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• Deflated preconditioned conjugate gradient solvers for the Pressure–Poisson equation

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

  Journal of Computational Physics (2008). Vol. 227 (24), pp. 10196-10208

  
  

  Abstract

  A deflated preconditioned conjugate gradient technique has been developed for the solution of the Pressure–Poisson equation within an incompressible flow solver. The deflation is done using a region-based decomposition of the unknowns, making it extremely simple to implement. The procedure has shown a considerable reduction in the number of iterations. For grids with large graph-depth the savings exceed an order of magnitude. Furthermore, the technique has shown a remarkable insensitivity to the number of groups/regions chosen, and to the way the groups are formed.

  Abstract
  A deflated preconditioned conjugate gradient technique has been developed for the solution of the Pressure–Poisson equation within an incompressible flow solver. [...]

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• A discontinuous Galerkin method using Taylor basis for computing shock waves on arbitrary grids

  H. Luo, J. Baum, R. Lohner

  (2009). Shock Waves. Springer, Berlin, Heidelberg

  
  

  Abstract

  The discontinuous Galerkin methods [1] (DGM) have recently become popular for the solution of systems of conservation laws to arbitrary order of accuracy. The DGM combine two advantageous features commonly associated to finite element and finite volume methods. As in classical finite element methods, accuracy is obtained by means of high-order polynomial approximation within an element rather than by wide stencils as in the case of finite volume methods. The physics of wave propagation is, however, accounted for by solving the Riemann problems that arise from the discontinuous representation of the solution at element interfaces.

  Abstract
  The discontinuous Galerkin methods [1] (DGM) have recently become popular for the solution of systems of conservation laws to arbitrary order of accuracy. The DGM combine [...]

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• A Hybrid Building-Block and Gridless Method for Computing Shock Waves

  H. Luo, J. Baum, R. Lohner

  (2009). Computational Fluid Dynamics 2006. Springer, Berlin, Heidelberg

  
  

  Abstract

  The numerical methods for the solution of the compressible Euler and Navier-Stokes equations can be classified by the mesh they use as structured grid methods, unstructured grid methods, Cartesian grid methods, and gridless methods. Each of these methods, advocated, promoted, developed, and used by their respective supporters, has its own advantages and disadvantages. The structured grid methods have a disadvantage in mesh generation for complex geometries. The main advantage of the unstructured grid methods is the ease of grid generation for complex configurations. However, the computational costs and memory requirements are generally higher than their structured grid counterparts. The advantages of the Cartesian grid methods include ease of grid generation, lower computational storage requirements, and significantly less operational count per cell.

  Abstract
  The numerical methods for the solution of the compressible Euler and Navier-Stokes equations can be classified by the mesh they use as structured grid methods, unstructured [...]

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• On 3D Anisotropic Local Remeshing for Surface, Volume and Boundary Layers

  A. Loseille, R. Lohner

  (2009). Proceedings of the 18th International Meshing Roundtable. Springer, Berlin, Heidelberg

  
  

  Abstract

  A simple strategy for generating anisotropic meshes is introduced. The approach belongs to the class of metric-based mesh adaptation procedures where a field of metric tensors governs the adaptation. This development is motivated by the need of generating anisotropic meshes for complex geometries and complex flows. The procedure may be used advantageously for cases where global remeshing techniques become either unfeasible or unreliable. Each of the local operations used is checked in a variety of ways by taking into account both the volume and the surface mesh. This strategy is illustrated with surface mesh adaptation and with the generation of meshes suited for boundary layers analysis. Two simple mesh operators are used to recursively modify the mesh: edge collapse and point insertion on edge. It is shown that using these operators jointly with a quality function allows to quickly produce an quality anisotropic mesh. Each adaptation entity, ie surface, volume or boundary layers, relies on a specific metric tensor field. The metric-based surface estimate is used to control the deviation to the surface and to adapt the surface mesh. The volume estimate aims at controlling the interpolation error of a specific field of the flow. The boundary layers metric-based estimate is deduced from a level-set distance function.

  Abstract
  A simple strategy for generating anisotropic meshes is introduced. The approach belongs to the class of metric-based mesh adaptation procedures where a field of metric tensors [...]

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• On the Computation of Steady-State Compressible Flows Using a DG Method

  H. Luo, J. Baum, R. Lohner

  (2006). Computational Fluid Dynamics 2006. Springer, Berlin, Heidelberg

  
  

  Abstract

  Most efforts in the development of the discontinuous Galerkin methods (DGM) in computational fluid dynamics are primarily focused on the time accurate compressible Euler and Navier-Stokes equations. Its accuracy, efficiency, capability, robustness for steady state flow problems are relatively unexplored. In order for DGM to become a viable, attractive, probably even better alternative to the more traditional, more elaborate, well established finite volume methods (FVM), and finite element methods (FEM) for steady state computations, the following three issues have to be addressed: 1) Lack of efficient flow solver for steady state computations: Most efforts in the development of the discontinuous Galerkin methods are primarily focused on the spatial discretization.

  Abstract
  Most efforts in the development of the discontinuous Galerkin methods (DGM) in computational fluid dynamics are primarily focused on the time accurate compressible Euler and [...]

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• Image-based analysis of blood flow modification in stented aneurysms

  J. Cebral, F. Mut, S. Appanaboyina, R. Lohner, C. Miranda, E. Escrivano, P. Lylyk, C. Putman

  (2009). Proceedings Volume 7262, Medical Imaging 2009: Biomedical Applications in Molecular, Structural, and Functional Imaging;

  
  

  Abstract

  Currently there is increased interest in the use of stents as flow diverters for the treatment of intracranial aneurysms, especially wide necked aneurysms that are difficult to treat by coil embolization or surgical clipping. This paper presents image-based patient-specific computational models of the hemodynamics in cerebral aneurysms before and after treatment with a stent alone, with the goal of better understanding the hemodynamic effects of these devices and their relation to the outcome of the procedures. Stenting of cerebral aneurysms is a feasible endovascular treatment option for aneurysms with wide necks that are difficult to treat with coils or by surgical clipping. However, this requires stents that are capable of substantially modifying the intra-aneurysmal flow pattern in order to cause thrombosis of the aneurysm. The results presented in this paper show that the studied stent was able to change significantly the hemodynamic characteristics of the aneurysm. In addition, it was shown that patient-specific computational models constructed from medical images are capable of realistically representing the in vivo hemodynamic characteristics observed during conventional angiography examinations before and after stenting. This indicates that these models can be used to better understand the effects of different stent designs and to predict the alteration in the hemodynamic pattern of a given aneurysm produced by a given flow diverter. This is important for improving current design of flow diverting devices and patient treatment plans.

  Abstract
  Currently there is increased interest in the use of stents as flow diverters for the treatment of intracranial aneurysms, especially wide necked aneurysms that are difficult [...]

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• Calculating Blast Loads for Civil Engineering Structures

  R. Lohner, J. Baum

  (2009). 100 Volumes of ‘Notes on Numerical Fluid Mechanics’. Notes on Numerical Fluid Mechanics and Multidisciplinary Design, vol 100. Springer, Berlin, Heidelberg

  
  

  Abstract

  A brief overview of the state of the art of computing blast loads on civil engineering structures is given. The general problem setting, requirements, main physical phenomena and timescales, as well as suitable numerical methods are described. Several examples show the power of blast loads calculations for civil engineering structures.

  Abstract
  A brief overview of the state of the art of computing blast loads on civil engineering structures is given. The general problem setting, requirements, main physical phenomena [...]

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• Numerical simulation of H2/air detonation using unstructured mesh

  F. Togashi, R. Lohner, N. Tsuboi

  Shock Waves (2009). Vol. 19 (2), pp. 151-162

  
  

  Abstract

  To explore the capability of unstructured mesh to simulate detonation wave propagation phenomena, numerical simulation of H2/air detonation using unstructured mesh was conducted. The unstructured mesh has several adv- antages such as easy mesh adaptation and flexibility to the complicated configurations. To examine the resolution dependency of the unstructured mesh, several simulations varying the mesh size were conducted and compared with a computed result using a structured mesh. The results show that the unstructured mesh solution captures the detailed structure of detonation wave, as well as the structured mesh solution. To capture the detailed detonation cell structure, the unstructured mesh simulations required at least twice, ideally 5times the resolution of structured mesh solution.

  Abstract
  To explore the capability of unstructured mesh to simulate detonation wave propagation phenomena, numerical simulation of H2/air detonation using unstructured mesh was conducted. [...]

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• Adjoint-Based Design of Shock Mitigation Devices

  A. Stück, F. Camelli, R. Lohner

  Int. J. Numer. Meth. Fluids (2009). Vol. 64 (4), pp. 443-472

  
  

  Abstract

  Unsteady Euler and adjoint Euler solvers have been combined in order to aid in the design of shock mitigation devices. The flowfield is integrated forward in time and stored. The adjoint is then integrated going backwards in time, restoring and interpolating the saved Euler solution to the current point in time. The gradient is obtained from a surface integral formulation during the adjoint run. Comparisons of adjoint‐based and finite‐differencing gradients for different verification cases show less than 10% deviation. The results obtained indicate that this is a very cost‐effective way to obtain the gradients of an objective function with respect to surface design changes. Moreover, as the sensitivity information is determined over a complete surface, the procedure provides considerable insight, and can efficiently facilitate the design of shock mitigation devices such as architecturally appealing blast walls.

  Abstract
  Unsteady Euler and adjoint Euler solvers have been combined in order to aid in the design of shock mitigation devices. The flowfield is integrated forward in time and stored. [...]

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• Large-Eddy Simulations of a Supersonic Jet and Its Near-Field Acoustic Properties

  J. Liu, K. Kailasanath, R. Ramamurti, R. Lohner, D. Munday, E. Gutmark

  AIAA Journal (2009). Vol. 47 (8), pp. 1849-1864

  
  

  Abstract

  Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a sharp contraction at the nozzle throat have been carried out. The flowfield and near-field acoustics for various total pressure ratios from overexpanded to underexpanded jet flow conditions have been investigated. The location and spacing of the shock cells are in good agreement with experimental data and previous theoretical results. The velocity profiles are also in good agreement with data from experimental measurements. A Mach disk is observed immediately downstream of the nozzle exit for overexpanded jet conditions with nozzle pressure ratios much lower than the fully expanded value. It is found that this type of nozzle with a sharp turning throat does not have a shock- free condition for supersonic jet flows. The near-field intensities of pressure fluctuations show wavy structures for cases in which screech tones are observed. The large-eddy simulations predictions of the near-field noise intensities show good agreement with those obtained from experimental measurements. This good agreement shows that large- eddy simulations and measurements can play complementary roles in the investigation of the noise generation from supersonic jet flows.  

  Abstract
  Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a sharp contraction at the nozzle throat have been carried out. The flowfield [...]

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