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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 83
Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Paper 175

Reordering for the Iterative Solution of Hybrid Elastic Structures

A. Comerlati, G. Gambolati and C. Janna

Department of Mathematical Methods and Models for Scientific Applications (DMMMSA), University of Padova, Italy

Full Bibliographic Reference for this paper
A. Comerlati, G. Gambolati, C. Janna, "Reordering for the Iterative Solution of Hybrid Elastic Structures", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Eighth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 175, 2006. doi:10.4203/ccp.83.175
Keywords: reordering, Krylov, preconditioning, conjugate gradient.

It is well known that the outcome of the finite element method in linear elastic structural problems is a linear system with the system matrix both symmetric and positive definite (SPD) [1,2]. In the present paper, the influence of matrix reordering on convergence of the preconditioned conjugate gradient (PCG) is addressed. The two preconditioners used herein are a variant of the incomplete Cholesky factorization with an user-specified fill-in degree [3]:
  1. the former is a variant of the ILUT preconditioner [4,3] (here denoted as SAADILUT), which is commonly used for nonsymmetric problems and firstly converts matrix from the symmetric sparse row (SSR) to the compressed sparse row (CSR) format;
  2. the latter is an incomplete factorization with a variable fill-in (denoted as SYMILUT) that is implemented so as to take the maximum advantage from the matrix symmetry and not to require the format conversion from SSR to CSR.
Within the two implementations the fill-in degree has a different meaning: in SAADILUT lfil1 denotes the maximum allowable number of nonzero elements for each row of the incomplete factors excluding the diagonal elements; in SYMILUT lfil2 represents the maximum allowable number of nonzero elements for each row of the factors excluding all the original nonzero elements.

As mentioned above our focus is on linear systems arising from the spatial discretization of linear elastic civil structures obtained by coupling different elements such as beams, trusses, and shells. The ordering schemes addressed are:

  1. SPARSKIT based on the Cuthill-McKee algorithm [5,3];
  2. METIS based on the multilevel nested dissection algorithm [6].
Both these reordering schemes were originally developed and implemented for matrix factorization. They basically act by reducing the matrix bandwidth, leading to a matrix structure consisting of little "arrow sub-matrices" pointing downward, thus yielding less fill-in in the Gaussian elimination. The best PCG performance is finally compared with a direct solver as is implemented in the MA57 routine of the Harwell Software library [7,8]. Numerical experiments are performed on a realistic cable-stayed bridge recently built in Porto Maghera near Venice. The main results are summarized below:
  1. when far nodes are connected, the native unknowns ordering yields SPD matrices which may not be suitable for the solution by PCG methods due to the poor preconditioner quality;
  2. Choleski-based symmetric preconditioners provide the best overall performance with matrices reordered with METIS;
  3. the direct solver MA57 outperforms the best PCG by a factor 3-4 and is more robust as it does not require the selection of any empirical parameter such as the most appropriate degree of fill-in in the preconditioner calculation.

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