Net Flow Rate Research Articles

Highly accelerated free-breathing real-time 2D flow imaging using compressed sensing and shared velocity encoding.

2D real-time (RT) phase-contrast (PC) MRI is a promising alternative to conventional PC MRI, which overcomes problems due to irregular heartbeats or poor respiratory control. This study aims to evaluate a prototype compressed sensing (CS)-accelerated 2D RT-PC MRI technique with shared velocity encoding (SVE) for accurate beat-to-beat flow measurements. The CS RT-PC technique was implemented using a single-shot fast RF-spoiled gradient echo with SVE by symmetric velocity encoding, and acquired with a temporal resolution of 51-56.5ms in 1-5 heartbeats. Both aortic dissection phantom (n = 8) and volunteer (n = 7) studies were conducted using the prototype CS RT (CS, R = 8), the conventional (GRAPPA, R = 2), and the fully sampled PC sequences on a 3T clinical system. Flow parameters including peak velocity, peak flow rate, net flow rate, and maximum velocity were calculated to compare the performance between different methods using linear regression, intraclass correlation (ICC), and Bland-Altman analyses. Comparisons of the flow measurements at all locations in the phantoms demonstrated an excellent correlation (all R2 ≥ 0.93) and agreement (all ICC ≥ 0.97) with negligible means of differences. In healthy volunteers, a similarly good correlation (all R2 ≥ 0.80) and agreement (all ICC ≥ 0.90) were observed; however, CS RT slightly underestimated the maximum velocities and flow rates (~ 12%). The highly accelerated CS RT-PC technique is feasible for the evaluation of flow patterns without requiring breath-holding, and it allows for rapid flow assessment in patients with arrhythmia or poor breath-hold capacity. The free-breathing real-time flow MRI technique offers improved spatial and temporal resolutions, as well as the ability to image individual cardiac cycles, resulting in superior image quality compared to the conventional PC technique when imaging patients with arrhythmias, especially those with atrial fibrillation. • The highly accelerated prototype CS RT-PC MRI technique with improved temporal resolution by the concept of SVE is feasible for beat-to-beat flow evaluation without requiring breath-holding. • The results of the phantom and in vivo quantitative flow evaluation show the ability of the prototype CS RT-PC technique to obtain reliable flow measurements similarly to the conventional PC MRI. • With less than 12% underestimation, excellent agreements between the two techniques were shown for the measurements of peak velocities and flow rates.

Discovery of mixing characteristics for enhancing coiled reactor performance through a Bayesian optimisation-CFD approach

Plug flow characteristics are advantageous in various manufacturing processes for fine/bulk chemicals, pharmaceuticals, biofuels, and waste treatment as they contribute to maximising product yield. One such versatile flow chemistry platform is the coiled tube reactor subjected to oscillatory motion, producing excellent plug flow qualities equivalent to well-mixed tanks-in-series ‘N’. In this study, we discover the critical features of these flows that result in high plug flow performance using a data-driven approach. This is done by integrating Bayesian optimisation, a surrogate model approach, with Computational fluid dynamics that we treat as a black-box function to explore the parameter space of the operating conditions, oscillation amplitude and frequency, and net flow rate. Here, we correlate the flow characteristics as a function of the dimensionless Strouhal, oscillatory Dean, and Reynolds numbers to the reactor plug flow performance value ‘N’. Under conditions of optimal performance (specific examples are provided herein), the oscillatory flow is just sufficient to limit axial dispersion through flow reversal and redirection, and to promote Dean vortices. This automated, open-source, integrated method can be easily adapted to identify the flow characteristics that produce an optimised performance for other chemical reactors and processes.

Performance Optimization of Nozzle-Diffuser Piezoelectric Micropump with Multiple Vibrating Membranes by Design of Experiment (DOE) Method

During recent years, microfluidics based microelectromechanical systems (MEMSs) have found multiple applications in biomedical engineering. One of their most important implementations is fluid transfer in microliter and nanoliter scales. Nowadays, micropumps are extensively used in various medical applications such as drug delivery. In this study, the performance of a piezoelectric micropump is investigated and optimized. This micropump consists of a pump chamber and three deformable walls in a nozzle-diffuser shape, which are used to create pressure gradient between the inlet and outlet. The performance of the micropump is evaluated by transient Computational Fluid Dynamics (CFD) simulation using dynamic mesh. Then its performance is optimized using the Design of Experiment (DOE) method based on mean net outlet mass flow rate and flow reversibility at the pump outlet. The results indicate an improvement of 34.5% in mean net outlet mass flow rate and a significant decrease in reversibility. The maximum mean net outlet mass flow rate and the minimum reversibility corresponding to the optimum geometries are 95.82 mL/min and 0.05%, respectively.

Continuous Contactless Measurement of Blood Volume Inside Venous Reservoirs for Cardiopulmonary Bypass

During a cardiopulmonary bypass (CPB) procedure using a heart-lung machine, the volume of blood inside the venous reservoir is a crucial piece of information for the perfusionist to make clinical decisions. In existing CPBs, venous reservoir blood volume can only be visually estimated by the perfusionists from the marks on the reservoirs. Such manual reading has poor resolution and it is unpractical to continuously track the blood volume over a long period. In this work, we introduce a blood volume sensor that can automatically measure and monitor the blood volume inside the venous reservoir in real time, with high accuracy and high resolution. Our sensor implements an optical scheme, in which the liquid level of blood is optically scanned by a contact image sensor comprised of a linear array of CMOS sensor pixels. The liquid level is then converted into blood volume. For a changing blood volume, the net flow rate into the reservoir is also instantly calculated from the recorded volumes. The demonstrated sensor prototype has been calibrated for a specific commercial reservoir and tested in a simulated CPB experiment using bovine blood. The results showed that the blood volume inside the reservoir and the flow rate can both be reliably monitored using our sensor. The reading can cover the entire operable range of venous blood volume (250 mL – 4000 mL), and the resolution has reached better than 2 mL. Our blood volume sensor shows great promise for improving the monitoring and management of patient blood volume during CPB procedures.

Cerebrospinal fluid production rate in various pathological conditions: a preliminary study

IntroductionThe cerebrospinal fluid (CSF) production rate in humans is not clearly defined but is estimated to be 18–24 ml/h (Trevisi et al Croat Med J 55(4):377–387 (24); Casey and Vries Childs Nerv Syst 5(5):332–334 (8)). A frequent clinical observation is that patients often drain higher volumes of CSF than can be explained by the assumed ‘normal’ CSF production rate (PRcsf). In the National Hospital for Neurology and Neurosurgery PRcsf was recorded in a variety of common neurosurgical pathologies using LiquoGuard7, an automated peristaltic pump that accurately controls CSF drainage and maintains a pre-set CSF pressure.MethodsA prospective observational study was performed from September 2021 onwards, on all patients in the National Hospital for Neurology and Neurosurgery who required CSF drainage as part of their ongoing treatment. The external drain was connected to a LiquoGuard7 pump (Möller Medical GmbH, Fulda, Germany), and the internal software of LiquoGuard7 was used to measure PRcsf. Statistical analysis used SPSS (version 25.0, IBM) by paired t test, comparing measured rates to hypothetical ‘normal’ CSF production rates calculated and published by Ekstedt (16–34ml/h) (Ekstedt J Neurol Neurosurg Psychiatry 41(4):345–353 (14)), assuming a similar distribution.ResultsPRcsf was calculated in 164 patients. Suspected normal pressure hydrocephalus (n=41): PRcsf of 79ml/h±20SD (p<0.0001). Post-surgical CSF leak (n=26): PRcsf of 90ml/h±20SD (p<0.0001). Subarachnoid haemorrhage (n=34): PRcsf of 143ml/h±9SD (p<0.0001). Intracerebral haemorrhage (n=22): PRcsf of 137ml/h±20SD (p<0.0001). Spinal lesions (n=7): PRcsf of 130ml/h±20SD (p<0.0032). Pituitary adenomas (n=10): PRcsf of 29 ml/h±9SD (p<0.049). Idiopathic intracranial hypertension (n=15): PRcsf of 86ml/h±10SD (p<0.0001). Decompensated long-standing overt ventriculomegaly (n=4): PRcsf of 65ml/h±10SD (p<0.0001). Cerebral infection (n=5): PRcsf of 90ml/h±20SD (p<0.0001).ConclusionNet CSF production rate may be higher than expected in many conditions, as measured with new device LiquoGuard7 through the study of net flow rate, which may have implications for clinical decisions on CSF diversion. The conventional understanding of CSF production and circulation does not explain the findings of this study. More extensive studies are needed to validate this technique.

Effects of magnetic field and porosity on compressible flow in an asymmetric channel

This study addressed the compressible flow of viscous fluid in an asymmetric channel under peristalsis. The difference in amplitudes and phase of traveling waves created the asymmetry of channel. Simultaneous effect of magnetic field is also incorporated. Fluid flows through a porous medium. The analytical treatment of the solution is carried out by considering upper wall amplitude as the small parameter. The expressions of flow rate and net axial velocity are constructed for the second-order approximation. Numerical integration is employed to calculate net flow rate. The role of sundry parameters is illustrated graphically. Trapping phenomenon is also taken into account by plotting streamlines against sundry parameters. The significant finding of this study is that flow rate and axial velocity enhance as fluid transitions from hydrodynamic to hydromagnetic. Enhancement in the compressibility parameter trims down the velocity and the flow rate as well. Also, asymmetry of the channel causes an enhancement in the flow rate. This model is the most prevailing version of compressible flow under peristalsis through an asymmetric channel. The findings of this study have worth mentioning yields, which can be applicable in numerous areas of fluid dynamics and aircraft industry.

Stationary flow driven by non-sinusoidal time-periodic pressure gradients in wavy-walled channels

The classical problem of secondary flow driven by a sinusoidally varying pressure gradient is extended here to address periodic pressure gradients of complex waveform, which are present in many oscillatory physiological flows. A slender two-dimensional wavy-walled channel is selected as a canonical model problem. Following standard steady-streaming analyses, valid for small values of the ratio ε of the stroke length of the pulsatile motion to the channel wavelength, the spatially periodic flow is described in terms of power-law expansions of ε, with the Womersley number assumed to be of order unity. The solution found at leading order involves a time-periodic velocity with a zero time-averaged value at any given point. As in the case of a sinusoidal pressure gradient, effects of inertia enter at the following order to induce a steady flow in the form of recirculating vortices with zero net flow rate. An improved two-term asymptotic description of this secondary flow is sought by carrying the analysis to the following order. It is found that, when the pressure gradient has a waveform with multiple harmonics, the resulting velocity corrections display a nonzero flow rate, not present in the single-frequency case, which enables stationary convective transport along the channel. Direct numerical simulations for values of ε of order unity are used to investigate effects of inertia and delineate the range of validity of the asymptotic limit ε≪1. The comparisons of the time-averaged velocity obtained numerically with the two-term asymptotic description reveals that the latter remains remarkably accurate for values of ε exceeding 0.5. As an illustrative example, the results of the model problem are used to investigate the cerebrospinal-fluid flow driven along the spinal canal by the cardiac and respiratory cycles, characterized by markedly non-sinusoidal waveforms.

Evolution of channel flow and Darcy’s law beyond the critical Reynolds number

For incompressible channel flow, there is a critical state, characterized by a critical Reynolds number Rec and a critical wavevector mc along the channel direction, beyond which the channel flow becomes unstable in the linear regime. In this work, we investigate the channel flow beyond the critical state and find the existence of a new fluctuating, quasi-stationary flow that comprises the laminar Poiseuille flow superposed with a counter-flow component, accompanied by vortices and anti-vortices. The net flow rate is reduced by ~ 15% from the linear, laminar regime. Our study is facilitated by the analytical solution of the linearized, incompressible, three-dimensional (3D) Navier–Stokes (NS) equation in the channel geometry, with the Navier boundary condition, alternatively denoted as the hydrodynamic modes (HMs). By using the HMs as the complete mathematical basis for expanding the velocity in the NS equation, the Rec is evaluated to 5-digit accuracy when compared to the well-known Orszag result, without invoking the standard Orr-Sommerfeld equation. Beyond Rec, the analytical solution is indispensable in offering physical insight to those features of the counter-flow component that differs from any of the pressure-driven channel flows. In particular, the counter flow is found to comprise multiple HMs, some with opposite flow direction, that can lead to a net boundary reaction force along the counter-flow direction. The latter is analyzed to be necessary for satisfying Newton’s law. Experimental verification of the predictions is discussed.Graphical

Experimental and Numerical Study on Cavitation Effects in Centrifugal Pumps

Experimental and numerical investigations of the centrifugal pump performance at non-cavitating and cavitating flow conditions were carried out in the present study. Experiments were performed by applying a vacuum to a closed-loop system to investigate the effects of the net positive suction head available (NPSHa), flow rate, water temperature and pump speed on the centrifugal pump performance. Accordingly, many of the important parameters concerning cavitation phenomenon were calculated. Also, the noise which is accompanied by cavitation was measured. Numerical analysis was implemented for two phase flow (the water and its vapor) using a 2-D simulation by ANSYS FLUENT software to investigate the internal flow of centrifugal pump under cavitating conditions. It was observed that with decreasing NPSHa, the values of the pump head, flow rate and efficiency initially remain constant, but with further reduction in NPSHa these parameters will decrease. Also, it was found that at 3% head drop the percentage drop of the flow rate is less than 2% whereas the percentage drop of the efficiency is greater than 3%. Numerically, it was noticed that the cavitation regions appear at the leading edge of suction side of the impeller blades which represents the lowest pressure area inside the computational domain of the centrifugal pump.

Computational and theoretical model of electro-osmotic flow pumping in a microchannel with squeezing walls

Numerical simulation and theoretical solution for the electro-osmotic pumping flow of electrolyte solution in a microchannel with squeezing and charged walls are developed in this study. The mathematical model is derived based on using a strong coupling between the nonlinear Poisson–Boltzmann equation and the flow lubrication theory. The governing equations are integrated numerically using the finite difference method. Moreover, an analytical solution to the problem is also obtained using the lubrication theory and is used to solve the Poisson–Boltzmann equation without any approximation technique. The effects of various parameters such as the wall zeta potential, Debye length, and electric field on the fluid pressure distribution, velocity field, and the net flow rate are investigated in detail. The results show that the induced pumping rate depends strongly on the combined effects of the Helmholtz–Smoluchowski, zeta potential, and electrical double layer. Moreover, the produced net flow directionality can be controlled efficiently by manipulating the Helmholtz–Smoluchowski and/or the wall zeta potential. The results obtained from the numerical simulation are then compared with the theoretical analysis and have shown to be in agreement with the proposed mathematical model.

Assessment of the Influence of Asymmetric Triacetate Cellulose Membrane on the Rate of Removal of Middle Molecular Weight Uremic Toxins in Patients Treated with Postdilution Online Hemodiafiltration

BACKGROUND: Postdilution online hemodiafiltration (OL-HDF) effectively removes uremic toxins of middle molecular weight from the blood of patients with end-stage chronic kidney disease. The rate of removal of uremic toxins depends on the type of dialysis membrane, blood flow rate (Qb), net ultrafiltration flow rate (Qnuf), and total convective volume (Vconv). AIM: The aim of this study was to examine the efficacy of asymmetric triacetate cellulose dialysis membrane in patients on post-dilution OL-HDF. METHODS: Thirty-five patients treated with post-dilution OL-HDF hemodiafiltration for at least 3 months were examined. The main parameters for assessing the efficiency of removal of uremic toxins of middle molecular weight are the concentration of β2-microglobulin (β2-M) and interleukin-6 (IL-6) in serum before and after a single session of post-dilution OL-HDF. The followings were used for statistical analysis: Kolmogorov–Smirnov test, Student’s T test for bound samples and Wilcoxon test. RESULTS: The average Vconv was 20.90 ± 3.30 liters/session. The β2-M reduction index during a single session of postdilution OL-HDF was 71.10 ± 6.39%, the IL-6 reduction index was 43.75 ± 15.60%, and the albumin reduction index was 4.55 ± 2.31%. CONCLUSION: The asymmetric triacetate cellulose dialysis membrane effectively removes β2-M and IL-6 during a single session of postdilution OL-HDF. The β2-M reduction index is ∼70%, the IL-6 reduction index is ∼40%, and albumin loss is &lt;4.0 g/4 h. The examined dialysis membrane and dialysis modality prevent the development of amyloidosis associated with dialysis, microinflammation and reduce the risk of developing atherosclerotic cardiovascular diseases in the population of patients treated with regular hemodiafiltration.

Multiobjective Bayesian Optimization Framework for the Synthesis of Methanol from Syngas Using Interpretable Gaussian Process Models.

Methanol production has gained considerable interest on the laboratory and industrial scale as it is a renewable fuel and an excellent hydrogen energy storehouse. The formation of synthesis gas (CO/H2) and the conversion of synthesis gas to methanol are the two basic catalytic processes used in methanol production. Machine learning (ML) approaches have recently emerged as powerful tools in reaction informatics. Inspired by these, we employ Gaussian process regression (GPR) to the model conversion of carbon monoxide (CO) and selectivity of the methanol product using data sets obtained from experimental investigations to capture uncertainty in prediction values. The results indicate that the proposed GPR model can accurately predict CO conversion and methanol selectivity as compared to other ML models. Further, the factors that influence the predictions are identified from the best GPR model employing "Shapley Additive exPlanations" (SHAP). After interpretation, the essential input features are found to be the inlet mole fraction of CO (Y(CO, in)) and the net inlet flow rate (Fin(nL/min)) for our best prediction GPR models, irrespective of our data sets. These interpretable models are employed for Bayesian optimization in a weighted multiobjective framework to obtain the optimal operating points, namely, maximization of both selectivity and conversion.

Electropumping of water in nanochannels using non-uniform electric fields

Pumping fluids at the nanoscale is of fundamental importance to nanofluidic systems. We perform molecular dynamics simulations and provide the extended Navier-Stokes (ENS) equation to prove the electropumping possibility of using non-uniform electric fields to induce the continuous and unidirectional net flow of water confined in nanochannels. The net flow can be seen as a result of the combined effects of the unidirectional orientation of water molecules and the net force induced by the electric field intensity difference between O and H atoms in a water molecule. Higher net flow rates can be achieved by enhancing the electric field intensity gradients through the characteristics parameter adjustment of external electric fields. Importantly, the electropumping is more effective than the pressure-driven flow due to the ordered arrangements of water molecules in electric fields. The ENS equation proposed is more applicable at the flow field with high order degree of water molecules.

Patterned convection in inclined slots

An analysis of laminar natural convection in inclined slots subjected to patterned heating has been performed. The imposed heating takes a simple form characterized by a single Fourier mode combined with uniform heating. It is shown that periodic heating applied at the lower plate produces no net flow when the slot is either horizontal or vertical, but a net upward flow is generated when the slot is tilted. Periodic heating applied at the upper plate produces net downward flow in the inclined situation. The addition of uniform heating promotes the upward flow while cooling has the opposite effect. There is a critical inclination angle at which the maximum net flow rate is greatest. Dynamic and thermal boundary layers are present when the wavenumber of the imposed heating is large. The use of heating at both plates, with the same wavenumber, leads to a flow dominated by the plate exposed to a more intense heating; when the two plates are heated equally no net flow is observed irrespective of the inclination angle. Changes of the relative positions of the two patterns can change the net flow rate by up to 50 %. The intensity of the flow increases with reduction of the Prandtl number. If the heating applied to the plates is of different wavelength, but of the same intensity, a wide range of behaviours of the flow system is possible. The details of this response are sensitive to the ratio of the two wavenumbers.

Modeling and Mathematical Investigation of Blood-Based Flow of Compressible Rate Type Fluid with Compressibility Effects in a Microchannel.

In this investigation, the compressibility effects are visualized on the flow of non-Newtonian fluid, which obeys the stress–strain relationship of an upper convected Maxwell model in a microchannel. The fundamental laws of momentum and mass conservation are used to formulate the problem. The governing nonlinear partial differential equations are reduced to a set of ordinary differential equations and solved with the help of the regular perturbation method assuming the amplitude ratio (wave amplitude/half width of channel) as a flow parameter. The axial component of velocity and flow rate is computed through numerical integration. Graphical results for the mean velocity perturbation function, net flow and axial velocity have been presented and discussed. It is concluded that the net flow rate and increase in case of the linear Maxwell model, while they decrease in case of the convected Maxwell model. The compressibility parameter shows the opposite results for linear and upper convected Maxwell fluid.

Ascending non-Newtonian long drops in vertical tubes

We report on theoretical and experimental studies describing the buoyancy-driven ascent of a Taylor long drop in a circular vertical pipe where the descending fluid is Newtonian, and the ascending fluid is non-Newtonian yield shear thinning and described by the three-parameter Herschel–Bulkley model, including the Ostwald–de Waele model as a special case for zero yield. Results for the Ellis model are included to provide a more realistic description of purely shear-thinning behaviour. In all cases, lubrication theory allows us to obtain the velocity profiles and the corresponding integral variables in closed form, for lock-exchange flow with a zero net flow rate. The energy balance allows us to derive the asymptotic radius of the inner current, corresponding to a stable node of the differential equation describing the time evolution of the core radius. We carried out a series of experiments measuring the rheological properties of the fluids, the speed and the radius of the ascending long drop. For some tests, we measured the velocity profile with the ultrasound velocimetry technique. The measured radius of the ascending current compares fairly well with the asymptotic radius as derived through the energy balance, and the measured ascent speed shows a good agreement with the theoretical model. The measured velocity profiles also agree with their theoretical counterparts. We have also developed dynamic similarity conditions to establish whether laboratory physical models, limited by the availability of real fluids with defined rheological characteristics, can be representative of real phenomena on a large scale, such as exchanges in volcanic conduits. Appendix B contains scaling rules for the approximated dynamic similarity of the physical process analysed; these rules serve as a guide for the design of experiments reproducing real phenomena.

Enhancing the flow efficiency of micropumping devices using a PDMS biomimetic diversion system

In this paper, a biomimetic diversion structure was proposed based on the contraction and expansion behavior of the heart valve, which was placed in the micropump of the circular chamber. The micropump is composed of a PZT-4A actuator and a PDMS diversion structure, whose role is to behave as both a control flow rate and a flow field rectification. The weak form "fluid solid coupling" modeling method is used to simulate the flow field characteristics of the micropump, and then the geometric parameters of the PDMS diversion structure is redesigned to achieve the flow rate basic requirements (3.5 ml min−1). The research results confirm that the proposed micropump has the best pump efficiency (3.34%) at a backpressure of 25 cmH2O and a flow rate of 1.79 mL min−1. In addition, the net flow rate was assessed over a range of operating frequencies (200–900 Hz, in 50 Hz increments). The maximum flow rate is approximately 3.53 ml min−1 at a resonance frequency of 700 Hz. This indicates that the presented micropump has an advantage in drug delivery applications owing to its frequency regulation characteristics, and that the release rate of drug delivery can be easily controlled to maintain therapeutic efficacy.

Net flow rates versus roll rates as non-performing consumer loans forecasting methodologies

Roll rates and net flow rates can be seen as the evolution of ageing of accounts receivable and Markov chains. They are accepted methodologies to model the behavior of non-performing consumer loans by buckets and to predict losses, but we find that quite often they are wrongly used as interchangeable concepts, although roll rates track individual accounts across buckets in consecutive months and net flow rates just compare consecutive buckets in consecutive months. We determine their matrices of transition probabilities and analyze them in both stationary and steady-state conditions. Net flow rates have many advantages over roll rates, but a quite important finding for financial institutions and supervisors is that historical flow rates are not conservative for forecasting: when the level of new delinquencies soars, contemporary flow rates will tend to be lower than they would be in steady-state conditions, creating a feeling of false confidence and leading to the underestimation of future losses.

Modeling the squeeze flow of droplet over a step

In this paper, we study the squeeze flow of a droplet confined between two plates in the presence of a step. Understanding this fluid mechanics problem is of the utmost importance particularly for nanoimprint lithography, wherein the photoresist droplets are dispensed on a substrate and imprinted and cured into a desired pattern. Often, the desired pattern includes various steps and trenches, and the droplets need to flow over. Here, we use the lubrication theory to find the instantaneous pressure and velocity fields. A volume-of-fluid advection algorithm is also used for evolving the volume fraction in time. The obtained results reveal that for step sizes comparable to the gap between plates, the squeeze flow characteristics become quite distinct across the step. Under such circumstances, the fluid finds it less expensive to reverse its flow direction toward the deep region to pass through the low-resistance zone, which leads to a net mass flow rate across the step from a shallow to deep region. Such a mass transfer is found to be enhanced by applying larger squeezing forces. This phenomenon becomes less noticeable for liquid film thicknesses much larger than the step size. As a result, it takes large droplets a longer time to reach to the regime wherein a substantial mass flow rate occurs. In addition, the results suggest that the dimensionless characteristic features, such as the ratios of volume and area of liquid in the deep (or shallow) region to those of the total liquid, collapse onto their corresponding master curves.

Impact of moving traffic on pollutant transport in street canyons under perpendicular winds: A CFD analysis using large-eddy simulations

• One-way traffic has stronger influences on pollutant dispersion than two-way traffic • The influence of traffic motion is noticeable under a low approaching wind speed • At U 10 = 2 m/s, the net pollutant flow rate at the side can be increased by 14 times • Traffic density plays a much less role than traffic direction and wind speed • Considering traffic motion is essential for accurate predictions of urban pollutant In urban street canyons, wind and vehicle motions jointly influence the pollutant dispersion. The present study systematically investigated the impact of (i) traffic direction, (ii) traffic density, and (iii) approaching wind speed on the mechanism of pollutant dispersion. Large-eddy simulations were performed for a street canyon with an aspect ratio of unity under two-lane traffic flow. Results show that two-lane traffic moving in the opposite direction insignificantly affects the mean pollutant concentrations inside the canyon, with a change in the net pollutant flow rate by less than 7%. However, traffic moving in the same direction strongly modify the local mean pollutant concentration, especially under a low approaching wind speed. Under the wind with a 10-meter speed of 2 m⋅s −1 , one-way traffic plays a dominant role in determining the concentration pattern, shifting the region with the maximum pollutant concentration from the center to the corner downstream of the traffic direction. The absolute net pollutant outflow rate at the canyon's lateral opening downstream of the traffic direction is increased by about 14 times, which is caused by the outflow convective pollutant flux. Findings here suggest numerical studies that neglect traffic motion can erroneously predict the local pollutant concentration in street canyons.