Mean Velocity Magnitude Research Articles

Detection of cerebral ischemic injury during extracorporeal membrane oxygenation using microvascular flows measured by ultrasound localization microscopy

Pediatric Extracorporeal Membrane Oxygenation (ECMO) is a life-saving therapy. Affecting 8–50% of patients, brain injury is among the most frequent complications. Non-invasive monitoring of brain perfusion during ECMO using contrast-enhanced ultrasound imaging can potentially improve the outcome. Here we utilize an in-house developed Ultrasound Localization Microscopy (ULM) procedure, which involves a super-resolution deep learning approach for subpixel localization of microbubbles and Kalman filter-based bubble tracking, to measure the brain blood perfusion in a pediatric pig model during ECMO. Parameters, including the mean velocity magnitude in large (>1mm) and medium (0.2–1 mm) vessels, as well as the cerebral microcirculation for micro (<0.2 mm) vessels, are analyzed and compared with the histological readings of brain ischemic injury levels. Data show significantly reduced micro perfusion in the cortex and thalamus with increasing levels of ischemic injury. Compared with large and medium vascular flows, microvascular flows are more sensitive in not only detecting injury but also evaluating injury levels. These results suggest that the management of ECMO patients could be guided by and benefit from the non-invasive monitoring of cerebral microvascular flows.

Wake characterization of tidal turbines in the Pentland Firth using vessel-mounted ADCP measurements

Knowledge of the extension and velocity deficit induced by tidal turbine wakes is crucial for the optimization of tidal farm layouts, as the wake induced by an upstream turbine may substantially affect the power and loadings of a device located downstream. The MeyGen project is the largest planned tidal stream energy project in the world: it aims to develop up to 398 MW of installed power in the Pentland Firth, Scotland, a site with current velocities reaching up to 5 m/s. During Phase 1 of the project, four 1.5 MW turbines were installed, providing a valuable opportunity to investigate turbine wake dynamics in full scale. As part of Phase 2, site characterization campaigns were carried out to plan the deployment of additional turbines, representing 28 MW of tidal power capacity. Hence, the influence of the existing devices on the downstream flow was investigated.
 This work introduces the method used for studying wakes downstream tidal turbines using vessel-mounted Acoustic Doppler Current Profiles (ADCP) measurements. During two spring tides, the data was collected using an ADCP Teledyne Workhorse 600 kHz configured for along-beam velocities recording with 1 m resolution, as well as a GNSS Hemisphere Vector V102. The aim of this study was to map the flow downstream the turbines already installed on site, in order to identify and characterize the wake. Cross-flow transect measurements were conducted at various along-flow distances downstream from the devices, and repeated several times for improved accuracy. The cycles of repeated transects were performed both at flood and ebb tides, with the turbine running or switched off to spot potential differences introduced by device operation. Mean flow velocity estimates in the cross-section were obtained from raw data using the location-based velocity solver developed by Vermeulen et al. (2014). The method provides improved flow velocity estimates from vessel-mounted ADCP measurements because it strongly reduces the spatial extent over which flow homogeneity must be assumed, thus decreasing the chances that this assumption will fail. The reduction of the volume across which homogeneity must be achieved may represent a significant advantage for the investigation of tidal turbine wakes. To the authors’ knowledge, this is the first time that this method has been applied to wake characterization in a tidal stream energy site.
 Outputs of the analysis include mean velocity magnitude and direction, as well as velocity deficits associated with the wakes. The mean velocity estimates obtained in each cross-section are compared for cases when the turbine was running or switched off. Findings reveal that vessel-mounted ADCP transects, coupled with the location-based methodology for velocity estimation, provide a powerful tool for tidal turbine wake characterization. The final presentation and paper will provide the results of this study.

Role of artificial intelligence based image-processing techniques for port development activities in physical tidal models

ABSTRACT The Cochin Port physical tidal model available at Central Water and Power Research Station (CWPRS), Ministry of Jal Shakti, Government of India, is used to simulate the tidal conditions of real-time Cochin Port. Analysis of physical tidal model takes time and requires specialised knowledge in order to analyse and identify the velocity profile through manual instrumentation. The use of artificial intelligence (AI) based image processing techniques may reduce the analysis time and improve the accuracy of physical tidal model. An attempt was made to determine the velocity profile of Cochin Port model (scaled down model) at CWPRS through AI-based video image-processing analysis using MATLAB. Based on the analysis, it was observed that magnitude of horizontal velocity is varying for 1579 times in about 30.36 sec duration from 0.01156 m/s to 0.32986 m/s with a mean velocity magnitude of about 0.20357 m/s. The results indicated that digital videography-based image processing approach with the assistance of trained AI algorithms gives the précised velocity profile having 578 unique magnitude variations out of 1579 readings in 30.36 sec, which is far better than the manual instrumentation, and therefore, manual instrumentation may be substituted with the developed artificial intelligence (AI) based image processing technique.

Experimental study on flow kinematics of dam-break induced surge impacting onto a vertical wall

Tsunami surges are frequently simulated by dam-break flows over dry beds. The purpose of this study is to quantitatively investigate the flow kinematics and turbulent characteristics of a surge impacting onto a vertical wall. To quantify the flow kinematics, the particle image velocimetry technique was used in the non-aerated region, while the bubble image velocimetry technique was employed to measure the impact-induced turbulent flow with air entrainment. The measured velocity fields of the impact-induced splash confirmed the feasibility of Ko and Yeh's [Coastal Eng. 131, 1–11 (2018)] model employing a solid-body motion assumption of splash that estimates the impact force by bores and surges at the initial impact stage. Velocity fields and streamlines revealed that the main water body overturned backward and formed a large two-phase vortex, while a small counter rotating vortex was also formed at the corner of the wall-bed junction. The mean velocity magnitude of the small corner vortex is about two-thirds that of the main water body. The mean turbulent intensity of these aerated regions is about 3.4 times that of the non-aerated regions. Based on a wavelet transform-based method, the result reveals that the mean turbulence length scale of the aerated region is about two-thirds that of the non-aerated region. This study reveals for the first time the quantitative flow field results of the surge impact process, which deepen insight of tsunami risk in coastal engineering, thus improving the accuracy of post-damage prediction in coastal areas.

Estimating the Average River Cross‐Section Velocity by Observing Only One Surface Velocity Value and Calibrating the Entropic Parameter

AbstractThe current research aims to predict the velocity distribution and discharge rates in rivers based on the entropy concept using only one surface velocity measurement. In this direction, first, the uncrewed aerial vehicle (UAV)‐based image acquisition technique was applied to collect the surface velocity distribution along two European rivers, the Sajó, and the Freiberger Mulde Rivers. Seven cross sections were chosen for the analysis. At each cross section, first, the entropic parameter Φ(M) was calibrated based on the maximum and mean velocity magnitudes, derived from Acoustic Doppler Current Profilers, respectively, showing a trend for all cross sections with a range of 0.6 < Φ(M) < 0.75. Next, the maximum surface velocity provided by the UAV was implemented as a single velocity input. Finally, the bathymetry data, herein collected by UAV, were considered as the input for the entropy approach. In this way, the entropy iterative method allowed estimating the mean flow velocity by identifying the location (dip) of maximum velocities across the river site and inferring the 2D velocity distribution. The results highlighted that the entropy approach can accurately predict the velocity distribution and discharge rates with a percentage error lower than 13%.

Spatiotemporal droplet dispersion measurements demonstrate face masks reduce risks from singing

COVID-19 has restricted singing in communal worship. We sought to understand variations in droplet transmission and the impact of wearing face masks. Using rapid laser planar imaging, we measured droplets while participants exhaled, said ‘hello’ or ‘snake’, sang a note or ‘Happy Birthday’, with and without surgical face masks. We measured mean velocity magnitude (MVM), time averaged droplet number (TADN) and maximum droplet number (MDN). Multilevel regression models were used. In 20 participants, sound intensity was 71 dB for speaking and 85 dB for singing (p < 0.001). MVM was similar for all tasks with no clear hierarchy between vocal tasks or people and > 85% reduction wearing face masks. Droplet transmission varied widely, particularly for singing. Masks decreased TADN by 99% (p < 0.001) and MDN by 98% (p < 0.001) for singing and 86–97% for other tasks. Masks reduced variance by up to 48%. When wearing a mask, neither singing task transmitted more droplets than exhaling. In conclusion, wide variation exists for droplet production. This significantly reduced when wearing face masks. Singing during religious worship wearing a face mask appears as safe as exhaling or talking. This has implications for UK public health guidance during the COVID-19 pandemic.

Impact of roof geometry on cross-ventilation in vaulted buildings: a CFD study

The roof geometry is an important parameter in determining the natural cross-ventilation in buildings. Although many studies have been performed on natural ventilation, the impact of the large set of vaulted roofs on the building cross-ventilation remains to be addressed. In this paper, high-fidelity computational fluid dynamics (CFD) simulations, validated with experimental data, are performed on five naturally ventilated buildings with two opposing windows, each with a specific type of vaulted roof and otherwise identical, in 0° incident flow angle, to assess the impact of vaulted roof geometry on the building cross-ventilation performance. The following vault geometries are investigated and compared against a flat-roof building with the same specifications: (i) segmental, (ii) low-rise pointed, (iii) mid-rise catenary (iv) high-rise pointed and (v) high-rise catenary. The results show that compared to the flat-roof building, the vaulted roofs cause a <5% decrease in volume flow rate and 16-29% increase in the average mean velocity magnitude at the occupants’ level. In vaulted buildings, the low-velocity regions of the flow are moved further away from the occupied zone towards the ceiling, thus providing a fresher air to the occupants. The results clarify the high potential of vaulted roofs for natural ventilation and sustainable design.

Numerical study of the effect of LVAD inflow cannula positioning on thrombosis risk

Left ventricular assist devices (LVADs) have been increasingly used as a therapy for patients with end-stage heart failure. However, a growing number of clinical observations have shown that LVADs are associated with thromboembolic events, which are potentially related to the changes in intraventricular flow. Particularly, the flow fields around the inflow cannula (IC) of the LVAD. In this study, a fluid structure interaction (FSI) simulation was conducted to evaluate the hemodynamics of a patient specific left ventricle (LV) with varying LVAD IC orientations. The LV model was obtained from computed tomography scans and modeled to have contraction and relaxation during cardiac cycles following available experimental data of LV volume changes. The LV of the patient was assumed to have an end systolic volume of 223.7 mL and a stroke volume of 46.4 mL. Four different IC positions were considered: towards the (1) septum; (2) aortic valve (AV); (3) mitral valve (MV) and (4) inferior wall (IW). The potential thrombus growth around the IC was assumed to be caused by blood stagnation regions with low velocity (<5 mm/s) and low shear rate (<60/s) flow. Mean velocity magnitudes and low blood velocity regions around the IC were numerically obtained. To quantitatively compare the thrombosis risks of the four simulation cases, the time-averaged volumes of the low-velocity regions and the low shear rate regions were calculated. The intraventricular volumes of low velocity zones based on IC orientation are 1.42 mL toward the septum, 1.14 mL toward the AV, 0.93 mL toward the MV, and 1.24 mL toward the IW. The intraventricular volumes of low shear regions based on IC orientation are 11.54 mL toward the septum, 11.15 mL toward the AV, 9.24 mL toward the MV, and 10.7 mL toward the IW. IC orientation toward the MV results in lower volumetric regions of low flow and low shear within the ventricle, which consequently may lead to a reduced risk of thrombus formation.

Physiological Stress Modelling and Hemolysis Prediction for High Shear Stress Flows using Computational Hemodynamics

Predicting hemolysis is a mandatory task when designing blood flow related mechanisms. For decades, researchers have tried to estimate trauma in red blood cell (RBC) for applying in assist mechanisms development, but the specificity and absence of more physical details have limited models for this purpose into ranges of applications. This work aims to present a new method for modelling hemolysis considering a stress threshold that RBC could stand and, bellow that, a Physiological Stress. Complementing this application, simulations in Ventricular Assist Device (VAD) was performed using Computational Fluid Dynamics (CFD) for the hemodynamics. For hemolysis risk analyses, critical regions were established by a mean stress magnitude, also purposed here. The mean stress magnitude is presented including turbulent parameters, trying to reduce the error in calculating the mean stress tensor by mean velocity magnitudes in Reynolds Average Navier-Stokes models for turbulent flows. Five turbulent models were tested: Standard κ-ε, κ-ε RNG, κ-ε Realizable, Standard κ-ω, κ-ω SST and Spalart-Allmaras models. Results indicate similar results for considering Physiological Stress compared to traditional model applications, even using adapted coefficients, what induces specific coefficients for models applying Physiological Stress might improve hemolysis estimations. The κ-ε RNG and κ-ω SST models had better agreement with data and physical expectations and the best scenarios for applying traditional and improved models purposed for future uses.

Image-based computational hemodynamic analysis of an anterior communicating aneurysm treated with the Woven EndoBridge device

BackgroundThe Woven EndoBridge (WEB) aneurysm embolization device is a relatively recent developed intrasaccular flow disruptor committed to intracranial wide-neck and complex aneurysm occlusion. In this study, we aimed to assess hemodynamic profiles of an AcomA aneurysm treated with WEB Single Layer (WEB-SL) to shed light on the understanding of hemodynamic alterations in the context of a recanalized aneurysm. MethodsImage-Based computational fluid dynamics (CFD) was used to simulate pre and post-treatment blood flow of an anterior communicating aneurysm treated with WEB-SL. The simulated velocity field was used to visualize and quantify velocity, wall shear stress (WSS), and pressure distributions. ResultsFollowing WEB deployment, infiltration of flow and mean velocity magnitude were decreased considerably within the aneurysm sac. WSS was the only measured index that demonstrated a dual hemodynamic pattern; it reduced along the superior half of the aneurysm sac while slightly increased in the neck region. At one-year follow-up, angiography showed recanalization of the aneurysm, and the WEB had been pushed to the dome. ConclusionsAll assessed hemodynamic parameters reduced significantly from pre to post-WEB placement in the sac of AcomA aneurysm; however, recanalization may happen secondary to filling the proximal portion of the WEB-SL and increased WSS in the neck of the aneurysm. Conduction of larger studies is warranted to elaborately investigate the potential role of CFD techniques for correlating the hemodynamics indices with clinical outcomes.

Window geometry impact on a room's wind comfort

PurposeNatural ventilation is an environmentally friendly effective way of improving thermal comfort and the quality of indoor conditions if applied properly. This study aims to investigate the physical mechanism of the air movement and also the influence of building geometry in a cross-ventilated room through a parametric study of window geometrical characteristics using computational fluid dynamics.Design/methodology/approachMomentum and continuity equations are solved by the control volume method using a commercially available software. Standard k−ɛ turbulence model is employed to simulate the incompressible airflow and SIMPLE algorithm to solve the conservation equations. Mean air velocity magnitude is measured at three different surfaces of different heights, and the effect of incoming wind velocity inside the building is studied.FindingsThe research concluded that window hood and sill projections reduce indoor wind velocity magnitude, play a major role in incoming wind direction and thus have a crucial impact on wind circulation and indoor air quality.Social implicationsThe paper has evaluated redesigning of a both practical and ornamental architectural element named Palekaneh, which is found in many historical buildings in several hot places in the world. Its optimal design could increase indoor natural ventilation quality and decrease a space's cooling load. Therefore, a new passive cooling architectural element could be re-introduced to the regions previously enjoying such ornaments. This is economically efficient because it eventually saves a considerable amount of energy in the long run and is socially important because of the revitalization of architectural identity.Originality/valueThe role of a building envelope's physical features, although being studied for solar absorption and daylight availability, has rarely been investigated for natural ventilation, especially in a small scale, thus making the paper novel in this regard. This provides a guideline for designers to assess the impact of their design on redirecting wind-induced natural ventilation the very early stages of design.

3D-printed, patient-specific intracranial aneurysm models: From clinical data to flow experiments with endovascular devices.

Flow models of intracranial aneurysms (IAs) can be used to test new and existing endovascular treatments with flow modulation devices (FMDs). Additionally, 4D flow magnetic resonance imaging (MRI) offers the ability to measure hemodynamics. This way, the effect of FMDs can be determined noninvasively and compared to patient data. Here, we describe a cost-effective method for producing flow models to test the efficiency of FMDs with 4D flow MRI. The models were based on human radiological data (internal carotid and basilar arteries) and printed in 3D with stereolithography. The models were printed with three different printing layers (25, 50, and 100µm thickness). To evaluate the models in vitro, 3D rotational angiography, time-of-flight MRI, and 4D flow MRI were employed. The flow and geometry of one model were compared with in vivo data. Two FMDs (FMD1 and FMD2) were deployed into two different IA models, and the effect on the flow was estimated by 4D flow MRI. Models printed with different layer thicknesses exhibited similar flow and little geometric variation. The mean spatial difference between the vessel geometry measured in vivo and in vitro was 0.7±1.1mm. The main flow features, such as vortices in the IAs, were reproduced. The velocities in the aneurysms were similar in vivo and in vitro (mean velocity magnitude: 5.4±7.6 and 7.7±8.6cm/s, maximum velocity magnitude: 72.5 and 55.1cm/s). By deploying FMDs, the mean velocity was reduced in the IAs (from 8.3±10 to 4.3±9.32cm/s for FMD1 and 9.9±12.1 to 2.1±5.6cm/s for FMD2). The presented method allows to produce neurovascular models in approx. 15 to 30h. The resulting models were found to be geometrically accurate, reproducing the main flow patterns, and suitable for implanting FMDs as well as 4D flow MRI.

Effects of Bulk Flow Pulsations on Film Cooling with Two Sister Holes

In a triple-hole system comprising a primary hole and two sister holes, when the sister holes are positioned slightly downstream of the main hole under steady flow conditions, their jets generate an anti-counter-rotating vortex pair. Vortex interactions between the jets increase the effectiveness of adiabatic film cooling. In this study, a series of large-eddy simulations were conducted to understand how pulsations in the main flow affect film cooling in a triple hole. To understand the effects of pulsations on film cooling performance is important for better cooling design of the gas turbine engines. The numerical simulations were carried out on a flat plate geometry with a triple cylindrical hole system at 35° injection angle. The pulsations were approximately sinusoidal, and their effect on film cooling was investigated at several frequencies (2, 16, and 32 Hz) and Strouhal numbers (Sr = 0.1005, 0.8043, and 1.6085) at an average blowing ratio of 0.5. The results for the triple-hole system were compared with those for a single hole for the same amount of cooling air and the same cross-sectional area of the holes. Increasing the Strouhal number of the main flow decreased η in both systems. However, at each Strouhal number, η was higher in the triple hole. Furthermore, the triple-hole system was found to be better for film cooling than a single-hole system for higher values of the pulsation Strouhal number. Contours of time-averaged film cooling effectiveness and dimensionless temperature, instantaneous film cooling effectiveness contours on a test plate, mean velocity magnitude contours in the hole, and Q-contours for the triple holes under the application of pulsations to the flow were investigated.

Direct numerical simulation of a non-equilibrium three-dimensional turbulent boundary layer over a flat plate

Abstract

Magnitude and Reliability of Velocity and Power Variables During Deadlifts Performed With and Without Lifting Straps.

Jukic, I, García-Ramos, A, Malecek, J, Omcirk, D, and Tufano, JJ. Magnitude and reliability of velocity and power variables during deadlifts performed with and without lifting straps. J Strength Cond Res 36(5): 1177-1184, 2022-This study aimed to compare the magnitude and reliability of mean velocity (MV), peak velocity (PV), mean power (MP), and peak power (PP) between deadlifts performed with (DLw) and without (DLn) lifting straps. Sixteen resistance-trained men performed a DLn 1-repetition maximum (1RM) session followed by 4 experimental sessions (2 with each deadlift variant in a randomized order). Each experimental session comprised lifts at 20, 40, 60, and 80% of the DLn 1RM. No significant differences were found between DLw and DLn for MV, MP, PV, and PP at any load (p = 0.309-1.00; g = 0.00-0.19). All mechanical variables showed an acceptable reliability for both deadlift conditions at each relative load (coefficient of variation [CV] <8%; intraclass correlation coefficient [ICC] > 0.70; g < 0.5) with the only exception of MV at 60% 1RM for DLw (ICC = 0.62) and at 40% 1RM for DLn (ICC = 0.65). Furthermore, MV and PV generally had lower within-subject CV (CV = 3.56-5.86%) than MP and PP (CV = 3.82-8.05%) during both deadlift conditions. Our findings suggest that sport professionals might not need to consider implementing lifting straps with the aim to maximize velocity and power outputs with submaximal loads in a deadlift exercise. Because all mechanical variables measured showed an acceptable level of reliability for both DLw and DLn, they can all be used to track changes in performance during the deadlift exercise. However, velocity variables were slightly more consistent (lower CV), which makes them more appropriate to track DLw and DLn performance changes.

Altered hemodynamics during arteriovenous fistula remodeling leads to reduced fistula patency in female mice

ObjectiveThe arteriovenous fistula (AVF) is the preferred method of dialysis access because of its proven superior long-term outcomes. However, women have lower rates of AVF patency and utilization than men. We used a novel mouse AVF model that recapitulates human AVF maturation to determine whether there are differences in AVF patency in female and male mice. MethodsAortocaval fistulas were created in female and male C57BL/6 mice (9-10 weeks). At days 0, 3, 7, and 21, infrarenal inferior vena cava (IVC) and aortic diameters and flow velocity were monitored by Doppler ultrasound and used to calculate the vessel diameter, blood flow, and shear stress. AVF were harvested, and expression of proteins was examined by proteomic analysis and immunofluorescence and of messenger RNA by quantitative polymerase chain reaction analysis. ResultsAt baseline, female mice weighed less and had lower IVC velocity and smaller magnitudes of shear stress, but there was no significant difference in IVC diameter and thickness. After AVF creation, both female and male mice had similar IVC dilation and thickening with no significant differences in IVC wall thickness at day 21. However, female mice had diminished AVF patency by day 42 (25.7% vs 64.3%; P = .039). During fistula remodeling, female mice had lower IVC mean velocity and shear stress magnitude and increased spectral broadening (days 0-21). Messenger RNA and protein expression of Krüppel-like factor 2, endothelial nitric oxide synthase, and vascular cell adhesion molecule 1 was similar at baseline in female and male mice but increased in the AVF only in male mice but not in female mice (day 21). Proteomic analysis of female and male mice detected 56 proteins expressed at significantly higher levels in the IVC of female mice and 67 proteins expressed at significantly higher levels in the IVC of male mice (day 7); function-specific analysis showed that the IVC of male mice overexpressed proteins that belong to pathways implicated in the regulation of vascular function, thrombosis, response to flow, and vascular remodeling. ConclusionsAVF in female mice have diminished patency, preceded by lower velocity, reduced magnitudes of shear stress, and less laminar flow during remodeling. There is also sex-specific differential expression of proteins involved in thrombosis, response to laminar flow, inflammation, and proliferation. These findings suggest that hemodynamic changes during fistula maturation may play an important role underlying the diminished rates of AVF utilization in women.

Examination of the physical assumptions of a quasi-steady vector model using the integral momentum equation

Quasi-Steady (QS) vector models have served as a convenient and effective tool for wind load estimations for low-rise buildings in the wind engineering community. In order to understand the applicability for practice, the physical assumptions of a QS vector model are investigated in this paper. The derivation is done through algebraic manipulation of the time-averaged integral momentum equation, which is used to relate mean, area-averaged, roof surface pressures to the mean flow and turbulence field above a roof. The two main assumptions of the QS model are revealed through this process: (i) The streamlines of an instantaneous flow near the roof are assumed to be the mean streamlines so that the instantaneous direction of the flow measured at the reference point is equivalent to the mean direction; (ii) The magnitude of the instantaneous flow is obtained by amplifying the magnitude of the mean flow at a spatially uniform rate such that the amplified magnitude of mean velocity is equivalent to the instantaneous magnitude measured at the reference point. Missing terms in the QS model are used to develop correction terms to improve QS model performance.

Comparison of K-E Turbulence Model Wall Functions Applied on a T-Junction Channel Flow

The flow acting in a T-junction channel is present in several industrial applications, such as air conditioning systems, water cooling circuits, gas exhaust systems and others. In order to numerically simulate this case, the Average Reynolds Navier-Stokes (RANS) equation is used for a two-dimensional stationary flow using the k-e model together with wall functions such as standard wall function, Enhanced and Menter-Lechner wall treatments. The moment ratio used is = 2 and the Reynolds number at the inlet of the flow parallel to the channel is Re = 15,000. The results were compared with the literature data using Large Scale Simulation (LES). The results obtained for k-e model Enhanced and Menter-Lechner wall treatment were satisfactory and close to that found by the LES simulation, however, results obtained from k-e model standard wall function presented large deviation to literature, mainly in the boundary layer and K production profiles. In general, the results presented small distortions for the profiles of turbulent kinetic energy production near walls, however, they illustrate in an analogous manner to the literature the production of turbulent kinetic energy K concentrated in the shear layers between flows. The main results analyzed in this paper are the length of the recirculation bubble, boundary layer profile, mean velocity magnitude and kinetic energy production k.

PIV measurements and numerical analysis of in-cylinder tumble flow in a motored engine

The purpose of this study is to present an experimental methodology and a numerical analysis to characterize the in-cylinder tumbling flow generated by a motored four-valve spark ignition single-cylinder optical engine. High-speed two-dimensional particle image velocimetry measurements were made in the symmetry vertical plane between the inlet and outlet valves. The velocity flow fields were recorded during the intake and compression strokes for three different engine speeds (1000, 1500, and 2000 rpm) at several crank angles. Computational fluid dynamics (CFD) simulations were carried out using the Star-CD software with es-ICE module, and the two different turbulence models, RNG–k–e and k–w SST, were analyzed with experimental data of the engine running at 1000 rpm in motored conditions. Regarding experimental results, vorticity, tumble ratio, and kinetic energy were calculated and compared. The initially large mean kinetic energy and tumble ratio, which is caused by the high-velocity jets of the inlet valves and in-cylinder bulk flow, decrease during intake in response to the piston motion. The air jet entering the cylinder space strikes the piston top surface generating a counter-clockwise rotating tumble vortex. Flow field cyclic variation could be noticed and quantified. In addition, an evolution analysis of the main vortex center and engine speed influence were made. Moreover, a qualitative analysis of the CFD scalar flow field in the region of interest is presented. The numerical results showed a better agreement with the RNG–k–e and experimental mean velocity magnitude curve than k–ω–SST. However, the qualitative scalar velocity field of the k–ω model captured more details of the flow than the k–e model. The methodology contributes to a better understanding of flow motion behavior, piston speed and valve timing influences, and, consequently, the mixture formation process in spark ignition engines.

Numerical and Experimental Studies on a Syngas-Fired Ultra Low NOx Combustor

Simulations and exhaust measurements of temperature and pollutants in a syngas-fired model trapped vortex combustor for stationary power generation applications are reported. Numerical simulations employing Reynolds-averaged Navier–Stokes (RANS) and large eddy simulations (LES) with presumed probability distribution function (PPDF) model were also carried out. Mixture fraction profiles in the trapped vortex combustor (TVC) cavity for nonreacting conditions show that LES simulations are able to capture the mean mixing field better than the RANS-based approach. This is attributed to the prediction of the jet decay rate and is reflected on the mean velocity magnitude fields, which reinforce this observation at different sections in the cavity. Both RANS and LES simulations show close agreement with the experimentally measured OH concentration; however, the RANS approach does not perform satisfactorily in capturing the trend of velocity magnitude. LES simulations satisfactorily capture the trend observed in exhaust measurements which is primarily attributed to the flame stabilization mechanism. In the exhaust measurements, mixing enhancement struts were employed, and their effect was evaluated. The exhaust temperature pattern factor was found to be poor for baseline cases, but improved with the introduction of struts. NO emissions were steadily below 3 ppm across various flow conditions, whereas CO emissions tended to increase with increasing momentum flux ratios (MFRs) and mainstream fuel addition. Combustion efficiencies ∼96% were observed for all conditions. The performance characteristics were found to be favorable at higher MFRs with low pattern factors and high combustion efficiencies.