Gábor Janiga

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

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Publications (98)98.47 Total impact

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    ABSTRACT: Sylvia Glaßer, Jan Hirsch, Philipp Berg, Patrick Saalfeld, Oliver Beuing, Gabor Janiga, Bernhard Preim Bildverarbeitung für die Medizin (BVM), in print, 2016
    No preview · Conference Paper · Mar 2016
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    ABSTRACT: The present paper analyzes numerically the entropy generation induced by forced convection in a canonical configuration. The configuration itself includes two well known fluid dynamic problems: (1) an external flow (flow around a cylinder, Kármán flow); and (2) an internal flow (flow between two concentric rotating cylinders, Couette flow). In many daily engineering issues (e.g., cooling of electric machines), a combination of these problems occurs and has to be investigated. Using the canonical configuration, the fields of entropy generation are analyzed in this work for a constant wall heat flux but varying two key parameters (Reynolds numbers Re∞ and Re0). The entropy generation due to conduction shows an absolute minimum around Re0 = 10, 000. The same minima can be found by a detailed analysis of the temperature profile. Thus, entropy generation seems to be a suitable indicator for optimizing heat exchange processes and delivers a large amount of information concerning fluid and heat transport.
    Full-text · Article · Dec 2015 · Entropy
  • Hai Yu · Gábor Janiga · Dominique Thévenin
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    ABSTRACT: An optimization method suitable for improving the performance of Archimedes screw axial rotary blood pumps is described in the present article. In order to achieve a more robust design and to save computational resources, this method combines the advantages of the established pump design theory with modern computer-aided, computational fluid dynamics (CFD)-based design optimization (CFD-O) relying on evolutionary algorithms and computational fluid dynamics. The main purposes of this project are to: (i) integrate pump design theory within the already existing CFD-based optimization; (ii) demonstrate that the resulting procedure is suitable for optimizing an Archimedes screw blood pump in terms of efficiency. Results obtained in this study demonstrate that the developed tool is able to meet both objectives. Finally, the resulting level of hemolysis can be numerically assessed for the optimal design, as hemolysis is an issue of overwhelming importance for blood pumps.
    No preview · Article · Nov 2015 · Artificial Organs
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    ABSTRACT: Although flow-diverting devices are promising treatment options for intracranial aneurysms, jailed side branches might occlude leading to insufficient blood supply. Especially differences in the local stent strut compression may have a drastic influence on subsequent endothelialization. To investigate the outcome of different treatment scenarios, over- and undersized stent deployments were realized experimentally and computationally. Two Pipeline Embolization Devices were placed in the right common carotid artery of large white swine, crossing the right ascending pharyngeal artery. DSA and PC-MRI measurements were acquired pre- and post-stenting and after three months. To evaluate the stent strut endothelialization and the corresponding ostium patency, the swine were sacrificed and scanning electron microscopy measurements were carried out. A more detailed analysis of the near-stent hemodynamics was enabled by a realistic virtual stenting in combination with highly resolved Computational Fluid Dynamics simulations using case-specific boundary conditions. The oversizing resulted in an elongated stent deployment with more open stent pores, while for the undersized case a shorter deployment with more condensed pores was present. In consequence, the side branch of the first case remained patent after three months and the latter almost fully occluded. The virtual investigation confirmed the experimental findings by identifying differences between the individual velocities as well as stent shear stresses at the distal part of the ostia. The choice of flow-diverting device and the subsequent deployment strategy strongly influences the patency of jailed side branches. Therefore, careful treatment planning is required, to guarantee sufficient blood supply in the brain territories supplied those branches.
    No preview · Article · Nov 2015 · Journal of Biomechanics
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    ABSTRACT: Background and purpose: A variety of approaches exists where numerical methods that underlie numerous assumptions are used in order to predict rupture in intracranial aneurysms. Since many hemodynamic parameters have been derived from these computations but no persistent explanation for rupture has been found, the acceptance of CFD among physicians is still limited in practice. Therefore, the International CFD Rupture Challenge 2013 was introduced to compare the rupture and blood flow predictions of the participants. Methods: 26 groups from 15 countries participated in the challenge. They were provided with surface models of two intracranial aneurysms and asked to carry out the corresponding hemodynamics simulations, free to choose their own mesh, solver and temporal discretization. As output, they had to submit velocity results along centerlines and on planes, and pressure results only along centerlines. The first phase of the challenge, described in a separate paper, was aimed at predicting which of the two aneurysms had previously ruptured and where the rupture site was located. The second phase, described in this paper, was aimed at assessing the variability of the solutions. Participants were free to choose boundary conditions in the first phase, whereas they were prescribed in the second phase. In order to compare the computational results with experimental results, steady flow measurements using Particle Image Velocimetry (PIV) were carried out in a silicone model of one of the provided aneurysms. Results: Approximately 80 % of the participating groups generated similar results. Both velocity and pressure computations were in a good agreement among each other for cycle-averaged and peak-systolic predictions. Most outliers underestimated the majority's solution but identified comparable flow structures. Regarding validation, the results of steady CFD simulations and PIV experiments were in a good agreement and occurring flow structures were detected with a high similarity. Conclusion: The comparisons demonstrated that a broad range of solution strategies lead to highly similar flow predictions. Both in-house and commercial solvers produced the few outliers, suggesting that verification procedures and/or solver settings, rather than the solver itself, were to blame. To further validate the computational results of the majority, time-dependent measurements need to be carried out. Overall, the challenge revealed that the physics of the blood flow can be predicted by different numerical approaches under certain assumptions. However, more biological aspects need to be considered in order to be able to virtually asses the rupture risk of an intracranial aneurysm.
    No preview · Article · Oct 2015 · Journal of Biomechanical Engineering
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    ABSTRACT: The optimal treatment of intracranial aneurysms using flow diverting devices is a fundamental issue for neuroradiologists as well as neurosurgeons. Due to highly irregular manifold aneurysm shapes and locations, the choice of the stent and the patient-specific deployment strategy can be a very difficult decision. To support the therapy planning, a new method is introduced that combines a three-dimensional CFD-based optimization with a realistic deployment of a virtual flow diverting stent for a given aneurysm. To demonstrate the feasibility of this method, it was applied to a patient-specific intracranial giant aneurysm that was successfully treated using a commercial flow diverter. Eight treatment scenarios with different local compressions were considered in a fully automated simulation loop. The impact on the corresponding blood flow behavior was evaluated qualitatively as well as quantitatively, and the optimal configuration for this specific case was identified. The virtual deployment of an uncompressed flow diverter reduced the inflow into the aneurysm by 24.4% compared to the untreated case. Depending on the positioning of the local stent compression below the ostium, blood flow reduction could vary between 27.3% and 33.4%. Therefore, a broad range of potential treatment outcomes was identified, illustrating the variability of a given flow diverter deployment in general. This method represents a proof of concept to automatically identify the optimal treatment for a patient in a virtual study under certain assumptions. Hence, it contributes to the improvement of virtual stenting for intracranial aneurysms and can support physicians during therapy planning in the future.
    No preview · Article · Oct 2015 · Journal of Biomechanics
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    ABSTRACT: Computational fluid dynamic (CFD) simulations of blood flow provide new insights into the hemodynamics of vascular pathologies such as cerebral aneurysms. Understanding the relations between hemodynamics and aneurysm initiation, progression, and risk of rupture is crucial in diagnosis and treatment. Recent studies link the existence of vortices in the blood flow pattern to aneurysm rupture and report observations of embedded vortices - a larger vortex encloses a smaller one flowing in the opposite direction - whose implications are unclear. We present a clustering-based approach for the visual analysis of vortical flow in simulated cerebral aneurysm hemodynamics. We show how embedded vortices develop at saddle-node bifurcations on vortex core lines and convey the participating flow at full manifestation of the vortex by a fast and smart grouping of streamlines and the visualization of group representatives. The grouping result may be refined based on spectral clustering generating a more detailed visualization of the flow pattern, especially further off the core lines. We aim at supporting CFD engineers researching the biological implications of embedded vortices.
    Full-text · Article · Aug 2015 · IEEE Transactions on Visualization and Computer Graphics
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    ABSTRACT: The lattice Boltzmann method (LBM) is a useful technique for simulating multiphase flows and modeling complex physics. Specifically, we use LBM combined with a direct-forcing (DF) immersed boundary (IB) method to simulate fluid-particle interactions in two-phase particulate flows. Two grids are used in the simulation: a fixed uniform Eulerian grid for the fluid phase and a Lagrangian grid that is attached to and moves with the immersed particles. Forces are calculated at each Lagrangian point. To exchange numerical information between the two grids, discrete delta functions are used. The resulting DF IB-LBM approach is then successfully applied to a variety of reference flows, namely the sedimentation of one and two circular particles in a vertical channel, the sedimentation of one or two spheres in an enclosure, and a neutrally buoyant prolate spheroid in a Couette flow. This last application proves that the developed approach can be used also for non-spherical particles. The three forcing schemes and the different factors affecting the simulation (added mass effect, corrected radius) are also discussed. © 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.
    No preview · Article · Aug 2015
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    ABSTRACT: The importance of wind energy has increased at a rapid pace in the last years. As a result, increasing efforts are taken to improve the efficiency and extend the applicability of wind turbines to all suitable locations. Although HAWTs (horizontal axis wind turbines) are clearly the most well-spread, VAWTs (vertical axis wind turbines) show several advantages. In this category, the H-Darrieus configuration is particularly popular.For evaluating the performance of an H-Darrieus wind turbine, two-dimensional simulations relying on CFD (computational fluid dynamics) have been widely employed, since detailed full three-dimensional simulations for an optimization remain beyond reach with current computer power. Unfortunately, there is up to now no agreement in the scientific literature concerning the best possible model and model parameters for such conditions. At the same time, different models, different solvers and different meshing strategies may yield very different results.The aim of the current study is to perform a systematic numerical analysis in order 1) to identify the necessary mesh resolution for the different turbulence models, 2) to choose a model suitable for optimization and 3) to compare the characteristic curves obtained with the different models for four different configurations.
    No preview · Article · Aug 2015 · Energy
  • Philipp Berg · A. Abdelsamie · G Janiga
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    ABSTRACT: Numerical methods are increasingly used to predict the blood flow in intracranial aneurysms. However, laminar flow in the cerebral vasculature is normally considered. Recent studies proposed that also transitional effects could be present in aneurysms. To further investigate this question, a high order direct numerical simulation solver carried out hemodynamic simulations within a complex patient-specific aneurysm. The considered case already served as basis for an international CFD challenge and hence several simulation results exist. Initial evaluations revealed no signs of unstable flow or increased frequencies within the aneurysm. Further refined simulations and quantitative analysis will follow to identify possible fluctuations.
    No preview · Conference Paper · Jun 2015
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    Full-text · Conference Paper · Jun 2015
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    G Janiga · P Berg · S Sugiyama · K Kono · D A Steinman
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    ABSTRACT: Rupture risk assessment for intracranial aneurysms remains challenging, and risk factors, including wall shear stress, are discussed controversially. The primary purpose of the presented challenge was to determine how consistently aneurysm rupture status and rupture site could be identified on the basis of computational fluid dynamics. Two geometrically similar MCA aneurysms were selected, 1 ruptured, 1 unruptured. Participating computational fluid dynamics groups were blinded as to which case was ruptured. Participants were provided with digitally segmented lumen geometries and, for this phase of the challenge, were free to choose their own flow rates, blood rheologies, and so forth. Participants were asked to report which case had ruptured and the likely site of rupture. In parallel, lumen geometries were provided to a group of neurosurgeons for their predictions of rupture status and site. Of 26 participating computational fluid dynamics groups, 21 (81%) correctly identified the ruptured case. Although the known rupture site was associated with low and oscillatory wall shear stress, most groups identified other sites, some of which also experienced low and oscillatory shear. Of the 43 participating neurosurgeons, 39 (91%) identified the ruptured case. None correctly identified the rupture site. Geometric or hemodynamic considerations favor identification of rupture status; however, retrospective identification of the rupture site remains a challenge for both engineers and clinicians. A more precise understanding of the hemodynamic factors involved in aneurysm wall pathology is likely required for computational fluid dynamics to add value to current clinical decision-making regarding rupture risk. © 2015 American Society of Neuroradiology.
    Full-text · Article · Dec 2014 · American Journal of Neuroradiology
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    Toni Eger · D. Thévenin · G. Janiga · T. Bol · R. Schroth
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    ABSTRACT: The demand for higher power density at low costs leads to an increasing importance of thermal management in electrical machines. Different approaches are employed to develop cooling strategies. However, a truly quantitative analysis is barely used, partly due to the huge data quantity generated by the flow simulation. In many cases only visual postprocessing under consideration of a few variables is implemented. The present paper provides an overview about a method suitable for investigating transport phenomena induced by forced convection. Different configurations are retained to check the generality of the approach. Based on the second law of thermodynamics, the entropy generation in the fluid field is considered. As an example of external flow, a semi-infinite plate is examined. For internal flows, two rotating, concentric cylinders are then analysed. Finally, transport processes in the flow between these cylinders located in a rectangular domain (Ω) are analyzed. In addition, the outer cylinder has two openings along the main stream direction. The straight flow imposed at the inlet velocity of the domain (Ω) is influenced by the rotational motion of the inner cylinder as well as by the opening angle (β) at the outer cylinder and the radius ratio between the two cylinders (Π). The thermal boundary conditions are isothermal or isoflux and isothermal or adiabatic for the inner and outer cylinder, respectively. In a first step, numerical calculations of the Nusselt number (Nu) are discussed. This methodical approach can be a first step to analysing transport phenomena under consideration of physically-based indicators.
    Full-text · Conference Paper · Dec 2014
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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
    No preview · Article · Nov 2014 · Crystal Growth & Design
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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.
    Full-text · Article · Aug 2014 · Annals of Biomedical Engineering
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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.
    Full-text · Conference Paper · Jul 2014

  • No preview · Conference Paper · Jun 2014
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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.
    Full-text · Article · May 2014 · IEEE Transactions on Visualization and Computer Graphics
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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.
    Full-text · Conference Paper · Apr 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.
    Full-text · Article · Mar 2014 · International Journal of Heat and Mass Transfer