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Acceleration on stretched meshes with line-implicit LU-SGS in parallel implementation
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aerodynamics.ORCID iD: 0000-0001-9902-6216
2015 (English)In: International journal of computational fluid dynamics (Print), ISSN 1061-8562, E-ISSN 1029-0257, Vol. 29, no 2, 133-149 p.Article in journal (Refereed) Published
Abstract [en]

The implicit lower-upper symmetric Gauss-Seidel (LU-SGS) solver is combined with the line-implicit technique to improve convergence on the very anisotropic grids necessary for resolving the boundary layers. The computational fluid dynamics code used is Edge, a Navier-Stokes flow solver for unstructured grids based on a dual grid and edge-based formulation. Multigrid acceleration is applied with the intention to accelerate the convergence to steady state. LU-SGS works in parallel and gives better linear scaling with respect to the number of processors, than the explicit scheme. The ordering techniques investigated have shown that node numbering does influence the convergence and that the orderings from Delaunay and advancing front generation were among the best tested. 2D Reynolds-averaged Navier-Stokes computations have clearly shown the strong efficiency of our novel approach line-implicit LU-SGS which is four times faster than implicit LU-SGS and line-implicit Runge-Kutta. Implicit LU-SGS for Euler and line-implicit LU-SGS for Reynolds-averaged Navier-Stokes are at least twice faster than explicit and line-implicit Runge-Kutta, respectively, for 2D and 3D cases. For 3D Reynolds-averaged Navier-Stokes, multigrid did not accelerate the convergence and therefore may not be needed.

Place, publisher, year, edition, pages
2015. Vol. 29, no 2, 133-149 p.
Keyword [en]
line-implicit, convergence acceleration, line-implicit LU-SGS, implicit time-stepping, parallelisation, LU-SGS, ordering techniques
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
URN: urn:nbn:se:kth:diva-156412DOI: 10.1080/10618562.2015.1021692ISI: 000352611900001ScopusID: 2-s2.0-84927796786OAI: diva2:766621

QC 20141201. Updated from accepted to published.

Available from: 2014-11-27 Created: 2014-11-27 Last updated: 2015-08-05Bibliographically 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
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.
TRITA-AVE, ISSN 1651-7660 ; 2014:72
Compressible CFD, Convergence Acceleration, Implicit Time-Stepping, LU-SGS, Upwind Type Dissipation, Line-implicit, Ordering, Parallelization, Parameters, Multigrid
National Category
Aerospace Engineering
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)

QC 20141201

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

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