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Documents published in Scipedia

• A Hybrid Grid Generation Method for Complex Geometries

  H. Luo, S. Spiegel, R. Lohner

  AIAA Journal (2010). Vol. 48 (11), pp. 2639-2647

  
  

  Abstract

  A hybrid mesh generation method is described to discretize complex geometries. The idea behind this hybrid method is to combine the orthogonality and directionality of a structured grid, the efficiency and simplicity of a Cartesian grid, and the flexibility and ease of an unstructured grid in an attempt to develop an automatic, robust, and fast hybrid mesh generation method for configurations of engineering interest. A semistructured quadrilateral grid is first generated on the wetted surfaces. A background Cartesian grid, covering the domain of interest, is then constructed using a Quadtree-based Cartesian Method. Those Cartesian cells overlapping with the semistructured grids or locating outside of computational domain are then removed using an Alternating Digital Tree method. Finally, an unstructured grid generation method is used to generate unstructured triangular cells to till all empty regions in the domain as a result of the trimming process. The automatic placement of sources at the geometrical irregularities is developed to render these regions isotropic, thus effectively overcoming the difficulty of generating highly stretched good-quality elements in these regions. The self-dividing of the exposed semistructured elements with high aspect ratio and the adaptation of the background mesh using the cell size information from the exposed semistructured elements for generating Cartesian cells are introduced to improve the quality of unstructured triangular elements and guarantee the success of the unstructured grid generation in the void regions. The developed hybrid grid generation method is used to generate a hybrid grid for a number of test cases, demonstrating its ability and robustness to mesh complex configurations.

  Abstract
  A hybrid mesh generation method is described to discretize complex geometries. The idea behind this hybrid method is to combine the orthogonality and directionality of a structured [...]

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• An assessment of architecturally appealing, semi-open shock mitigation devices

  R. Lohner, J. Baum

  Engineering Computations (2010). Vol. 29 (1), pp. 19-30

  
  

  Abstract

  Purpose – Limitations in space and city planning constraints have led to the search for alternative shock mitigation devices that are architecturally appealing. The purpose of this paper is to consider a compromise solution which consists of partially open, thick, bending-resistant shapes made of acrylic material that may be Kevlar- or steel-reinforced. Seven different configurations were analyzed numerically. Design/methodology/approach – For the flow solver, the FEM-FCT scheme as implemented in FEFLO is used. The flowfields are initialized from the output of highly detailed 1-D (spherically symmetric) runs. Peak pressure and impulse are stored and compared. In total, seven different configurations were analyzed numerically. Findings – It is found that for some of these, the maximum pressure is comparable to usual, closed walls, and the maximum impulse approximately 50 percent higher. This would indicate that such designs offer a blast mitigation device eminently suitable for built-up city environments. Research limitations/implications – Future work will consider fully coupled fluid-structure runs for the more appealing designs, in order to assess whether such devices can be manufactured from commonly available materials such as acrylics or other poly-carbonates. Practical implications – This would indicate that such designs offer a blast mitigation device eminently suitable for built-up city environments. Originality/value – This is the first time such a semi-open blastwall approach has been tried and analyzed.

  Abstract
  Purpose – Limitations in space and city planning constraints have led to the search for alternative shock mitigation devices that are architecturally appealing. The [...]

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• Numerical modeling of blast wave propagation in a generic facility

  F. Togashi, J. Baum, O. Soto, R. Lohner

  (2011). Proceedings of the 9th International Conference on Shock and Impact Loads on Structures. pp. 659-667.

  
  

  Abstract

  Accurate evaluation of interior wall response to blast in a building requires the use of a coupled fluid-structure dynamics methodology. The reason is that the walls respond to the blast loading on the time-scale of the blast reverberation within the building. When the wall fails due to blast loading, the debris as well as the airblast propagate into adjacent rooms to load the next layer of walls. To provide better understanding this phenomenon, a program has been initiated which combines experimental and computational efforts. The program objective is to improve the understandings of internal blast damage and fragment dispersion phenomena under the transient/quasi-static pressure condition, so as to improbé the modeling of these scenarios in fast-running codes. The paper will describe the numerical simulations conducted, in which we modeled CMU walls response to blast in a generic facility. The results show that the numerical code was able to accurately predict wall breach due to nearby charge, the complete wall movement, and the loading of the second wall first by the airblast, then by the high-inertia slower-moving wall. Comparison of pressure time histories between the measured and predicte data show good agreement both in detonation room as well as the adjacent bay.

  Abstract
  Accurate evaluation of interior wall response to blast in a building requires the use of a coupled fluid-structure dynamics methodology. The reason is that the walls respond [...]

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• Semi-automatic porting of a general fortran CFD code to GPUS: The difficult modules

  R. Lohner, A. Corrigan

  Aerospace Research Central (2011). Vol. 20, pp. 1-19

  
  

  Abstract

  Over the last year, a considerable portion of FEFLO, a general-purpose legacy CFD code operating on unstructured grids, was ported to run on GPUs. Like so many other legacy codes, FEFLO is an adaptive, edge-based finite element code for the solution of compressible and incompressible flow, which was primarily written in Fortran 77 and has previously been ported to vector, shared memory parallel and distributed memory parallel machines. Due to the large size of FEFLO and the likelihood of human error in porting, as well as the desire for continued development within a single codebase, a specialized Python script, based on FParser (Peterson, 2009), was written to perform automated translation from the OpenMP-parallelized loops to GPU kernels implemented in CUDA, along with GPU memory management, while integrating with the existing framework for distributed memory parallelism via MPI. The present paper describes extensions of the script and algorithmic techniques that enable the efficient running on GPUs of the modules that are not straightforward to port. In particular, we consider LU-SGS algorithms, linelet preconditioning and particle-mesh algorithms.

  Abstract
  Over the last year, a considerable portion of FEFLO, a general-purpose legacy CFD code operating on unstructured grids, was ported to run on GPUs. Like so many other legacy [...]

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• Generation of Unstructured Grids Suitable for Rans Calculations

  R. Lohner

  (2011). Computational Aerosciences in the 21st Century. ICASE LaRC Interdisciplinary Series in Science and Engineering, vol 8. Springer, Dordrecht

  
  

  Abstract

        The present Conference was intended to provide a forum to discuss future trends and developments that may affect the way Computational Aerosciences are conducted. One way to venture statements about the future is via exptrapolations from historical trends. In each of the individual disciplines that comprise the aerosciences - aerodynamics, structures, propulsion, control, stealth, etc. - one observes the typical bottom-up trend. Starting from sufficiently simple geometries and equations to have an impact and be identified as `computational’ (e.g., potential flow in 2-D for aerodynamics), more and more realism is added at the geometrical and PDE level. While the engineering process (Figure 1) follows the line: project, objectives, optimization goals, discipline, problem definition, gridding, solution of the PDE and evaluation, the developments in the Computational Sciences tend to run in the opposite direction: solvers, mesh generators, preprocessors, multi-disciplinary links, complete database. At present (1998), for commercial software we are at the threshold mesh generators/ preprocessors, so we can estimate that sometime during the next decade we will see the widespread use of multi-disciplinary links and towards the end of the decade the appearance of integrated analysis and simulation databases. At the research level, we are already entering the complete database approach.  

  Abstract
        The present Conference was intended to provide a forum to discuss future trends and developments that may affect the way Computational Aerosciences [...]

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• Validation of the computational fluid–structure interaction simulation at real-scale tests of a flexible 29 m umbrella in natural wind flow

  A. Michalski, P. Kermel, E. Haug, R. Lohner, R. Wüchner, K. Bletzingerm

  Journal of Wind Engineering and Industrial Aerodynamics (2011). Vol. 99 (4), pp. 400-413

  
  

  Abstract

  The sensitivity of membrane structures to wind loads due to their flexibility and small inertial masses raises the question of their behavior under natural wind conditions. Particularly transient wind loads could lead to dynamic amplification of the structural response. The assessment of the dynamic response of membrane structures is complex due to their special load carrying behavior, their material properties, and their distinct structural interaction with flow induced effects. Computationally intensive fluid–structure interaction simulation could overcome simplifications and limitations of existing approaches, especially small scale wind tunnel tests, and allow the assessment of all relevant structural and fluid phenomena. This paper outlines a virtual design methodology for lightweight flexible membrane structures under the impact of fluctuating wind loads and provides results on the unique validation of the method at real-scale tests of a highly flexible 29 m umbrella.

  Abstract
  The sensitivity of membrane structures to wind loads due to their flexibility and small inertial masses raises the question of their behavior under natural wind conditions. [...]

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• Running unstructured grid‐based CFD solvers on modern graphics hardware

  A. Corrigan, F. Camelli, R. Lohner, J. Wallin

  Int. J. Numer. Meth. Fluids (2011). Vol. 66 (2), pp. 221-229

  
  

  Abstract

  Techniques used to implement an unstructured grid solver on modern graphics hardware are described. The three‐dimensional Euler equations for inviscid, compressible flow are considered. Effective memory bandwidth is improved by reducing total global memory access and overlapping redundant computation, as well as using an appropriate numbering scheme and data layout. The applicability of per‐block shared memory is also considered. The performance of the solver is demonstrated on two benchmark cases: a NACA0012 wing and a missile. For a variety of mesh sizes, an average speed‐up factor of roughly 9.5 × is observed over the equivalent parallelized OpenMP code running on a quad‐core CPU, and roughly 33 × over the equivalent code running in serial.

  Abstract
  Techniques used to implement an unstructured grid solver on modern graphics hardware are described. The three‐dimensional Euler equations for inviscid, compressible flow [...]

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• Computational Hemodynamics Framework for the Analysis of Cerebral Aneurysms

  F. Mut, R. Lohner, A. Chien, S. Tateshima, F. Viñuela, C. Putman, J. Cebral

  Int. J. Numer. Meth. Bio. Engng. (2010). Vol. 27 (6), pp. 822-839

  
  

  Abstract

  Assessing the risk of rupture of intracranial aneurysms is important for clinicians because the natural rupture risk can be exceeded by the small but significant risk carried by current treatments. To this end numerous investigators have used image‐based computational fluid dynamics models to extract patient‐specific hemodynamics information, but there is no consensus on which variables or hemodynamic characteristics are the most important. This paper describes a computational framework to study and characterize the hemodynamic environment of cerebral aneurysms in order to relate it to clinical events, such as growth or rupture. In particular, a number of hemodynamic quantities are proposed to describe the most salient features of these hemodynamic environments. Application to a patient population indicates that ruptured aneurysms tend to have concentrated inflows, concentrated wall shear stress distributions, high maximal wall shear stress, and smaller viscous dissipation ratios than unruptured aneurysms. Furthermore, these statistical associations are largely unaffected by the choice of physiologic flow conditions. This confirms the notion that hemodynamic information derived from image‐based computational models can be used to assess aneurysm rupture risk, to test hypotheses about the mechanisms responsible for aneurysm formation, progression, and rupture, and to answer specific clinical questions.

  Abstract
  Assessing the risk of rupture of intracranial aneurysms is important for clinicians because the natural rupture risk can be exceeded by the small but significant risk carried [...]

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• Clinical application of image‐based CFD for cerebral aneurysms

  J. Cebral, F. Mut, D. Sforza, R. Lohner, E. Scrivano, P. Lylyk, C. Putman

  Int. J. Numer. Meth. Bio. Engng. (2011). Vol. 27 (7), pp. 977-992

  
  

  Abstract

  During the last decade, the convergence of medical imaging and computational modeling technologies has enabled tremendous progress in the development and application of image‐based computational fluid dynamics modeling of patient‐specific blood flows. These techniques have been used for studying the basic mechanisms involved in the initiation and progression of vascular diseases, for studying possible ways to improve the diagnosis and evaluation of patients by incorporating hemodynamics information to the anatomical data typically available, and for the development of computational tools that can be used to improve surgical and endovascular treatment planning. However, before these technologies can have a significant impact on the routine clinical practice, it is still necessary to demonstrate the connection between the extra information provided by the models and the natural progression of vascular diseases and the outcome of interventions. This paper summarizes some of our contributions in this direction, focusing in particular on cerebral aneurysms.

  Abstract
  During the last decade, the convergence of medical imaging and computational modeling technologies has enabled tremendous progress in the development and application of image‐based [...]

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• Deflated preconditioned conjugate gradient solvers for the pressure‐Poisson equation: Extensions and improvements

  R. Lohner, F. Mut, J. Cebral, R. Aubry, G. Houzeaux

  Int. J. Numer. Meth. Engng. (2011). Vol. 87 (1-5), pp. 2-14

  
  

  Abstract

  Extensions and improvements to a deflated preconditioned conjugate gradient technique for the solution of the pressure‐Poisson equation within an incompressible flow solver are described. In particular, the use of the technique for embedded grids, for cases where volume of fluid or level set schemes are required and its implementation on parallel machines are considered. Several examples are included that demonstrate a considerable reduction in the number of iterations and a remarkable insensitivity to the number of groups/ regions chosen and/or to the way the groups are formed.

  Abstract
  Extensions and improvements to a deflated preconditioned conjugate gradient technique for the solution of the pressure‐Poisson equation within an incompressible flow solver [...]

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