Optimization of Anastomotic Geometry for Vascular Access Fistula

Arteriovenous fistula (AVF) is one type of vascular access which is a surgically created vein used to remove and return blood during hemodialysis [1]. It is a long-term treatment for kidney failure. Although clinical treatment and technology have both achieved great improvements in recent years, the vascular access for hemodialysis still has significant early failure rates after the insertion of AVF in patients [2]. Studies have shown that stenosis in the vascular access circuit is the single major cause for access morbidity. Majority of efforts to understand the mechanisms of stenosis formation, and its prevention and management have largely focused on understanding and managing this complication based on the pathophysiology, tissue histology and molecular biology; however these efforts have not resulted in significant progress to date. We believe that the major impact in this area will come from continued and accurate understanding of the hemodynamics, and by development of techniques of intervention to modulate factors such as flow rates, pressures and compliance of the circuit. The goal of this paper is to study anastomotic models of AV access using Computational Fluid Dynamics (CFD) and optimize them to minimize the wall shear stress (WSS). In order to achieve this goal, the commercial CFD software FLUENT [3] is employed in conjunction with a single objective genetic algorithm [4]. Computations for two types of AVF currently in use in clinical practice are performed. AVF with 25° angle/3–4mm diameter and 90° angle/3–5mm diameter are selected to conduct the optimization. A single-objective genetic algorithm is employed in the optimization process and a k-kl-ω turbulence model is employed in CFD simulations; this model can accurately compute transitional/turbulent flows. In order to optimize for the same flow conditions, a fixed boundary condition is used during the optimization process. Computations for 16 to 20 generations of the selected AVFs are obtained from the genetic algorithm solver. The maximum WSS in the two AVFs considered are 6997.8 and 7750 dynes/cm2; however, the maximum WSS in the shape-optimized AVFs are reduced to 3511.2 and 4293.9 dynes/cm2 respectively, which have decreased by 49.82% and 44.59% respectively. Thus, the probability of the formation of stenosis in AVFs and early failure rates of vascular access are reduced by using the optimized AVFs.

Near Wall Turbulence Effects on Particle Transport and Deposition in Human Tracheobronchial Multi-Level Bifurcation Model

Transport and deposition of micro and nano-particles in the upper tracheobronchial tree were analyzed using a multi-level asymmetric lung bifurcation model. The multi-level lung model is flexible and computationally efficient by fusing sequence of individual bifurcations with proper boundary conditions. Trachea and the first two generations of the tracheobronchial airway were included in the analysis. In these regions, the airflow is in turbulent regime due to the disturbances induced by the laryngeal jet. Anisotropic Reynolds stress transport turbulence model (RSTM) was used for mean the flow simulation, together with the enhanced two-layer model boundary conditions. Particular attention is given to evaluate the importance of the “quadratic variation of the turbulent fluctuations perpendicular to the wall” on particle deposition in the upper tracheobroncial airways.

Measurement of 3D Flow Structure of Viscoelastic Fluid Using Digital Holographic Microscope

This paper aims to develop a three-dimensional measurement approach to investigate the flow structures of viscoelastic fluid in the curved microchannel by using digital holographic microscope (DHM). With the advantage of DHM, the real-time three-dimensional measurement for the complex flow field can be accomplished. The measurment system uses off-axis holographic / interferometric optical setup for the target, and 3D3C particle tracking velocimetry (PTV) can be achieved based on the analysis of phase information of holograms. To diagnose the chaotic flow inside the microchannel, the 3D temporal positions of tracer particles in the volume of 282μm × 282μm × 60μm have been detected and real-time velocity vectors were calculated based on the PTV algorithm. The measured flow field was then compared with the results obtained by using confocal micro particle image velocimetry (PIV). This technique is proven to be successful for the measurements of microfluidic flow, especially for the truly real-time 3D motions.

The Effects of Fuel and Air Mixtures in Non-Premixed Combustion for a Bluff-Body Flame

The bluff-body stabilized flame is used in a numerical study of the non-premixed flames. This paper shows numerical investigations on the effects of hydrogen compositions and nonflammable diluent mixtures on the combustion and NO emission characteristics of syngas non-premixed flames for a bluff-body burner. The assessment of turbulent non-premixed combustion modeling techniques is presented and discussed. The simulations study the predictive capabilities of five turbulence models and are compared with the experiments of Correa and Gulati [1] for a non-premixed flame of 27.5%CO/32.3%H2/40.2%N2 and air. The Realizable k-ε and the Reynolds Stress (RSM) models were found to perform the best. Based on this, a numerical study to assess the effects of hydrogen component on syngas non-premixed combustion was performed. As a result, hydrogen addition caused the radial velocity and strain rate to decrease, which was important for mixing to decrease NO. Also, the effectiveness of nonflammable diluent mixtures, including N2, CO2 and H2O, were characterized in terms of the ability to reduce NO emission in syngas non-premixed flames. Results indicated that CO2 was the most effective diluent to reduce NO emission and H2O was more effective than N2. CO2 diluent produced low levels of OH radical, which makes CO2 the most effective diluent. Although H2O increased OH radicals, it was still effective to decrease the thermal NO because of its high specific heat.

The Design of a Degassing Pump Based on Numerical and Experimental Research

In the food industry, it is necessary to remove the gas in liquid, in order to prevent the juice from oxidation and prolong guarantee period. At present, vacuum tanks are widely used in beverage productive process to remove air. However, the vacuum tanks are usually bulky and of high energy consumption. In this study, the purposes of degassing pump design are: (1) effectively removing free gas and dissolved gas in liquid, (2) conveying the liquid, (3) more energy efficient than vacuum tank. In this process, numerical method is adopted to simulate the gas-liquid separation with eulerian multiphase model and population balance model (PBM) using the commercial code FLUENT. Structures of pulp pump and gas-liquid cylindrical cyclone separator are used for reference. The degassing pump mainly consists of inlet tube, centrifuge drum, exhaust tube, impeller, volute and outlet tube etc. The numerical simulation results show that there is a huge low pressure area in the center of the drum. Free gas and dissolved gas are separated into the low pressure region under the effect of centrifugal force and local low pressure. Bubble diameter has a great impact on the degassing effect. PBM which considers bubbles coalescence and breakup is adopted here to calculate the diameter of bubbles. The drum diameter has an extremely influence on the inlet pressure of degassing pump. The centrifuge drum is installed in front of impeller, so pre-swirl of the fluid inside impeller inlet is strong. The hydraulic performance of pump slightly declines when centrifuge drum is added in numerical calculation. The degassing pump is in manufacturing and will be tested in future.

Heat Transfer Enhancement of Elastic Turbulence in Curved Microchannel

Flow instabilities of generalized turbulence can prompt heat transfer enhancement in Newtonian fluids. In curved microchannel, viscoelastic fluid also leads to complex behavior even flow instabilities at very low Reynolds number (Re) conditions, and hereby the potential heat transfer enhancement. The primary purpose of this paper is to obtain the influence factors on heat transfer in virtue of viscoelastic instabilities. Microelectronics manufacturing technology was taken to fabricate the microchips of curvilinear microchannels. Platinum films were used as micro heater and temperature measurement sensor, which were embedded in the polydimethylsiloxane (PDMS) microchannel. In our experiments, the polymer solution flow with large enough elasticity becomes quite irregular; even gets into elastic turbulence compared with sucrose solution flow. With a lower thermal conductivity, the polymer solutions show higher heat transfer intensity than sucrose solutions. Heat transfer enhancement due to elastic turbulence in a curved is hereby identified. Furthermore, viscoelastic effects act to augment the heat transfer effect in terms of the convective heat transfer coefficient.

Spent Flow Effects of Multiple Micro-Jet Impingement Cooling Models

The current study investigated comparative performance of wall-confined and unconfined multiple micro-jet impingement heat sink models for electronic cooling applications. The pressure-drop and thermal characteristics were determined for steady incompressible and laminar flow by solving three-dimensional Navier–Stokes equations. Several parallel and staggered micro-jet configurations consisting of a maximum of 16 jet impingements were tested. The effectiveness of various micro-jet configurations, i.e., inline 2×2, 3×3 and 4×4 jets, and staggered 5-jet and 13-jet arrays with nozzle diameters 50, 76, and 100 μm, were analyzed at various flow rates for the maximum temperature-rise, pressure-drop and heat transfer coefficient characteristics. Two design parameters, the ratio of jet diameter to height of the channel and jet distribution, were chosen for comparative performance analysis.

Multiphase Computation of Cavitation Breakdown in Model and Prototype Scale Francis Turbines

Steady-periodic multiphase Computational Fluid Dynamics (CFD) simulations were conducted to capture cavitation breakdown in a Francis hydroturbine due to large-scale vaporous structures. A reduced-scale model and a full-scale prototype were investigated to display differences in vapor content and machine performance caused by lack of Reynolds and Froude similarity. The model scale efficiencies compared favorably (within 3%) to the experimental cavitation tests. The CFD model and prototype displayed distinct qualitative and quantitative differences as σ was reduced. A stage-by-stage analysis was conducted to assess the effect of cavitation on loss distribution throughout the machine. Furthermore, a formal mesh refinement study was conducted on efficiency and volume of vapor, with three mesh levels and Richardson extrapolation, to ensure convergence.

Numerical Study of Flow Through Models of Aortic Valve Stenoses and Assessment of Gorlin Equation

Gorlin equation has been applied in clinical practice for evaluating the aortic valve area (AVA) of vascular and aortic valve stenosis for past sixty years [1]. It was derived using the Bernoulli equation across the stenosis with the assumption that the velocity of the fluid behind the stenosis is much greater than the velocity upstream of the stenosis. Because of this assumption, the calculated stenosed area may have large error if the flow rate across the valve is low or the stenosis is mild [2]. In a recent paper, Okpara and Agarwal [3] proposed a new equation (Agarwal – Okpara equation) which significantly decreases the evaluation error compared to the Gorlin Equation. The purpose of this paper is to modify the Agarwal – Okpara equation to generalize its applicability based on additional data calculated from the commercial CFD software FLUENT as well as the clinical data obtained from the literature. A total of ten cases are computed using CFD to assess the range of validity of the Gorlin equation and the Agarwal – Okpara equation. In addition, eighty clinical data points were obtained from the papers of Minners et al. [4] and Hakii et al. [5] covering a large range of severity of stenosis. The error in AVA computed from Gorlin equation and Agarwal – Okpara equation varied from 7.44 to 82.14% and 0.06 to 27.26% respectively compared to the CFD simulation data, and 41.47 to 83.60% and 8.88 to 33.98% respectively compared to the clinical data; however, AVA calculated using the Agarwal – Okpara – Bao equation presented in this paper gives results within 0.42 to 9.6% error compared to the exact AVA used in FLUENT simulations, and from 4.76 to 24.13% compared to the clinical data. The Agarwal – Okpara – Bao equation agrees with the clinical data for all relevant flow rates and full range of severity of stenosis. Thus, the use of Agarwal – Okpara – Bao equation to evaluate AVA in clinical practice is suggested.

Implementation of Infinite Height Levee in CaFunwave Using an Immersed Boundary Method

Numerical simulations of storm-surge-wave actions on coastal highways and levees are very important research topics for coastal Louisiana. In a large scale region hydrodynamic model, highways and levees are often complicated in geometry and much smaller in size compared to the grid size. The immersed boundary method (IBM) allows for those complicated geometries to be modeled in a less expensive way. It can allow very small geometries to be modeled in a large scale simulation, without requiring them to be explicitly on the grid. It can also allow for complicated geometries not collocated on the grid points. CaFunwave is a project that uses the Cactus Framework for modeling a solitary coastal wave impinging on a coastline, and is the wave solver in this research. The IBM allows for a levee with different geometries to be implemented on a simple Cartesian grid in the CaFunwave package. The IBM has not been used previously for this type of application. Implementing an infinite height levee using the IBM in the Cactus CaFunwave code involves introducing IB forcing terms into the standard 2-D depth averaged shallow water equation set. These forcing terms cause the 2-D solitary wave to experience a virtual force at the grid points surrounding the immersed boundary levee. In this paper the levee was implemented and tested using two different immersed boundary methods. The first method was a feedback-force method, which proved to be more effective at modeling the levee than the second method, the direct-forcing method. In this study, the results of the two methods, as well as the shape effects on the flow, are presented and discussed.