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# Efficient Uncertainty Quantification with the Polynomial Chaos Method for Stiff Systems

Cheng, Haiyan and Sandu, Adrian (2007) Efficient Uncertainty Quantification with the Polynomial Chaos Method for Stiff Systems. Technical Report TR-07-19, Computer Science, Virginia Polytechnic Institute and State University.

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## Abstract

The polynomial chaos method has been widely adopted as a computationally feasible approach for uncertainty quantification. Most studies to date have focused on non-stiff systems. When stiff systems are considered, implicit numerical integration requires the solution of a nonlinear system of equations at every time step. Using the Galerkin approach, the size of the system state increases from $n$ to $S \times n$, where $S$ is the number of the polynomial chaos basis functions. Solving such systems with full linear algebra causes the computational cost to increase from $O(n^3)$ to $O(S^3n^3)$. The $S^3$-fold increase can make the computational cost prohibitive. This paper explores computationally efficient uncertainty quantification techniques for stiff systems using the Galerkin, collocation and collocation least-squares formulations of polynomial chaos. In the Galerkin approach, we propose a modification in the implicit time stepping process using an approximation of the Jacobian matrix to reduce the computational cost. The numerical results show a run time reduction with a small impact on accuracy. In the stochastic collocation formulation, we propose a least-squares approach based on collocation at a low-discrepancy set of points. Numerical experiments illustrate that the collocation least-squares approach for uncertainty quantification has similar accuracy with the Galerkin approach, is more efficient, and does not require any modifications of the original code.

Item Type: Departmental Technical Report Uncertainty quantification, polynomial chaos, least-squares collocation, Smolyak algorithm, low-discrepancy data sets Computer Science > Numerical Analysis 978 Cheng, Haiyan 30 May 2007