# Accession Number:

## ADA625374

# Title:

## Matrix-Free Polynomial-Based Nonlinear Least Squares Optimized Preconditioning and its Application to Discontinuous Galerkin Discretizations of the Euler Equations

# Descriptive Note:

# Corporate Author:

## NAVAL POSTGRADUATE SCHOOL MONTEREY CA DEPT OF APPLIED MATHEMATICS

# Personal Author(s):

# Report Date:

## 2015-06-01

# Pagination or Media Count:

## 26.0

# Abstract:

We introduce a preconditioner that can be both constructed and applied using only the ability to apply the underlying operator. Such a preconditioner can be very attractive in scenarios where one has a highly e cient parallel code for applying the operator. Our method constructs a polynomial preconditioner using a nonlinear least squares NLLS algorithm. We show that this polynomial-based NLLS-optimized PBNO preconditioner signi cantly improves the performance of a discontinuous Galerkin DG compressible Euler equation model when run in an implicit-explicit time integration mode. The PBNO preconditioner achieves signi cant reduction in GMRES iteration counts and model wall-clock time, and signi cantly outperforms several existing types of generalized linear least squares GLS polynomial preconditioners. Comparisons of the ability of the PBNO preconditioner to improve DG model performance when employing the Stabilized Biconjugate Gradient algorithm BICGS and the basic Richardson RICH iteration are also included. In particular, we show that higher order PBNO preconditioning of the Richardson iteration run in a dot product free mode makes the algorithm competitive with GMRES and BICGS in a serial computing environment. Because the NLLS-based algorithm used to construct the PBNO preconditioner can handle both positive de nite and complex spectra without any need for algorithm modi cation we suggest that the PBNO preconditioner is, for certain types of problems, an attractive alternative to existing polynomial preconditioners based on linear least-squares methods.

# Descriptors:

# Subject Categories:

- Numerical Mathematics