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• A Fast, Matrix-free Implicit Method for Computing Low Mach Number Flows on Unstructured Grids

  H. Luo, J. Baum, R. Lohner

  Int. J. Numer. Meth. Fluids (2000). Vol. 14 (2), pp. 133-157

  
  

  Abstract

  A fast, matrix-free implicit method has been developed to solve low Mach number flow problems on unstructured grids. The preconditioned compressible Euler and Navier-Stokes equations are integrated in time using a linearized implicit scheme. A newly developed fast, matrix-free implicit method, GMRES + LU−SGS, is then applied to solve the resultant system of linear equations. A variety of computations has been made for a wide range of flow conditions, for both in viscid and viscous flows, in both 2D and 3D to validate the developed method and to evaluate the effectiveness of the GMRES + LU−SGS method. The numerical results obtained indicate that the use of the GMRES + LU−SGS method leads to a significant increase in performance over the LU−SGS method, while maintaining memory requirements similar to its explicit counterpart. An overall speedup factor from one to more than two order of magnitude for all test cases in comparison with the explicit method is demonstrated.

  Abstract
  A fast, matrix-free implicit method has been developed to solve low Mach number flow problems on unstructured grids. The preconditioned compressible Euler and Navier-Stokes [...]

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• Flow and dispersion around buildings: An application with FLEFLO

  F. Camelli, R. Lohner

  (2000). European Congress on Computational Methods in Applied Sciences and Engineering

  
  

  Abstract

  A series of simulations were performed in order to test the accuracy of FEFLO for problems of flow and dispersion around buildings. The major case was extracted from the Evaluation of Modelling Uncertainty (project EMU)[1]. The simulations were run in "black box" mode, i.e. no tuning of parameters was allowed. The results obtained were in very close agreement with the available wind-tunnel data for the case selected. The accuracy is comparable with other codes. The results show that an Euler run with proper profile yields a fairly accurate answer.

  Abstract
  A series of simulations were performed in order to test the accuracy of FEFLO for problems of flow and dispersion around buildings. The major case was extracted from the Evaluation [...]

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• Measurement of stenosis from Magnetic resonance angiography using vessel skeletons

  P. Yim, R. Mullick, R. Summers, H. Marcos, J. Cebral, R. Lohner, P. Choyke

  (2000). Proceedings Volume 3978, Medical Imaging 2000: Physiology and Function from Multidimensional

  
  

  Abstract

  Measurement of stenosis due to atherosclerosis is essential for interventional planning. Currently, measurement of stenosis from magnetic resonance angiography (MRA) is made based on 2D maximum intensity projection (MIP) images. This methodology, however, is subjective and does not take full advantage of the 3D nature of MRA. To address these limitations we present a deformable model for reconstructing the vessel surface with particular application to the carotid artery. The deformable model is based on a cylindrical coordinate system of a curvilinear axes. In this coordinate system, the location of each point on the surface of the deformable model is described by its axial, circumferential and radial position. The points on the surface deform in the radial direction so as to minimize discontinuity in radial position between adjacent points while maximizing the proximity of the surface to local edges in the image. The algorithm has no bias towards either narrower or wider cross- sectional shapes and is thus appropriate for the measurement of stenosis. Axes of the vessels are indicated manually or determined by axes detection methods. Once completed, the surface reconstruction lends itself directly to 3D methods for measuring cross-sectional diameter and area.

  Abstract
  Measurement of stenosis due to atherosclerosis is essential for interventional planning. Currently, measurement of stenosis from magnetic resonance angiography (MRA) is made [...]

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• Generation of non‐isotropic unstructured grids via directional enrichment

  R. Lohner, J. Cebral

  Int. J. Numer. Meth. Engng. (2000). Vol. 49 (1-2), pp. 219-232

  
  

  Abstract

  A procedure for the generation of highly stretched grids suitable for Reynolds‐averaged Navier–Stokes (RANS) calculations is presented. In a first stage, an isotropic (Euler) mesh is generated. In a second stage, this grid is successively enriched with points in order to achieve highly stretched elements. The element reconnection is carried out using a constrained Delaunay approach. Points are introduced from the regions of lowest stretching towards the regions of highest stretching. The procedure has the advantages of not requiring any type of surface recovery, not requiring extra passes or work to mesh concave ridges/corners, and guarantees a final mesh, an essential requirement for industrial environments. Given that point placement and element quality are highly dependent for the Delaunay procedure, special procedures were developed in order to obtain optimal point placement.

  Abstract
  A procedure for the generation of highly stretched grids suitable for Reynolds‐averaged Navier–Stokes (RANS) calculations is presented. In a first stage, an isotropic [...]

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• CFD Applications in Support of the Space Shuttle Risk Assessment

  J. Baum, E. Mestreau, H. Luo, D. Sharov, R. Lohner

  (2000). 1st Modeling and Simulation Subcommittee; November 13, 2000 - November 17, 2000; Monterey, CA; United States

  
  

  Abstract

  The paper describes a numerical study of a potential accident scenario of the space shuttle, operating at the same flight conditions as flight 51L, the Challenger accident. The interest in performing this simulation is derived by evidence that indicates that the event itself did not exert large enough blast loading on the shuttle to break it apart. Rather, the quasi-steady aerodynamic loading on the damaged, unbalance vehicle caused the break-up. Despite the enormous explosive potential of the shuttle total fuel load (both liquid and solid), the post accident explosives working group estimated the maximum energy involvement to be equivalent to about five hundreds of pounds of TNT. This understanding motivated the simulation described here. To err on the conservative side, we modeled the event as an explosion, and used the maximum energy estimate. We modeled the transient detonation of a 500 lbs spherical charge of TNT, placed at the main engine, and the resulting blast wave propagation about the complete stack. Tracking of peak pressures and impulses at hundreds of locations on the vehicle surface indicate that the blast load was insufficient to break the vehicle, hence demonstrating likely crew survivability through such an event. 

  Abstract
  The paper describes a numerical study of a potential accident scenario of the space shuttle, operating at the same flight conditions as flight 51L, the Challenger accident. [...]

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• On the Computation of Compressible Turbulent Flows on Unstructured Grids

  H. Luo, D. Sharov, J. Baum, R. Lohner

  Int. J. Numer. Meth. Fluids (2001). Vol. 14 (4), pp. 253-270

  
  

  Abstract

  An accurate, fast, matrix-free implicit method has been developed to solve compressible turbulent How problems using the Spalart and Allmaras one equation turbulence model on unstructured meshes. The mean-flow and turbulence-model equations are decoupled in the time integration in order to facilitate the incorporation of different turbulence models and reduce memory requirements. Both mean flow and turbulent equations are integrated in time using a linearized implicit scheme. A recently developed, fast, matrix-free implicit method, GMRES+LU-SGS, is then applied to solve the resultant system of linear equations. The spatial discretization is carried out using a hybrid finite volume and finite element method, where the finite volume approximation based on a containment dual control volume rather than the more popular median-dual control volume is used to discretize the inviscid fluxes, and the finite element approximation is used to evaluate the viscous flux terms. The developed method is used to compute a variety of turbulent flow problems in both 2D and 3D. The results obtained are in good agreement with theoretical and experimental data and indicate that the present method provides an accurate, fast, and robust algorithm for computing compressible turbulent flows on unstructured meshes.

  Abstract
  An accurate, fast, matrix-free implicit method has been developed to solve compressible turbulent How problems using the Spalart and Allmaras one equation turbulence model [...]

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• Practical CFD Applications to Design of a Wave Cancellation Multihull Ship

  C. Yang, F. Noblesse, R. Lohner, D. Hendrix

  (2001). Twenty-Third Symposium on Naval Hydrodynamics, pp. 206-222

  
  

  Abstract

  Four methods of analysis — a nonlinear method based on Euler's equations and three linear potential flow methods — are used to determine the optimal lo- cation of the outer hulls for a wave cancellation mul- tihull ship that consists of a main center hull and two outer hulls. The three potential flow methods corre- spond to a hierarchy of simple approximations based on the Fourier-Kochin representation of ship waves and the slender-ship approximation.

  Abstract
  Four methods of analysis — a nonlinear method based on Euler's equations and three linear potential flow methods — are used to determine the optimal lo- cation [...]

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• Time-Accurate Implicit ALE Algorithm for Shared-Memory Parallel Computers

  D. Sharov, H. Luo, J. Baum, R. Lohner

  (2001). Computational Fluid Dynamics 2000. Springer, Berlin, Heidelberg

  
  

  Abstract

  An unstructured grid matrix-free GMRES+LU-SGS scheme is used to simulate unsteady CFD problems with moving bodies. The method is applied on sharedmemory, cache-based parallel machines. A special grid renumbering technique is used for the parallelization rather than the traditional method of partitioning the computational domain. Moving mesh with recurrent remeshing models the body motion.

  Abstract
  An unstructured grid matrix-free GMRES+LU-SGS scheme is used to simulate unsteady CFD problems with moving bodies. The method is applied on sharedmemory, cache-based parallel [...]

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• A Coupled CFD/CSD Methodology for Simulating Structural Response to Airblast and Fragment

  J. Baum, H. Luo, E. Mestreau, R. Lohner, D. Pelessone, C. Charman

  (2001). Computational Fluid Dynamics 2000. Springer, Berlin, Heidelberg

  
  

  Abstract

  Several classes of important engineering problems require the concurrent application of CFD and CSD techniques. Currently, attempts to model these problems are solved either iteratively, requiring several cycles of CFD run followed by CSD run, or by assuming that the CFD and CSD solutions can be decoupled. The various efforts to develop a fluid/structure coupling can be classified according to the complexity level of the approximations used for each of the domains. These range from simple 6 DOF integration to finite elements with complex models for elasto-plastic materials with rupture laws and contact. Similarly, the fluid dynamics approximations range from the potential flow (irotational, inviscid, isentropic flows) to the full Navier-Stokes set of equations. The present research interests focus on non-linear applications, in particular, structures that experience severe deformations due to blast loads. Hence, the fluid applies either the Euler or Reynolds-Averaged Navier-Stokes equations, while elasto-plastic materials with rupture criteria are used for the structural modeling.

  Abstract
  Several classes of important engineering problems require the concurrent application of CFD and CSD techniques. Currently, attempts to model these problems are solved either [...]

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• An accurate, fast, matrix-free implicit method for computing unsteady flows on unstructured grids

  H. Luo, J. Baum, R. Lohner

  Computers and Fluids (2001). Vol. 30 (2), pp. 137-159

  
  

  Abstract

  An accurate, fast, matrix-free implicit method has been developed to solve the three-dimensional compressible unsteady flows on unstructured grids. A nonlinear system of equations as a result of a fully implicit temporal discretization is solved at each time step using a pseudo-time marching approach. A newly developed fast, matrix-free implicit method is then used to obtain the steady-state solution to the pseudo-time system. The developed method is applied to compute a variety of unsteady flow problems involving moving boundaries. The numerical results obtained indicate that the use of the present implicit method leads to a significant increase in performance over its explicit counterpart, while maintaining a similar memory requirement.

  Abstract
  An accurate, fast, matrix-free implicit method has been developed to solve the three-dimensional compressible unsteady flows on unstructured grids. A nonlinear system of equations [...]

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