors in the heat pulse velocity (Vh). Furthermore, probe misalignment had a large influence on thermal diffusivity (κ) and the determination of zero flow points. Assuming uniform axial thermal diffusivity and zero sap flow conditions, we developed a new in situ probe spacing correction method based on three temperature sensor measurements inside the same temperature probe, which can be used for correcting both linear and nonlinear misalignments. Laboratory experimental results verified that our method could significantly reduce the errors in determinations of J by using the Compensation Heat Pulse velocity (CHP) method, the Heat Ratio (HR) method and the T-max method.

Excitation condition analysis of guided wave on PFA tubes for ultrasonic flow meter

Impurity accumulation, which decreases the accuracy of flow measurement, is a critical problem when applying Z-shaped or U-shaped ultrasonic flow meters on straight PFA tubes. It can be expected that the guided wave can be used to implement flow measurement on straight PFA tubes. In this paper, the propagation of guided wave is explained by finite element simulations for the flow meter design. Conditions of guided wave generation, including the excitation frequency and the wedge structure, are studied in the simulations. The wedge is designed as a cone which is friendly to be manufactured and installed. The cone angle, the piezoelectric wafer’s resonant frequency and the vibration directions are studied in the simulations. The simulations shows that the propagation of guided wave in thin PFA tubes is influenced by the piezoelectric wafers’ resonant frequency and the vibration direction when the mode is on the ‘water line’. Based on the results of the simulations, an experiment is conducted to verify the principles of excitation conditions, which performs flow measurement on a straight PFA tube well.

Effects of Abrupt Pipe Diameter Changes on Venturi Flowmeters

Although accurate flow measurement is a requirement in many applications, flowmeters are often installed in unfavorable conditions. Any time a flowmeter is installed close to a flow disturbance, such as a pipe fitting or a valve, the installation should be investigated to ensure there will be no negative effects on the meter's performance. This study investigated the use of computational fluid dynamics (CFD) coupled with laboratory data to determine the effects that sudden changes in pipe diameter can have on the discharge coefficients of Venturi flowmeters. The study determined how accurately CFD can match laboratory data, then used CFD to determine the distance needed between a sudden change in diameter and a Venturi flowmeter so that the meter is no longer affected by the disturbance. The results show that CFD can be an effective tool for investigating installation effects for flowmeters.

Novel monorail infusion catheter for volumetric coronary blood flow measurement in humans: in vitro validation.

The aim of this study is to validate a novel monorail infusion catheter for thermodilution-based quantitative coronary flow measurements. Based on the principles of thermodilution, volumetric coronary flow can be determined from the flow rate of a continuous saline infusion, the temperature of saline when it enters the coronary artery, and the temperature of the blood mixed with the saline in the distal part of the coronary artery. In an in vitro set-up of the systemic and coronary circulation at body temperature, coronary flow values were varied from 50-300 ml/min in steps of 50 ml/min. At each coronary flow value, thermodilution-based measurements were performed at infusion rates of 15, 20, and 30 ml/min. Temperatures and pressures were simultaneously measured with a pressure/temperature sensor-tipped guidewire. Agreement of the calculated flow and the measured flow as well as repeatability were assessed. A total of five catheters were tested, with a total of 180 measurements. A strong correlation (ρ=0.976, p<0.0001) and a difference of -6.5±15.5 ml/min were found between measured and calculated flow. The difference between two repeated measures was 0.2%±8.0%. This novel infusion catheter used in combination with a pressure/temperature sensor-tipped guidewire allows accurate and repeatable absolute coronary flow measurements. This opens a window to a better understanding of the coronary microcirculation.

Unilateral fetal-type circle of Willis anatomy causes right–left asymmetry in cerebral blood flow with pseudo-continuous arterial spin labeling: A limitation of arterial spin labeling-based cerebral blood flow measurements?

The accuracy of cerebral blood flow measurements using pseudo-continuous arterial spin labeling can be affected by vascular factors other than cerebral blood flow, such as flow velocity and arterial transit time. We aimed to elucidate the effects of common variations in vascular anatomy of the circle of Willis on pseudo-continuous arterial spin labeling signal. In addition, we investigated whether possible differences in pseudo-continuous arterial spin labeling signal could be mediated by differences in flow velocities. Two hundred and three elderly participants underwent magnetic resonance angiography of the circle of Willis and pseudo-continuous arterial spin labeling scans. Mean pseudo-continuous arterial spin labeling-cerebral blood flow signal was calculated for the gray matter of the main cerebral flow territories. Mean cerebellar gray matter pseudo-continuous arterial spin labeling-cerebral blood flow was significantly lower in subjects having a posterior fetal circle of Willis variant with an absent P1 segment. The posterior fetal circle of Willis variants also showed a significantly higher pseudo-continuous arterial spin labeling-cerebral blood flow signal in the ipsilateral flow territory of the posterior cerebral artery. Flow velocity in the basilar artery was significantly lower in these posterior fetal circle of Willis variants. This study indicates that pseudo-continuous arterial spin labeling measurements underestimate cerebral blood flow in the posterior flow territories and cerebellum of subjects with a highly prevalent variation in circle of Willis morphology. Additionally, our data suggest that this effect is mediated by concomitant differences in flow velocity between the supplying arteries.

Characterization of flow homogeneity downstream of a slotted orifice plate in a two-phase flow using electrical resistance tomography

Two-phase flows are complex and unpredictable in nature, commonly encountered in a majority of fluid transport systems. The accurate measurement of two-phase flow is critical for a wide range of applications from wet stream to multiphase flows. There are different methods to meter two-phase flow in various industries. One approach is to produce a flow meter that does not require the individual flow components to be separated and measured separately. This goal can be met if a homogenized mixture is produced which can be measured by a standard single phase flow meter. The slotted orifice plate was invented as a flow meter for single phase flows, it is independent upon upstream flow conditions. Slotted orifice plate flow meter's utilization in two-phase flow revealed that it is highly capable of working as a flow conditioner transforming most of the multiphase flow regimes into a fairly uniform mixture. This study measures how the relative homogeneity of an air/water mixture varies downstream of the slotted plate in a horizontal pipe for various upstream conditions including elongated bubble and slug flow regimes using electrical resistance tomography (ERT). According to this study, the optimal location with a maximum homogeneity was determined to be between 1.5 and 2.5 pipe diameters downstream of the slotted orifice plate. This indicates that placing a slotted orifice plate at the obtained distance upstream of another flow meter such as a venturi coupled with a density measuring device like a radiation based densitometer or an electrical impedance device will help in obtaining accurate multiphase flow rate measurement.

Recommended Practice for Flow Control and Measurement in Electric Propulsion Testing

Accurate control and measurement of propellant flow to a thruster is one of the most basic and fundamental requirements for operation of electric propulsion systems, whether they be in the laboratory or on flight spacecraft. Hence, it is important for the electric propulsion community to have a common understanding of typical methods for flow control and measurement. This paper addresses the topic of propellant flow primarily for the gaseous propellant systems that have dominated laboratory research and flight application over the last few decades, although other types of systems are also briefly discussed. Although most flight systems have employed a type of pressure-fed flow restrictor for flow control, both thermal-based and pressure-based mass flow controllers are routinely used in laboratories. Fundamentals and theory of operation of these types of controllers are presented, along with sources of uncertainty associated with their use. Methods of calibration and recommendations for calibration processes are presented. Finally, details of uncertainty calculations are presented for some common calibration methods and for the linear fits to calibration data that are commonly used.

1614not just 2d but also 4d flow measurements in pulsatile phantom are accurate and reproducible

Introduction: 4DFlow is an emerging technique with high potential in the evaluation of congenital and acquired heart diseases. The validation of accuracy and reproducibility of flow measurements in 4DFlow is of key importance for the clinical use of this technique. Here, we compare 2D and 4DFlow using a robust pulsatile flow phantom setup with different tube configurations and heart rates. Methods:Acquisition: A closed-circuit pulsatile flow phantom was built; composed by an industrial membrane flow pump, two counter-flow parallel silicone tubes (inflow tube at isocenter and center of FoV and outflow tube out of isocenter) and a Coriolis flow meter (Endress + Hauser), to measure a ground-truth net flow measure with a precision of about ±1ml/s. Two configurations were tested: 1) with tubes parallel to the z axis of the scanner (axial), and 2) with tubes placed in a double oblique. Flow phantom scans were performed on a GE 3T MR750w MR scanner (Waukesha, WI). Retrospectively gated PC; “breathhold” and “free-breathing” (with 3 averages) 2D through-plane FastCINE and kat ARC 4D Flow were scanned [1]. Table 1 summarizes the flow phantom configuration setups and the main MRI protocol parameters. Analysis: PC net flow measurements were evaluated using CVI42 software (Calgary, Canada) for 2D and Arterys (San Francisco, CA) for 4DFlow. For both, background phase correction (BPC) using static phantom correction [2] and image-based correction [3] was applied. Statistical analyses were performed in Microsoft Excel. Intraclass Correlation Coefficients (ICC) was used to evaluate reproducibility. The Bland-Altmann plot was used to compare the obtained net flow results using MR to the ground-truth value measured by the flow meter in percentage difference as a measure of accuracy. Results: At the position of the outflow tube (>8cm from isocenter) before BPC the highest background phase error of 13% for the lower VENC was found. However, Figure 1 shows a mean percentage error of less than 8% (less than 6% for 4DFlow) with respect to the ground-truth flow for both tubes and for all configurations after BPC. Reproducibility was excellent obtaining ICC = 0.97 for 20 datasets. Discussion: 2D PC and 4DFlow across multiple imaging conditions and setup configurations for a pulsatile flow phantom setup are reliable and accurate.

Accuracy of 4D Flow Measurement of Cerebrospinal Fluid Dynamics in the Cervical Spine: An In Vitro Verification Against Numerical Simulation.

Abnormal alterations in cerebrospinal fluid (CSF) flow are thought to play an important role in pathophysiology of various craniospinal disorders such as hydrocephalus and Chiari malformation. Three directional phase contrast MRI (4D Flow) has been proposed as one method for quantification of the CSF dynamics in healthy and disease states, but prior to further implementation of this technique, its accuracy in measuring CSF velocity magnitude and distribution must be evaluated. In this study, an MR-compatible experimental platform was developed based on an anatomically detailed 3D printed model of the cervical subarachnoid space and subject specific flow boundary conditions. Accuracy of 4D Flow measurements was assessed by comparison of CSF velocities obtained within the in vitro model with the numerically predicted velocities calculated from a spatially averaged computational fluid dynamics (CFD) model based on the same geometry and flow boundary conditions. Good agreement was observed between CFD and 4D Flow in terms of spatial distribution and peak magnitude of through-plane velocities with an average difference of 7.5 and 10.6% for peak systolic and diastolic velocities, respectively. Regression analysis showed lower accuracy of 4D Flow measurement at the timeframes corresponding to low CSF flow rate and poor correlation between CFD and 4D Flow in-plane velocities.

SIMULATION OF ELECTRODE FOR DUAL-MODALITY ELECTRICAL RESISTANCE TOMOGRAPHY AND ULTRASONIC TRANSMISSION TOMOGRAPHY FOR IMAGING TWO-PHASE LIQUID AND GAS

Accurate multiphase flow measurement of gas/liquid, liquid/solid and liquid/liquid flow is still challenging for researchers in process tomography. The reconstructed images are poor particularly in the center area because of ill-posed inverse problems and limited of measurements data. Dual-modality tomography has been introduced to overcome the problem by means each modality is sensitive to specific properties of materials to be imaged. This paper proposed combination of ultrasonic transmission tomography (UTT) and electrical resistance tomography (ERT) for imaging two phase gas/liquid. In the proposed combination, detection ability in the medium of interest improved because two different images in the same space can be obtained simultaneously. This paper presents 3D numerical modeling approach using COMSOL software for ERT excitation strategy and electrode pre-designed geometry. Electrical resistance tomography (ERT) can be implemented for gas/liquid flow if the liquid is conductive. The objectives of this work is to analyze the optimum electrode dimension and shape in order to improve the situation of: (1) gas bubble detection located in the centre of the medium, (2) potential distribution and current density in a conductive medium, the developed numerical model simulated the changes in resistivity of the conductive material, with variations of electrode sizes, with opposite current excitation implemented into the region of interest. Simulation results show that the electrode size of 12 mm (w) × 40 mm (h) is suitable, which gives a good detection of center gas bubble with diameter 10mm in 100-mm-diameter acrylic vessel. Finally the findings are verified with Image reconstruction using Linear Back Projection (LBP) which gives good indication of the 10mm gas bubble.

Fluid flow measurements by means of vibration monitoring

The achievement of accurate fluid flow measurements is fundamental whenever the control and the monitoring of certain physical quantities governing an industrial process are required. In that case, non-intrusive devices are preferable, but these are often more sophisticated and expensive than those which are more common (such as nozzles, diaphrams, Coriolis flowmeters and so on). In this paper, a novel, non-intrusive, simple and inexpensive methodology is presented to measure the fluid flow rate (in a turbulent regime) whose physical principle is based on the acquisition of transversal vibrational signals induced by the fluid itself onto the pipe walls it is flowing through. Such a principle of operation would permit the use of micro-accelerometers capable of acquiring and transmitting the signals, even by means of wireless technology, to a control room for the monitoring of the process under control. A possible application (whose feasibility will be investigated by the authors in a further study) of this introduced technology is related to the employment of a net of micro-accelerometers to be installed on pipeline networks of aqueducts. This apparatus could lead to the faster and easier detection and location of possible leaks of fluid affecting the pipeline network with more affordable costs. The authors, who have previously proven the linear dependency of the acceleration harmonics amplitude on the flow rate, here discuss an experimental analysis of this functional relation with the variation in the physical properties of the pipe in terms of its diameter and constituent material, to find the eventual limits to the practical application of the measurement methodology.

Quantification of errors in cerebral blood flow measurements due to dispersion in arterial spin labelling.

The accuracy of cerebral blood flow (CBF) measurements using arterial spin labelling (ASL) is particularly affected by dispersion. In spite of this, however, the current recommended implementation of ASL - the white paper (WP) - does not account for dispersion, which leads to the introduction of errors in CBF. In fact, these errors are also likely to vary with the arterial transit time (ATT), which is the transport time from the labelling region to the tissue. Using pseudo-continuous ASL, this study assesses a variety of dispersion models in comparison with the WP quantification formula, enabling the errors introduced by the WP to be quantified. In particular, this study shows that the WP quantification only holds for ATTs below 1.25s - and that this ATT value reduces further as dispersion occurs. The levels of dispersion beyond which the WP introduces significant error are also quantified, such that provided the dispersion levels fall below the thresholds determined in this study, the WP can still measure CBF with reasonable accuracy.

Low-Cost Pressure Probe Sensor for Predicting Turbulence-Induced Vibration From Invasive Low-Velocity Turbulent Flow Measurements

Measuring high-intensity turbulence with fast response pressure probe sensors has proved to be an effective and accurate technique; unfortunately, according to the available literature, most pressure probe research focuses primarily on high-velocity (>100 m/s) applications. Commercially available pressure probe sensor systems are available; however, most designs are intended for not only high-velocity flow but also for high bandwidth ( $\sim 10$ kHz) and multidirectional applications, which drastically increase cost and complexity. Extremely limited options exist for those seeking a simple yet sensitive and accurate unidirectional turbulent flow measurement method; therefore, the intended purpose of this paper is to provide a means for future scientists and engineers to develop a pressure probe sensor tailored to their specific needs. Research presented in this paper focuses on the design and analysis of two high-sensitivity, low-cost pressure probes for invasive measurements in low-velocity (<20 m/s) highly turbulent air flow. The pressure probe design, calibration techniques, and modeling are discussed and experimentally validated.

Research of the Thermal Parameters and the Accuracy of Flow Measurement of the Biological Fuel

Research of the Thermal Parameters and the Accuracy of Flow Measurement of the Biological Fuel

Discharge coefficient of compound triangular–rectangular sharp-crested side weirs in subcritical flow conditions

Side weirs are among the important hydraulic structures for flow diversion in flood schemes or sewer networks. Side weirs are generally rectangular or triangular in shape and have a limitation for precise measurement of low and high flow. Therefore compound side weirs are suggested for accurate flow measurement in a wide range of discharges. In this study, the discharge coefficient of compound sharp-crested side weir consisting of rectangular and triangular part was studied experimentally. The results are in agree with previous study which show the discharge coefficient of a compound side weir is a function of the upstream Froude number ((Fr1), the ratio of weighted crest height of the weir to upstream water depth (w¯/y1) and the ratio of weir length to upstream water depth (L/y1). Based on the experimental data and regression modeling, a dimensionless equation has been developed for estimation of the discharge coefficient and two equations have been proposed for estimation of flow discharge in compound triangular–rectangular sharp-crested side weirs. The results also show that there is a good agreement between estimated and measured data.

Effects of Pipe Wall Offsets on Differential Pressure Meter Accuracy

Accurate flow measurement is essential for the management of any type of fluid system. This research investigated the effect on accuracy that five types of 12‐in. differential‐pressure flow meters have as a result of being installed in pipelines of differing inside diameter. The types of meters chosen for this research were the classical Venturi meter, Halmi Venturi Tube, wedge meter, V‐cone meter, and the X‐meter. Each meter was tested for accuracy with10 pipe wall offsets varying from a 0.125‐in. sudden contraction to a 0.937‐in. sudden expansion of the pipe radius. The meters' reading during each test series was compared with the meters' reading with no pipe wall offset at the same Reynolds number. This research demonstrated that for accurate flow measurement, most flow meters require that the inside diameter of the piping be the same as the inside diameter of the meter.

An analysis of Granier sap flow method, its sensitivity to heat storage and a new approach to improve its time dynamics

Granier sap flow method is a simple and easily applicable method to monitor sap flow in trees in field conditions, and is thus in wide use. However, it has been suggested that the method is slow to capture transient changes in actual sap flux density due to heat storage and release within the stem. We show here how this may lead to biases in the estimation of the dynamics of sap flux density especially at low flow rates when thermal diffusivity is low. We also demonstrate how the traditional Granier sap flow method could be modified to improve the temporal precision of the sap flow measurement. In the new system, the temperature difference between the heated and the reference needle is kept constant by varying the heating power and the sap flux density is calculated from the power consumption. This leads to reduced changes in the heat content of stem. These modifications also make the method more robust in terms of stability of power supply and reduce power consumption during low flow conditions. The time dynamics of the Granier method and the new “steady temperature method” are simulated with a previously published numerical model of xylem heat balance and tested in a laboratory experiment with cut pieces of stem. The numerical model is also used to demonstrate that the relation between parameter K, calculated from instantaneous sensor temperature and maximum sensor temperature, and actual sap flux density is not constant for either the traditional Granier or the new modified sensor, but is dependent on the range of sapflow rates examined and on the value of thermal diffusivity. Continuous measurements of thermal diffusivity of the sapwood along with the needle temperature/power consumption could help to improve the accuracy of the sap flow measurements.

Volumetric blood flow measurement using Doppler ultrasound: concerns about the technique.

Volumetric blood flow measurement using Doppler ultrasound: concerns about the technique.

Measurement of Retinal Blood Flow Using Fluorescently Labeled Red Blood Cells

Blood flow is a useful indicator of the metabolic state of the retina. However, accurate measurement of retinal blood flow is difficult to achieve in practice. Most existing optical techniques used for measuring blood flow require complex assumptions and calculations. We describe here a simple and direct method for calculating absolute blood flow in vessels of all sizes in the rat retina. The method relies on ultrafast confocal line scans to track the passage of fluorescently labeled red blood cells (fRBCs). The accuracy of the blood flow measurements was verified by (1) comparing blood flow calculated independently using either flux or velocity combined with diameter measurements, (2) measuring total retinal blood flow in arterioles and venules, (3) measuring blood flow at vessel branch points, and (4) measuring changes in blood flow in response to hyperoxic and hypercapnic challenge. Confocal line scans oriented parallel and diagonal to vessels were used to compute fRBC velocity and to examine velocity profiles across the width of vessels. We demonstrate that these methods provide accurate measures of absolute blood flow and velocity in retinal vessels of all sizes.