Commercial Finite Volume Solver Research Articles

A hemodynamic study of blood flow models on various stent graft configurations during aorto-iliac reconstruction.

To compare the hemodynamic performance of three (Bottom Up non-ballet, Top-Down non-ballet, Top Down ballet) idealized stent graft configurations used during endovascular repair of abdominal aortic aneurysms, under the influence of various rheological models. Ten rheological models are assumed and a commercial finite volume solver is employed for the simulation of blood flow under realistic boundary conditions. An appropriate mesh convergence study is performed and five hemodynamic variables are computed: the time average wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), endothelial cell activation potential and displacement force (DF) for all three configurations. The choice of blood flow model may affect results, but does not constitute a significant determinant on the overall performance of the assumed stent grafts. On the contrary, stent graft geometry has a major effect. Specifically, the Bottom Up non-ballet type is characterized by the least favorable performance presenting the lowest TAWSS and the highest OSI, RRT and ECAP values. On the other hand, the Top Down ballet type presents hemodynamic advantages yielding the highest TAWSS and lowest OSI, RRT and ECAP average values. Furthermore, the ballet type is characterized by the lowest DF, although differences observed are small and their clinical relevance uncertain. The effect of the assumed rheological model on the overall performance of the grafts is not significant. It is thus relatively safe to claim that it is the type of stent graft that determines its overall performance rather than the adopted blood flow model.

Reduced order 1 + 3D numerical model for evaluating the performance of solar borehole thermal energy storage systems

This study presents a computationally efficient numerical model for solving the fluid flow and heat transfer phenomena in co-axial boreholes heat exchangers arranged on a rectangular grid (N × N) drill hole pattern. Such systems of boreholes are an integral component of a Solar-Borehole Thermal Energy Storage System for seasonal thermal storage application. The numerical model couples a one-dimensional fluid flow and convective heat transfer model in the heat exchanger with a three-dimensional model of conductive heat transfer in the strata. In addition to solving mass and heat transfer in borehole systems, it integrates a solar thermal collector system and building dynamic thermal load to design a Solar- Borehole Thermal Energy Storage system for a 182-unit residential apartment building. Simulation also considers and evaluates the conductive, convective, and radiative thermal losses of the system. The model is validated using experimental data from a coaxial borehole heat exchanger and compared to the results of a commercial finite volume solver for multiple co-axial boreholes. The computational cost of developed numerical model is reduced by a factor of 194 compared to a commercial finite volume solver. Solar- Borehole Thermal Energy Storage system simulation is performed for a period of five years considering hourly fluctuations in solar irradiance and building dynamic thermal energy demand. Parametric study is performed on depth and spacing of boreholes, and mass flowrate in the solar thermal collectors. It is found that a system of 100 boreholes, each 100 m deep, linked to 674 solar thermal collectors can provide the heating for the building with a total thermal demand of 5600 GJ per year.

Numerical study of the intensification of single-phase heat transfer in a sandwich-like channel using staggered miniature-pin fins

This study presents the numerical analysis of single-phase forced convection heat transfer and pressure drop during internal flow in a shallow, rectangular sandwich-like copper channel heated from both the top and the bottom. Simulations were conducted using a commercial finite volume solver that solves governing continuity, momentum, and energy equations simultaneously. The bottom side of the channel was considered with arrays of rectangular miniature-pin fins. The miniature-pin fin geometry was modeled in a staggered configuration with tip clearance from the opposite surface while the orientation of the fins was directed parallel to the flow direction of the working fluid. Water was used as a working fluid with a fully developed velocity profile at the channel entry. Simulations were carried out for varying inlet temperatures and mass flow rates. All calculations were performed with the RNG k-ε model to model complex turbulent flows appropriately. Grid independence and validation analyses were also conducted. With the validated model, additional analyses were performed for 0, 0.1, 0.3, 0.6, and 1 mm tip clearances, and correlations were proposed after performing regression analyses for the friction factor and heat transfer coefficient. The calculations revealed that in contrast to a straight (nonfinned) surface, up to 220% larger heat transfer coefficients could be obtained for the finned surfaces under comparable operating conditions. It was also shown that tip clearance in turbulent flow affects heat transfer negatively. With increasing clearance, the Nusselt number decreased; in other words, heat transfer worsened. On the other hand, the use of miniature-pin fins led to a rise in the pressure drop over the entire heated length. For the Reynolds number spectrum of 4000–20000, the ratio of the friction coefficient of the finned surface to the friction coefficient of the nonfinned surface varied from 6 to 5 while with a further reduction in pressure loss a value of approximately 4 could be obtained for larger tip clearances. Hence, with larger tip clearances, the friction factor decreased, reflecting the abatement of pressure loss.

Classification of Blood Rheological Models through an Idealized Symmetrical Bifurcation

The assumed rheological behavior of blood influences the hemodynamic characteristics of numerical blood flow simulations. Until now, alternative rheological specifications have been utilized, with uncertain implications for the results obtained. This work aims to group sixteen blood rheological models in homogeneous clusters, by exploiting data generated from numerical simulations on an idealized symmetrical arterial bifurcation. Blood flow is assumed to be pulsatile and is simulated using a commercial finite volume solver. An appropriate mesh convergence study is performed, and all results are collected at three different time instants throughout the cardiac cycle: at peak systole, early diastole, and late diastole. Six hemodynamic variables are computed: the time average wall shear stress, oscillatory shear index, relative residence time, global and local non-Newtonian importance factor, and non-Newtonian effect factor. The resulting data are analyzed using hierarchical agglomerative clustering algorithms, which constitute typical unsupervised classification methods. Interestingly, the rheological models can be partitioned into three homogeneous groups, whereas three specifications appear as outliers which do not belong in any partition. Our findings suggest that models which are defined in a similar manner from a mathematical perspective may behave substantially differently in terms of the data they produce. On the other hand, models characterized by different mathematical formulations may belong to the same statistical group (cluster) and can thus be considered interchangeably.

Comparison of the Haemodynamic Performance of Various Treatment Options for Aorto-Iliac Occlusive Disease

Several surgical and endovascular techniques are used during the treatment of aorto-iliac occlusive disease. Aortobifemoral bypass (AoBFB) is the standard of care, but other options such as axillobifemoral (AxBFB) bypass, aorto-iliac kissing stents (KS), and covered endovascular reconstruction of aortic bifurcation (CERAB) are also available. This study aimed to perform a computational comparison of these four modalities to investigate their haemodynamic performance. Eight patient specific anatomies were analysed, with each of the abovementioned techniques used to treat two anatomies. The CT angiograms were segmented from the renal (or axillary) to common femoral arteries and the 3D geometries were exported. A commercial finite volume solver was implemented for numerical simulations. Outcomes that were assessed were pressure drop (ΔP) between the inlet and the outlet for every configuration and haemodynamic indices of Time Average Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI), and Relative Residence Time (RRT) as markers of a thrombogenic environment. The results indicate that maximum ΔP was observed at peak systole for all models, with values ranging between 12mmHg and 21mmHg for the AoBFB, 64mmHg and 96mmHg for the AxBFB, 31mmHg and 46mmHg for the KS, and 43mmHg and 46mmHg for the CERAB configuration. TAWSS, OSI, and RRT varied among different configurations, mostly presenting values well above thrombogenic thresholds. Regarding RRT, the percentage of total surface area presenting such values is 2.5%, 3.2%, 2%, and 4.3% for the AoBFB, AxBFB, KS, and CERAB configurations, respectively. Computational modelling indicates a favourable haemodynamic performance of AoBFB compared with the other configurations. This leads to a smaller pressure drop and sconsequently a higher pressure in the outlet of the conduit, which is the perfusion pressure of the limb. Notably, lower patency rates of the latter modalities cannot be explained based on haemodynamic indices.

Evaluation of Second- and High-Order Solvers in Wall-Resolved Large-Eddy Simulation

In the context of wall-resolved industrial large-eddy simulation, a comparison is made between a high-order flux reconstruction (FR)/correction procedure via reconstruction (CPR) solver (hpMusic) with refinement and a commercial second-order finite volume solver (Fluent) with mesh refinement ( refinement). A well-known benchmark problem in turbomachinery is employed: transonic flow over a von Karman Institute high-pressure turbine vane at a Reynolds number of . All of the meshes originated from the same coarse mesh, a mixed unstructured mesh, generated through global uniform refinement for the purpose of evaluating the solution dependence on mesh and polynomial order. Because the meshes used for hpMusic and Fluent belong to the same family, useful information about solution accuracy and efficiency can be obtained. Detailed comparisons are made in mean surface loading, heat transfer, power spectral density of pressure at selected monitor points, mean boundary-layer velocity and total temperature profiles, and wake loss. Numerical results are compared with experimental data, when available. The high-order FR/CPR method is shown to achieve a higher accuracy at a reduced cost than the second-order finite volume method.

Investigation of Airfoil Aeroacoustics with Blowing Control Mechanism

In this investigation, it is dealt with computational aero-acoustic analysis of an airfoil with jet blowing. The airfoil shape is selected as NACA0015 profile with jet blowing on upper surface. The calculations of analysis are done by using commercial finite volume solver. The k-e turbulence model is used for the turbulence modeling and the Ffowcs Williams and Hawking acoustic analogy model is run for determination of acoustic data. The numerical results are compared with experimental data for computed Sound Pressure Level without jet blowing and well agreement is observed. In the case of jet blowing, the effects of different jet angle, velocity ratio and angle of attack on airfoil are investigated and noise levels of non jet cases and jet blowing cases are studied.

Validation of the Lattice Boltzmann Method Implementation in a Drip Emitter

Abstract. For computational microfluidics, the lattice Boltzmann method (LBM) may present some advantages, including physical representation of microscopic interactions, a uniform algorithm for multiphase flows, and ease in dealing with complex boundaries. In addition, LBM algorithms have been developed to solve microfluidics-related processes and phenomena, such as heat transfer, electric/magnetic fields, and diffusion. This article investigates the unsteady flow inside the complex 3D geometry of an irrigation drip emitter by employing LBM. Grid convergence analysis is performed to obtain adequate mesh resolution. The numerical results obtained by LBM are validated in comparison with PIV measurements and the results of a commercial finite volume solver. The results indicate that LBM can calculate a reliable simulation in comparison with the finite volume solver. The LBM results are also consistent with the experimental results. LBM offers an alternative to continuum-based CFD in simulating the fluid dynamics of drip emitters.

Computational fluid dynamic analysis of the effect of morphologic features on distraction forces in fenestrated stent grafts

Secure fixation of endovascular stent grafts is essential for successful endovascular aneurysm repair. Hemodynamic distraction forces are generated by blood pressure and blood flow and act against fixation force to encourage migration that may eventually lead to late stent graft failure. The aim of this in silico study was to determine which morphologic features were associated with greater distraction force. Computer models of 54 in situ fenestrated stent grafts were constructed from postoperative computed tomography scans by use of image processing software. Computational fluid dynamic analysis was then performed by use of a commercial finite volume solver with boundary conditions representative of peak systole. Distraction force results were obtained for each component of the stent graft. Distraction force was correlated with lumen cross-sectional area (XSA) at the inlet and outlet of components and was compared between groups of components, depending on the magnitude of four predefined angles within the aortoiliac territory that we describe in detail. Median total resultant distraction force (RDF) acting on the fenestrated proximal bodies was 4.8 N (1.3-15.7 N); bifurcated distal bodies, 5.6N (1.0-8.0 N); and limb extensions, 1.7 N (0.6-8.4N). Inlet XSA exhibited strong, positive correlation with total RDF in proximal body and distal body components (Spearman correlation coefficient ρ, 0.883 and 0.802, respectively). Outlet XSA exhibited a similarly strong, positive correlation with total RDF in limb extension components (ρ, 0.822). Outlet angulation ≥ 45 degrees was associated with greater total RDF in the limb extension components only (P = .004). For a given blood pressure, XSA was the most important morphologic determinant of total RDF. Angulation within the aorta was not large enough to influence this, whereas iliac angulation affecting outlet angulation of limb extension components was associated with significantly greater total RDF.

Numerical simulation of two dimensional vortex-induced vibrations of an elliptic cylinder at low Reynolds numbers

Flow-induced vibrations (FIV) of an elastically supported rigid elliptical cylinder, which is free to move in cross-flow and in-line directions, with equal natural frequencies in both directions, is numerically simulated. Laminar flow regime (60<Re<400) for selected cylinder cross sectional aspect ratios and inclination angles (a/b=0.5, 1, 2; α=0°, 45°, 90°), with a low non-dimensional mass (m∗=10), are considered. Also, to enhance high amplitude oscillations, the structural damping coefficient is supposed to be zero (ξ=0). The natural structural frequency of the oscillator is assumed to match the associated vortex shedding frequency of a stationary cylinder with zero inclination angle at Re=100. The unsteady fluid/cylinder interactions are accurately captured by implementing the moving mesh scheme via integration of an in-house developed User Define Function (UDF) into the main code of the commercial finite volume solver Fluent. The temporal discretization is based on the computationally efficient non-iterative Fractional Step time marching procedure. Numerical results reveal the decisive effects of cross sectional aspect ratio and inclination angle on the cylinder VIV characteristics (i.e., transverse/in-line response amplitudes, aerodynamic coefficients, Strouhal number, phase portrait, time response, orbital trajectory, and wake mode pattern). In particular, it is observed that the fluid/structure interaction effects notably amplify with the decrease (increase) in the cylinder cross sectional aspect ratio (inclination angle), linked to the decrease in the added mass effect. The validity of the simulations is established by comparison with the available experimental/numerical data.

Validation of a lattice Boltzmann method implementation for a 3D transient fluid flow in an intracranial aneurysm geometry

The unsteady fluid flow inside the complex geometry of an intracranial aneurysm during a cardiac cycle was simulated. The results obtained by several versions of a lattice Boltzmann based code were compared with the results of PIV and LDA measurements and with those of a commercial finite volume solver. The best version from the tested lattice Boltzmann codes was chosen for further comparisons and grid convergence measurement. It was shown that the lattice Boltzmann method is capable of producing results with an accuracy comparable to the finite volume solver in a 3D unsteady flow and also that the results are in good agreement with the experimental results. It was also shown that using a GPU implementation of the lattice Boltzmann method it is possible to significantly reduce the run-time length compared to the CPU only implementation of the lattice Boltzmann method.

Effect of Bend Geometry on Heat Transfer and Pressure Drop in a Two-Pass Coolant Square Channel for a Turbine

This paper presents a comparative numerical study of turbulent flow inside a two-pass internal cooling channel with different bend geometries. The goal is to find a geometry that reduces the bend related pressure loss and enhances overall heat transfer coefficient. A square channel with a round U-bend is taken as a baseline case and the heat transfer and pressure drop for nine different bend geometries are compared with the baseline. Modifications for the bend geometry are made along the channel divider wall and at the end wall of the 180 deg bend. The bend geometries studied include: (1) a turning vane geometry, (2) an asymmetrical bulb, (3) three different symmetrical bulbs, (4) two different bow shaped geometries at the end wall, (5) a bend with an array of dimples in the bend region, and (6) finally a combination of bow geometry and dimples. The solution procedure is based on a commercial finite volume solver using the Reynolds averaged Navier–Stokes (RANS) equation and a turbulence model. A two equation realizable k-ɛ model with enhanced wall treatment is used to model the turbulent flow. It was found that the bend geometry can have a significant effect on the overall performance of a two-pass channel. The modified bend geometries are compared with the baseline using Nusselt number ratios, friction factor ratios, and thermal performance factors (TPF) as the metrics. All the modified bend geometries show increase in the TPF with the symmetrical bulb configuration showing nearly a 40% reduction in friction factor ratio and a 30% increase in thermal performance. The highest TPF (41% increase over baseline) is observed for the symmetrical bulb combined with a bow along the outer walls and surface dimples.

Numerical Study of Creeping Liquid Metal Flow in the Presence of a Permanent Magnet

AbstractIn this paper, we present the numerical study of interaction of a liquid metal flow with a small permanent magnet. Such an analysis is of fundamental interest for applications involving Lorentz Force Velocimetry (LFV). As a canonical problem, we consider the flow of liquid metal in the creeping flow regime (Re=0.01) through a square duct with electrically insulating walls. For this setup, we perform magnetohydrodynamic (MHD) simulations by coupling the commercial finite volume solver FLUENT and finite element solver COMSOL Multiphysics. Parametric analyses are performed at different relative positions of the permanent magnet with respect to the duct and with different magnetisation directions. The analyses provide good reference results for qualitative understanding of MHD flows exposed to non‐uniform magnetic fields. (© 2011 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The Effect of Side Wall Mass Extraction on Pressure Loss and Heat Transfer of a Ribbed Rectangular Two-Pass Internal Cooling Channel

Gas turbine blades are often cooled by using combined internal and external cooling methods where for internal cooling purposes, usually, serpentine passages are applied. In order to optimize the design of these serpentine passages it is inevitable to know the influence of mass extraction due to film cooling holes, dust holes, or due to side walls for feeding successive cooling channels as for the trailing edge on the internal cooling performance. Therefore, the objective of the present study was to analyze the influence of side wall mass extraction on pressure loss and heat transfer distribution in a two-pass internal cooling channel representing a cooling scheme with flow towards the trailing edge. The investigated rectangular two-pass channel consisted of an inlet and outlet duct with a height-to-width ratio of H/W=2 connected by a 180 deg sharp bend. The tip-to-web distance was kept constant at Wel/W=1. The mass extraction was realized using several circular holes in the outlet pass side wall. Two geometric configurations were investigated: A configuration with mass extraction solely in the outlet pass and a configuration with mass extraction in the bend region and outlet pass. The extracted mass flow rate was 0%, 10%, and 20% of the inlet channel mass flow. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal technique. Pressure losses were determined in separate experiments by local static pressure measurements. Furthermore, a computational study was performed solving the Reynolds-averaged Navier–Stokes equations using the commercial finite-volume solver FLUENT. The numerical grids were generated using the hybrid grid generator CENTAUR. Three different turbulence models were considered: the realizable k-ε model with two-layer wall treatment, the k-ω-SST model, and the v2-f model. The experimental data of the investigation of side wall ejection showed that the heat transfer in the bend region slightly increased when the ejection were in operation, while the heat transfer in the section of the outlet channel with side wall ejection was nearly not affected. After this section, a decrease in heat transfer was observed, which can be attributed to the decreased mainstream mass flow rate.

RESEARCH OF THE ROTATIONAL EFFECTS ON THE BOUNDARY LAYER OF WIND TURBINE BLADES

The flow field past the rotating blade of a horizontal axial wind turbine has been modeled with a full 3–D steady–RANS approach. Flow computations have been performed using the commercial finite–volume solver Fluent. The NREL phase VI wind turbine blade sections from the 3–D rotating geometry were chosen and the corresponding 2–D flow computations have been carried out for comparison with different angles of attack and in stalled conditions. The simulation results are analyzed. The main features of the boundary layer flow are described, for both the rotating blade and the corresponding 2–D profiles. Computed pressure distributions and aerodynamic coefficients show evidence of less lift losses after separation in the 3–D rotating case, mostly for the inward sections of the blade and the highest angles of attack, which is in agreement with the literature.

Simulation of Turbulent Airflow Using a CT Based Upper Airway Model of a Racehorse

Computational model for airflow through the upper airway of a horse was developed. Previous flow models for human airway do not hold true for horses due to significant differences in anatomy and the high Reynolds number of flow in the equine airway. Moreover, models that simulate the entire respiratory cycle and emphasize on pressures inside the airway in relation to various anatomical diseases are lacking. The geometry of the airway was created by reconstructing images obtained from computed tomography scans of a thoroughbred racehorse. Different geometries for inhalation and exhalation were used for the model based on the difference in the nasopharynx size during the two phases of respiration. The Reynolds averaged Navier-Stokes equations were solved for the isothermal flow with the standard k-epsilon model for turbulence. Transient pressure boundary conditions for the entire breathing cycle were obtained from past experimental studies on live horses. The flow equations were solved in a commercial finite volume solver. The flow rates, computed based on the applied pressure conditions, were compared to experimentally measured flow rates for model validation. Detailed analysis of velocity, pressure, and turbulence characteristics of the flow was done. Velocity magnitudes at various slices during inhalation were found to be higher than corresponding velocity magnitudes during exhalation. The front and middle parts of the nasopharynx were found to have minimum intraluminal pressure in the airway during inhalation. During exhalation, the pressures in the soft palate were higher compared to those in the larynx, epiglottis, and nasopharynx. Turbulent kinetic energy was found to be maximum at the entry to the airway and gradually decreased as the flow moved inside the airway. However, turbulent kinetic energy increased in regions of the airway with abrupt change in area. Based on the analysis of pressure distribution at different sections of the airway, it was concluded that the front part of the nasopharynx requires maximum muscular activity to support it during inhalation. During exhalation, the soft palate is susceptible to displacements due to presence of high pressures. These can serve as critical information for diagnosis and treatment planning of diseases known to affect the soft palate and nasopharynx in horses, and can potentially be useful for human beings.

CFD–RANS analysis of the rotational effects on the boundary layer of wind turbine blades

The flow field past the rotating blade of a horizontal axis wind turbine has been modeled with a full 3–D steady–RANS approach. Flow computations have been performed using the commercial finite–volume solver Fluent. A number of blade sections from the 3–D rotating geometry were chosen and the corresponding 2–D flow computations have been carried out for comparison, for different angles of attack and in stalled conditions. In order to investigate the effects of rotation a postprocessing tool has been developed, allowing the evaluation of the terms in the boundary layer equations. Examples of the output are proposed for the analyzed flow situations. The main features of the boundary layer flow are described, for both the rotating blade and the corresponding 2–D profiles. Computed pressure distributions and aerodynamic coefficients evidence less lift losses after separation in the 3–D rotating case, mostly for the inward sections of the blade and the highest angles of attack, in agreement with the literature.