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Convergence Acceleration of the CFD Code Edge by LU-SGS
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aerodynamics.ORCID iD: 0000-0001-9902-6216
(Department Aeronautics & Systems Technology, FOI, Swedish Defense Research Institute, Stockholm, Sweden)
2011 (English)In: 3rd CEAS European Air & Space Conference, Venice, 24-28 October 2011, CEAS/AIDAA , 2011, 606-611 p.Conference paper, Published paper (Refereed)
Abstract [en]

Edge is a flow solver for unstructured grids based on a dual grid and edge-based formulation. The standard dual-time stepping methods for compressible unsteady flows are inadequate for large-scale industrial problems. This has motivated the present work, in which an implicit Lower-Upper Symmetric Gauss-Seidel (LU-SGS) type of relaxation has been implemented in the code Edge with multigrid acceleration. Two different types of dissipation, a scalar and a matrix model, have been constructed which increase the diagonal dominance of the system matrix but not the numerical viscosity of the computed solution. A parametric study demonstrates convergence accelerations by a factor of three for inviscid transonic flows compared to explicit Runge-Kutta smoothing for multigrid acceleration.

Place, publisher, year, edition, pages
CEAS/AIDAA , 2011. 606-611 p.
Keyword [en]
compressible CFD, convergence acceleration, implicit time-stepping, matrix dissipation
National Category
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-78943OAI: oai:DiVA.org:kth-78943DiVA: diva2:493047
Conference
3rd CEAS European Air & Space Conference
Note
QC 20120209Available from: 2012-02-09 Created: 2012-02-08 Last updated: 2014-12-01Bibliographically approved
In thesis
1. Acceleration of Compressible Flow Simulations with Edge using Implicit Time Stepping
Open this publication in new window or tab >>Acceleration of Compressible Flow Simulations with Edge using Implicit Time Stepping
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Computational fluid dynamics (CFD) has become a significant tool routinely used in design and optimization in aerospace industry. Typical flows may be characterized by high-speed and compressible flow features and, in many cases, by massive flow separation and unsteadiness. Accurate and efficient numerical solution of time-dependent problems is hence required, and the efficiency of standard dual-time stepping methods used for unsteady flows in many CFD codes has been found inadequate for large-scale industrial problems. This has motivated the present work, in which major effort is made to replace the explicit relaxation methods with implicit time integration schemes. The CFD flow solver considered in this work is Edge, a node-based solver for unstructured grids based on a dual, edge-based formulation. Edge is the Swedish national CFD tool for computing compressible flow, used at the Swedish aircraft manufacturer SAAB, and developed at FOI, lately in collaboration with external national and international partners. The work is initially devoted to the implementation of an implicit Lower-Upper Symmetric Gauss-Seidel (LU-SGS) type of relaxation in Edge with the purpose to speed up the convergence to steady state. The convergence of LU-SGS has been firstly accelerated by basing the implicit operator on a flux splitting method of matrix dissipation type. An increase of the diagonal dominance of the system matrix was the principal motivation. Then the code has been optimized by means of performance tools Intel Vtune and CrayPAT, improving the run time. It was found that the ordering of the unknowns significantly influences the convergence. Thus, different ordering techniques have been investigated. Finding the optimal ordering method is a very hard problem and the results obtained are mostly illustrative. Finally, to improve convergence speed on the stretched computational grids used for boundary layers LU-SGS has been combined with the line-implicit method.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. x, 35 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2012:25
Keyword
compressible CFD, convergence acceleration, implicit time-stepping, matrix dissipation, line-implicit, performance optimization
National Category
Engineering and Technology Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-97455 (URN)
Presentation
2012-06-11, Entrepl (E2), Lindstedsvägen 3, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20120626Available from: 2012-06-26 Created: 2012-06-13 Last updated: 2012-06-26Bibliographically approved
2. Acceleration of Compressible Flow Simulations with Edge Using  Implicit Time Stepping
Open this publication in new window or tab >>Acceleration of Compressible Flow Simulations with Edge Using  Implicit Time Stepping
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Computational fluid dynamics (CFD) is a significant tool routinely used indesign and optimization in aerospace industry. Often cases with unsteadyflows must be computed, and the long compute times of standard methods hasmotivated the present work on new implicit methods to replace the standardexplicit schemes. The implementation and numerical experiments were donewith the Swedish national flow solver Edge, developed by FOI,universities, and collaboration partners.The work is concentrated on a Lower-Upper Symmetric Gauss-Seidel (LU-SGS)type of time stepping. For the very anisotropic grids needed forReynolds-Averaged Navier-Stokes (RANS) computations of turbulent boundary layers,LU-SGS is combined with a line-implicit technique.  The inviscid flux Jacobians which contribute to the diagonalblocks of the system matrix are based on a flux splitting method with upwind type dissipation giving  control over diagonal dominance and artificial dissipation.The method is  controlled by several parameters, and comprehensivenumerical experiments were carried out to identify their influence andinteraction so that close to optimal values can be suggested. As an example,the optimal number of iterations carried out in a time-step increases with increased resolution of the computational grid.The numbering of the unknowns is important, and the numberings produced by mesh generators of Delaunay- and advancing front-type wereamong the best.The solver has been parallelized with the Message Passing Interface (MPI) for runs on multi-processor hardware,and its performance scales with the number of processors at least asefficiently as the explicit methods. The new method saves typicallybetween 50 and 80 percent of the runtime, depending on the case, andthe largest computations have reached 110M grid nodes. Theclassical multigrid acceleration for 3D RANS simulations was foundineffective in the cases tested in combination with the LU-SGS solverusing optimal parameters. Finally, preliminary time-accurate simulations for unsteady flows have shown promising results.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xv, 97 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2014:72
Keyword
Compressible CFD, Convergence Acceleration, Implicit Time-Stepping, LU-SGS, Upwind Type Dissipation, Line-implicit, Ordering, Parallelization, Parameters, Multigrid
National Category
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-156414 (URN)978-91-7595-370-0 (ISBN)
Public defence
2014-12-12, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20141201

Available from: 2014-12-01 Created: 2014-11-27 Last updated: 2014-12-01Bibliographically approved

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