Gábor Janiga

Otto-von-Guericke-Universität Magdeburg, Magdeburg, Saxony-Anhalt, Germany

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Publications (77)51.55 Total impact

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    ABSTRACT: Computational fluid dynamics (CFD) coupled with the discrete element method (DEM) has been used to investigate numerically crystal dynamics in an existing pilot-scale batch crystallizer. The CFD-DEM combination provides a detailed description of crystal dynamics considering a four-way coupling. In a previous analysis,1 CFD had been coupled with a discrete phase model (DPM) using a simple one-way coupling. The corresponding predictions are then compared with those obtained through four-way coupling considering KH2PO4 crystals in water. From the CFD-DEM simulation, it is possible to investigate quantitatively the driving force controlling crystal growth and the interaction of crystals with reactor walls, baffles, and impellers. This delivers essential data for process improvement. Different seeding procedures were also compared. The seed crystals have been injected either within the complete liquid volume or, as in the experiments, through a funnel. By varying the most important crystallization process p
    Crystal Growth & Design 11/2014; · 4.69 Impact Factor
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    ABSTRACT: Validation studies are prerequisites for computational fluid dynamics (CFD) simulations to be accepted as part of clinical decision-making. This paper reports on the 2011 edition of the Virtual Intracranial Stenting Challenge. The challenge aimed to assess the reproducibility with which research groups can simulate the velocity field in an intracranial aneurysm, both untreated and treated with five different configurations of high-porosity stents. Particle imaging velocimetry (PIV) measurements were obtained to validate the untreated velocity field. Six participants, totaling three CFD solvers, were provided with surface meshes of the vascular geometry and the deployed stent geometries, and flow rate boundary conditions for all inlets and outlets. As output, they were invited to submit an abstract to the 8th International Interdisciplinary Cerebrovascular Symposium 2011 (ICS'11), outlining their methods and giving their interpretation of the performance of each stent configuration. After the challenge, all CFD solutions were collected and analyzed. To quantitatively analyze the data, we calculated the root-mean-square error (RMSE) over uniformly distributed nodes on a plane slicing the main flow jet along its axis and normalized it with the maximum velocity on the slice of the untreated case (NRMSE). Good agreement was found between CFD and PIV with a NRMSE of 7.28%. Excellent agreement was found between CFD solutions, both untreated and treated. The maximum difference between any two groups (along a line perpendicular to the main flow jet) was 4.0 mm/s, i.e. 4.1% of the maximum velocity of the untreated case, and the average NRMSE was 0.47% (range 0.28-1.03%). In conclusion, given geometry and flow rates, research groups can accurately simulate the velocity field inside an intracranial aneurysm-as assessed by comparison with in vitro measurements-and find excellent agreement on the hemodynamic effect of different stent configurations.
    Annals of biomedical engineering. 08/2014;
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    ABSTRACT: Cerebral aneurysms are dangerous dilatations of the intracranial vasculature. As these aneurysms are located in the Circle of Willis, which is the essential vessel structure supplying the brain with blood, a rupture leads to drastic consequences for the patient. Physicians are faced with the decision to leave the aneurysm untouched or to intervene. This decision is strongly related to the assessment of rupture risk. It is assumed that hemodynamic factors, i.e. forces resulting from the blood flow itself, play an important role. Thus, a full understanding of correlations between blood flow characteristics and rupture risk is the long-term, ambitious objective of our research. A second aspect that has to be considered is the method of treatment. Several options (clip, coil, flow diverter stenting) are available. A patient-specific analysis and optimization of interventional devices would be ultimately very advantageous. Fluid dynamical simulations provide the most promising tool to reach such ambitious objectives, but require validation. In this contribution, image-based flow investigations of a steady flow through the silicone phantom model of an anatomically realistic giant aneurysm are presented and used as validation source for computational fluid dynamics (CFD). Therefore, the vessel geometry was segmented and reconstructed from angiography data delivering a geometry suitable for simulations as well as for casting a silicone block with included hollow vessels for optical measurements. A stereoscopic particle image velocimetry (PIV) arrangement featuring an index-matched artificial blood liquid is used to obtain averaged velocity vectors in ten parallel planes through the aneurysm sac. From this data streamlines and isocontours of velocity are presented, which show the three-dimensional rolling motion inside the sac. Several PIV planes are compared to their simulation counterparts and show excellent agreement proving the ability of CFD and its assumptions to reliably capture the essential flow features in such cases. As an outlook, the stereo-PIV setup has been slightly modified to enable 3D Particle Tracking Velocimetry (PTV) at low seeding concentrations.
    International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon; 07/2014
  • IFF-Wissenschaftstage "Digitales Engineering zum Planen, Testen und Betreiben technischer Systeme", Magdeburg, Germany; 06/2014
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    ABSTRACT: Obtaining vascular pressure information is of major interest for physicians since vascular diseases can be diagnosed much faster. However, complex catheter measurements are difficult to carry out and relatively time-consuming. Therefore, non-invasive techniques would significantly enhance this procedure. Using phase-contrast magnetic resonance imaging all velocity components were measured as a function of time in two neck vessels and two intracranial vessels, respectively. A script was developed that calculates the patient-specific volume flow rates through arbitrary planes within the region of interest. Afterwards, an iterative scheme based on the pressure Poisson equation was applied in order to estimate the corresponding relative pressure up to a constant defined by the user. A verification of the introduced method succeeded using 2D as well as 3D test cases from computational fluid dynamics. The calculation of the volume flow rates in the patient-specific neck vessels shows a good agreement for the considered planes and all values lie in a physiological range. Also the corresponding pressure curves appear with a realistic course. The coupling of pressure and velocity was successful and therefore relative pressure fields in the human vasculature can be calculated non-invasively. However, the accuracy of the obtained results strongly depends on the quality of the measured velocity field. Since the resolution is limited and intracranial vessels are not covered sufficiently, improvements with higher magnetic strength are needed.
    International Symposium on Biomedical Imaging, Beijing, China; 04/2014
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    ABSTRACT: Optimization of heat exchangers, as a consequence of their vital role in several industries and applications, has attracted a lot of interest in the last years, and in particular the necessity of improving their performances is well recognized. The coupling of optimization techniques with Computational Fluid Dynamics (CFD) has demonstrated to be a valid methodology for easily explore this work, a CFD-based shape optimization of a tube bundle in crossflow is presented, as a natural extension of the work of Hilbert et al. (2006) [1]. In this study, also the flow inside the tubes has been computed, and the coupled simulation of the external flow and thermal field is performed also on a periodic domain. Two genetic algorithms have been tested and compared, NSGA-II and FMOGA-II: the latter makes an internal use of surrogate models to speed up and improve the optimization process, and proved to be a promising algorithm. The results demonstrate how the search for efficient geometric configurations should also take into account the internal flow field.
    International Journal of Heat and Mass Transfer 03/2014; 68:585–598. · 2.52 Impact Factor
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    ABSTRACT: A two-dimensional cross-flow tube bank heat exchanger arrangement problem with internal laminar flow is considered in this work. The objective is to optimize the arrangement of tubes and find the most favorable geometries, in order to simultaneously maximize the rate of heat exchange while obtaining a minimum pressure loss. A systematic study was performed involving a large number of simulations. The global optimization method NSGA-II was retained. A fully automatized in-house optimization environment was used to solve the problem, including mesh generation and CFD (computational fluid dynamics) simulations. The optimization was performed in parallel on a Linux cluster with a very good speed-up.The main purpose of this article is to illustrate and analyze a heat exchanger arrangement problem in its most general form and to provide a fundamental understanding of the structure of the Pareto front and optimal geometries. The considered conditions are particularly suited for low-power applications, as found in a growing number of practical systems in an effort toward increasing energy efficiency. For such a detailed analysis with more than 140 000 CFD-based evaluations, a design-of-experiment study involving a response surface would not be sufficient. Instead, all evaluations rely on a direct solution using a CFD solver.
    Energy 02/2014; 65:364–373. · 4.16 Impact Factor
  • Gábor Janiga
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    ABSTRACT: This work investigates the flow in a benchmark nozzle model of an idealized medical device proposed by the FDA using computational fluid dynamics (CFD). It was in particular shown that a proper modeling of the transitional flow features is particularly challenging, leading to large discrepancies and inaccurate predictions from the different research groups using Reynolds-averaged Navier-Stokes (RANS) modeling. In spite of the relatively simple, axisymmetric computational geometry, the resulting turbulent flow is fairly complex and non-axisymmetric, in particular due to the sudden expansion. The resulting flow cannot be well predicted with simple modeling approaches. Due to the varying diameters and flow velocities encountered in the nozzle, different typical flow regions and regimes can be distinguished, from laminar to transitional and to weakly turbulent. The purpose of the present work is to re-examine the FDA-CFD benchmark nozzle model at a Reynolds number of 6500 using large eddy simulation (LES). The LES results are compared with published experimental data obtained by Particle Image Velocimetry (PIV) and an excellent agreement can be observed considering the temporally averaged flow velocities. Different flow regimes are characterized by computing the temporal energy spectra at different locations along the main axis.
    Computers in biology and medicine 01/2014; 47C:113-119. · 1.27 Impact Factor
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    ABSTRACT: Understanding the hemodynamics of blood flow in vascular pathologies such as intracranial aneurysms is essential for both their diagnosis and treatment. Computational Fluid Dynamics (CFD) simulations of blood flow based on patient-individual data are performed to better understand aneurysm initiation and progression and more recently, for predicting treatment success. In virtual stenting, a flow-diverting mesh tube (stent ) is modeled inside the reconstructed vasculature and integrated in the simulation. We focus on steady-state simulation and the resulting complex multiparameter data. The blood flow pattern captured therein is assumed to be related to the success of stenting. It is often visualized by a dense and cluttered set of streamlines. We present a fully automatic approach for reducing visual clutter and exposing characteristic flow structures by clustering streamlines and computing cluster representatives. While individual clustering techniques have been applied before to streamlines in 3D flow fields, we contribute a general quantitative and a domain-specific qualitative evaluation of three state-of-the-art techniques. We show that clustering based on streamline geometry as well as on domain-specific streamline attributes contributes to comparing and evaluating different virtual stenting strategies. With our work, we aim at supporting CFD engineers and interventional neuroradiologists.
    IEEE Transactions on Visualization and Computer Graphics 01/2014; · 1.90 Impact Factor
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    ABSTRACT: Background: Computational fluid dynamics (CFD) opens up multiple opportunities to investigate the hemodynamics of the human vascular system. However, due to numerous assumptions the acceptance of CFD among physicians is still limited in practice and validation is mandatory. Method of approach: Therefore, time-dependent quantitative phase contrast MRI measurements in a healthy volunteer and two intracranial aneurysms were carried out at 3- and 7-Tesla. Based on the acquired images, 3D models of the aneurysms were reconstructed and used for the numerical simulations. Flow information from the MR measurements were applied as boundary conditions. The 4D velocity fields obtained by CFD and MRI were qualitatively as well as quantitatively compared including cut planes and vector analyses. Results: For all cases a high similarity of the velocity patterns was observed. Additionally, the quantitative analysis revealed a good agreement between CFD and MRI. Deviations were caused by minor differences between the reconstructed vessel models and the actual lumen. The comparisons between diastole and systole indicate that relative differences between MRI and CFD are intensified with increasing velocity. Conclusions: The findings of this study lead to the conclusion that CFD is able to predict accurately intracranial velocities when realistic geometries and boundary conditions are provided. Due to the considerably higher temporal and spatial resolution of CFD compared to MRI, complex flow patterns can then be further investigated in order to evaluate their role with respect to aneurysm formation or rupture. Nevertheless, special care is required regarding the vessel reconstruction since the geometry has a major impact on the subsequent numerical results. Keywords: Computational Fluid Dynamics, Intracranial Aneurysm, 7 Tesla Phase contrast MR Imaging, Validation.
    Journal of Biomechanical Engineering 11/2013; · 1.52 Impact Factor
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    ABSTRACT: Proper Orthogonal Decomposition (POD) is an effective tool in fluid dynamics for investigation of complex, transitional or turbulent flows. In POD the transient vector or scalar field (velocity, concentration, temperature, etc.) is decomposed into a sum of spatial modes multiplied with time coefficients (Fourier-splitting method). However, these spatial modes and time coefficients can in practice be obtained by different methods. Even if POD has been used in numerous fluid dynamical studies, there are only few publications describing the relationship between the different methods and comparing the results. In the present case the POD basis functions are calculated either by Singular Value Decomposition (SVD) or by the Snapshot-POD approach. The results are compared in order to understand similarities and differences between the methods, as well as advantages and drawbacks. Comparisons between the obtained spatial modes, time coefficients, required computational effort, and complexity of calculation are presented and discussed. The influence of the numerical settings is also investigated, in particular the impact of the number of snapshots on the results. Finally, the differences obtained when analyzing a vector field globally or component-wise are discussed in detail.
    International Journal of Heat and Fluid Flow 10/2013; 43:204–211. · 1.58 Impact Factor
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    ABSTRACT: Intracranial aneurysms are abnormal dilatations of the cerebral arteries that are, in case of a rupture, highly lifethreatening. Numerical investigations carried out to support clinical physicians are commonly assuming laminar flow conditions. However, especially in diseased arteries transition to turbulence has been observed. To identify a possible transition, the effect of geometry as well as of blood cells were investigated in the present work. Therefore, hemodynamic simulations were carried out under realistic flow conditions in an idealized basilar tip aneurysm. In addition, the impact of blood cells is examined using DNS with a pseudo-spectral code, adding Lagrangian spherical particles (using a point force approach) mimicking the suspension. The statistical analyses revealed that even under normal flow conditions fluctuations are observed during the cardiac cycle. These appear at relatively high frequencies, around 100 Hz. Additionally, the particulate phase significantly influenced the flow stability. Hence, the results indicate that transitional effects might indeed play a role to understand hemodynamics and rupture of intracranial aneurysms, and should be accordingly taken into account.
    8th International Symposium on Turbulence and Shear Flow Phenomena, Poitiers, France; 08/2013
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    ABSTRACT: The number of numerical studies predicting blood flow in intracranial aneurysms is rapidly increasing over the last years. Due to a high spatial as well as temporal resolution, computational fluid dynamics (CFD) approaches offer a high potential to investigate flow interaction within the human vascular system. However, state-of-the-art methods still underlie several assumptions, e.g., rigid vessel walls, analytical boundary conditions or the consideration of blood as a single-phase continuous fluid. In consequence, the acceptance of CFD is still limited among a majority of physicians [1]. In order to overcome these reasonable doubts, simulations need to be validated via experiments. Therefore, two patient-specific intracranial aneurysms were measured by means of 7-Tesla magnetic resonance imaging (MRI). Afterwards, highly resolved numerical simulations were carried out and the peak-systolic velocity fields compared in a qualitative manner.
    ASME 2013 Summer Bioengineering Conference; 06/2013
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    ABSTRACT: Die numerische Strömungsmechanik (Computational Fluid Dynamics) eröffnet zahlreiche Möglichkeiten zur Erforschung der Hämodynamik im menschlichen Gefäßsystem. Allerdings unterliegen die Methoden zahlreichen Annahmen, weshalb die Akzeptanz bei Medizinern oftmals begrenzt ist. Da Validierungen notwendig sind, wurden zeitabhängige 7- Tesla Phasenkontrast-MR-Messungen in zwei intrakraniellen Aneurysmen durchgeführt. Dreidimensionale Modelle wurden auf Basis der aufgenommenen Schichtdaten rekonstruiert und für die numerischen Simulationen verwendet. Gemessene Flussinformationen dienten dabei als Randbedingungen. Die mittels CFD und MRT bestimmten 4D Geschwindigkeitsfelder wurden qualitativ mithilfe von Schnittebenen verglichen. Für beide Fälle konnte eine hohe Ähnlichkeit der Flussmuster festgestellt werden und es ergab sich eine gute Übereinstimmung zwischen CFD und MRT. Auftretende Abweichungen wurden hauptsächlich auf geringfügige Differenzen zwischen den rekonstruierten Gefäßen und dem tatsächlichem Lumen zurückgeführt. Die Ergebnisse dieser Studie führen zu der Schlussfolgerung, dass die numerische Strömungsmechanik sich als Methode eignet, um realistische Blutflussinformationen zu generieren. Aufgrund der hohen zeitlichen und räumlichen Auflösung können komplexe hämodynamische Effekte erforscht werden, um schließlich die behandelnden Ärzte in ihren Tätigkeiten zu unterstützen.
    IFF-Wissenschaftstage "Digitales Engineering zum Planen, Testen und Betreiben technischer Systeme, Magdeburg, Germany; 06/2013
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    ABSTRACT: In this work, we investigate numerically the hydrodynamics and crystal motion in a draft tube batch crystallizer using Computational Fluid Dynamics (CFD) coupled with the Discrete Phase Model (DPM). The flow generated by the rotating impeller interacts with baffles and generates a complex, unsteady, three-dimensional turbulent flow with large-scale recirculations. To investigate this flow, a Multiple Reference Frame approach is first used as initial condition for a more accurate Sliding Mesh Model. Turbulence is modeled with the k–ε model considering the Unsteady Reynolds-Averaged Navier–Stokes formulation. The computational model is first used to analyze the impact of the liquid volume in the vessel on hydrodynamics. Coupling CFD with DPM, crystal motion is then investigated. For this purpose, crystals are introduced in the flow at eight different positions. Crystal motion has been investigated in a Lagrangian manner through one-way coupling considering drag and buoyancy forces. Deposition probabilities have been calculated for different crystal types and interpreted as an indicator for unfavorable crystallization conditions, allowing identifying suitable liquid volumes and seeding positions.
    Journal of Crystal Growth 06/2013; 372:219–229. · 1.55 Impact Factor
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    ABSTRACT: Abstract The number of scientific publications dealing with stented intracranial aneurysms is rapidly increasing. Powerful computational facilities are now available; an accurate computational modeling of hemodynamics in patient-specific configurations is, however, still being sought. Furthermore, there is still no general agreement on the quantities that should be computed and on the most adequate analysis for intervention support. In this article, the accurate representation of patient geometry is first discussed, involving successive improvements. Concerning the second step, the mesh required for the numerical simulation is especially challenging when deploying a stent with very fine wire structures. Third, the description of the fluid properties is a major challenge. Finally, a founded quantitative analysis of the simulation results is obviously needed to support interventional decisions. In the present work, an attempt has been made to review the most important steps for a high-quality computational fluid dynamics computation of virtually stented intracranial aneurysms. In consequence, this leads to concrete recommendations, whereby the obtained results are not discussed for their medical relevance but for the evaluation of their quality. This investigation might hopefully be helpful for further studies considering stent deployment in patient-specific geometries, in particular regarding the generation of the most appropriate computational model.
    Biomedizinische Technik/Biomedical Engineering 05/2013; · 1.16 Impact Factor
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    ABSTRACT: Cerebral aneurysms are a pathological vessel dilatation that bear a high risk of rupture. For the understanding of this risk, the analysis of hemodynamic information plays an important role in clinical research. These infor-mation are obtained by computational fluid dynamics (CFD) simulations. Thus, an effective visual exploration of patient-specific blood flow behavior in cerebral aneurysms was developed to support the domain experts in their investigation process. We present advanced visualization and interaction techniques, which provide an overview, focus-and-context views as well as multi-level explorations. Moreover, an automatic extraction process of quali-tative flow characteristics, which are correlated with the risk of rupture is introduced. Although not established in clinical routine yet, interviews and informal user studies confirm the usefulness of these methods. 1. Motivation Cerebral aneurysms represent a threatening vascular disease which bears a high risk of rupture with an annual rupture rate of 1% to 2%, a mortality rate in case of rupture of 40% to 50% [BSH * 10]. The aneurysm's size is correlated with the risk of rupture: larger aneurysms are more likely to rupture. The majority of all ruptured aneurysms, however, are small (5-10 mm). The hemodynamics within the aneurysm plays also an important role in aneurysm progression and rupture. Complex and instable flow is correlated with an increased risk [CCAea05]. Thus, biomedical engineers and neuroradi-ologists who are involved in clinical research, are interested in the analysis of hemodynamic characteristics obtained by patient-specific CFD simulation. They focus on assessing the correlations between flow characteristics (e.g., flow ve-locity, WSS, and inflow jet) and the risk of rupture in order to support treatment decisions. Neuroradiologists want to al-ter the flow in a defined manner with an appropriate stent or coiling to induce thrombosis. The choice of a stent and the exact placement belong to the treatment decisions. Since treatment carries a significant risk of complication,
    Proc. of EG Medical Price; 05/2013
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    ABSTRACT: Stimulated by a recent controversy regarding pressure drops predicted in a giant aneurysm with a proximal stenosis, the present study sought to assess variability in the prediction of pressures and flow by a wide variety of research groups. In phase I, lumen geometry, flow rates, and fluid properties were specified, leaving each research group to choose their solver, discretization, and solution strategies. Variability was assessed by having each group interpolate their results onto a standardized mesh and centerline. For phase II, a physical model of the geometry was constructed, from which pressure and flow rates were measured. Groups repeated their simulations using a geometry recon-structed from a micro-computed tomography (CT) scan of the physical model with the measured flow rates and fluid properties. Phase I results from 25 groups demonstrated remarkable consistency in the pressure patterns, with the majority predicting peak systolic pressure drops within 8% of each other. Aneurysm sac flow patterns were more variable with only a few groups reporting peak systolic flow instabilities owing to their use of high temporal resolutions. Variability for phase II was comparable, and the median predicted pressure drops were within a few millimeters of mercury of the measured values but only after accounting for submillimeter errors in the reconstruction of the life-sized flow model from micro-CT. In summary, pressure can be predicted with consistency by CFD across a wide range of solvers and solution strategies, but this may not hold true for specific flow patterns or derived quantities. Future challenges are needed and should focus on hemodynamic quantities thought to be of clinical interest. [
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    ABSTRACT: Stimulated by a recent controversy regarding pressure drops predicted in a giant aneurysm with a proximal stenosis, the present study sought to assess variability in the prediction of pressures and flow by a wide variety of research groups. In phase I, lumen geometry, flow rates, and fluid properties were specified, leaving each research group to choose their solver, discretization, and solution strategies. Variability was assessed by having each group interpolate their results onto a standardized mesh and centerline. For phase II, a physical model of the geometry was constructed, from which pressure and flow rates were measured. Groups repeated their simulations using a geometry reconstructed from a micro-computed tomography (CT) scan of the physical model with the measured flow rates and fluid properties. Phase I results from 25 groups demonstrated remarkable consistency in the pressure patterns, with the majority predicting peak systolic pressure drops within 8% of each other. Aneurysm sac flow patterns were more variable with only a few groups reporting peak systolic flow instabilities owing to their use of high temporal resolutions. Variability for phase II was comparable, and the median predicted pressure drops were within a few millimeters of mercury of the measured values but only after accounting for submillimeter errors in the reconstruction of the life-sized flow model from micro-CT. In summary, pressure can be predicted with consistency by CFD across a wide range of solvers and solution strategies, but this may not hold true for specific flow patterns or derived quantities. Future challenges are needed and should focus on hemodynamic quantities thought to be of clinical interest.
    Journal of Biomechanical Engineering 02/2013; 135(2):021016. · 1.52 Impact Factor
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    Chemie Ingenieur Technik 01/2013; 85(9). · 0.70 Impact Factor