Secondary Flow Region Research Articles

CFD analysis of non-Newtonian blood flow through human carotid artery bifurcation: Carotid sinus susceptible to atherosclerosis

Fat accumulation on the arteries' walls has become a serious health concern. This study conducted an in-depth hemodynamic analysis to identify the regions of a human carotid artery most susceptible to fat accumulation or atherosclerosis, considering the following factors: blood velocity, secondary flow regions, pressure, and wall shear stress. The hemodynamic analysis used a generalized geometric model to analyze these factors at various locations within the carotid bifurcation over different time points. Results showed that the bifurcation region is comparatively more affected by the factors contributing to atherosclerosis. Especially, the sinus region experiences all the key factors: a larger secondary flow, higher blood pressure, lower mean WSS, and lower fluctuation in temporal WSS. The findings indicate that the bifurcation region, particularly the sinus is prone to fat accumulation. These results are consistent with clinical observations, emphasizing the bifurcation and sinus regions as key sites for plaque accumulation. Insights from this analysis will serve as a foundation for optimizing geometric models of bifurcated arteries to improve blood flow and, consequently, reduce the risk of atherosclerosis.

Numerical study on the gas-solid diffusion characteristics of airflow and dust particles in metal mine blasting excavation under three hybrid ventilation modes

Dust pollution has long been a challenge in the blasting excavation of underground metal mines, and uncontrolled diffusion of dust particles seriously endangers the health and safety of workers as well as the environment in underground space. Most existing research on dust diffusion focuses on fully mechanized coal mining face or borehole drilling, where the dust source emission characteristics differ significantly from metal mine blasting excavation. Furthermore, it is still unclear how the ventilation modes of Far-Pressing-Far-Absorption (FPFA), Near-Pressing-Far-Absorption (NPFA), and Far-Pressing-Near-Absorption (FPNA) affect the mechanism of dust diffusion in the breathing zone. In this work, gas-solid flow characteristics in a typical metal mine blasting tunnel are numerically studied based on Euler-Lagrange method. The interphase forces between airflow and dust particles are comprehensively modeled, and the particle diffusion effect caused by fluid turbulence is described by the discrete random walk model. Five typical regions for airflow disorder phenomena in the working tunnel are predicted, including the jet development region, the return airflow core region, the airflow reversal region, the airflow recirculation region, and the secondary flow region. The spatial distribution of dust exhibits nonuniform characteristics under turbulent airflow transport. The time results of the monitored dust concentration reveal that the junction of the working tunnel and the return airway is a key area of focus for the prevention and control of dust pollution. Among the three ventilation modes, the Far-Pressing-Near-Absorption (FPNA) ventilation mode has the best dust removal efficiency. The supply vent further from the blasting face allows the jet to fully develop, while the exhaust vent closer to the blasting face allows dust particles to be drawn in and discharged earlier. Under the condition of fixed total power consumption for the supply and exhaust fans, a larger exhaust airflow velocity helps remove dust from the breathing zone.

Uncertainty quantification of turbine endwall aero-thermal performance under combustor-turbine interface louver coolant and cavity

The uncertainty characteristics have a significant influence on turbine cooling performance and its reliability, especially for the combustor-turbine interface region which faces more complicated flow interactions. Previous researches under the certainty framework are not able to reflect the realistic cooling characteristics of the interface region under real operation circumstances. Considering the random fluctuations of the major structure and flow parameters, uncertainty quantification of turbine endwall cooling performance under realistic combustor-turbine interface conditions is conducted in this paper using the polynomial chaos expansion method. The mix characteristic of combustor louver coolant and interface cavity leakage in the turbine endwall region is investigated, and the influences of parameter uncertainties on endwall cooling, vane phantom cooling, and cascade aerodynamic characteristics are analyzed. The results indicate that the uncertainties at the combustor-turbine interface largely influence the endwall cooling by modifying the strength and position of the secondary flows. Under the random fluctuations of interface geometry parameters, the local mean value of endwall cooling effectiveness can reach up to 0.25, and the standard deviation of endwall cooling effectiveness can reach 0.1. The geometrical parameters of interface width and misalignment height are the decisive factors for endwall cooling uncertainty. The random fluctuations of interface width and misalignment height separately cause great cooling effectiveness uncertainties on the pressure side endwall and suction side hot ring. The uncertainties in mainstream and structure parameters exhibit a reduced propensity to propagate onto vane phantom cooling. The decisive factors for vane phantom cooling uncertainty are interface width and misalignment height, and the major influenced area is the vane middle part. Four mainstream and structure parameters all make great contributions to the aerodynamic uncertainty at the strong secondary flow region and vane wake flow region. This paper can provide a theoretical basis for the design of combustor-turbine interface cooling structures to accommodate the actual turbine operation uncertainty conditions.

Characterization of the Endwall Flow in a Low-Pressure Turbine Cascade Perturbed by Periodically Incoming Wakes, Part 2: Unsteady Blade Surface Measurements Using Pressure-Sensitive Paint

Unsteady pressure-sensitive paint (i-PSP) measurements were performed at a sampling rate of 30 kHz to investigate the near-endwall blade suction surface flow inside a low-pressure turbine cascade operating at engine-relevant high-speed and low-Re conditions. The investigation focuses on the interaction of periodically incoming bar wakes at 500 Hz with the secondary flow and the blade suction surface. The results build on extensive PIV measurements presented in the first part of this two-part publication, which captured the ’negative-jet-effect’ of the wakes throughout the blade passage. The surface pressure distributions are combined with CFD to analyze the flow topology, such as the passage vortex separation line. By analyzing data from phase-locked PIV and PSP measurements, a wake-induced moving pressure gradient negative in space and positive in time is found, which is intensified in the secondary flow region by 33% with respect to midspan. Furthermore, two methods of frequency-filtering based on FFT and SPOD are compared and utilized to associate a pressure fluctuation peak around 678 Hz with separation bubble oscillation.

Gas-Solid Flow Characteristics of Airflow and Dust Particles in Blasting Excavation of Underground Metal Mine Tunnels.

Effective dust removal has long been a challenge in the blasting mining of underground metal mine tunnels, and uncontrolled dust diffusion seriously endangers workers' respiratory systems and the underground space safety environment. However, the vast majority of existing numerical studies on dust diffusion are focused on coal mine fully mechanized mining, which is different from metal mine blasting excavation in terms of stope structure and dust properties. Furthermore, the mechanism by which the forced and exhaust ventilation modes affect the diffusion characteristics of inhalable particles is unclear. In this work, gas-solid flow characteristics for dust diffusion in a typical metal mine blasting tunnel were numerically investigated based on the Euler-Lagrange method, where the blasting face instantly released 6.37 × 107 particles with 100 different sizes, ranging from 0.8 to 200 μm. The interphase forces between airflow and dust particles are comprehensively modeled, and the particle diffusion effect caused by fluid turbulence is described by a discrete random walk model. Detailed information for airflow turbulence and particle migration was revealed, and dust removal efficiencies for inhalable particulate matter (PM10) by forced, exhaust, and hybrid ventilation were analyzed. Numerical results predict a complex airflow pattern in the working roadway, including the jet-flow region, return airflow core region, airflow disorder region, and secondary flow region. Dust diffusion temporal characteristics can be divided into three stages, namely, the initial stage of dust generation, the efficient ventilation and dust removal stage, and the later stage of dust diffusion. Dust diffusion spatial characteristics indicate that under the Coanda wall attachment effect, the dust concentration exhibits nonuniform distribution in both vertical and horizontal directions of the return air roadway. The dust removal efficiency of hybrid ventilation on inhalable particles above respiratory height is better than that of forced ventilation, especially in the return air roadway. The additional exhaust air duct based on forced ventilation can discharge more inhalable particles from the tunnel.

Particle Image Velocimetry Measurements in a High-Speed Low-Reynolds Low-Pressure Turbine Cascade

Abstract Particle image velocimetry (PIV) measurements in the blade-to-blade (B2B) plane and cascade outlet plane (COP) of a high-speed low-pressure turbine (LPT) cascade were performed at engine-representative outlet Mach number (0.70–0.95), and Reynolds number (70–120 k) under steady flow conditions. The freestream turbulence characteristics were imposed by means of a passive turbulence grid. The PIV results on the B2B plane were compared against five-hole probe (5HP) and Reynolds-averaged Navier–Stokes (RANS) computations to assess the validity of measurement and simulation techniques in the engine-relevant LPT cascade flows. The PIV captured the wake depth and width measured by the 5HP whereas the RANS displayed an overprediction of the wake Mach number deficit. The 5HP was found to impose sinewave fluctuations of the measured flow angle downstream, around three times higher than PIV. Additionally, PIV estimated turbulence intensity (TI) in the cascade, showing TI decay along a streamline. At the highest Mach number, a peak TI occurred past a shock wave. Measurements of the outlet flow field highlighted a high TI in the secondary flow region whereas high degree of anisotropy (DA) was registered in the boundary of the secondary flow and freestream regions. The contribution of the streamwise fluctuation component was found to be less than the crosswise and radial components in the freestream region. Increasing the cascade outlet Mach number, the contribution of streamwise fluctuation to the DA was observed to decrease.

Impact of Unsteady Wakes on the Secondary Flows of a High-Speed Low-Pressure Turbine Cascade

The aerodynamics of a high-speed low-pressure turbine (LPT) cascade were investigated under steady and unsteady inlet flows. The tests were performed at outlet Mach (M) and Reynolds numbers (Re) of 0.90 and 70k, respectively. Unsteady wakes were simulated by means of a wake generator equipped with bars. A bar reduced frequency (f+) of ∼0.95 was used for the unsteady case. The inlet flow field was characterized in terms of the total pressure profile and incidence. The blade aerodynamics at midspan and the secondary flow region were investigated by means of pneumatic taps and hot-film sensors. The latter provided a novel view into the impact of the secondary flows on the heat transfer topology on the blade suction side (SS). The cascade performance was quantified in terms of the outlet flow angle and losses by means of a directional multi-hole probe. The results report the phase-averaged impact of unsteady wakes on the secondary flow structures in an open test case high-speed LPT geometry.

Investigation on multiphase flow of multi-size cuttings particles and non-Newtonian drilling fluids in oil and gas horizontal well drilling using kinetic theory of granular flow

Effective cuttings transport has long been a challenge in oil and gas horizontal drilling, and inadequate wellbore cleaning can result in severe drilling accidents including stuck pipe. The transportation of multi-size cuttings by non-Newtonian drilling fluids in the horizontal wellbore with a rotating drill pipe was systematically investigated using the kinetic theory of granular flow. Collisions between particles and their two-way coupling with drilling fluid turbulence were modeled. Flow regime of multi-size cuttings has been predicted, and effects of particle size composition characteristics were studied. The spatial inhomogeneity of the non-Newtonian fluid viscosity and the sedimentation of particles result in a complex stratified flow regime for multi-size cuttings transport. Cuttings concentration and drilling fluid axial velocity distributions both show swaying phenomena with drill pipe rotation. Moreover, the secondary flow regions appear with negative tangential velocities. The cuttings transport ratio (CTR = 0.846) is the best in the case of cuttings with different particle sizes injected in a uniform concentration. While in the case where the concentration of injected multi-size cuttings is negatively correlated with particle size, the cuttings velocity fluctuations caused by collisions and frictions are the most severe, and the borehole cleaning is the worst (CTR = 0.803).

Experimental and numerical analysis of convective heat transfer and entropy generation of graphene/water nanofluid in AEAOT heat exchanger

BackgroundEffective utilization of nanofluid's potential and optimal heat exchanger design is paramount in addition to heat transfer enhancement. A novel alternate elliptical tube is fabricated, and graphene nanofluids are chosen to enhance the convective heat transfer characteristics. MethodsThe experimental setup is validated with circular pipe results before conducting graphene nanofluid experiments. The finite volume method was adopted to conduct simulation results, and its validation was done with the experimental results. The results of numerical simulations are compared with experiments, and the obtained mean percentage error (MPE) is 1.17%, 8.71%, 8.05% and 8.85% for water, G0.05%, G0.1% and G0.2%, respectively. Experiments are conducted at different mass flow rates and inlet temperatures of nanofluid. Significant findingsThe oval tube in an alternate direction improves the secondary flow region with a high swirl, leading to a smaller thermal boundary layer thickness. Nusselt number enhances as an average of 10.3%, 29.2% and 39.1% for graphene nanofluids prepared using weight concentrations 0.05%, 0.1% and 0.2%, respectively, at 80 °C. The effect of inlet temperature reveals heat transfer coefficient and Nusselt number increase and 1.57, 1.88, 2.16 times than water is observed for G0.05%, G0.1% and G0.2%, respectively. Pertinent to entropy generation, Bejan number (Be) decrease with graphene nanoparticle concentration; as inlet temperature increases, corresponding fluid friction irreversibilities are observed as 18.4%, 24.6% and 36.4% for G0.05%, G0.1% and G0.2% nanofluids, respectively. The present work caters for the importance of AEAOT design which prompts heat transfer enhancement; indeed, optimization of transition length and aspect ratio are essential to overcome the pressure loss cost.

Enhanced performance of a microchannel with rectangular vortex generators

Microchannel heat sinks and heat exchangers are widely used in the cooling of electronic systems. However, it is still important to enhance the heat transfer in the microchannel so that the intense heat generated can be removed. Vortex generators (VGs) create secondary flow structures in the flow, increasing the fluid mixing, thinning the thermal boundary layer, and ultimately boosting heat transfer. Here, we have controlled the flow structure and improved the heat transfer with the lowest possible pressure loss by placing VGs of different sizes, numbers, and angles of attack in a microchannel. The improvement in heat transfer is accelerated as vortex intensity increases. The angle of attack has a significant impact on vortex formation lengths, which reach high dimensions around 90°. Furthermore, increasing the VG length significantly increases the vortex formation lengths. The number of VG pairs has a significant impact on heat transfer and pressure losses. As the number of VG pairs increases, so does the area occupied by the secondary flow regions in the microchannel, increasing the fluid mixture and boosting heat transfer. The highest enhancement in heat transfer using VGs is obtained at around 230%, while the corresponding increase in pressure loss is 950%. According to the JF factor which we consider a performance evaluation criteria, the best result is around 1.38. The Genetic Aggregation Response Surface Methodology has been applied to numerical results. The related method is realized to produce results that are consistent with the numerical results within a ±5% error interval. All the input parameters considered in the sensitivity analysis have an impact of at least 10% on the output parameters.

Quasi-static secondary flow regions formed by microfluidic contraction flows of wormlike micellar solutions

This study investigates the effects of yield stress (τ0) and shear banding on the fluidic behaviors of cetyltrimethylammonium bromide/sodium salicylate wormlike micellar solutions flowing through a microfluidic planar contraction (8:1) geometry. Test solutions with different surfactant concentrations (Cd = 75, 87.5, and 100 mM) at a fixed molar ratio of salt to surfactant (R = 0.32) were characterized by shear and extensional rheometry. While the lower concentrated test solution (Cd = 75 mM) with low τ0 (≈ 0.02 Pa) and no shear banding showed a Newtonian-like flow behavior for Mach number, Ma < 1, the flow with corner vortices was formed when Ma exceeds unity. For higher Cd (87.5 and 100 mM), new fluidic phenomena are documented: (i) even at a low volumetric flow rate (Q), the fluid velocity at upstream corners was slower than that of Newtonian-like flows and (ii) at higher Q, the secondary flow with a quasi-static condition was formed at Ma well lower than unity. Micro-particle image velocimetry showed the lower shear rates at upstream corners, which can be understood by the effects of contraction entry, shear thinning, and high yield stress. The quasi-static secondary flow region was not induced by generation of elastic shock waves; instead the shear banding was found to be the underlying mechanism for the separation of the region from the main flow. In addition, the length of secondary flow regions showed a close correlation with the Deborah number, which was calculated using the extensional relaxation time.

Numerical study on the influence of shroud cavity in the high-pressure centrifugal compressor for compressed air energy storage system

As a key energy equipment of the compressed air energy storage (CAES) system, the centrifugal compressor with shroud cavity is employed to avoid the leakage flow from the rotor, especially in the high-pressure stages. In order to clarify the leakage characteristics and its influences, the high-pressure centrifugal compressor with shroud cavity is taken as the research object in the present work. First, the leakage flow across the shroud cavity has been studied numerically, it’s found that there are negative effects on the high-pressure compressor’s aerodynamic performance by the shroud cavity. And then, the flow characteristics inside the shroud cavity are showed by finding several secondary flow regions and cavity vortices. In addition, through the entropy contours and streamlines, the influence on the main flow by the leakage are analysed in detail. By analysing the influence of the shroud cavity on the high-pressure compressor, this paper can provide a practical guidance for the design of the impeller in the CAES system.

Effect of Non-Newtonian Fluid Behavior on Forced Convection from a Cluster of Four Circular Cylinders in a Duct, Part I: Power-Law Fluids

Forced convection heat transfer in power-law fluids has been investigated numerically around four identical circular cylinders in a diamond array in a square enclosure. For the laminar flow, the governing equations have been solved numerically over the following ranges of parameters: Reynolds number (5 − 200), Prandtl number (0.7 − 100), power-law index (0.2 − 2) and center-to center gap between cylinders (0.3 − 0.7) to elucidate their influence. The detailed kinematics and engineering parameters are influenced by the gap between the cylinders via the development of multiple secondary flow regions and/or the splitting of incoming fluid stream. The drag on the trailing cylinders with reference to the lead cylinder can be up to ± ∼ 80% higher or lower depending upon the gap between the cylinders, power-law index and Reynolds numbers. In compact arrays, the drag becomes slightly negative due to the reverse flow for certain combinations of power-law index (< 1) and high Reynolds numbers (100 and 200). Similarly, the heat transfer affected by the subsequent cylinders ranges from 50-60% for the second cylinder which drops to ∼10% for the last cylinder. Finally, the functional dependence of the Nusselt number has been consolidated in terms of Reynolds number, Prandtl number, power-law index and the gap ratio.

Experimental Study on Deteriorated Performance, Vibration, and Geometry Changes of an Electrical Submersible Pump Under Sand Water Flow Condition

AbstractIn the oil and gas industry, the proppant backflow from fracturing wells severely reduces the lifespan of widely used downhole electrical submersible pumps (ESPs). In field applications, a minimal sand concentration may cause severe damage to ESPs in a short time. In order to resist the three-body abrasive sand wear, flanged tungsten carbide sleeves are used in ESPs. However, the wear-resistant performance of different pump geometry is not well analyzed and understood. Therefore, a 64 h pump erosion and abrasion test was conducted with water at the pump's best efficient flowrate with a sand concentration of 1 wt% to imitate the damage caused by short-term proppant backflow. The test was divided into several periods, after which the pump performance, paint-removal wear pattern, eroded pump geometries, and stage vibration were measured and recorded. The wear-rate on pump stage geometries gradually decreased at the beginning of 8–16 h. Then, the carbide sleeves started to help sustain the pump rotation. As a result, the wear-rate and pump vibration became relatively stable. Therefore, the wear mechanism in the secondary flow region (seal rings and sleeves) is believed to change from abrasive wear to the combined erosive-abrasive wear. The pump overall performance decreased by approximately 10% after the 64 h test. The performance, pump geometry, and vibration data are previous for understanding wear mechanism, predicting failures, improving pump design, and saving the well service cost.

Entropy Study on the Enhanced Heat Transfer Mechanism of the Coupling of Detached and Spiral Vortex Fields in Spirally Corrugated Tubes

The present work numerically studied the enhanced heat transfer mechanism of the coupled fields of detached and spiral vortices in symmetrical and asymmetrical spirally corrugated tubes. The heat transfer and viscous dissipation were analyzed by evaluating the entropy combined with the flow patterns, and the global entropy generation rate was analyzed by considering the local Nusselt number and friction factor. The results indicated that, both the leeside and windward corrugation angles had obvious effects on the strength of detached vortex and spiral flow, and also the location of the vortex. The maximum values of the heat transfer entropy were located at the boundary layers, and the heat transfer entropy of the secondary flow region was more distinguished than in other parts of the main flow region. The maximum values of friction entropy generation were located at both the boundary layers and the core of detached vortex. The performance evaluation criterion (PEC) presents nearly the same values for the four cases, when the Reynolds number (Re) is less than 6,300. In addition, the PEC of symmetrical spirally corrugated tube with corrugation angle equal to 25° showed the best performance, when Re is greater than 6,300. To keep the PEC above 1, Re should not exceed 33,000.

Low-frequency modes in the fluid-structure interaction of a U-tube model for the steam generator in a PWR

Abstract In the SG (steam generator) of PWR (pressurized water reactor) for a nuclear plant, hundreds of U-shaped tubes are used for the heat exchanger system. They interact with primary pressurized cooling water flow, generating flow-induced vibration in the secondary flow region. A simplified U-tube model is proposed in this study to apply for experiment and its counterpart computation. Using the commercial code, ANSYS-CFX, we first verified the Moody chart, comparing the straight pipe theory with the results derived from CFD (computational fluid dynamics) analysis. Considering the virtual mass of fluid, we computed the major modes with the low natural frequencies through the comparison with impact hammer test, and then investigated the effect of pump flow in the frequency domain using FFT (fast Fourier transform) analysis of the experimental data. Using two-way fluid-structure interaction module in the CFD code, we studied the influence on mean flow rate to generate the displacement data. A feasible CFD method has been setup in this research that could be applied potentially in the field of nuclear thermal-hydraulics.

Motion, deformation, and aggregation of multiple red blood cells in three-dimensional microvessel bifurcations

Vessel bifurcation is a place where many diseases start, such as the atherosclerosis, because the flow in this place is complex involving secondary flow and/or stagnation regions, and the cells are often blocked to aggregate together. A numerical study has been conducted to analyze three-dimensional motion, deformation, and aggregation of multiple red blood cells (RBCs) in a microvessel bifurcation. A smoothed dissipative particle dynamics model is used to simulate the fluids inside and outside of the RBCs. The RBC membrane is modeled as a triangular network, associated with a deformation potential energy and an aggregation potential energy to describe the RBC deformation and aggregation, respectively. The interaction between the fluid and the RBCs is modeled by the immersed boundary method. The numerical models are first validated by examining the rheology of multiple RBCs in a cylindrical tube. Then, we investigate the effect of number of RBCs, mechanical properties, and interaction strength on their motion, deformation, and aggregation. The simulation results showed that the leading RBC has more deformation, compared with subsequent RBCs. The larger the RBC number, the easier the RBCs aggregate. The RBC deformation has an obvious effect on the RBC aggregation, whereas the RBC aggregation has a slight effect on the RBC deformation. Both the RBC deformation and aggregation can cause the RBC centroid to deviate at the apex of bifurcation, and this determines which branch the RBCs move into.

Impact of temperature and concentration dispersion on the physiology of blood nanofluid: links to atherosclerosis

A theoretical model of blood-silver nanofluid flow through an \(\omega \)-shaped tapered stenotic artery in the presence of a catheter is understood in detail. The rheology of the blood is considered as that of a micro-polar fluid. Suspension of the silver nano-particles in the micro-polar fluid is proposed to investigate the temperature and concentration dispersion from the immersed temperature-sensitive drug-coated nano-particles. The effects of velocity discontinuity at the arterial wall in the stenotic and non-stenotic regions are considered. The outer surface of the catheter is layered with the temperature-sensitive, drug-coated nano-particles. The resulting mathematical formulation involving the coupled non-linear momentum, temperature and concentration equations is solved using the Homotopy Perturbation Method. The efficiency and convergence of the method to the modelled equations are discussed in detail. The consequent effects of the fluid and geometric parameters on pressure drop, flow rate, impedance and wall shear stress of the fluid flow are computed. It is noticed that the high volume fraction of the nano-particles in the blood results in high flow velocity, contributing to secondary flow regions, thus resulting in higher shear stress. Such high volume fraction of the nano-particle may lead to the pathological disorder called aneurysm. This physical model has an important application of drug delivery in biomedical and pharmaceutical industry to prevent obstructions in arteries. Further, the results obtained could be very useful in the manufacturing of related artificial devices.

A Study of the Flow Patterns Between Two Corrugated Plates With an Egg-Carton Configuration

Enhancing mixing in heat exchangers for low Re regimes is vital. A better mixing may be achieved by using corrugated plates. In this work, the flow patterns between corrugated plates with a novel egg-carton geometry were studied. Three-dimensional (3D) numerical models were developed for the steady laminar flow between two corrugated plates having 180 deg or 0 deg phase angles. The Reynolds number (Re ≤ 600) was defined as a function of the average distance between the corrugated plates. The numerical models were strictly developed and corroborated to achieve global convergence, local convergence, and grid-size independence. For both phase angles, it was determined that “close recirculations” decrease in size downstream and finally disappear becoming “open recirculations” due to the flow developing characteristics; the secondary flow regions were found to grow downstream; interestingly, increments on the Reynolds number favor recirculation growth and early flow detachment; the behavior and geometry of the recirculation were in line with previous flow visualization results. The recirculations were determined to be z-symmetric with respect to the channel center only for the 180 deg model. The recirculations in the 0 deg model were smaller and became “open recirculations” earlier than in the 180 deg model. Convex geometries on the transversal direction were found to favor detachment, while concave geometries inhibit it. The capability of the numerical methods to track flow paths in any direction showed a complex three-dimensional flow causing 3D-interaction among secondary flows and the main flow not reported before for these channels and just hinted by previous flow visualization studies.

Stereoscopic particle image velocimetry of the impinging venous needle jet during hemodialysis

Stereoscopic particle image velocimetry (S-PIV) was used to quantify the dynamic flow field caused by the venous needle jet (VNJ) in an idealised model of hemodialysis cannulation. Scaling based on Reynolds number ensured dynamic similarity with physiological conditions. Measurements taken along the center plane indicate the presence of a steady secondary flow region, which develops downstream of the impingement region. Upon impingement, a wall jet forms on the floor of the vein and spreads along the curvature of the vessel. Circulating flow forms due to the interaction between the jet spreading and the wall jet. This secondary flow region represents a potential site of stenosis on the roof of the vein where flow is reversed. The effects of the circulating flows can be minimized by using shallow needle angles, low needle flow rates and placement of the needle away from the walls of the vein.