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Geoid models around Sognefjord using depth data (1998)

Dahl, O. C. ; Forsberg, R.

Springer

Journal of geodesy 72 (1998), S. 547-556

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ISSN: 1432-1394

Keywords: Key words. Bathymetry ; Gravity ; Terrain effects ; Quasigeoid minus geoid ; Geoid ; GPS/levelling

Source: Springer Online Journal Archives 1860-2000

Topics: Architecture, Civil Engineering, Surveying

Notes: Abstract. Bathymetry data from Sognefjord, Norway, have been included in a terrain model, and their influence on the geoid has been calculated. The test area, located in the western part of Norway, was chosen due to its deep fjords and high mountains. Inclusion of bathymetry data in the terrain model altered the computed gravimetric geoid by as much as a few decimeters. The effect was detectable to a distance of more than 100 km. All calculated geoids, both with and without bathymetry data in the terrain model, fit the geoidal heights determined by available Global Positioning System (GPS) and levelling heights at the sub-decimetre level. Contrary to expectations, the accuracy in geoid prediction was reduced when using bathymetric data. The geoid changes were largest over the fjord where no GPS points were located. Different methods on the same area [isostatic and Residual Terrain Model (RTM)-terrain reductions] showed differences of approximately 1 m. Rigorous distinction between quasigeoid and geoid was found to be essential in this kind of area.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s001900050193

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An ILU preconditioner with coupled node fill-in for iterative solution of the mixed finite element formulation of the 2D and 3D Navier-Stokes equations (1992)

Dahl, O. ; Wille, S. Ø

Chichester : Wiley-Blackwell

International Journal for Numerical Methods in Fluids 15 (1992), S. 525-544

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ISSN: 0271-2091

Keywords: Navier-Stokes ; Mixed formulation ; Orthomin ; Bi-CGSTAB ; Jacobi preconditioning ; SSOR ; Incomplete LU ; Inner iterations ; Engineering ; Engineering General

Source: Wiley InterScience Backfile Collection 1832-2000

Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Notes: In the present paper, preconditioning of iterative equation solvers for the Navier-Stokes equations is investigated. The Navier-Stokes equations are solved for the mixed finite element formulation. The linear equation solvers used are the orthomin and the Bi-CGSTAB algorithms. The storage structure of the equation matrix is given special attention in order to avoid swapping and thereby increase the speed of the preconditioner. The preconditioners considered are Jacobian, SSOR and incomplete LU preconditioning of the matrix associated with the velocities. A new incomplete LU preconditioning with fill-in for the pressure matrix at locations in the matrix where the corner nodes are coupled is designed. For all preconditioners, inner iterations are investigated for possible improvement of the preconditioning. Numerical experiments are executed both in two and three dimensions.

Additional Material: 5 Ill.