Average Cross-sectional Velocity Research Articles

Development and validation of non-invasive endothelial shear stress (ESS) estimations of coronary artery haemodynamics using computational modelling

Abstract Background Vascular remodelling is influenced by various biomechanical forces such as endothelial shear stress (ESS) and is associated with development and progression of coronary artery disease (CAD). ESS is the parallel frictional force exerted by blood flow on the endothelial luminal surface of the arterial wall. While ESS may not be the only cause of atherosclerosis, it is postulated that low ESS creates a pro-atherogenic environment and high ESS is athero-protective. Computational fluid dynamics (CFD) is an engineering method used to analyse fluid flow and has been increasingly applied to simulate ESS in cardiovascular research. However, current CFD-derived ESS estimates have never been validated and can display significant variability between research groups. Purpose This study aims to provide a comparative analysis of coronary flow using in-silico modelling of coronary flow and particle image velocimetry (PIV). Methods A proximal left anterior descending (pLAD) coronary phantom model was constructed from a patient who had received same day cardiac computed tomography angiography (CTCA) and invasive coronary haemodynamic (Combowire XT) assessment. Patient-specific coronary flow data was applied to CFD boundary conditions and a mock circulatory loop. CFD was validated using PIV under steady flow in a 4-times-scaled model. Same plane velocity field and flow patterns (at mid-luminal of pLAD) from both in-silico and in-vivo data were compared and analysed. Results Mean velocity contours and magnitude were analysed from CFD and PIV. Patient specific average peak velocity obtained from invasive assessment was 0.21m/s. Patient-specific velocity average from CFD was 0.22m/s. The approximated magnitude difference is 4.7%. Both same-axial average cross-sectional velocity estimated by CFD-pLAD and PIV-pLAD are 0.087m/s. Velocity contours and flow patterns simulated in the CFD were also comparable with the PIV results. Conclusion Simulation data from CFD correlate well with experimental PIV results. This early data allows for a non-invasive approach to determine patient-specific coronary haemodynamics (such as velocity profiling and ESS) and form the basis of personalising cardiovascular risk. Insights from this work may aid future development of a novel tool that incorporates CFD-derived ESS estimates with patient-specific risk factors to better predict rapid CAD progression. Funding Acknowledgement Type of funding sources: Public Institution(s). Main funding source(s): Cardiac Society of Australia and New Zealand Monash Institute of Medical Engineering

Study of the anisotropic permeability of proton exchange membrane fuel cell gas diffusion layer by lattice Boltzmann method

In proton exchange membrane fuel cells (PEMFCs), the gas diffusion layers (GDLs) are located between the flow channel and the catalytic layer, which not only provides a transport channel for the reaction gas and the output water, but also its fibrous skeleton is the key structure for the electron load transfer, and its transport performance is an important factor that affects the performance of the cell. In this paper, GDLs are stochastically reconstructed and the Lattice Boltzmann method (LBM) is adopted to study permeability. The changes of through-plane permeability and in-plane permeability are studied by varying the porosity, fiber diameter, and pressure drop. It is found that the porosity plays a dominant role in the variation of permeability, and the through-plane permeability at a porosity of 0.85 is 24.78 times greater than that at 0.5. The influence of fiber diameter can not be ignored. When the diameter is larger than 8 μm, the influence on permeability is more obvious, and the permeability is no longer sensitive to changes in diameter after a certain degree of porosity has been reached. The pressure drop only changes the magnitude of the average cross-sectional velocity of the gas diffusion layer, and the effect on the permeability is negligible. The in-plane permeability is larger than the through-plane permeability. In the scope of the paper research, the ratio of the two values ranges from 1.3 to 1.7.

Formation mechanism for emitter composite-clogging in drip irrigation system

Revealing the mechanism and influential path of emitter clogging in drip irrigation is the basis to control this issue. Eight commonly used flat emitters were tested to analyze the effects of emitter flow path geometric parameters on the hydraulic indexes, clogging performance, and composition of clogging substances, and the influential path was established through the path analysis method. The results indicated that the average cross-sectional velocity (v) was significantly correlated to clogging degrees and clogging substance formation. v directly affected the formation of clogging substance components, and was influenced by the width–depth ratio (W/D) and the relative radius (A1/2/L). Among all the components of the clogging substances, solid particle (SP), and CaCO3 and MgCO3 precipitates (C–MP) directly affected emitter clogging process, while phospholipid fatty acids (PLFAs) and extracellular polymeric substance (EPS) mainly relied on the indirect impacts. Therefore, emitter with larger v rather than that with larger Q should be a better choice from the perspective of the anti-clogging performance, and larger v could be obtained through the optimized design of emitter structure by controlling the appropriate ranges of W/D and A1/2/L.

Modeling congenital nasal pyriform aperture stenosis using computational fluid dynamics

ObjectivesCongenital nasal pyriform aperture stenosis (CNPAS) is a rare cause of airway obstruction in the neonate. Computational airway modeling has not been done in neonates and young infants to understand the impact of stenosis on functional nasal airflow. In this study, we 1) applied computational fluid dynamics (CFD) model to the airway of a neonate with CNPAS and 2) compare airflow dynamics of a normal and CNPAS airway. MethodsThree-dimensional models of the nasal airway of a normal neonate and a neonate with CNPAS were created using computed tomography scans of the facial bones. Measured anatomic parameters included volume, surface area, and cross-sectional area. CFD simulation was then performed. Simulated flow parameters included pressure, average velocity, and resistance. ResultsThe neonate with CNPAS had a lesser volume (2.74 cm3 vs. 4.50 cm3) and surface area (18.8 cm2 vs. 45.5 cm2) than the normal airway. The CNPAS airway had a lesser bilateral cross-sectional area and average cross-sectional velocity throughout the length of the model. While there is a large pressure drop in the normal airway immediately after the entry point, the pressure drop in the CNPAS airway occurs more posteriorly. The total nasal resistance was approximately eight-fold greater in the CNPAS airway than the normal. ConclusionsCFD analysis can be performed on airways of neonates with nasal obstruction, such as in CNPAS. A CFD model may help characterize severity of airway obstruction as it can predict the three-dimensional pattern of airflow. Determining the role of CFD in clinical management of CNPAS requires further investigation.

Computational optimization in simulating velocities and water-surface elevations for habitat–flow functions in low-slope rivers

ABSTRACTThe impact of varying computational mesh discretization on the accuracy of simulating velocities and water-surface elevations was investigated using the River2D model and data from 10 study reaches in three low-slope (<0.2%) Canadian rivers. A wide range of computational aspects were examined, including node spacing (1.35 to 60 m), number of mesh nodes (1250 to 58,000) and domain widths (60 to 800 m). Computed values for average cross-sectional velocities and water-surface profiles were compared with corresponding field survey data. The statistical mean absolute and root-mean-square errors were used to evaluate the discrepancy between measured and simulated values due to the mesh discretization characteristics. The results showed that mesh design discretization had a pronounced effect on the precision of the velocity and water-surface elevation predictions. Although the ratio of node spacing to river reach width may be site specific, it was found that optimal values should be <0.022 to obtain the highest accuracy. Regression equations of optimal ratios of node spacing to river reach width and the corresponding minimum mesh resolution errors were estimated. An example is provided that illustrates how the choice of computational mesh properties can affect habitat characterization for fish.

The effect of the pool and riffle on dissolved, non-conservative mass transport in rivers

One-dimensional substance transport models assume that the river reach modelled has a uniform cross-sectional shape which manifests as a constant average velocity in the model equations. Rarely do rivers meet this criterion. Their channels are seldom uniform in shape but rather alternate in a quasi-periodic manner between pool and riffle sections. This bedform sequencing imparts a corresponding variation in the average cross-sectional velocity which is not accounted for in constant velocity transport models. The literature points out that the pool and riffle planform may be the reason for the sometimes poor predictions obtained from these models. This paper presents a new variable velocity transport model and confirms that the fluctuation in average cross-sectional velocity caused by the pool and riffle planform does have a marked effect on transport in rivers. The pool and riffle planform promotes an enhanced decay of a substance when a first-order biochemical reaction is simulated with the new transport equation. Investigation of the analytical solution shows that the enhanced decay is the result of the overall lower velocity experienced in a pool and riffle channel as opposed to a uniform channel. This difference in transport velocity between a pool and riffle channel and a uniform channel becomes more pronounced as flow declines a critical finding for total maximum daily load calculations because these regulatory limits are usually determined for low flow levels by models that do not account for this phenomenon.

The velocity-reversal hypothesis revisited

In 1971, Edward Keller proposed the velocity-reversal hypothesis to describe observations where near-bed velocities in pools increased at a faster rate than in riffles. The reversal in conditions was used to explain the maintenance of pool-riffle sequences. The hypothesis continues to draw interest, and research shows that velocity reversals occur in limited conditions, but average cross-sectional velocities in pools do not universally exceed those in riffles at near-bankfull flows. A consensus is now beginning to form that flow convergence in pools is common at high flow and helps to form an isolated region of jet flow with high velocities that maintains the characteristics of the pool-riffle sequence.

Onset of turbulence in a regular porous medium: An experimental study

This paper presents the results from an experimental study on turbulent flows through a regular porous matrix of spheres packed in a cubic arrangement. The special pore geometry made it possible to measure the velocity within individual pores, using the ultrasonic velocity profiler (UVP). The UVP recorded the time series of the average cross-sectional velocity at a series of points along the pore. The measurements were performed for various hydraulic gradients resulting in a pore Reynolds number between 70 and 430. Three different regimes were detected: unsteady laminar, transition to turbulence, and turbulent. An additional measurement was performed using particle image velocimetry (PIV), which recorded two velocity components within a window which covered only the widest pore section. The PIV test had a Reynolds number of 370, which corresponds to transitional flow just before the onset of turbulence. The PIV result revealed the spatial distribution of mean velocities and turbulence intensities, as well as the shapes of coherent flow structures during significant turbulent events.

Flow in Trapezoidal Channels

Four charts are presented for the selection of standard concrete trapezoidal canals. The charts cover Systeome International (SI)– and English-dimensioned canals. If the channel invert slope and flow are known, a standard section can be chosen (bottom width and side slope) and the flow depth determined. Alternately, the invert slope and average cross-sectional velocity can be used as a basis of selection. Procedures are given for using the charts for channels of Manning roughnesses not included in the standards.

Effects of acceleration on the accuracy of MR phase velocity measurements.

Acceleration in blood flow can affect the accuracy of phase velocity measurements. Convective acceleration is due to changes in flow geometry and is independent of the time-varying acceleration caused by flow pulsatility. To analyze the effects of convective acceleration on flow velocity measurements, phase velocity measurements were obtained in steady laminar flow in the convergent segment of a 90%, hourglass-shaped stenosis phantom at a Reynolds number of 1,500. Measurements at the stenosis indicated that convective acceleration caused the measured values of average cross-sectional velocity to deviate as much as 37% from the theoretical values. The magnitude of the error could be accounted for by including the convective acceleration term in the phase shift equation. Convective acceleration effects should not be ignored in flow velocity measurements through stenoses, even when time-dependent acceleration due to flow pulsatility can be neglected.

Estimating Water Volume Discharge through Salt-Marsh Tidal Channels: An Aspect of Material Exchange

Water volume discharge estimates were made at a sample cross-section of the Canary Creek salt marsh (Lewes, Delaware). To account for cross-sectional velocity variations, a dense spatial array of current meters was used. Simultaneous measurements were taken at hourly intervals over three complete tidal cycles. A practical method is presented whereby instantaneous average cross-sectional velocity, instantaneous cross-sectional discharge and tidal cycle discharge can be estimated from a dense current meter array, orideal data set. Based on theseideal estimates, it is shown that a simplified spatial array can be used to estimate water discharges within acceptable error limits. Instantaneous cross-sectional discharges and total tidal cycle discharges were estimated with uncertainties of ±11% and ±7%, respectively. To minimize errors when estimating material exchange between salt marshes and adjacent waters, it is suggested that a comprehensive assessment of cross-sectional lateral current velocity variations be conducted.