Skewed Velocity Profiles Research Articles

Superiority of a Representative MRI Flow Waveform over Doppler Ultrasound for Aortic Wave Reflection Assessment in Children and Adolescents With/Without a History of Heart Disease

Wave separation analysis (WSA) reveals the impact of forward- and backward-running waves on the arterial pressure pulse, but the calculations require a flow waveform. This study investigated (1) the variability of the ascending aortic flow waveform in children and adolescents with/without a childhood heart disease history (CHD); (2) the accuracy of WSA obtained with a representative flow waveform (RepFlow), compared with the triangulation method and published ultrasound-derived adult representative flow; (3) the impact of limitations in Doppler ultrasound on WSA; and (4) generalizability of results to adults with a history of CHD. Phase contrast MRI was performed in youth without (n = 45, Group 1, 10–19 years) and with CHD (n = 79, Group 2, 7–18 years), and adults with CHD history (n = 29, Group 3, 19–59 years). Segmented aortic cross-sectional area was used as a surrogate for the central pressure waveform in WSA. A subject-specific virtual Doppler ultrasound was performed on MRI data by extracting velocities from a sample volume. Time/amplitude-normalized ascending aortic flow waveforms were highly consistent amongst all groups. WSA with RepFlow therefore yielded errors < 10% in all groups for reflected wave magnitude and return time. Absolute errors were typically 1.5–3 times greater with other methods, including subject-specific (best-case/virtual) Doppler ultrasound, for which velocity profile skewing introduced waveform errors. Our data suggest that RepFlow is the optimal approach for pressure-only WSA in children and adolescents with/without CHD, as well as adults with CHD history, and may even be more accurate than subject-specific Doppler ultrasound in the ascending aorta.

An Electrohydrodynamic analysis for the characterisation of fluid flow within conducting and non-conducting channels

This paper presents a generalized non-dimensional theoretical formulation with boundary conditions to analyze the EHD flow within the conducting and non-conducting channels. The substantial effect of normal and tangential components of electric field on fluid flow within the channel is presented through simulation results based on the non-dimensional parameters of strength, penetration, and repulsion of electric forces. The simulation results reveal that the normal component of induced electric field transits the flow regime within surfaces of a channel. The tangential component of applied electric field increases the thickness of viscous boundary layer, leading to skewed velocity profiles.

Effects of inflow boundary layer on the wake of a radially non-uniform porous disk

This study presents the results of an experimental investigation focusing on the effects of the inflow boundary layer on the wake characteristics of a 0.12 m diameter porous disk with radially non-uniform porosity in terms of mean flow, turbulence, and wake scaling. Two-dimensional two-component particle image velocimetry measurements within the wake are performed up to 7.5 diameters downstream as the disk is lowered deeper into a boundary layer that is representative of a neutral atmospheric boundary layer over a flat terrain. Results show that otherwise symmetrical wake velocity profiles that exist outside the boundary layer get skewed and sheared around the disk centerline in the boundary layer due to the inflow wind shear. The turbulent kinetic energy, its production, and Reynolds shear stress levels in the wake get asymmetrical around the centerline of the disk such that the production of turbulent kinetic energy is observed to be higher above centerline. Due to the inflow shear, the wake centerline gets shifted downwards (i.e., toward the wind tunnel wall), which is in contrast to the observations on real wind turbine wakes in the literature where the wake actually lifts up. The asymmetrical and skewed velocity profiles both in the streamwise and cross-stream directions can be collapsed onto a single function by using proper wake scaling parameters based on the ratio of local strain to average strain within the velocity profile calculated separately for either side of the wake.

Inconsistency in aortic stenosis severity between CT and echocardiography: prevalence and insights into mechanistic differences using computational fluid dynamics

ObjectivesThe aims of this study were to evaluate the inconsistency of aortic stenosis (AS) severity between CT aortic valve area (CT-AVA) and echocardiographic Doppler parameters, and to investigate potential underlying...

Hydrodynamics of a weakly curved channel

The underlying mechanism of curvature induced helicoidal flow in a weakly curved channel intrigues researchers. Here, we explore the hydrodynamics of weakly curved channels, defined by the limiting values of the curvature ratio (ratio of channel half-width to radius of curvature) and aspect ratio (ratio of channel half-width to average flow depth) as 0.1 and 10, respectively. The three-dimensional continuity and momentum equations are solved analytically, involving the appropriate boundary conditions and closing the system by means of the turbulence closure model and the indispensable fluid constitutive formulations. The skewed filament of the azimuthal velocity component, emanating from the effects of curvilinear streamlines, is introduced into the analysis, for the first time, to address the flow asymmetry across the flow cross section. The modification of the radial slope due to the presence of the stress term in the radial momentum balance is accomplished by a slope correction factor, which turns out to be a weak function of the reciprocal of the power-law exponent. The attenuation of the azimuthal shear stress component, resulting from the skewed velocity profile, is characterized by the damping function to provide a quantitative insight into the redistribution of the primary flow momentum. The velocity field reveals that the flow circulation (on the flow cross-sectional plane) about the azimuthal axis and the flow helicity strengthen with an increase in the curvature ratio. The variation of the radial free surface profile is more sensitive to the flow Froude number than to the curvature ratio. The evolutions of the stress field with several key parameters are also examined.

Afferent motor feedback determines the perceived location of tactile stimuli in the external space presented to the moving arm.

People make systematic errors when localizing a brief tactile stimulus in the external space presented on the index finger while moving the arm. Although these errors likely arise in the spatiotemporal integration of the tactile input and information about arm position, the underlying arm position information used in this process is not known. In this study, we tested the contributions of afferent proprioceptive feedback and predictive arm position signals by comparing localization errors during passive vs. active arm movements. In the active trials, participants were instructed to localize a tactile stimulus in the external space that was presented to the index finger near the time of a self-generated arm movement. In the passive trials, each of the active trials was passively replayed in randomized order, using a robotic device. Our results provide evidence that the localization error patterns of the passive trials are similar to the active trials and, moreover, did not lag but rather led the active trials, which suggests that proprioceptive feedback makes an important contribution to tactile localization. To further test which kinematic property of this afferent feedback signal drives the underlying computations, we examined the localization errors with movements that had differently skewed velocity profiles but overall the same displacement. This revealed a difference in the localization patterns, which we explain by a probabilistic model in which temporal uncertainty about the stimulus is converted into a spatial likelihood, depending on the actual velocity of the arm rather than involving an efferent, preprogrammed movement.NEW & NOTEWORTHY We show that proprioceptive feedback of arm motion rather than efferent motor signals contributes to tactile localization during an arm movement. Data further show that localization errors depend on arm velocity, not displacement per se, suggesting that instantaneous velocity feedback plays a role in the underlying computations. Model simulation using Bayesian inference suggests that these errors depend not only on spatial but also on temporal uncertainties of sensory and motor signals.

Computationally Efficient Fluid-Particle Dynamics Simulations of Arterial Systems

AbstractRealistic and accurate computer simulations of the particle-hemodynamics in arterial systems can be a valuable tool for numerous biomedical applications. Examples include optimal by-pass grafting and optimal drug-delivery, as well as best medical management concerning the cardio-vascular system. However, such numerical analyses require large computer resources which may become prohibitive for extended sets of arterial bifurcations. A remedy is to develop a hybrid model where the first few generations of the bifurcating arteries of interest are simulated in full 3-D, while a 1-D model is then coupled for subsequent bifurcations. Alternatively, a 1-D computer model can be directly employed to simulate fluid-particle transport in complex bifurcating networks.Relying on a representative axial velocity profile, a physiological 1-D model has been developed and validated, which is capable of predicting with reasonable accuracy arterial flow, pressure field and elastic wall interaction as well as particle transport. The usefulness of the novel 1-D simulation approach is demonstrated via a comparison to 3-D blood flow and microsphere transport in a hepatic artery system, featuring as outlets one major branch and four small daughter vessels. Compared to the 3-D simulation, the 1-D analysis requires only about 1% of computational time. The hybrid modeling approach would be also applicable to the human respiratory tract to evaluate the fate of inhaled aerosols.A simple and cost-effective way to simulate particle-hemodynamics is using a 1-D model for simulating arterial pressures and flow rates as well as microsphere transport, based on assumptions involving the use of a simple algebraic pressure-area relation, an exponential elasticity model for the vessels, and considering only unidirectional flow with a representative skewed velocity profile. In summary, the novel contributions are:• Particle tracking in arteries via 1-D fluid modeling and selection of an averaged, skewed velocity profile based on 3-D simulation results to provide more realistic friction and inertia term values for modeling a flow system with bifurcations.• The 1-D model can be coupled to a 3-D model so that simulations can be run for larger regions of vascular or lung-airway systems.

Effect of Velocity Profile Skewing on Blood Velocity and Volume Flow Waveforms Derived From Maximum Doppler Spectral Velocity

Given evidence that fully developed axisymmetric flow may be the exception rather than the rule, even in nominally straight arteries, maximum velocity (Vmax) can lie outside the Doppler sample volume (SV). The link between Vmax and derived quantities, such as volume flow (Q), may therefore be more complex than commonly thought. We performed idealized virtual Doppler ultrasound on data from image-based computational fluid dynamics (CFD) models of the normal human carotid artery and investigated how velocity profile skewing and choice of sample volume affected Vmax waveforms and derived Q variables, considering common assumptions about velocity profile shape (i.e., Poiseuille or Womersley). Severe velocity profile skewing caused substantial errors in Vmax waveforms when using a small, centered SV, although peak Vmax was reliably detected; errors with a long SV covering the vessel diameter were orientation dependent but lower overall. Cycle-averaged Q calculated from Vmax was typically within ±15%, although substantial skewing and use of a small SV caused 10%–25% underestimation. Peak Q derived from Womersley's theory was generally accurate to within ±10%. Vmax pulsatility and resistance indexes differed from Q-based values, although the Q-based resistance index could be predicted reliably. Skewing introduced significant error into Vmax-derived Q waveforms, particularly during mid-to-late systole. Our findings suggest that errors in the Vmax and Q waveforms related to velocity profile skewing and use of a small SV, or orientation-dependent errors for a long SV, could limit their use in wave analysis or for constructing characteristic or patient-specific flow boundary conditions for model studies.

Errors in the estimation of wall shear stress by maximum Doppler velocity

ObjectiveWall shear stress (WSS) is an important parameter with links to vascular (dys)function. Difficult to measure directly, WSS is often inferred from maximum spectral Doppler velocity (Vmax) by assuming fully-developed flow, which is valid only if the vessel is long and straight. Motivated by evidence that even slight/local curvatures in the nominally straight common carotid artery (CCA) prevent flow from fully developing, we investigated the effects of velocity profile skewing on Vmax-derived WSS. MethodsVelocity profiles, representing different degrees of skewing, were extracted from the CCA of image-based computational fluid dynamics (CFD) simulations carried out as part of the VALIDATE study. Maximum velocities were calculated from idealised sample volumes and used to estimate WSS via fully-developed (Poiseuille or Womersley) velocity profiles, for comparison with the actual (i.e. CFD-derived) WSS. ResultsFor cycle-averaged WSS, mild velocity profile skewing caused ±25% errors by assuming Poiseuille or Womersley profiles, while severe skewing caused a median error of 30% (maximum 55%). Peak systolic WSS was underestimated by ∼50% irrespective of skewing with Poiseuille; using a Womersley profile removed this bias, but ±30% errors remained. Errors were greatest in late systole, when skewing was most pronounced. Skewing also introduced large circumferential WSS variations: ±60%, and up to ±100%, of the circumferentially averaged value. ConclusionVmax-derived WSS may be prone to substantial variable errors related to velocity profile skewing, and cannot detect possibly large circumferential WSS variations. Caution should be exercised when making assumptions about velocity profile shape to calculate WSS, even in vessels usually considered long and straight.

A computational fluid dynamics study on geometrical influence of the aorta on haemodynamics

Cardiovascular diseases, such as atherosclerosis and aneurysm, are closely associated with haemodynamic factors that are governed by luminal geometry. The present work aimed to study the effect of geometrical variation of aging aortas on haemodynamics. Six aged subjects with intricate geometrical features, such as bulging or twisted supra-aortic arteries, sharply curved arch and double-curved descending aorta, were chosen from our medical database. These six geometrically variant aortas were reconstructed and the pulsatile nature of the blood flow of these subject-specific aorta models investigated using computational fluid dynamics simulations. Realistic time-dependent boundary conditions are prescribed for various arteries of the aorta models. This study suggests that haemodynamics in the human aorta is highly dependent on geometrical features. The positioning and contouring of the supra-aortic arteries may be associated with the skewness of velocity profiles. The flow profiles in the aortic arch or bends are generally skewed towards the inner curvature wall and this skewness may give rise to the formation of secondary flow in the inner curvature wall of the distal arch. The degree of vorticity in the distal aortic arch is found to be related to the arch curvature. The helical nature of aortic haemodynamics is predominant in the systole phrase when it begins with a left-handed rotation and then vanishes in the ascending aorta, whereas a right-handed rotation persists in the distal aortic arch. Lower wall shear stress is also found in the ascending regions where secondary flow is present. The aorta with an irregular contour and large degree of curvature at its arch favours the development of the intra-aortic secondary flow that subsequently relates to the pathogenesis of atheroma. The present study identifies the general trend of haemodynamic behaviours associated with various local geometrical features. Combining the knowledge of the correlation between haemodynamics and the underlying risks in the development of cardiovascular diseases, our study hopes to provide a better understanding of the relationship between aortic morphology and developing pathobiology of cardiovascular diseases. As such, early medical planning as well as surgical interventions can be designed to retard or prevent the development of cardiovascular diseases.

On the shape of the common carotid artery with implications for blood velocity profiles

Clinical and engineering studies typically assume that the common carotid artery (CCA) is straight enough to assume fully developed flow, yet recent studies have demonstrated the presence of skewed velocity profiles. Toward elucidating the influence of mild vascular curvatures on blood flow patterns and atherosclerosis, this study aimed to characterize the three-dimensional shape of the human CCA. The left and right carotid arteries of 28 participants (63 ± 12 years) in the VALIDATE (Vascular Aging-–The Link that Bridges Age to Atherosclerosis) study were digitally segmented from 3D contrast-enhanced magnetic resonance angiograms, from the aortic arch to the carotid bifurcation. Each CCA was divided into nominal cervical and thoracic segments, for which curvatures were estimated by least-squares fitting of the respective centerlines to planar arcs. The cervical CCA had a mean radius of curvature of 127 mm, corresponding to a mean lumen:curvature radius ratio of 1:50. The thoracic CCA was significantly more curved at 1:16, with the plane of curvature tilted by a mean angle of 25° and rotated close to 90° with respect to that of the cervical CCA. The left CCA was significantly longer and slightly more curved than the right CCA, and there was a weak but significant increase in CCA curvature with age. Computational fluid dynamic simulations carried out for idealized CCA geometries derived from these and other measured geometric parameters demonstrated that mild cervical curvature is sufficient to prevent flow from fully-developing to axisymmetry, independent of the degree of thoracic curvature. These findings reinforce the idea that fully developed flow may be the exception rather than the rule for the CCA, and perhaps other nominally long and straight vessels.

Influence from bends on a pressure-time measurement

One of the criteria for using the pressure-time method is that there is a straight pipe between the measurement cross sections. If the physical influences from a bend were known the method could be expanded for use over bends and hence on more power plants. At the Norwegian University of Science and Technology (NTNU) a test rig suited for experiments on the pressure-time method has been built. In addition to earlier experiments performed on straight pipe and double 90 degree bend, experiments on double 45 degree bend has been performed. This is done in order to try to find out if there is a clear relationship between bend geometry and its influence on the pressure time curve. To further develop the knowledge a full scale test in the field has been conducted on a 5 MW 58 meter head machine with a vertical penstock and a 90 degree bend. In the laboratory a double 90 degree bend caused an underestimation of the flow of about 1 % compared to a straight pipe, while for a double 45 degree bend a conclusion can not be found as the spread around the mean and uncertainty bands is within the range for the straight pipe. The field measurements with a 90 degree bend showed that the underestimation became 8.5%. When measuring pressure difference over bends the resulting calculation will give an underestimation of the flow. This should imply that the measured transient pressure is damped. (Regardless if this is the correct term it is here chosen to call this damping.) The question is if the damping is caused by friction or kinetic effects. The rapid deceleration of the water masses will give transient friction. How transient friction behaves is a topic of interest in hydraulic research. Bends will also cause skewed velocity and pressure profile on inlet and outlet of the bend as well as secondary flows. How this affects the transient pressure is also an interesting parameter.

Airway Wall Stiffening Increases Peak Wall Shear Stress: A Fluid–Structure Interaction Study in Rigid and Compliant Airways

The airflow characteristics in a computed tomography (CT) based human airway bifurcation model with rigid and compliant walls are investigated numerically. An in-house three-dimensional (3D) fluid-structure interaction (FSI) method is applied to simulate the flow at different Reynolds numbers and airway wall stiffness. As the Reynolds number increases, the airway wall deformation increases and the secondary flow becomes more prominent. It is found that the peak wall shear stress on the rigid airway wall can be five times stronger than that on the compliant airway wall. When adding tethering forces to the model, we find that these forces, which produce larger airway deformation than without tethering, lead to more skewed velocity profiles in the lower branches and further reduced wall shear stresses via a larger airway lumen. This implies that pathologic changes in the lung such as fibrosis or remodeling of the airway wall-both of which can serve to restrain airway wall motion-have the potential to increase wall shear stress and thus can form a positive feed-back loop for the development of altered flow profiles and airway remodeling. These observations are particularly interesting as we try to understand flow and structural changes seen in, for instance, asthma, emphysema, cystic fibrosis, and interstitial lung disease.

Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) Modelling of Carotid Artery Haemodynamics under Steady Flow: A Validation Study

Particle image velocimetry (PIV) and computational fluid dynamics (CFD) modelling of blood flow through a carotid artery bifurcation has been carried out in order to assess the role of haemodynamics in atherosclerosis and validate a novel wall shear stress (WSS) measurement method via detailed quantitative data analysis. Velocity and WSS data, obtained by PIV was compared against CFD-predicted data obtained for the same geometry and boundary conditions. The results demonstrate that both methods are capable of capturing essential flow characteristics, and are in good agreement. The axial velocity data clearly show the important flow features such as skewed velocity profiles and a low-momentum region near the sinus outer wall, although with slight errors between the two techniques in the order of 6.1-13.7%. The secondary velocity plots and three-dimensional particle traces reveal complex flow features within the sinus, such as Dean vortices and helicoidal flow. WSS data obtained by both techniques reveal the presence of a low-momentum and reversed-flow regions, and show agreement with errors in the order of 1% in the common carotid artery and 0.9-9.8% in the sinus. The error sources in both the experimental and numerical methodology are discussed and recommendations for future applications are made.

Noninvasive determination of spatial distribution and temporal gradient of wall shear stress at common carotid artery

Wall shear stress (WSS) has been proved to play a critical role in formation and development of atherosclerotic plaques. Our objective was to quantify local WSS in vivo in normal subjects, and to analyze spatial distribution patterns and determine the temporal gradient of WSS. Seventy-eight CCAs of 42 healthy volunteers at common carotid arteries (CCAs) were studied. Cine phase-contrast MR sequence was used to acquire the flow velocity information. Three-dimensional paraboloid modeling was applied to fit the velocity profiles and WSS values were calculated. Mean WSS value for CCAs was 0.783±0.209, with the range of WSS value from −0.541 to 3.464 N/m 2. The 95% confidence interval for mean WSS value in CCA was (0.736–0.830) N/m 2. Different WSS spatial distribution patterns were classified into three types according to the location of low WSS values during a cardiac cycle. Mean value of maximum temporal gradient of WSS was 14.12±5.46, with the range from 5.87 to 33.23 N/m 2 s −1. Skewed velocity profiles were displayed in most CCAs, indicating the flow patterns in CCA were more complicated than commonly assumed. Obvious inter-subject variation were found in magnitude, spatial distribution and the temporal gradient of WSS in CCAs, and the blood flow patterns as well.

NUMERICAL CHARACTERIZATION OF THE FLOW FIELD IN A FOUR-GENERATION AIRWAY

In this paper, various aspects of respiratory airflow generated from the branching network of tubes that make up the tracheal-bronchial tree are numerically analyzed using the computational fluid dynamics (CFD) package CFX. The model used is a four-generation airway that is geometrically similar to Weibel's symmetrical model. In the present analysis, two different models (in-plane and off-plane) are examined for a wide range of Reynolds numbers that correspond to human breathing conditions. The findings indicate that the secondary flow patterns generated become more significant as the flow passes from the trachea to the fourth-generation airway. Moreover, comparison between in-plane and off-plane models shows that the skewed velocity profiles and secondary flows for the in-plane model are more prominent than those for the off-plane one. In general, the model developed in this study is capable of providing an overall insight into the effect of fluid flow in multiple generations of the human upper respiratory airways.

Fluid flow and particle deposition analysis in a realistic extrathoracic airway model using unstructured grids

An efficient Lagrangian particle tracking module is developed for unstructured grids to analyze the deposition patterns in a CT based realistic extrathoracic airway model. Inhalation flow rates of 15, 30 and 60 l/min are considered with particle diameters ranging between 2 and 20 μ m . The complex flow patterns with skewed velocity profiles and flow separations are discussed. While most idealized mouth–throat models predict no transition for a flow rate of 15 l/min, we observe increase in kinetic energy levels soon after glottis, indicating the sensitivity to geometrical complexity at low flow rates. Mouth cavity acts as an effective filter and is responsible for a major percentage of total deposition for all three flow rates. Combining our model depositions with central bronchial tree depositions indicates that for normal breathing conditions, inspiratory deposition for the particle size 5 μ m , which is usually referred to as the upper limit of the respirable range for inhalation drugs, does not exceed 12%.

Particle deposition in obstructed airways

One approach to tackle the particle deposition in human lungs in close proximity is to develop an understanding of the particle motion in bifurcation airways. Chronic obstructive pulmonary disease (COPD) is one of the most common diseases in humans. COPD always results in inflammation that leads to narrowing and obstructing of the airways. The obstructive airways can alter the respiratory flow and particle deposition significantly. In order to study the effect of obstruction on particle deposition, four three-dimensional four-generation lung models based on the 23-generation model of Weibel [1963. Morphometry of the Human Lung. New York Academic Press, Springer, Berlin.] have been generated. The fully three-dimensional incompressible laminar Navier–Stokes equations are solved using computational fluid dynamics (CFD) solver on structured hexahedral meshes. Subsequently, a symmetric four-generation airway model serves as the reference and the other three models are considered to be obstructed at each generation, respectively. The calculation results show that the obstructive airway has significant influence on the particle deposition down-stream of the obstruction. The skewed velocity profile in the bifurcation airway is modified by the throat; consequently, more particles impact on the divider which results in higher deposition efficiency.

Evaluating Large Saccades in Patients with Brain‐Stem or Cerebellar Disorders

Clinicians conventionally test saccades at the bedside by noting the accuracy, initiation time, and speed of large movements, with the patient's head stationary. Partly for methodological reasons, laboratory analysis of saccades has mainly focused on movements of 20 degrees or less. By measuring the velocity waveform of large saccades, it is possible to examine more closely the way in which brain stem and cerebellum guide the eye to the target. Large saccades made by healthy humans show a positively skewed velocity profile. Slow saccades made by patients with brain-stem disorders show a prolonged plateau of low velocity. Some patients with cerebellar disorders may show increased acceleration and deceleration of saccades. Each of these velocity waveforms can be modeled by changing the parameters that describe medium-lead burst neuron firing. In certain other brain-stem and cerebellar disorders, transient decelerations or premature terminations of saccades occur; such velocity waveforms cannot be modeled solely by changing the parameters that describe burst neuron firing. Instead, it is necessary to postulate dysfunction of the mechanism that normally inhibits pontine omnipause neurons, thereby permitting burst neurons to discharge until the saccade is completed. Analysis of large, abnormal saccades calls for application of novel techniques to identify the beginning and end of the saccadic pulse command.

Skewed velocity profile effect on turbine flowmeter performance

Turbine flowmeters (TFMs) are mechanical flow metering devices and are being used extensively in various industries to measure the flowrate of a wide variety of fluids. The TFMs are known for their excellent repeatability, higher accuracy, wide operating range, high shock capability, low pressure loss and wide temperature and pressure limits. The performance characteristics of TFMs depend greatly on the upstream flow condition, the fluid properties and the geometrical parameters of the rotor. Hence, a systematic study has been carried out to establish the effect of a skewed velocity profile on the lower limit of the linear range, usable range, meter factor and pressure drop across the meter. Alteration of the velocity profile has been achieved by providing a gate valve about four pipe diameters upstream of the meter. From the result, it can be seen that the average meter factor does not change for a slightly skewed velocity profile but increases drastically for a highly skewed velocity profile. To overcome the skewed velocity profile effect, three types of flow straightener have been provided upstream of the rotor. It is found that a ten-bladed flat plate straightener is most effective in killing the skewness in the upstream velocity profile and keeping the variation in meter factor within ±1.25 per cent of the standard values.