COLLECTIONS

Documents published in Scipedia

• Three-dimensional fluid-structure interaction using a finite element solver and adaptive remeshing

  R. Lohner

  Computing Systems in Engineering (1990). Vol. 1 (2), pp. 257-272

  
  

  Abstract

  The combination of adaptive remeshing techniques, flow solvers for transient problems with moving grids, and consistent rigid body motion integrators in three dimensions is presented. The resulting scheme allows the economical simulation of fully coupled fluid-rigid body interaction problems of arbitrary geometric complexity. Several results, showing three-dimensional store separation, are given to demonstrate the capabilities developed.

  Abstract
  The combination of adaptive remeshing techniques, flow solvers for transient problems with moving grids, and consistent rigid body motion integrators in three dimensions is [...]

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• Electromagnetic PIC simulations using finite elements on unstructured grids

  J. Ambrosiano, S. Brandon, R. Lohner

  (1991). Proceeding 14th Conference on Numerical Simulation of Plasmas, Annapolis

  
  

  Abstract

  There is an increasing demand to use kinetic plasma simulations to model real plasma devices such as e-beam diodes, plasma torches, opening switches, and so on. With this demands comes the realization that standard simulation tools based on rectangular structured meshes are often too inflexible to accommodate real device geometries. To gain geometric flexibility, one typically must abandon structured rectangular meshes. Some have had considerable success by distorting the grid to conform to boundary-fitted curvilinear coordinates. This gives substantial flexibility while retaining a logically rectangular data structure. However, to achieve the greatest flexibility, an unstructured mesh is necessary. For the last several years, we have been engaged in designing electromagnetic particle-in-cell (PIC) simulations that can be performed on unstructured grids of triangles. We feel this is a powerful strategy for matching kinetic plasma simulations to real problem geometries. Unstructured meshes not only accommodate complicated boundary shapes with ease, but also allow extreme local refinement without affecting resolution elsewhere. The basic challenges to overcome in formulating EM-PIC on these new meshes are: (1) grid generation; (2) particle interpolation and tracking, and (3) field solution. In this paper we briefly describe our techniques and give a simple example.

  Abstract
  There is an increasing demand to use kinetic plasma simulations to model real plasma devices such as e-beam diodes, plasma torches, opening switches, and so on. With this [...]

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• Simple Elements and Linelets for Incompressible Flows

  R. Lohner

  (1991). The finite element method in the 1990’s. Springer, Berlin, Heidelberg

  
  

  Abstract

  The current trends for the simulation of large-scale incompressible flow fields using finite elements are discussed. The main items are: a) the use of simple elements through stabilization and analogy with LBB-satisfying elements, and b) the development of fast solvers for general grids.

  Abstract
  The current trends for the simulation of large-scale incompressible flow fields using finite elements are discussed. The main items are: a) the use of simple elements through [...]

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• Three-dimensional space-marching algorithm on unstructured grids

  M. McGrory, R. Lohner, R. Walters

  AIAA Journal (1991). Vol. 29 (1), pp. 1844-1849

  
  

  Abstract

  A three-dimensional space-marching algorithm using an unstructured discretization is proposed. The method utilizes a two-dimensional unstructured grid generator to construct grids in a crossflow plane while maintaining structure in the marching direction. The spatial discretization is obtained by applying a characteristic-based, upwind, finite volume scheme for the solution of the Euler equations. Solutions are presented for several different geometries comparing the results with existing numerical techniques and experiment.

  Abstract
  A three-dimensional space-marching algorithm using an unstructured discretization is proposed. The method utilizes a two-dimensional unstructured grid generator to construct [...]

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• Formation of shocks within axisymmetric nozzles

  E. Loth, J. Baum, R. Lohner

  AIAA Journal (1992). Vol. 30 (1), pp. 268-270

  
  

  Abstract

  The formation of shocks within axisymmetric supersonic nozzles has received considerable attention in the past, both experimentally and computationally. The presence of undesirable oblique shocks can significantly alter the downstream flowfield, reduce the thrust efficiency, and affect both the external acoustic signature and base pressure. In this paper numerical simulations of axisymmetric supersonic nozzle flow have been accomplished using an axisymmetric version of the finite element method/flux corrected transport algorithm. The adaptive unstructured gridding and the conservative nonlinear FCT scheme predicted shock formation that agreed with experimental data.

  Abstract
  The formation of shocks within axisymmetric supersonic nozzles has received considerable attention in the past, both experimentally and computationally. The presence of undesirable [...]

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• Parallel unstructured grid generation

  R. Lohner, J. Camberos, M. Merriam

  Computer Methods in Applied Mechanics and Engineering (1992). Vol. 95 (3), pp. 343-357

  
  

  Abstract

  A parallel unstructured grid generation algorithm is presented and implemented on the INTEL hypercube. Different processor hierarchies are discussed, and the appropriate hierarchies for mesh generation and mesh smoothing are selected. A domain-splitting algorithm for unstructured grids, which tries to minimize the surface-to-volume ratio of each subdomain, is described. This splitting algorithm is employed both for grid generation and grid smoothing. Results obtained on the INTEL hypercube demonstrate the effectiveness of the algorithms developed.

  Abstract
  A parallel unstructured grid generation algorithm is presented and implemented on the INTEL hypercube. Different processor hierarchies are discussed, and the appropriate hierarchies [...]

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• Vorticity produced by shock wave diffraction

  S. Siver, E. Loth, J. Baum, R. Lohner

  Shock Waves (1992). Vol. 2, pp. 31-41

  
  

  Abstract

  Numerical simulations with a monotonicity preserving flow solver have been performed to study shock diffraction phenomena and shock wave generated vorticity. The computations were performed using the conservative Finite Element Method-Flux Corrected Transport (FEM-FCT) scheme, which has been shown to have an excellent predictive capability for various compressible flows with both strong and weak shocks. An adaptive unstructured methodology based on adapting to high density and entropy gradients was used in conjunction with a conservative shock-capturing scheme to adequately resolve strong and weak flowfield gradients. The chief interest was the formation of vorticity arising from shock wave propagation over a sharp corner and the high accuracy and resolution of the interacting compressible wave features. Numerical simulations were compared with previous experimental results and exhibited remarkably good agreement in terms of compressible wave propagation, as well as vorticity development and transport. The computations also allowed insight into the fundamental fluid dynamics, specifically shock diffraction, vortex convection and shock-vortex interactions.

  Abstract
  Numerical simulations with a monotonicity preserving flow solver have been performed to study shock diffraction phenomena and shock wave generated vorticity. The computations [...]

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• Finite element methods in CFD: Grid generation, adaptivity, and parallelization

  R. Lohner

  (1992). Special Course on Unstructured Grid Methods for Advection Dominated Flows

  
  

  Abstract

  Numerical methods for the solution of field problems using unstructured grids have reached a high degree of maturity. Although computational fluid dynamics (CFD) have been dominated by structured grids, with the increase in computational ability, unstructured grids have started to have an impact on complex geometrical problems in CFD. The following paper describes grid generation, adaptive refinement schemes, visualization and parallelization issues. It also includes a short chapter on CFD among related disciplines that will help the newcomer with flow simulation codes

  Abstract
  Numerical methods for the solution of field problems using unstructured grids have reached a high degree of maturity. Although computational fluid dynamics (CFD) have been [...]

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• Supersonic Flow over an Axisymmetric Backward-Facing Step

  E. Loth, K. Kailasanth, R. Lohner

  Journal of Spacecraft and Rockets (1992). Vol. 29 (3), pp. 352-359

  
  

  Abstract

  Large eddy numerical simulations of supersonic flows over an axisymmetric backward-facing step have been completed using a recently developed axisymmetric version of the finite element method-flux corrected transport algorithm, FEM-FCT. The code is based on the mixing layer and recompression premises of the Chapman-Korst model. It solves the time-accurate Euler equations utilizing 1) adaptive unstructured gridding to resolve flow feature details and 2) a conservative nonlinear scheme to capture features in the compressible flowfield. This approach may be employed since the location of the separation point is fixed at the sharp corner. The simulation allows the large-scale structures present in the mixing layer to interact dynamically with the recompression region. Comparisons have been made with available experimental data for a range of Mach numbers and step height to step radius ratios for which boundary-layer effects are not pronounced. In general, good agreement between predictions and measurements was found for 1) the time-averaged surface pressure distributions along the backstep and the reattachment wall, 2) the flowfield structure in general, and 3) the downstream reattachment lengths. Effects of axisymmetry were noted in flowfield characteristics such as increased base pressure, curvature of expansion fans, stronger recompression shocks, and higher Mach number levels in the subsonic recirculating region.

  Abstract
  Large eddy numerical simulations of supersonic flows over an axisymmetric backward-facing step have been completed using a recently developed axisymmetric version of the finite [...]

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• Adaptive h‐refinement on 3D unstructured grids for transient problems

  R. Lohner, J. Baum

  International Journal for Numerical Methods in Fluids (1992). Vol. 14 (12), pp. 1407-1419

  
  

  Abstract

  An adaptive finite element scheme for transient problems is presented. The classic h‐enrichment/coarsening is employed in conjunction with a tetrahedral finite element discretization in three dimensions. A mesh change is performed every n time steps, depending on the Courant number employed and the number of ‘protective layers’ added ahead of the refined region. In order to simplify the refinement/coarsening logic and to be as fast as possible, only one level of refinement/coarsening is allowed per mesh change. A high degree of vectorizability has been achieved by pre‐sorting the elements and then performing the refinement/coarsening groupwise according to the case at hand. Further reductions in CPU requirements arc realized by optimizing the identification and sorting of elements for refinement and deletion. The developed technology has been used extensively for shock‐shock and shock‐object interaction runs in a production mode. A typical example of this class of problems is given.

  Abstract
  An adaptive finite element scheme for transient problems is presented. The classic h‐enrichment/coarsening is employed in conjunction with a tetrahedral finite element [...]

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