Specific Computational Fluid Dynamics Research Articles

Performance Evaluation of the Pressurized Synhelion Absorbing Gas Receiver

The pressurized design of the Synhelion absorbing gas receiver concept has been presented. Despite its intrinsic increased design complexity, foreseen advantages such as receiver downscaling and more compact piping and insulation systems were the drivers for the initial development of the 250 kWth receiver design operating at high pressure (10 bar absolute). The latter was driven by the results of specific computational fluid dynamics (CFD) simulations aimed at evaluating the receiver thermo-fluid dynamics behaviour along with the relative performance. This paper shows the results of the two initial CFD simulations campaigns aimed at evaluating the effect, on the receiver performance, of the integration of absorbing inserts (i.e., a series of concentric disks specifically arranged into the cavity to capture the incoming concentrated solar radiation) and the position and shape of the inlet section of the heat transfer fluid. The simulations results allowed not only to evaluate the receiver performance, in terms of thermal efficiency, but also to observe some criticalities related to the motion of the HTF through the receiver.

Three-Dimensional Simulations of Viscous Incompressible Fluid Flows on Grids with Unmatched Grid Interfaces

The paper describes a numerical method that considers specific computational fluid dynamics (CFD) aspects of viscous incompressible flow simulations in the vicinity of interfaces between unmatched fragments of unstructured grids. The method is based on the general grid interface (GGI) principle, which involves conservative flux interpolation and does not require original grid modification at unmatched interfaces. A method is presented which combines adjacent unmatched fragments of an unstructured grid into a single domain by means of virtual interfaces considering connections between adjacent cells through virtual faces. The performance of the method is illustrated by the finite-volume discretization of the transport equation in the region of matched interfaces and its modification for the case of unmatched interfaces. The efficiency of the proposed method is demonstrated by three-dimensional CFD simulations with grid models composed of unmatched unstructured grid fragments. The simulation results are compared with equivalent simulations on matched grids. The influence of unmatched interfaces on the convergence rate and accuracy of the solution is assessed.

Wall Shear Stress Topological Skeleton Variability Predicts Myocardial Infarction

Objective: Coronary atherosclerotic plaques experience a variety of haemodynamic stimuli linked to plaque progression and destabilisation that increases the risk of occlusion.1 Peculiar wall shear stress (WSS) features obtained from patient specific computational fluid dynamics (CFD) simulations have been associated with myocardial infarction (MI).2 Therefore, WSS based quantities may prove to have predictive capability for MI, allowing identification of lesions prone to rupture and improving the medical management of patients with coronary artery disease (CAD).

Ultrasound Based Computational Fluid Dynamics Assessment of Brachial Artery Wall Shear Stress in Preeclamptic Pregnancy.

Preeclampsia (PE) is a pregnancy complication of abnormally elevated blood pressure and organ damage where endothelial function is impaired. Wall shear stress (WSS) strongly effects endothelial cell morphology and function but in PE the WSS values are unknown. WSS calculations from ultrasound inaccurately assume cylindrical arteries and patient specific computational fluid dynamics (CFD) typically require time-consuming 3D imaging such as CT or MRI. Two-dimensional (2D) B-mode ultrasound images were lofted together to create simplified three-dimensional (3D) geometries of the brachial artery (BA) that incorporate artery curvature and non-circular cross sections. This process was efficient and on average took 120±10 s. Patient specific CFD was then performed to quantify BA WSS for a small cohort of PE (n=5) and normotensive pregnant patients (n=5) and compared against WSS calculations assuming a cylindrical artery. For several WSS metrics (time averaged WSS (TAWSS), peak systolic WSS, oscillatory shear index (OSI), OSI/TAWSS and relative residence time) CFD on the simplified arterial geometries calculated large spatial differences in WSS that assuming a cylindrical artery cannot calculate. Bland-Altman and intra-class correlation (ICC) analyses found assuming a cylindrical artery both underestimated (p<0.05) and had poor agreement (ICC<0.5) with the maximum WSS values from CFD. WSS values that were abnormal compared to the normotensive patients (OSI=0.014±0.026) appear related to the pregnancy complications fetal growth restriction (n=2, OSI=0.14, 0.25) and gestational diabetes (n=1, OSI=0.23). Creating 3D artery geometries from 2D ultrasound images can be used for CFD simulations to calculate WSS from ultrasound without assuming cylindrical arteries. This approach requires minimal time for both medical imaging and CFD analysis.

Towards quantitative evaluation of wall shear stress from 4D flow imaging

Wall shear stress (WSS) is a relevant hemodynamic indicator of the local stress applied on the endothelium surface. More specifically, its spatiotemporal distribution reveals crucial in the evolution of many pathologies such as aneurysm, stenosis, and atherosclerosis. This paper introduces a new solution, called PaLMA, to quantify the WSS from 4D Flow MRI data. It relies on a two-step local parametric model, to accurately describe the vessel wall and the velocity-vector field in the neighborhood of a given point of interest. Extensive validations have been performed on synthetic 4D Flow MRI data, including four datasets generated from patient specific computational fluid dynamics simulations on carotids. The validation tests are focused on the impact of the noise component, of the resolution level, and of the segmentation accuracy concerning the vessel position in the context of complex flow patterns. In simulated cases aimed to reproduce clinical acquisition conditions, the WSS quantification performance reached by PaLMA is significantly higher (with a gain in RMSE of 12 to 27%) than the reference one obtained using the smoothing B-spline method proposed by Potters et al. (2015) method, while the computation time is equivalent for both WSS quantification methods.

Study of the Influence of Geometric Parameters in an Innovative Scraped Surface Heat Exchanger With Helical Ribbon

Abstract This work aims to characterize the hydrodynamic and thermal behaviors in an innovative scraped surface heat exchanger (SSHE) equipped with helical ribbon by means of the numerical simulation approach. In this study, the conservation equations of continuity, momentum, and energy in the laminar, steady-state and isothermal conditions are resolved using a specific computational fluid dynamics (CFD) code based on the 3D finite volume method. The effects of the gap between the exchanger wall and the tip of the ribbon, the ribbon width, and the number of turns in the ribbon on the hydrodynamic and thermal behaviors are studied. Varying the gap values leads to reveal an optimum value giving the highest heat transfer coefficient. Moreover, numerical results have shown that increasing the ribbon width improves the heat transfer. Furthermore, the influence of the number of turns is carried out for Reynolds number ratios (Rer/Rea) inferior and superior to 1. Results revealed that increasing the number of turns avoids the back-mixing phenomenon and thus improves the heat transfer. In this study, the establishment of correlation is determined with the introduction of dimensionless and geometrical groups to predict the heat transfer coefficient in SSHE.

Assessing the Risk of Intracranial Aneurysm Rupture Using Morphological and Hemodynamic Biomarkers Evaluated from Magnetic Resonance Fluid Dynamics and Computational Fluid Dynamics.

Purpose:Evaluate in vivo hemodynamic and morphological biomarkers of intracranial aneurysms, using magnetic resonance fluid dynamics (MRFD) and MR-based patient specific computational fluid dynamics (CFD) in order to assess the risk of rupture.Methods:Forty-eight intracranial aneurysms (10 ruptured, 38 unruptured) were scrutinized for six morphological and 10 hemodynamic biomarkers. Morphological biomarkers were calculated based on 3D time-of-flight magnetic resonance angiography (3D TOF MRA) in MRFD analysis. Hemodynamic biomarkers were assessed using both MRFD and CFD analyses. MRFD was performed using 3D TOF MRA and 3D cine phase-contrast magnetic resonance imaging (3D cine PC MRI). CFD was performed utilizing patient specific inflow–outflow boundary conditions derived from 3D cine PC MRI. Univariate analysis was carried out to identify statistically significant biomarkers for aneurysm rupture and receiver operating characteristic (ROC) analysis was performed for the significant biomarkers. Binary logistic regression was performed to identify independent predictive biomarkers.Results:Morphological biomarker analysis revealed that aneurysm size [P = 0.021], volume [P = 0.035] and size ratio [P = 0.039] were statistically significantly different between the two groups. In hemodynamic biomarker analysis, MRFD results indicated that ruptured aneurysms had higher oscillatory shear index (OSI) [OSI.max, P = 0.037] and higher relative residence time (RRT) [RRT.ave, P = 0.035] compared with unruptured aneurysms. Correspondingly CFD analysis demonstrated significant differences for both average and maximum OSI [OSI.ave, P = 0.008; OSI.max, P = 0.01] and maximum RRT [RRT.max, P = 0.045]. ROC analysis revealed AUC values greater than 0.7 for all significant biomarkers. Aneurysm volume [AUC, 0.718; 95% CI, 0.491–0.946] and average OSI obtained from CFD [AUC, 0.774; 95% CI, 0.586–0.961] were retained in the respective logistic regression models.Conclusion:Both morphological and hemodynamic biomarkers have significant influence on intracranial aneurysm rupture. Aneurysm size, volume, size ratio, OSI and RRT could be potential biomarkers to assess aneurysm rupture risk.

Denoising and spatial resolution enhancement of 4D flow MRI using proper orthogonal decomposition and lasso regularization

4D-Flow MRI has emerged as a powerful tool to non-invasively image blood velocity profiles in the human cardio-vascular system. However, it is plagued by issues such as velocity aliasing, phase offsets, acquisition noise, and low spatial and temporal resolution. In imaging small blood vessel malformations such as intra-cranial aneurysms, the spatial resolution of 4D-Flow is often inadequate to resolve fine flow features. In this paper, we address the problem of low spatial resolution and noise by combining 4D-Flow MRI and patient specific computational fluid dynamics using Least Absolute Shrinkage and Selection Operator. Extensive experiments using numerical phantoms of two actual intra-cranial aneurysms geometries show the applicability of the proposed method in recovering the flow profile. Comparisons with the state-of-the-art denoising methods for 4D-Flow show lower error metrics. This method can enable more accurate computation of flow derived patho-physiological parameters such as wall shear stresses, pressure gradients, and viscous dissipation.

Learning-Based Cell Injection Control for Precise Drop-on-Demand Cell Printing.

Drop-on-demand (DOD) printing is widely used in bioprinting for tissue engineering because of little damage to cell viability and cost-effectiveness. However, satellite droplets may be generated during printing, deviating cells from the desired position and affecting printing position accuracy. Current control on cell injection in DOD printing is primarily based on trial-and-error process, which is time-consuming and inflexible. In this paper, a novel machine learning technology based on Learning-based Cell Injection Control (LCIC) approach is demonstrated for effective DOD printing control while eliminating satellite droplets automatically. The LCIC approach includes a specific computational fluid dynamics (CFD) simulation model of piezoelectric DOD print-head considering inverse piezoelectric effect, which is used instead of repetitive experiments to collect data, and a multilayer perceptron (MLP) network trained by simulation data based on artificial neural network algorithm, using the well-known classification performance of MLP to optimize DOD printing parameters automatically. The test accuracy of the LCIC method was 90%. With the validation of LCIC method by experiments, satellite droplets from piezoelectric DOD printing are reduced significantly, improving the printing efficiency drastically to satisfy requirements of manufacturing precision for printing complex artificial tissues. The LCIC method can be further used to optimize the structure of DOD print-head and cell behaviors.

Optimization of lift force for a bio-inspired flapping wing model in hovering flight

The lift force is an important component that affects the aerodynamic performance of bio-inspired flapping-wing micro aerial vehicles. However, there is a lack of endeavors in the optimization of the flapping wing parameters that affect the lift force of micro aerial vehicles. This research is therefore to establish a methodology that combines computational fluid dynamic simulation for the evaluation of the lift generation and wing parameters. The Taguchi’s framework for design of experiments, combined with the computational fluid dynamics simulations, is performed on a simplified robotic fly wing model used in the experiment found in Dickinson et al. (1999), under the hovering flight mode, to identify the most influential parameters on the lift generation of micro aerial vehicles. A commercial computational fluid dynamics code, ANSYS/FLUENT, along with a three-dimensional Navier–Stokes solver is used to simulate the unsteady flow field. With the optimization of the time-averaged lift force as the optimization objective, five typical parameters (flapping frequency, maximum angle of attacks during the upstroke and downstroke motions, stroke amplitude, and rotation type) in the flapping trajectory equation are selected as the input factors with each having four levels. Specific computational fluid dynamics cases are simulated in accordance with the chosen orthogonal arrays. By conducting statistical analyses with analyses of means and variance, the flapping frequency and the stroke amplitude are determined to be the two most influential parameters. The response surface of the time-averaged lift force and the power consumption contour are constructed with respect to these two parameters to determine the optimal combination for the generation of lift forces under a specific power constrain and provide a guideline for bio-inspired micro aerial vehicle designs.

Improvement in field synergy principle: More rigorous application, better results

Basing on the original derivation of field synergy principle, we propose two rigorous rules for its applications with respect to incompressible flow in conventional-scale duct: (1) the synergy angle should be evaluated in the thermal boundary layer for laminar flow; (2) the FSP analysis must be refined to the viscous sublayer for turbulent flow in the strict sense. These two rules are verified by two specific computational fluid dynamics cases of forced convection in two-parallel plates with irregular boundary condition in laminar and turbulent flow regimes, respectively. By making elaborate post-processing, the local synergy angles in the very near-wall region are clearly visualized. It is found that the patterns of synergy angle distribution in laminar boundary layer/turbulent viscous sublayer are able to accurately reflect the local heat transfer ability, which provides an opportunity to directly visualize the local variation of convective heat transfer coefficient. Quantitative analysis also shows that the local Nusselt number variations are in perfect agreement with the local synergy angle variations according to the rules we proposed. This work also can be regarded as a verification for field synergy principle from a new aspect.

(668) - Correlation of LVAD Outflow Graft Orientation with Aortic Root Thrombosis Rationalized by Patient Specific Computational Fluid Dynamics Simulations

(668) - Correlation of LVAD Outflow Graft Orientation with Aortic Root Thrombosis Rationalized by Patient Specific Computational Fluid Dynamics Simulations

CFD simulation of flow-pressure characteristics of a pressure control valve for automotive fuel supply system

This study aims to elaborate on specific computational fluid dynamics (CFD) simulation methods for fitting the flow-pressure curve of a pressure control valve, which is spring-load valve widely used in the automotive fuel supply system. Given that the couple mechanism exists between the flow field in the valve and the spring system, numerous researchers chose to fit the characteristic curve with experimental approaches but scarcely focused on CFD methods. A direct CFD method is introduced in this study to solve this problem. Two evaluation criteria are used to determine whether the internal flow is physically real. An experiment is conducted to verify the simulation results, and the accuracy of this CFD method is proved. However, it is designed to solve one operating condition with fixed spring parameters and the accuracy depends on the amount of operating conditions. Thus, an indirect CFD method is developed based on the well-elaborated valve-governing equations to improve the efficiency and broaden the application extension. This method aims to simulate the exact value of the equation constants to uncouple the flow by numerical method. It is capable of dealing with changed operating conditions and varied spring parameters, and the results are also verified. The visualization of the internal flow provides a better understanding of the flow fields in the valve. The valve gap directly influences the hydraulic force distribution on the spool and causes most pressure loss. The physical meaning of the function constants are explained based on the flow analysis.

Numerical analysis of Venous External Scaffolding Technology for Saphenous Vein Grafts

This paper presents a method for analyzing and comparing numerically Saphenous Vein Grafts (SVGs) following Coronary Artery Bypass Graft surgery (CABG). The method analyses the flow dynamics inside vein grafts with and without supporting using Venous External Scaffolding Technology (VEST). The numerical method uses patients׳ specific computational fluid dynamics (CFD) methods to characterize the relevant hemodynamic parameters of patients׳ SVGs. The method was used to compare the hemodynamics of six patient׳s specific model and flow conditions of stented and non-stented SVGs, 12 months post-transplantation. The flow parameters used to characterize the grafts׳ hemodynamics include Time Averaged Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI) and Relative Residence Time (RRT). The effect of stenting was clearly demonstrated by the chosen parameters. SVGs under constriction of VEST were associated with similar spatial average of TAWSS (10.73 vs 10.29dyn/cm2), yet had fewer lesions with low TAWSS, lower OSI (0.041 vs 0.08) and RRT (0.12 vs 0.24), and more uniform flow with less flow discrepancies. In conclusion, the suggested method and parameters well demonstrated the advantage of VEST support. Stenting vein grafts with VEST improved hemodynamic factors which are correlated to graft failure following CABG procedure.

An open-source toolbox for multiphase flow in porous media

Multiphase flow in porous media provides a wide range of applications: from the environmental understanding (aquifer, site-pollution) to industrial process improvements (oil production, waste management). Modeling of such flows involves specific volume-averaged equations and therefore specific computational fluid dynamics (CFD) tools. In this work, we develop a toolbox for modeling multiphase flow in porous media with OpenFOAM®, an open-source platform for CFD. The underlying idea of this approach is to provide an easily adaptable tool that can be used in further studies to test new mathematical models or numerical methods. The package provides the most common effective properties models of the literature (relative permeability, capillary pressure) and specific boundary conditions related to porous media flows. To validate this package, solvers based on the IMplicit Pressure Explicit Saturation (IMPES) method are developed in the toolbox. The numerical validation is performed by comparison with analytical solutions on academic cases. Then, a satisfactory parallel efficiency of the solver is shown on a more complex configuration. Program summaryProgram title: porousMultiphaseFoamCatalogue identifier: AEUR_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEUR_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public LicenseNo. of lines in distributed program, including test data, etc.: 36355No. of bytes in distributed program, including test data, etc.: 1106569Distribution format: tar.gzProgramming language: C++.Computer: Any x86.Operating system: Generic Linux.Classification: 12.External routines: OpenFOAM® (version 2.0 and superior)Nature of problem:This software solves multiphase flow in porous media.Solution method:The numerical approach is based on the finite-volume method (FVM). Mass conservation equations for each fluid phase are reformulated into a pressure–saturation system. The generalized Darcy’s law is used to compute phase velocities in the porous medium. The pressure–saturation is solved using a segregated method based on the IMPES method. Parallel computing can be performed using the standard domain decomposition method of OpenFOAM®.Running time:The Buckley–Leverett examples provided only take a few seconds to run.

Fluid Mechanics of Mixing in the Vertebrobasilar System: Comparison of Simulation and MRI

Recent magnetic resonance imaging (MRI) studies have demonstrated that perfusion to the posterior fossa of the brain can be surprisingly unilateral, with specific vascular territories supplied largely by a single vertebral artery (VA) rather than a mixture of the two. It has been hyposthesized that this is due to a lack of mixing in the confluence of the VA into the basilar artery (BA), however the local mechanisms of mixing (or lack thereof) have not been previously examined in detail. This study aims to assess the mixing characteristics and hemodynamics of the vertebrobasilar junction using subject specific computational fluid dynamics (CFD) simulations, and perform quantitative comparisons to arterial spin labeling (ASL) MRI measurements. Subject specific CFD simulations and unsteady particle tracking were performed to quantitatively evaluate vertebrobasilar mixing in four subjects. Phase-contrast MRI was used to assign inflow boundary conditions. A direct comparison of the fractional flow contributions from the VAs was performed against perfusion maps generated via vessel-encoded pseudo-continuous arterial spin labeling (VEPCASL) MRI. The laterality of VA blood supply in 7/8 simulated cerebellar hemispheres and 5/7 simulated cerebral hemispheres agree with ASL data. Whole brain laterality of the VA supply agrees within 5% for measured and computed values for all patients. However, agreement is not as strong when comparing perfusion to individual regions. Simulations were able to accurately predict laterality of VA blood supply in four regions of interest and confirm ASL results, showing that very little mixing occurs at the vertebrobasilar confluence. Additional particle tracking analysis using Lagrangian coherent structures is used to augment these findings and provides further physical insight that complements current in vivo imaging techniques. A quantitative mix-norm measure was used to compare results, and sensitivity analysis was performed to assess changes in the results with pertubations in the boundary condition values.

Computational studies of the pitched blade turbines design effect on the stirred tank flow characteristics

A specific computational fluid dynamics (CFD) code is developed to predict the turbulent flow field in a stirred tank equipped with the pitched blade turbines (PBT) and to choose the most effective agitation system. The computer method permits the numerical analyses of turbines with complex geometries. After defining the list of nodes belonging to the interface separating the turbine shape and the flow domain, the meshes in the flow domain are automatically generated on a finite volume discretization. The three-dimensional flow of a fluid is numerically analyzed using the Navier–Stokes equations in conjunction with the standard k – ε turbulence model. The effects of inclined angle on the local and global flow characteristics have been particularly determined. To verify our computer simulations, the power numbers were measured and compared with computer results. Also, the flow patterns have been compared with the ones found by other experimental results. These matching results indicate the validity of our computer method.

Preliminary Study on Biomechanics of Vertebroplasty

Algorithm development for the automatic finite element modeling of patient vertebra. To present a technique for automatic generation of patient specific computational fluid dynamics (CFD) models for intraosseous PMMA cement flow simulation. The secondary objective is to demonstrate the possibility of using resultant PMMA cement distribution for post-PVP stress-strain analyses. There are no noninvasive methods for the visualization of PMMA cement flow. In addition, optimum volume and distribution of PMMA cement are still not known. Computational models that allow patient specific intraosseous PMMA cement flow visualization as well as postvertebroplasty mechanical evaluation would be advantageous. We developed an algorithm coded into a custom platform that inputs patient CT datasets. Hounsfield unit values were used to assign permeability values as well as modulus to the finite element model before analyses. Several user inputs are required, and these reflect the decisions made by physicians that practice vertebroplasty. As a case study, we isolated a single L1 vertebra from patient CT dataset and used our platform for model generation. Simulated vertebroplasty was performed for different PMMA cement volumes and at different placements to study the effects of varying distribution. Increased needle injection pressure was observed as the volume of PMMA increases and as the distribution of PMMA is in close proximity to the cortical walls. Stiffness of augmented vertebral body also increases with increased volume of PMMA administered. Varying distributions, for the same volume, of PMMA cement did not alter stiffness drastically. Our custom platform and technique for modeling vertebral bodies may contribute significantly to the science of vertebroplasty. Intraosseous PMMA cement flow can be visualized before vertebroplasty, and needle position altered for optimization. Also, parametric computational studies on the postvertebroplasty biomechanical effects of vertebroplasty are now enhanced with such a modeling capability.