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• Simulation of intracranial aneurysm stenting: Techniques and challenges

  S. Appanaboyina, F. Mut, R. Lohner, C. Putman, J. Cebral

  Comput. Methods Appl. Mech. Engrg., (2009). Vol. 198 (45-46), pp. 3567-3582

  
  

  Abstract

  Recently, there has been considerable interest in the use of stents as endovascular flow diverters for the treatment of intracranial aneurysms. Simulating this novel method of treatment is essential for understanding the intra-aneurysmal hemodynamics in order to design better stents and to personalize and optimize the endovascular stenting procedures. This paper describes a methodology based on unstructured embedded grids for patient-specific modeling of stented cerebral aneurysms, demonstrates how the methodology can be used to address specific clinical questions, and discusses remaining technical issues. In particular, simulations are presented on a number of patient-specific models constructed from medical images and using different stent designs and treatment alternatives. Preliminary sensitivity analyses with respect to stent positioning and truncation of the stent model are presented. The results show that these simulations provide useful and valuable information that can be used during the planning phase of endovascular stenting interventions for the treatment of intracranial aneurysms.

  Abstract
  Recently, there has been considerable interest in the use of stents as endovascular flow diverters for the treatment of intracranial aneurysms. Simulating this novel method [...]

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• The journal Communications in Numerical Methods in Engineering with Biomedical Applications becomes the International Journal for Numerical Methods in Biomedical Engineering (IJNMBE) from 1st January 2010

  P. Nithiarasu, R. Lohner

  Int. J. Numer. Meth. Fluids (2009). Vol. 61 (10), pp. 1179-1180

  
  

  Abstract

  Communications in Numerical Methods in Engineering , founded by Roland W. Lewis in 1985, will change its title to the ‘International Journal for Numerical Methods in Biomedical Engineering ’ and has a revised Aims and Scope.

  Abstract
  Communications in Numerical Methods in Engineering , founded by Roland W. Lewis in 1985, will change its title to the ‘International Journal for Numerical Methods [...]

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• Fast numerical solutions of patient‐specific blood flows in 3D arterial systems

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

  Int. J. Numer. Meth. Bio. Engng. (2010). Vol. 26 (1), pp. 73-86

  
  

  Abstract

  The study of hemodynamics in arterial models constructed from patient‐specific medical images requires the solution of the incompressible flow equations in geometries characterized by complex branching tubular structures. The main challenge with this kind of geometries is that the convergence rate of the pressure Poisson solver is dominated by the graph depth of the computational grid. This paper presents a deflated preconditioned conjugate gradients (DPCG) algorithm for accelerating the pressure Poisson solver. A subspace deflation technique is used to approximate the lowest eigenvalues along the tubular domains. This methodology was tested with an idealized cylindrical model and three patient‐specific models of cerebral arteries and aneurysms constructed from medical images. For these cases, the number of iterations decreased by up to a factor of 16, while the total CPU time was reduced by up to 4 times when compared with the standard PCG solver.

  Abstract
  The study of hemodynamics in arterial models constructed from patient‐specific medical images requires the solution of the incompressible flow equations in geometries characterized [...]

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• On the modeling of pedestrian motion

  R. Lohner

  Applied Mathematics Modelling (2010). Vol. 34 (2), pp. 366-382

  
  

  Abstract

  A model for the simulation of pedestrian flows and crowd dynamics has been developed. The model is based on a series of forces, such as: will forces (the desire to reach a place at a certain time), pedestrian collision avoidance forces, obstacle/wall avoidance forces; pedestrian contact forces, and obstacle/wall contact forces. Except for the will force, it is assumed that for any given pedestrian these forces are the result of only local (nearest neighbour) situations. The near-neighbour search problem is solved by an efficient incremental Delaunay triangulation that is updated at every timestep. In order to allow for general geometries a so-called background triangulation is used to carry all geographic information. At any given time the location of any given pedestrian is updated on this mesh. The results obtained to date show that the model performs well for standard benchmarks, and allows for typical crowd dynamics, such as lane forming, overtaking, avoidance of obstacles and panic behaviour.

  Abstract
  A model for the simulation of pedestrian flows and crowd dynamics has been developed. The model is based on a series of forces, such as: will forces (the desire to reach a [...]

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• An efficient fluid–solid coupled finite element scheme for weapon fragmentation simulations

  O. Soto, J. Baum, R. Lohner

  Engineering Fracture Mechanics (2010). Vol. 77 (3), pp. 549-564

  
  

  Abstract

  An efficient finite element (FE) scheme to deal with a class of coupled fluid–solid problems is presented. The main ingredients of such methodology are: an accurate Q1/P0 solid element (trilinear in velocities and constant piecewise-discontinuous pressures); a large deformation plasticity model; an algorithm to deal with material failure, cracking propagation and fragment formation; and a fragment rigidization methodology to avoid the possible numerical instabilities that may produce pieces of material flying away from the main solid body. All the mentioned schemes have been fully parallelized and coupled using a loose-embedded procedure with a well-established and validated computational fluid dynamics (CFD) code (FEFLO). A CSD and a CFD/CSD coupled case are presented and analyzed.

  Abstract
  An efficient finite element (FE) scheme to deal with a class of coupled fluid–solid problems is presented. The main ingredients of such methodology are: an accurate [...]

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• Cache‐efficient renumbering for vectorization

  R. Lohner

  Int. J. Numer. Meth. Bio. Engng. (2008). Vol. 26 (5), pp. 628-636

  
  

  Abstract

  A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring techniques, the new renumbering strategy reduces the spread in point‐data access for edge‐based solvers by more than an order of magnitude. The technique is particularly suited for multicore, cache‐based machines that allow for vectorization or pipelining.

  Abstract
  A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring [...]

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• Selective edge removal for unstructured grids with Cartesian cores

  R. Lohner, H. Luo, J. Baum

  Journal of Computational Physics (2005). Vol. 26 (1), pp. 208-226

  
  

  Abstract

  Several rules for redistributing geometric edge-coefficient obtained for grids of linear elements derived from the subdivision of rectangles, cubes or prisms are presented. By redistributing the geometric edge-coefficient, no work is carried out on approximately half of all the edges of such grids. The redistribution rule for triangles obtained from rectangles is generalized to arbitrary situations in 3-D, and implemented in a typical 3-D edge-based flow solver. The results indicate that without degradation of accuracy, CPU requirements can be cut considerably for typical large-scale grids. This allows a seamless integration of unstructured grids near boundaries with efficient Cartesian grids in the core regions of the domain.

  Abstract
  Several rules for redistributing geometric edge-coefficient obtained for grids of linear elements derived from the subdivision of rectangles, cubes or prisms are presented. [...]

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• Numerical Simulation of Long-Duration Blast Wave Evolution in Confined Facilities

  F. Togashi, J. Baum, E. Mestreau, R. Lohner, D. Sunshine

  Shock Waves (2010). Vol. 20 /5(, pp. 409-424

  
  

  Abstract

  The objective of this research effort was to investigate the quasi-steady flow field produced by explosives in confined facilities. In this effort we modeled tests in which a high explosive (HE) cylindrical charge was hung in the center of a room and detonated. The HEs used for the tests were C-4 and AFX 757. While C-4 is just slightly under-oxidized and is typically modeled as an ideal explosive, AFX 757 includes a significant percentage of aluminum particles, so long-time afterburning and energy release must be considered. The Lawrence Livermore National Laboratory (LLNL)-produced thermo-chemical equilibrium algorithm, “Cheetah”, was used to estimate the remaining burnable detonation products. From these remaining species, the afterburning energy was computed and added to the flow field. Computations of the detonation and afterburn of two HEs in the confined multi-room facility were performed. The results demonstrate excellent agreement with available experimental data in terms of blast wave time of arrival, peak shock amplitude, reverberation, and total impulse (and hence, total energy release, via either the detonation or afterburn processes. KeywordsDetonation-Blast wave-EOS-After burning-CFD

  Abstract
  The objective of this research effort was to investigate the quasi-steady flow field produced by explosives in confined facilities. In this effort we modeled tests in which [...]

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• Hemodynamic Analysis of Intracranial Aneurysms with Moving Parent Arteries: Basilar Tip Aneurysms

  D. Sforza, R. Lohner, C. Putman, J. Cebral

  Int. J. Numer. Meth. Bio. Engng. (2010). Vol. 26 (10), pp. 1219-1227

  
  

  Abstract

  The effects of parent artery motion on the hemodynamics of basilar tip saccular aneurysms and its potential effect on aneurysm rupture were studied. The aneurysm and parent artery motions in two patients were determined from cine loops of dynamic angiographies. The oscillatory motion amplitude was quantified by registering the frames. Patient‐specific computational fluid dynamics (CFD) models of both aneurysms were constructed from 3D rotational angiography images. Two CFD calculations were performed for each patient, corresponding to static and moving models. The motion estimated from the dynamic images was used to move the surface grid points in the moving model. Visualizations from the simulations were compared for wall shear stress (WSS), velocity profiles, and streamlines. In both patients, a rigid oscillation of the aneurysm and basilar artery in the anterio‐posterior direction was observed and measured. The distribution of WSS was nearly identical between the models of each patient, as well as major intra‐aneurysmal flow structures, inflow jets, and regions of impingement. The motion observed in pulsating intracranial vasculature does not have a major impact on intra‐aneurysmal hemodynamic variables. Parent artery motion is unlikely to be a risk factor for increased risk of aneurysmal rupture.

  Abstract
  The effects of parent artery motion on the hemodynamics of basilar tip saccular aneurysms and its potential effect on aneurysm rupture were studied. The aneurysm and parent [...]

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• On the simulation of highly nonlinear wave-breakwater interactions

  C. Yang, H. Lu, R. Lohner

  Journal of Hydrodynamics (2010). Vol. 22 (5), pp. 975-981

  
  

  Abstract

  A numerical time domain simulation model has been developed to study the highly nonlinear interactions between waves and rubble mound breakwaters. In this model, a volume of fluid (VOF) technique is used to capture the violent free surface motion. The incompressible Euler/Navier-Stokes equations, written in an arbitrary Lagrangian-Eulerian (ALE) frame, are solved using projection schemes and a finite element method on unstructured grids. A general advancing front technique for filling space with arbitrary separated objects is developed to model the rubbles that are laid down on the sloped surface of the breakwater in a random way. Three case studies are performed to study the effects of rubbles and rubble types on the wave dissipation and wave overtopping.

  Abstract
  A numerical time domain simulation model has been developed to study the highly nonlinear interactions between waves and rubble mound breakwaters. In this model, a volume [...]

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