Presence Of Recirculation Research Articles

A coherent composite approach for the continuous circular hydraulic jump and vortex structure

We examine the structure of the continuous circular hydraulic jump and recirculation for a jet impinging on a disk. We use a composite mean-field thin-film approach consisting of subdividing the flow domain into three regions of increasing gravity strength: a developing boundary layer near impact, an intermediate supercritical viscous layer and a region comprising the jump and subcritical flow. Unlike existing models, the approach does not require any empirically or numerically adjusted boundary conditions. We demonstrate that the stress or corner singularity for a film draining at the edge is equivalent to an infinite slope of the film surface, which we impose as the downstream boundary condition. The model is validated against existing experiment and numerical simulation of the boundary-layer and Navier–Stokes equations. We find that the flow in the supercritical region remains insensitive to the change in gravity level but is greatly affected by viscosity. The existence of the jump is not necessarily commensurate with the presence of recirculation, which is strongly dependent on the upstream curvature and steepness of the jump.

Evaluation of partial pressure CO2 change in the dialyzer blood inlet during hemodialysis as a measure of vascular access recirculation.

Vascular access recirculation during hemodialysis is associated with reduced effectiveness and worse survival outcomes. To evaluate recirculation, an increase in pCO2 in the blood of the arterial line during hemodialysis (threshold of 4.5 mmHg) was proposed. The blood returning from the dialyzer in the venous line has significantly higher pCO2 , so in the presence of recirculation, pCO2 in the arterial blood line may increase (ΔpCO2 ) during hemodialysis sessions. The aim of our study was to evaluate ΔpCO2 as a diagnostic tool for vascular access recirculation in chronic hemodialysis patients. We evaluated vascular access recirculation with ΔpCO2 and compared it with the results of a urea recirculation test, which is the gold standard. ΔpCO2 was obtained from the difference in pCO2 in the arterial line at baseline (pCO2 T1) and after 5 min of hemodialysis (pCO2 T2). ∆pCO2 = pCO2 T2-pCO2 T1. In 70 hemodialysis patients (mean age: 70.52 ± 13.97 years; hemodialysis vintage of 41.36 ± 34.54, KT/V 1.4 ± 0.3), ∆pCO2 was 4 ± 4 mmHg, and urea recirculation was 7% ± 9%. Vascular access recirculation was identified using both methods in 17 of 70 patients, who showed a ∆pCO2 of 10 ± 5 mmHg and urea recirculation of 20% ± 9%; time in months of hemodialysis was the only difference between vascular access recirculation and non-vascular access recirculation patients (22 ± 19 vs. 46 ± 36, p: 0.05). In the non-vascular access recirculation group, the average ΔpCO2 was 1.9 ± 2 (p: 0.001), and the urea recirculation % was 2.8 ± 3 (p: 0.001). The ΔpCO2 correlated with the urea recirculation % (R: 0.728; p < 0.001). ΔpCO2 in the arterial blood line during hemodialysis is an effective and reliable diagnostic tool for identifying recirculation of the vascular access but not its magnitude. The ΔpCO2 test application is simple and economical and does not require special equipment.

CFD based analysis of chlorination contact tank design

Chlorination is the most widely used technique for disinfecting water. Disinfection by chlorination inactivates pathogenic micro-organisms, preventing water-borne diseases. The design of a chlorination contact tank (CCT) requires precise residence time during which the chlorine passes through the tank and the consumption of a substantial quantity of chlorine during the disinfection process. The arrangements of baffles, as well as the inlet–outlet configurations, are also important which leads to the formation of a complex flow pattern causing the short-circuiting of the fluid flow and dead zone formation inside the CCT. Short-circuiting results in inefficient mixing, whereas, dead zones indicate the presence of residual chlorine in the tank. The objective of the present work is to report the preliminary results of the Computational Fluid Dynamics (CFD) analysis undertaken to investigate the hydrodynamics, turbulent transport and mixing inside the chlorination tank. Two different types of CCT geometry (rectangular and spiral) are considered for comparing the hydraulic and mixing efficiencies. Two-dimensional steady-state and time-variable numerical simulations are carried out. Species Transport modelling provides the distribution of residence time as well as chlorine concentration inside the tank. The presence of recirculation and dead zones in the chlorination tank are analysed from the velocity contour using image analysis. It was observed that amount of dead zones is significantly higher in the case of a rectangular tank when compared to a spiral one. Different hydraulic and mixing indexes were analysed in this study to compare the disinfection efficiency of the two tanks. The presence of secondary flow due to the centrifugal force facilitates the mixing process in spiral tanks.

Different Microfluidic Environments for In Vitro Testing of Lipid Nanoparticles against Osteosarcoma.

The use of lipid nanoparticles as biodegradable shells for controlled drug delivery shows promise as a more effective and targeted tumor treatment than traditional treatment methods. Although the combination of target therapy with nanotechnology created new hope for cancer treatment, methodological issues during in vitro validation of nanovehicles slowed their application. In the current work, the effect of methotrexate (MTX) encapsulated in different matrices was evaluated in a dynamic microfluidic platform. Effects on the viability of osteosarcoma cells in the presence of recirculation of cell media, free MTX and two types of blank and drug-containing nanoparticles were successfully assessed in different tumor-mimicking microenvironments. Encapsulated MTX was more effective than the equal dose free drug treatment, as cell death significantly increased under the recirculation of both types of drug-loaded nanoparticles in all concentrations. In fact, MTX-nanoparticles reduced cell population 50 times more than the free drug when 150-µM drug dose was recirculated. Moreover, when compared to the equivalent free drug dose recirculation, cell number was reduced 60 and 100 points more under recirculation of each nanoparticle with a 15-µM drug concentration. Thus, the results obtained with the microfluidic model present MTX-lipid nanoparticles as a promising and more effective therapy for pediatric osteosarcoma treatment than current treatment options.

Arteriovenous Fistula Surveillance Using Tomographic 3D Ultrasound

A well functioning arteriovenous fistula (AVF) is essential for haemodialysis. Despite regular duplex ultrasound (DUS) a significant number of AVFs fail. Tomographic 3D ultrasound (tUS) creates a 3D image of the AVF that can be interpreted by the clinician. DUS, tUS, and fistulograms were compared for the identification and measurement of flow limiting stenosis. Patients with AVF dysfunction on routine Transonic surveillance, defined as (1) > 15% reduction in flow on two consecutive occasions, (2) > 30% reduction in flow on one occasion, (3) flow of < 600 mL/sec, (4) presence of recirculation, underwent DUS. AVF tUS imaging was performed prior to fistulography. All fistulograms were reported by the same consultant radiologist and tUS images by the same vascular scientist blinded to the fistulogram results. Maximum diameter reduction in all stenoses were measured using all three imaging techniques. In 97 patients with 101 stenoses, the mean (± standard deviation [SD]) severity of stenosis was 63.0 ± 13.9%, 65.0 ± 11.6%, and 64.8 ± 11.7% for the fistulograms, DUS, and tUS respectively. The mean (± SD) time between ultrasound and fistulography imaging was 15.0 ± 14.5 days. Assuming the fistulogram as the "gold standard", Bland-Altman agreement for DUS was -1.9 ± 15.5% (limit of agreement [LOA] -32.2 - 28.4) compared with -1.7 ± 15.4% (LOA -31.9 - 28.4) for tUS. Median (± interquartile range) time to complete the investigation was 09:00 ± 03:19 minutes for DUS and 03:13 ± 01:56 minutes for tUS (p < .001). DUS and tUS were equally accurate at detecting AVF complications but tUS investigation requires less skill and was significantly quicker than DUS.

A numerical study of mass transfer from laminar liquid films

Abstract

Arterio-Venous Fistula Surveillance Using Tomographic 3D Ultrasound

Introduction: A well-functioning Arterio-Venous Fistula (AVF) is essential for haemodialysis. Despite regular Duplex Ultrasound (DUS), a significant number of AVF's fail within the first year. Tomographic 3D ultrasound (tUS) creates a 3D CT-like image of the AVF that can be interpreted by the Clinician. We compared DUS, tUS and fistulograms for the identification and estimation of flow-limiting stenosis. Methods: Patients with suspected AVF dysfunction at routine Transonic® surveillance, meeting certain criteria, underwent DUS. Criteria for DUE were; 1) >15% reduction in access flows on two consecutive occasions, 2) >30% reduction in access flow on one occasion, 3) access flow of < 600ml, 4) presence of recirculation. On the day of fistulography, DUS was repeated, in addition to tUS imaging of the fistula including the stenosis. Delayed tUS measurements were performed by a Vascular Scientist blinded to the fistulography result, reported by one Consultant Radiologist, and the DUS. Maximum diameter reduction at any stenosis was measured using all three techniques. Observer agreement was also measured. Results: In forty patients with thirty two stenoses, mean (sd) stenosis was measured as 61.9(±16.6)%, 66.9(±9.51)% and 67.1(±11.9)% for the fistulograms, tUS and DUS respectively. tUS observer agreement was excellent [intra ICC = 0.94 (95%CI 0.86-0.97) and inter ICC = 0.87 (95%CI 0.72-0.94), p< 0.001]. Bland-Altman agreement for tUS was -5.1(±17.9)% (95%CI -40.2-30.1) compared with -5.2(±17.3)% (95%CI -39.1-28.6) for DUS with the fistulogram as the index standard. Mean (sd) tUS scan time was 3.7(±2.2) min, compared to 9.9(±3.7) min for duplex, p< 0.001. Conclusion: DUS and tUS were equally accurate at detecting complications compared to fistulography. tUS has excellent observer agreement, takes less skill and is significantly quicker than DUS in detecting stenoses in an AVF. There is a risk reduction for repetitive strain injury when people use tUS for AVF surveillance compared with DUS due to the shorter scan time. Disclosure: This work is funded by the European Commissions Horizon 2020 scheme.

CFD simulation of flow through an orifice plate

In this present paper, the commercial Computational Fluid Dynamics (CFD) is used to predict the flow features in the orifice flow meter. Outcomes of the CFD simulations in terms of profiles of velocity and pressure are discussed in detail. It is observed that the flow is jet-like flow in the core region and the presence of recirculation, reattachment and shear layer regions flow features downstream the orifice. The location of vena-contracta was also estimated from CFD simulations. These results are consistent with other published data.

Cannula Design and Recirculation During Venovenous Extracorporeal Membrane Oxygenation

Extracorporeal membrane oxygenation (ECMO) is used as a lifesaving rescue treatment in refractory respiratory or cardiac failure. During venovenous (VV) ECMO, the presence of recirculation is known, but quantification and actions to minimize recirculation after measurement are to date not routinely practiced. In the current study, we investigated the effect of draining cannula design on recirculation fraction (Rf) during VV ECMO; conventional mesh cannula was compared with a multistage cannula. The effect of adjusting cannula position was also studied. Recirculation was measured with ultrasound dilution technique at different ECMO flows and after cannula repositioning. All patients who were admitted to our unit between October 2014 and July 2015 catheterized by the atrio-femoral single lumen method were included. A total of 108 measurements were conducted in 14 patients. The multistage cannula showed significantly less recirculation (19.0 ± 12.2%) compared with the conventional design (38.0 ± 13.7). Pooled data in cases improved from adjustment showing reduced Rf by 7%. In conclusion, the choice of cannula matters, as does adjustment of the draining cannula position during atrio-femoral VV ECMO. By utilizing the ultrasound dilution technique to measure Rf before and after repositioning, effective ECMO flow can be improved for a more effective ECMO treatment.

Identification of the Cold Flow Perturbation Sources in a Dump Combustor With a Tapered Exit

This paper reports on the experimental investigation of the flow inside a low aspect ratio (length less than the reattachment length of separated flow) dump combustor with a tapered exit. The flow field in the combustor is evaluated using wall pressure and velocity measurements at varying flow Reynolds numbers. The mean velocity and turbulence variation closer to the wall of the combustor was found to be different from that at other radial locations due to the presence of recirculation and possible thickening of the shear layer caused by a decrease in the strength of the potential core. The power spectral study of the wall pressure and velocity fluctuations suggested the dominant presence of acoustic perturbations with amplitude modulation of such perturbations due to viscous dissipation in the shear layer.

Remediation of chlorophenol- and phenol-contaminated groundwater by a sequencing batch biofilm reactor

The paper describes the results of an investigation aimed at evaluating suitability of a lab-scale Sequencing Batch Biofilm Reactor (SBBR) for the remediation of groundwater contaminated by phenol (P) and 2-chlorophenol (2-CP). Kinetics of compound degradation was determined along the bed height in the absence and in presence of effluent recirculation, and with different influent composition (compounds fed separately or in combination in the same stream). SBBR performances with and without recirculation were very satisfactory for all the influent compositions: the system showed 99% removal efficiencies for both phenol and 2-CP and their complete removal was always achieved far before the end of react. In the presence of recirculation, the concentration gradient established during fill was rapidly eliminated and an even biomass distribution along the bed height was formed. Consequently, an acceleration of the elimination process was observed, particularly for phenol that was mostly removed in the first hour of the cycle. When the compounds were fed simultaneously, 2-CP removal kinetics improved probably due to cometabolism. The adsorption phenomena of the toxic compounds on the packing material were studied also, showing about 50% COD removal after 7 hours contact time.

Carousel Effect in Alveolar Models

Experimental work over the past decade has shown that recirculation in alveoli substantially increases the transport of particles. We have previously shown that, for nondiffusing passive particles, this can be understood with the aid of Moffatt's famous corner flow model. Without wall motion, passive particles recirculate in a regular fashion and no chaos exists; however, wall motion produces extensive chaotic flow. Aerosols typically do not follow this flow as they are fundamentally different from fluid particles. Here, we construct a simple model to study diffusing particles in the presence of recirculation. We assume that all particles are passive, that is to say that they do not significantly alter the underlying flow. In particular, we consider particles with high Peclet number and neglect inertial effects. We modify the Lagrangian system for corner eddies to accommodate diffusing particles. Particle transport is governed by Langevin equations. Ensembles of diffusing particles are tracked by numerical integration. We show that transport of diffusing particles is enhanced by sufficiently strong underlying recirculation through a mechanism that we call the "carousel effect." However, as the corner is approached, the recirculation rapidly decreases in intensity, favoring motion by diffusion. Far from the corner's apex, recirculation dominates. For real alveoli, the model indicates that sufficiently strong recirculation can enhance transport of diffusing particles through the carousel effect.

Experimental heat transfer and flow analysis of a vented brake rotor

Brake rotors generate an opposing torque to a shaft, converting kinetic energy to heat thus necessitating heat transfer considerations. To better understand convection through the fins of a brake rotor, experimental and analytical methods were employed. The experimental approach involved two aspects, assessment of both heat transfer and fluid motion. A transient experiment was conducted to quantify the internal (fin) convection and external (rotor surface) convection terms for three nominal speeds. For the given experiment, conduction and radiation were determined to be negligible. Rotor rotational speeds of 342, 684, and 1025 [rpm] yielded fin convection heat transfer coefficients of 27.0, 52.7, 78.3 [W m −2 K −1], respectively, indicating a linear relationship. At the slowest speed, the internal convection represented 45.5% of the total heat transfer, increasing to 55.4% at 1025 [rpm]. Analytical solutions were evaluated where possible for the convective terms and found to be comparable to the experimental results. The flow aspect of the experiment involved the determination of the velocity field through the internal passages formed by the radial fins. Utilizing PIV (particle image velocimetry), the phase-averaged velocity field was determined. A number of detrimental flow patterns were observed, notably entrance effects and the presence of recirculation on the suction side of the fins.

Development of an adaptive Discontinuous-Galerkin finite element method for advection–reaction equations

This paper presents an adaptive Discontinuous-Galerkin finite element method to solve linear advection–reaction equations. The method was developed for the prediction of hemolysis in prosthetic devices, where the flow may include recirculation zones. To ensure the well-posedness of the problem, we solve the transient form of the PDE. Time discretization is achieved with an implicit Euler scheme in order to obtain the steady-state solution rapidly. Linear interpolation functions are used to approximate the hemolysis field. The a posteriori estimation of the error is obtained by solving an advection–reaction equation for the error, which is approximated with quadratic interpolation functions. Then, elementary estimations of the error are computed using a semi-norm developed to achieve the best possible results, even in the presence of recirculation. Code verification – to assess the convergence and conservation properties of the method – is performed with the method of manufactured solutions and with a conservation test using a sharp gaussian function as a reaction term. Excellent results are obtained in both cases. Predictions of hemolysis in a dialysis cannula are presented as an engineering application of our method.

Can blood flow surveillance and pre-emptive repair of subclinical stenosis prolong the useful life of arteriovenous fistulae? A randomized controlled study

Background. Stenosis is the main cause of arteriovenous fistula (AVF) failure. It is unclear, however, if surveillance for stenosis enhances AVF function and longevity and if there is an ideal time for intervention. Methods. In a 5-year randomized, controlled, open trial we compared blood flow surveillance and preemptive repair of subclinical stenoses (one or both of angioplasty and open surgery) with standard monitoring and intervention based upon clinical criteria alone to determine if the former prolonged the longevity of mature forearm AVFs. Surveillance with blood pump flow (Qb) monitoring during dialysis sessions and quarterly shunt blood flow (Qa) or recirculation measurements identified 79 AVFs with angiographically proven, significant (>50%) stenosis. The AVFs were randomized to either a control group (intervention done in response to a decline in the delivered dialysis dose or thrombosis; n ¼ 36) or to a pre-emptive treatment group (n ¼ 43). To evaluate a possible relationship between outcome and haemodynamic status of the access, AVFs were divided into functional and failing subgroups, according to Qa values higher or lower than 350 ml/min or the absence or presence of recirculation. Results. A Kaplan–Meier analysis showed that preemptive treatment reduced failure rate (P ¼ 0.003) and the Cox hazards model identified treatment (P ¼ 0.009) and higher baseline Qa (P ¼ 0.001) as the only variables associated with favourable outcome. Primary patency rates were higher in treatment than in control AVFs in both functional (P ¼ 0.021) and failing subgroups (P ¼ 0.005). They were also higher in functional than in failing AVFs in both control (P 350 ml/min portends a superior outcome with pre-emptive action in AVFs.

Estimation of organ transport function for recirculating indicator dilution curves.

The transport function of an indicator through an organ allows the calculation of important physiological parameters, but its estimation, especially in the presence of recirculation, can be difficult. In this paper, we estimate the transport function of 3H-mannitol (an extracellular tracer of glucose) in the human leg skeletal muscle. To do so, an indicator bolus is administered into the femoral artery and its recirculating dilution curves are nonuniformly sampled in both the femoral artery and the femoral vein. A new deconvolution-based method is used to simultaneously estimate the indicator transport function and the organ plasma flow. Subsequently, the indicator mean transit time and distribution volume are calculated. The reliability of the method is assessed by Monte Carlo simulation. The ability to estimate parameters, like mean transit time and extracellular distribution volume, is critical to the study of pathophysiologic states such as diabetes, insulin resistance, and hypertension.

Modelling engineering flows with Reynolds stress turbulence closure

A Reynolds stress model consists of differential or algebraic equations each governing the balance between generation, dissipation, redistribution and transport of a related Reynolds stress. More generally, “second-moment closure” combines the above stress equations with additional transport equations for heat or scalar fluxes. This paper argues the potential advantages of using such closures in preference to eddy viscosity models in the presence of recirculation, swirl and buoyancy. Some basic forms of Reynolds stress closure are introduced and their numerical implementation is outlined. Results are then presented for five out of some eighteen complex flow types computed at UMIST over the past six years. These demonstrate that significant improvements in predictive accuracy can be achieved in flows containing large recirculation regions and involving swirl. In all cases, such improvements appear to be rooted in the ability of the closures to mimic the strong response of the turbulence structure to streamline curvature.

Modelling engineering flows with Reynolds stress turbulence closure

A Reynolds stress model consists of differential or algebraic equations each governing the balance between generation, dissipation, redistribution and transport of a related Reynolds stress. More generally, “second-moment closure” combines the above stress equations with additional transport equations for heat or scalar fluxes. This paper argues the potential advantages of using such closures in preference to eddy viscosity models in the presence of recirculation, swirl and buoyancy. Some basic forms of Reynolds stress closure are introduced and their numerical implementation is outlined. Results are then presented for five out of some eighteen complex flow types computed at UMIST over the past six years. These demonstrate that significant improvements in predictive accuracy can be achieved in flows containing large recirculation regions and involving swirl. In all cases, such improvements appear to be rooted in the ability of the closures to mimic the strong response of the turbulence structure to streamline curvature.

Estimation of regional myocardial blood flow using computed tomography: a stochastic formulation.

Previously, we have presented a deterministic formulation for estimation of regional myocardial blood flow (RMBF) using X-ray computed tomography (CT) [1]. Quantitation of RMBF with the deterministic theory requires computing the zeroth and first moments of extrapolated myocardial contrast enhancement (CT numbers versus time) curves. This extrapolation is a potential source of error in the presence of recirculation, especially in myocardial regions with reduced flows. In the present paper, a stochastic approach for parameter estimation is undertaken, which renders an optimal RMBF estimate based upon the least squares error criteria. Random measurement errors are minimized, curve extrapolation is avoided, and the accuracy of RMBF estimates is predicted. The advantages of the stochastic versus the deterministic approach are demonstrated in the results obtained from the in vivo estimation of RMBF in normal and acutely ischemic myocardium of dogs.

Cold flow mixing rates with recirculation for pulverized coal reactors

AbstractThe mixing of particles and gases in confined, coaxial jets in the presence of recirculation was studied. Particle and gas mixing rates were measured for conditions simulating processes involving pulverized coal combustion and entrained gasification. The effects of mixing chamber diameter, gas velocity, particle size, and particle loading level on mixing rates were determined from particle mass flux and gas tracer measurements. In every case, the gas mixed much faster than did the particles. Introduction of an expanded mixing chamber of larger diameter (with recirculation zone) significantly increased both gas and particle mixing rates over that for a smaller mixing chamber without recirculation. However, this observed increase was not strongly dependent upon the diameter of the larger mixing chamber. Increasing secondary velocity also increased mixing rates significantly.