Mean Velocity Measurements Research Articles

Variability of 4D flow parameters when subjected to changes in MRI acquisition parameters using a realistic thoracic aortic phantom.

To assess the variability of peak flow, mean velocity, stroke volume, and wall shear stress measurements derived from 3D cine phase contrast (4D flow) sequences under different conditions of spatial and temporal resolutions. We performed controlled experiments using a thoracic aortic phantom. The phantom was connected to a pulsatile flow pump, which simulated nine physiological conditions. For each condition, 4D flow data were acquired with different spatial and temporal resolutions. The 2D cine phase contrast and 4D flow data with the highest available spatio-temporal resolution were considered as a reference for comparison purposes. When comparing 4D flow acquisitions (spatial and temporal resolution of 2.0 × 2.0 × 2.0 mm3 and 40 ms, respectively) with 2D phase-contrast flow acquisitions, the underestimation of peak flow, mean velocity, and stroke volume were 10.5, 10 and 5%, respectively. However, the calculated wall shear stress showed an underestimation larger than 70% for the former acquisition, with respect to 4D flow, with spatial and temporal resolution of 1.0 × 1.0 × 1.0 mm3 and 20 ms, respectively. Peak flow, mean velocity, and stroke volume from 4D flow data are more sensitive to changes of temporal than spatial resolution, as opposed to wall shear stress, which is more sensitive to changes in spatial resolution. Magn Reson Med 79:1882-1892, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

The role of vorticity in the near field development of sharp-edged, rectangular, wall jets

Experimental results on the near field development of a turbulent rectangular wall jet with aspect ratio 10 that issues from a sharp-edged orifice at Reh∼23,000 are presented and discussed, in comparison with results obtained in a free jet with identical initial conditions. Hot wire X-probe measurements on cross plane grids provide information on the 3D characteristics of the flow field. This work, besides presenting the main features of the jet, focuses on the effect of vorticity on the development of specific flow field characteristics. Mean vorticity components were estimated by interpolation and derivation from the mean and turbulent velocity measurements and the symmetries of the flow field were imposed by suitable averaging. Several terms of the axial vorticity equation are presented and discussed to uncover some complex flow physics, related e.g. to axis switching and the formation of a dumbbell shape of the jet outline, in the early stages of development.

An experimental study of the flow of nonspherical grains in a rotating cylinder

The effect of particle aspect ratio on the rheology of the flow of granular materials is studied experimentally in a quasi–two‐dimensional rotating cylinder, using two varieties of prolate spheroidal grains with different aspect ratios. Image analysis of high speed videos is used to obtain the flow profiles near the centre of the cylinder. The dynamic angle of repose and apparent viscosity in the medium show significant increase with increasing aspect ratio. The mean velocity, root mean square velocity and shear rate profiles are qualitatively similar for nonspherical and spherical particles, however, their magnitudes increase with increasing aspect ratio. A simple scaling is shown to predict the maximum thickness of the flowing layer for all the particles. The predictions of a model for the flow match with the measured mean velocity profiles and layer thickness. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4307–4315, 2017

Doppler Measurement of Parasternal Aortic Blood Flow Velocity

Echocardiographic Doppler is capable of continuously measuring blood velocity, and stroke volume at rest and during exercise. Doppler measurements require a near parallel angle between the flow of blood and the ultrasound signal necessitating apical cardiac views to obtain accurate absolute values. However, apical cardiac views are technically challenging during long serial recordings. Parasternal views are less challenging, but do not provide a parallel angle to the aorta. If a relationship exists between the aortic Doppler measurements from these two cardiac windows, a correction calculation can be established to estimate stroke volume from the parasternal view. PURPOSE: To determine if a relationship exists between Doppler measurements made at the aorta in the parasternal long axis and apical three chamber echocardiographic views. METHODS: Healthy college age participants underwent Doppler ultrasound measurements at the aorta in the parasternal long axis and apical three chamber view. Linear cross sectional analysis was performed to assess aortic annulus area and Teichholz stroke volume estimates in the parasternal long axis. Only participants with complete data sets are included in this analysis. Bivariate correlations and t-test were used as appropriate to analyze the data. RESULTS: Of the 18 participants studied, only 12 (66.7%; 6 males, 6 females, age 25.5±1.1yrs, height 156±14.2cm, weight 69.8±4.5kg, body fat 20.6±2%) had adequate Doppler signal strength in the parasternal long axis view for software automated signal tracing. No relationship was found between the time averaged mean blood flow velocity measurements obtained in the parasternal long axis and apical three chamber views (r=-0.216, p=0.498). Parasternal long axis Doppler based stroke volume (25.9±4.9mL/beat) measurements were lower than parasternal long axis Teichholz estimated stroke volume (57.8±4.9mL/beat) measurements (p<0.001), with no correlation between the two (r=0.402, p=0.190). CONCLUSIONS: While this data is preliminary, it suggests that aortic Doppler measurements in the parasternal long axis view suffer from low Doppler signal strength, and are inaccurate when compared to more conventional measurements.

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.

Experimental investigation on heat transfer from square jets issuing from perforated nozzles

This paper reports the results of an experimental investigation of fluid flow and heat transfer carried out with square jets issuing from perforated nozzles. This is accomplished by an impinging square jet on a uniformly heated plate of finite thickness (5 mm). The medium under consideration is air. Three different nozzle configurations are used in the study namely a single nozzle and perforated nozzles with four and nine holes, which are accommodated in the same available jet area 4.6 mm × 4.6 mm. This arrangement is akin to introducing a wire mesh at the nozzle exit plane. The effects of dimensionless jet-to-plate distance (2–9) and the mass flow rate of the jet fluid on the heat transfer rate are studied. Jet centerline mean velocity and turbulence intensity measurements are made with a hot-wire anemometer. The pressure drop across the orifice nozzle plate is measured and corresponding pumping power values are calculated. A comparison of the heat transfer performance and pumping power penalty of the three nozzle configurations is done.

Validity and reliability of a novel iPhone app for the measurement of barbell velocity and 1RM on the bench-press exercise

ABSTRACTThe purpose of this study was to analyse the validity and reliability of a novel iPhone app (named: PowerLift) for the measurement of mean velocity on the bench-press exercise. Additionally, the accuracy of the estimation of the 1-Repetition maximum (1RM) using the load–velocity relationship was tested. To do this, 10 powerlifters (Mean (SD): age = 26.5 ± 6.5 years; bench press 1RM · kg−1 = 1.34 ± 0.25) completed an incremental test on the bench-press exercise with 5 different loads (75–100% 1RM), while the mean velocity of the barbell was registered using a linear transducer (LT) and Powerlift. Results showed a very high correlation between the LT and the app (r = 0.94, SEE = 0.028 m · s−1) for the measurement of mean velocity. Bland–Altman plots (R2 = 0.011) and intraclass correlation coefficient (ICC = 0.965) revealed a very high agreement between both devices. A systematic bias by which the app registered slightly higher values than the LT (P < 0.05; mean difference (SD) between instruments = 0.008 ± 0.03 m · s−1). Finally, actual and estimated 1RM using the app were highly correlated (r = 0.98, mean difference (SD) = 5.5 ± 9.6 kg, P < 0.05). The app was found to be highly valid and reliable in comparison with a LT. These findings could have valuable practical applications for strength and conditioning coaches who wish to measure barbell velocity in the bench-press exercise.

Automated Generation of Reliable Blood Velocity Parameter Maps from Contrast-Enhanced Ultrasound Data.

Objectives The purpose of this study was the automated generation and validation of parametric blood flow velocity maps, based on contrast-enhanced ultrasound (CEUS) scans. Materials and Methods Ethical approval for animal experiments was obtained. CEUS destruction-replenishment sequences were recorded in phantoms and three different tumor xenograft mouse models. Systematic pixel binning and intensity averaging was performed to generate parameter maps of blood flow velocities with different pixel resolution. The 95% confidence interval of the mean velocity, calculated on the basis of the whole tumor segmentation, served as ground truth for the different parameter maps. Results In flow phantoms the measured mean velocity values were only weakly influenced by the pixel resolution and correlated with real velocities (r2 ≥ 0.94, p < 0.01). In tumor xenografts, however, calculated mean velocities varied significantly (p < 0.0001), depending on the parameter maps' resolution. Pixel binning was required for all in vivo measurements to obtain reliable parameter maps and its degree depended on the tumor model. Conclusion Systematic pixel binning allows the automated identification of optimal pixel resolutions for parametric maps, supporting textural analysis of CEUS data. This approach is independent from the ultrasound setup and can be implemented in the software of other (clinical) ultrasound devices.

Effects of different porous beds on turbulent characteristics in an open channel above the porous bed

An analysis of the turbulent characteristics of the flow in an open channel with horizontal porous beds using two-dimensional (2D) Particle Image Velocimetry (PIV) is presented. Hydraulic characteristics such as velocity distributions, turbulent intensities and Reynolds stress are investigated at a fine resolution using the PIV. Experiments were carried out, with four types of porous bed with the same porosity ε=0.70 and the same porous thickness (3cm): (a) porous filter, (b) rods bundle, (c) grass vegetation and (d) gravel bed. Turbulent characteristics are measured above the porous bed for the same different water heights (h΄=3, 5, 7 and 9cm) and for the same slope S=1.5‰. Main emphasis is given to the effects of relative porous thickness s΄/h (s΄=porous thickness, h=total flow height) on the flow characteristics over the porous beds. The relative thickness s΄/h varies between 0.250 and 0.500. The discharge ranges between 3.0 and 24.6 lt/s. Measurements of mean velocity and turbulence characteristics indicate the effects of the bed material used on the flow characteristics. The presence of rods bundle influences with different way the turbulent characteristics of the flow as regards the other porous beds.

Examining the Generality of τ0/TKE for Gravel and Cobble Beds with Sand Fill

AbstractTurbulence measurements over rough beds are used for a variety of purposes, including studies of habitat diversity for aquatic organisms, stream restoration efforts, and assessment of drag induced by vegetation. Turbulent kinetic energy (TKE) has been used to estimate bed shear stress by use of a proportionality constant of 0.19–0.21. The TKE method of bed shear stress estimation is less sensitive to flow misalignment relative to Reynolds stress or mean velocity measurements, but it has not been tested across a broad range of hydraulic conditions. In the work reported here, TKE was compared to bed shear stresses for gravel (D50=35 mm) and cobble (D50=156 mm) beds with varying discharges and levels of sand fill (D50=0.3 mm). The ratio of bed shear stress (τ0) with TKE was not strongly affected by mean velocity for either the gravel or cobble beds, but it was affected by bed roughness. The mean of the median values of τ0/TKE across three mean velocities was 0.24 for gravel with no sand in the bed...

The effect of Tai Chi exercises on postural stability and control in older patients with knee osteoarthritis

A few studies have examined the effect of Tai Chi on balance in elder patients with knee osteoarthritis (OA). The aim of this study was to determine the balance measures in elder patients with knee OA after Tai Chi exercises. For this purpose 14 females and 6 males with knee OA were chosen. Area and mean velocity of the center of pressure movements (CoP) were measured by force plate in standing positions (on foam and rigid surfaces). The measurements of area and mean velocity of CoP were performed before and after 60min of Tai Chi sessions (twice a week for 8 weeks). The results showed that the area of CoP in standing position on rigid surface was significantly decreased (P<0.01) after Tai Chi exercises. Furthermore, the mean velocity of CoP was significantly decreased after Tai Chi exercises on both rigid and foam surfaces (P<0.001). Our study also indicated that changes in surfaces (rigid and foam) would cause significant differences regarding the area of CoP in standing positions. However, similar findings were not found regarding the mean velocity of CoP. Considering the effects of Tai Chi on mean velocity of CoP, it might be concluded that motor control and postural stability improvements have occurred. Therefore, based on these results, Tai Chi exercises could be recommended for elder patients with knee OA as part of their rehabilitation and physical therapy protocols.

Validity and Reliability of the PUSH Wearable Device to Measure Movement Velocity During the Back Squat Exercise.

Balsalobre-Fernández, C, Kuzdub, M, Poveda-Ortiz, P, and Campo-Vecino, Jd. Validity and reliability of the PUSH wearable device to measure movement velocity during the back squat exercise. J Strength Cond Res 30(7): 1968-1974, 2016-The purpose of this study was to analyze the validity and reliability of a wearable device to measure movement velocity during the back squat exercise. To do this, 10 recreationally active healthy men (age = 23.4 ± 5.2 years; back squat 1 repetition maximum [1RM] = 83 ± 8.2 kg) performed 3 repetitions of the back squat exercise with 5 different loads ranging from 25 to 85% 1RM on a Smith Machine. Movement velocity for each of the total 150 repetitions was simultaneously recorded using the T-Force linear transducer (LT) and the PUSH wearable band. Results showed a high correlation between the LT and the wearable device mean (r = 0.85; standard error of estimate [SEE] = 0.08 m·s) and peak velocity (r = 0.91, SEE = 0.1 m·s). Moreover, there was a very high agreement between these 2 devices for the measurement of mean (intraclass correlation coefficient [ICC] = 0.907) and peak velocity (ICC = 0.944), although a systematic bias between devices was observed (PUSH peak velocity being -0.07 ± 0.1 m·s lower, p ≤ 0.05). When measuring the 3 repetitions with each load, both devices displayed almost equal reliability (Test-retest reliability: LT [r = 0.98], PUSH [r = 0.956]; ICC: LT [ICC = 0.989], PUSH [ICC = 0.981]; coefficient of variation [CV]: LT [CV = 4.2%], PUSH [CV = 5.0%]). Finally, individual load-velocity relationships measured with both the LT (R = 0.96) and the PUSH wearable device (R = 0.94) showed similar, very high coefficients of determination. In conclusion, these results support the use of an affordable wearable device to track velocity during back squat training. Wearable devices, such as the one in this study, could have valuable practical applications for strength and conditioning coaches.

Vortical structures and pressure pulsations in draft tube of a Francis-99 turbine at part load: RANS and hybrid RANS/LES analysis

Recognizing the limitations of the conventional linear-eddy-viscosity (LEVM) Reynolds-averaged Navier–Stokes (RANS) models to reproduce complex three-dimensional unsteady flows in hydraulic machinery, we performed a comparative assessment of a second-moment (Re-stress model, RSM) RANS closure and a hybrid RANS/LES method in capturing the flow and vortical structures in the draft tube of a Francis hydroturbine at off-design conditions. Considered is a case of part load (PL) at a flow rate of only 35% of the best efficiency point (BEP) characterised by multiple unsteady vortex systems. Despite some remaining uncertainties in generating the inflow conditions, both approaches reproduced reasonably well the measured mean velocity and the rms of its fluctuations, as well as the pressure spectrum with peaks detecting the precessing vortex core. In contrast to the common LEVMs, the Re-stress closure showed sufficient receptivity to intrinsic unsteadiness and reproduced well the overall flow and vortical patterns as well as the associated pressure pulsations in accord with the experiments. The hybrid RANS/LES method gave similar predictions as the RSM, but resolving a wider range of scales, which however, showed no significant effect on the dynamics of the dominant processing vortex core and the pressure pulsations.

Experimental and Numerical Investigation of the Precessing Helical Vortex in a Conical Diffuser, With Rotor–Stator Interaction

The flow unsteadiness generated in a swirl apparatus is investigated experimentally and numerically. The swirl apparatus has two parts: a swirl generator and a test section. The swirl generator which includes two blade rows, one stationary and one rotating, is designed such that the emanating flow at free runner rotational speed resembles that of a Francis hydroturbine operated at partial discharge. The test section consists of a conical diffuser similar to the draft tube cone of a Francis turbine. Several swirling flow regimes are produced, and the laser Doppler anemometry (LDA) measurements are performed along three survey axes in the test section for different runner rotational speeds (400–920 rpm), with a constant flow rate, 30 l/s. The measured mean velocity components and its fluctuating parts are used to validate the results of unsteady numerical simulations, conducted using the foam-extend-3.0 CFD code. Furthermore, phase-averaged pressure measured at two positions in the draft tube is compared with those of numerical simulations. A dynamic mesh is used together with the sliding general grid interfaces (GGIs) to mimic the effect of the rotating runner. The delayed detached-eddy simulation method, conjugated with the Spalart–Allmaras turbulence model (DDES–SA), is applied to achieve a deep insight about the ability of this advanced modeling technique and the physics of the flow. The RNG k−ε model is also used to represent state-of-the-art of industrial turbulence modeling. Both models predict the mean velocity reasonably well while DDES–SA presents more realistic flow features at the highest and lowest rotational speeds. The highest level of turbulence occurs at the highest and lowest rotational speeds which DDES–SA is able to predict well in the conical diffuser. The special shape of the blade plays more prominent role at lower rotational speeds and creates coherent structures with opposite sign of vorticity. The vortex rope is captured by both turbulence models while DDES–SA presents more realistic one at higher rotational speeds.

Large Eddy Simulation of a 100 kWth swirling oxy-coal furnace

Large Eddy Simulation (LES) has been applied to the swirling 100kWth OXYCOAL-AC test facility of Aachen University. A set of models to represent devolatilisation, volatile combustion, char combustion and radiation for oxy-coal combustion in an LES framework has been implemented and tested. A qualitative analysis of the flow behaviour and the overall coal combustion processes occurring within the furnace was made. The LES results for the flow field were compared to axial and tangential mean velocity measurements, showing good agreement, particularly in the upstream regions of the flame. The LES results were also compared to oxygen concentrations (vol.) and gas temperature. Overall good agreement was observed in the upstream central regions of the flame, whilst downstream the LES overestimated the combustion rates. It was also found that the recirculation zones are sensitive to char combustion, not just to the rate of devolatilisation as one might expect. An interesting problem occured in the prediction of the velocity profiles, for which the measurements were taken based on coal-particles, so that the outer-most stream remained invisible in the experiments (but not the LES), due to being free from particles. The results show the potential of using LES for more complex oxy-coal combustion burners and opens the way for applications to industrial furnaces.

Ion properties in a Hall current thruster operating at high voltage

Operation of a 5 kW-class Hall current Thruster for various voltages from 400 V to 800 V and a xenon mass flow rate of 6 mg s−1 have been studied with a quasi-neutral hybrid model. In this model, anomalous electron transport is fitted from ion mean velocity measurements, and energy losses due to electron–wall interactions are used as a tuned parameter to match expected electron temperature strength for same class of thruster. Doubly charged ions production has been taken into account and detailed collisions between heavy species included. As the electron temperature increases, the main channel of Xe2+ ion production becomes stepwise ionization of Xe+ ions. For an applied voltage of 800 V, the mass utilization efficiency is in the range of 0.8–1.1, and the current fraction of doubly charged ions varies between 0.1 and 0.2. Results show that the region of ion production of each species is located at the same place inside the thruster channel. Because collision processes mean free path is larger than the acceleration region, each type of ions experiences same potential drop, and ion energy distributions of singly and doubly charged are very similar.

Experimental and numerical investigation of coherent structure dynamics on mass transfer in a separated cavity flow

This study presents the experimental and numerical investigation of coherent structure dynamics on mass transfer in a separated cavity flow. The flow field dynamics of a cavity type stagnation zone with a length to width ratio of 2 were studied. The cavity was driven by a channel flow with a Reynolds number of 1.8E5. The study utilised flow visualisation, laser Doppler velocimetry (LDV) and electrical conductivity probe measurements. Measurements of mean velocity and turbulence intensity profiles across the separated flow field showed the development of the shear layer and the recirculating flow pattern in the cavity. The numerical simulations were also performed for comparison with the experimental results considering the solution of the RANS, LES model and two-layer turbulence model. LES produced the formation and dynamics of coherent structures within the separated shear layer. Flow visualisation revealed a pulsatile motion in the separated flow region associated with the formation and passage of coherent structures and was supported by measurements of tracer concentration. The study showed how coherent structures in the separated shear layer enhanced the mass transport process from the stagnation zone and how the recirculating flow in the cavity influenced the development of those structures.

Extracranial internal carotid artery stenosis in children with sickle cell disease - Which transducer, what measurement?

Transcranial Doppler ultrasound is used to screen and assess the intracranial arteries of children with sickle cell disease. Recent findings suggest that extracranial internal carotid artery (eICA) stenosis is also a contributing factor to silent cerebral infarction. Stenosis has been measured using phased array transducers with no beam/flow angle correction and linear arrays with angle correction. A total of 124 children undergoing TCD assessment were investigated for eICA velocities. Manual measurements of peak systolic velocity and TCD mean velocity were made with phased and linear array transducers. Peak systolic velocities ranged from 60 to 534 cm/s (median 126 cm/s) using the linear array and 53 to 394 cm/s (median 115 cm/s) using the phased array transducers. TCD mean ranged from 39 to 419 cm/s (median 81 cm/s) using the linear array and 34 to 295 cm/s (median 72 cm/s) using the phased array transducers. There are advantages and disadvantages of each method, but stenoses were readily identified as focal velocity increases. We suggest thresholds for each transducer and recommend that imaging of the eICA forms part of screening for this group of children.

Transient tracer distributions in the Fram Strait in 2012 and inferred anthropogenic carbon content and transport

Abstract. The storage of anthropogenic carbon in the ocean's interior is an important process which modulates the increasing carbon dioxide concentrations in the atmosphere. The polar regions are expected to be net sinks for anthropogenic carbon. Transport estimates of dissolved inorganic carbon and the anthropogenic offset can thus provide information about the magnitude of the corresponding storage processes. Here we present a transient tracer, dissolved inorganic carbon (DIC) and total alkalinity (TA) data set along 78°50′ N sampled in the Fram Strait in 2012. A theory on tracer relationships is introduced, which allows for an application of the inverse-Gaussian–transit-time distribution (IG-TTD) at high latitudes and the estimation of anthropogenic carbon concentrations. Mean current velocity measurements along the same section from 2002–2010 were used to estimate the net flux of DIC and anthropogenic carbon by the boundary currents above 840 m through the Fram Strait. The new theory explains the differences between the theoretical (IG-TTD-based) tracer age relationship and the specific tracer age relationship of the field data, by saturation effects during water mass formation and/or the deliberate release experiment of SF6 in the Greenland Sea in 1996, rather than by different mixing or ventilation processes. Based on this assumption, a maximum SF6 excess of 0.5–0.8 fmol kg−1 was determined in the Fram Strait at intermediate depths (500–1600 m). The anthropogenic carbon concentrations are 50–55 µmol kg−1 in the Atlantic Water/Recirculating Atlantic Water, 40–45 µmol kg−1 in the Polar Surface Water/warm Polar Surface Water and between 10 and 35 µmol kg−1 in the deeper water layers, with lowest concentrations in the bottom layer. The net fluxes through the Fram Strait indicate a net outflow of ∼ 0.4 DIC and ∼ 0.01 PgC yr−1 anthropogenic carbon from the Arctic Ocean into the North Atlantic, albeit with high uncertainties.

Effects of Reynolds Number on the Energy Conversion and Near-Wake Dynamics of a High Solidity Vertical-Axis Cross-Flow Turbine

Experiments were performed with a large laboratory-scale high solidity cross-flow turbine to investigate Reynolds number effects on performance and wake characteristics and to establish scale thresholds for physical and numerical modeling of individual devices and arrays. It was demonstrated that the performance of the cross-flow turbine becomes essentially R e -independent at a Reynolds number based on the rotor diameter R eD ≈ 106 or an approximate average Reynolds number based on the blade chord length R ec ≈ 2 × 105 . A simple model that calculates the peak torque coefficient from static foil data and cross-flow turbine kinematics was shown to be a reasonable predictor for Reynolds number dependence of an actual cross-flow turbine operating under dynamic conditions. Mean velocity and turbulence measurements in the near-wake showed subtle differences over the range of R e investigated. However, when transport terms for the streamwise momentum and mean kinetic energy were calculated, a similar R e threshold was revealed. These results imply that physical model studies of cross-flow turbines should achieve R eD ∼ 106 to properly approximate both the performance and wake dynamics of full-scale devices and arrays.