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Implementation of Implicit LU-SGS method with Line-implicit scheme on Stretched Unstructured Grids
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)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

A first implementation of line-implicit Lower-Upper Symmetric Gauss-Seidel (LU-SGS) in Edge has been considered in this paper for convergenceacceleration on stretched meshes. The motivation comes from the lack of efficiency of the implicit LU-SGS when running in RANS meshes. Edge is a flow solver for unstructured grids based on a dual grid and edge-based formulation. The line-implicit method has been implemented in the code Edge has an acceleration method when computing explicit schemes in stretched meshes. This methods works only in regions of stretched grid where the flow equations are integrated implicitly in time along the structured lines. The combination with an implicit LU-SGS should remove the restriction of the time step for explicit schemes and accelerate the convergence to steady state. The results have shown that for highly stretched meshes, line-implicit LU-SGS could perform much faster than LU-SGS.

National Category
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-98475OAI: oai:DiVA.org:kth-98475DiVA: diva2:537246
Note
QS 2012Available from: 2012-06-26 Created: 2012-06-26 Last updated: 2012-06-26Bibliographically 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

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