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• Regridding Surface Triangulations

  R. Lohner

  Journal of Computational Physics (1996). Vol. 126 (1), pp. 1-10

  
  

  Abstract

      An advancing front surface gridding technique that operates on discretely defined surfaces (i.e. triangulations) is presented. Different aspects that are required to make the procedure reliable for complex geometries are discussed. Notable among these are (a) the recovery of surface features and discrete surface patches from the discrete data, (b) filtering based on point and side-normals to remove undesirable data close to cusps and corners, (c) the proper choice of host faces for ridges, and (d) fast interpolation procedures suitable for complex geometries. Post-generation surface recovery or repositioning techniques are discussed. Several examples ranging from academic to industrial demonstrate the utility of the proposed procedure forab initiosurface meshing from discrete data, such as those encountered when the surface description is already given as discrete, the improvement of existing surface triangulations, as well as remeshing applications during runs exhibiting significant change of domain.  

  Abstract
      An advancing front surface gridding technique that operates on discretely defined surfaces (i.e. triangulations) is presented. Different aspects [...]

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• Progress in grid generation via the advancing front technique

  R. Lohner

  Engineering with Computers (1996). Vol. 12, pp. 186-210

  
  

  Abstract

  We describe recent extensions and improvements to the advancing front grid generation technique. These improvements target a range of applicability, speed and user friendliness. The range of applicability is enlarged by the ability to produce volumetric grids around thin surfaces (such as shells, membranes, fabrics or surfaces with cusps), the generation of high aspect ratio grids for Navier-Stokes applications, the generation of higher order triangular and tetrahedral elements, and the generation of quadrilateral and hexahedral elements. Speed improvements are the result of reduced search overheads, as well as vectorization and parallelization. User friendliness is enhanced by the ability to grid directly discrete data and simpler ways of specifying the desired element size and shape in space. Numerous examples are included that demonstrate the versatility and maturity that advancing front grid generators have achieved.

  Abstract
  We describe recent extensions and improvements to the advancing front grid generation technique. These improvements target a range of applicability, speed and user friendliness. [...]

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• Improved ALE mesh velocities for moving bodies

  R. Lohner, C. Yang

  Commun. Numer. Meth. Engng (1996). Vol. 12 (10), pp. 599-608

  
  

  Abstract

  A Laplacian smoothing of the mesh velocities with variable diffusivity based on the distance from moving bodies is introduced. This variable diffusivity enforces a more uniform mesh velocity in the region close to the moving bodies. Given that in most applications these are regions where small elements are located, the new procedure decreases element distortion considerably, reducing the need for local or global remeshing, and in some cases avoiding it altogether.

  Abstract
  A Laplacian smoothing of the mesh velocities with variable diffusivity based on the distance from moving bodies is introduced. This variable diffusivity enforces a more uniform [...]

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• Extensions and improvements of the advancing front grid generation technique

  R. Lohner

  Comput. Methods Appl. Mech. Engrg., (1996). Vol. 5 (10), pp. 119-132

  
  

  Abstract

  We describe extensions and improvements to the advancing front grid generation technique that have proven useful over the years. The following areas are treated in detail: situations with thin or crossing surfaces, meshing of surfaces defined by triangulations, and ease of user input to define the desired element size in space. The first extension is important if one considers the generation of volumetric grids around shells, membranes, fabrics, or CAD-data that exhibit cusps. Traditional advancing front generators are likely to fail in these situations. We propose the introduction of a crossing environment variable attached to faces and points in order to filter out undesired or incorrect information during the grid generation process. The second extension is required for situations where the surfaces to be gridded are not defined analytically, but via a triangulation. Typical cases where such triangulations are used to define the domain are geophysical problems, climate modelling and medical problems. The third topic deals with the reduction of manual labour to specify element size in space. Sources, element size attached directly to CAD-data, and adaptive background grids are discussed. Adaptive background grids, in combination with surface deviation tolerances, are used to obtain surface triangulations that represent the geometry faithfully, and at the same time enable a smooth transition to volumetric meshes.

  Abstract
  We describe extensions and improvements to the advancing front grid generation technique that have proven useful over the years. The following areas are treated in detail: [...]

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• Improved ALE mesh velocities for moving bodies

  R. Lohner, C. Yang

  Commun. Numer. Meth. Engng (1996). Vol. 12 (10), pp. 599-608

  
  

  Abstract

  A Laplacian smoothing of the mesh velocities with variable diffusivity based on the distance from moving bodies is introduced. This variable diffusivity enforces a more uniform mesh velocity in the region close to the moving bodies. Given that in most applications these are regions where small elements are located, the new procedure decreases element distortion considerably, reducing the need for local or global remeshing, and in some cases avoiding it altogether.

  Abstract
  A Laplacian smoothing of the mesh velocities with variable diffusivity based on the distance from moving bodies is introduced. This variable diffusivity enforces a more uniform [...]

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• Simulation of the magnetosphere with a new three dimensional MHD code and adaptive mesh refinement: Preliminary results

  D. Spicer, S. Zalesak, R. Lohner, S. Curtis

  Advances in Space Research (1996). Vol. 18 (8), pp. 253-262

  
  

  Abstract

  We present the first results from a new unstructured mesh three dimensional finite element MHD code which uses dynamic solution-adaptive mesh refinement in a manner similar to our two dimensional finite element MHD code /31/. The problem being considered here is the interaction of the solar wind with the earth's magnetosphere, using a three-dimensional Cartesian approximation. Our results strongly indicate that such adaptive mesh techniques have the ability to resolve structures in the three dimensional MHD flow field that would otherwise be possible only with orders of magnitude greater cost and that are most likely beyond the capability of present supercomputers.

  Abstract
  We present the first results from a new unstructured mesh three dimensional finite element MHD code which uses dynamic solution-adaptive mesh refinement in a manner similar [...]

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• Comparisons of Preconditioned Iterative Methods in the Implicit Unstructured Solver of Compressible Flow

  M. Nagaoka, R. Lohner

  Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers (1997). Vol. 63 (605), pp. 153-160

  
  

  Abstract

  Preconditioned iterative methods in an implicit unstructured grid solver of compressible flows are compared. The three-dimensional Euler and Navier-Stokes equations are discretized by the edge-based finite volume method. The artificial dissipation scheme for the inviscid numerical fluxes is developed for the unstructured grid from Jameson's scheme for the structured grid. The following methods are compared to the supersonic and transonic inviscid flows and subsonic viscous laminar and turbulent flows: the central difference methods with the scalar and matrix dissipation scheme and Roe's upwind method with the MUSCL scheme for the inviscid fluxes; the scalar and matrix dissipation method for the left-hand side operator in the implicit scheme; the ILU, DILU and SGS factorizations for the preconditioner; and GMRES and Bi-CGSTAB methods for the linear system solver. The results show that a combination of the matrix dissipation in the inviscid fluxes and the scalar dissipation in the left-hand side works better than the other *schemes. In the preconditioned iterative methods, a combination of DILU or SGS and the Bi-CGSTAB is recommended for the aspect of required memory.

  Abstract
  Preconditioned iterative methods in an implicit unstructured grid solver of compressible flows are compared. The three-dimensional Euler and Navier-Stokes equations are discretized [...]

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• Automatic unstructured grid generators

  R. Lohner

  Finite Elements in Analysis and Design (1997). Vol. 25 (1-2), pp. 111-134

  
  

  Abstract

  A review of automatic unstructured grid generators is given. These types of grids have found widespread use in computational fluid dynamics, computational structural dynamics, computational electro-magnetics and computational thermodynamics. The following topics are treated: the methods most commonly used, the specification of desired element size/shape and surface definition/meshing. Finally, the use of automatic grid generators as an enabling technology for moving body simulations and adaptive remeshing techniques is discussed.

  Abstract
  A review of automatic unstructured grid generators is given. These types of grids have found widespread use in computational fluid dynamics, computational structural dynamics, [...]

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• Conservative Load Projection and Tracking for Fluid-Structure Problems

  J. Cebral

  AIAA Journal (1997). Vol. 35 (4), pp. 687-692

  
  

  Abstract

  The loose coupling of computational fluid dynamics and computational structural dynamics solvers introduces some problems related to the information transfer between the codes. Some techniques developed to solve the problems of the load transfer and interface surface tracking are presented. The main criterion is to achieve conservation of total loads and total energy. The load projection scheme is based on Gaussian integration and fast interpolation algorithms for unstructured grids. The surface tracking algorithm, also based on interpolation, is important for many applications, including aeroelastic deformation of wings due to aerodynamic loads. The methodologies not only improve present fluid-structure interaction simulations, but also increase the range of their applicability. These techniques are of general character and can be used in other multidisciplinary applications as well.

  Abstract
  The loose coupling of computational fluid dynamics and computational structural dynamics solvers introduces some problems related to the information transfer between the codes. [...]

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• On Some Outstanding Issues in CFD

  R. Lohner

  (1998). ICASE/LaRC Interdisciplinary Series in Science and Engineering. Vol 6, pp. 195-212. Springer, Dordrecht

  
  

  Abstract

  Against the general feeling that ‘CFD is solved’, I offer a few remarks from the trenches. When performing large-scale simulations of geometrically and physically complex flows, the notion of reliable, cost-effective, widely applicable and easy-to-use CFD still appears more as fiction than reality. The present monograph addresses some major areas where progress is urgently required. These areas are listed in the order a typical CFD run proceeds: model import/creation, grid generation, flow solvers, mesh adaptation and quality control. Finally, with a view towards the coming decade, code, data and project management are discussed.

  Abstract
  Against the general feeling that ‘CFD is solved’, I offer a few remarks from the trenches. When performing large-scale simulations of geometrically and physically [...]

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