Lisandro Dalcin — Page 2

arXiv2015

An energy-stable convex splitting for the phase-field crystal equation

The phase-field crystal equation, a parabolic, sixth-order and nonlinear partial differential equation, has generated considerable interest as a possible solution to problems arising in molecular dynamics. This is because the phase-field crystal model can capture atomic-scale effects at time-scales that are orders of magnitude larger than what molecular dynamics simulations can afford presently. Nonetheless, solving this equation is not a trivial task, as a non-increasing free energy and mass conservation need to be verified for the numerical solution to be valid. This work focuses on these issues, and proposes a novel algorithm that guarantees mass conservation, unconditional energy stability and is second-order accurate in time. This is achieved through a convex-concave splitting of the nonlinearity present in the equation, along with the use of a stabilization term that bounds possible increases in free energy. We present numerical results that validate our mathematical proofs, and show two and three dimensional simulations involving crystal growth that showcase the robustness of the method.

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arXiv2012

The cost of continuity: performance of iterative solvers on isogeometric finite elements

In this paper we study how the use of a more continuous set of basis functions affects the cost of solving systems of linear equations resulting from a discretized Galerkin weak form. Specifically, we compare performance of linear solvers when discretizing using $C^0$ B-splines, which span traditional finite element spaces, and $C^{p-1}$ B-splines, which represent maximum continuity. We provide theoretical estimates for the increase in cost of the matrix-vector product as well as for the construction and application of black-box preconditioners. We accompany these estimates with numerical results and study their sensitivity to various grid parameters such as element size $h$ and polynomial order of approximation $p$. Finally, we present timing results for a range of preconditioning options for the Laplace problem. We conclude that the matrix-vector product operation is at most $\slfrac{33p^2}{8}$ times more expensive for the more continuous space, although for moderately low $p$, this number is significantly reduced. Moreover, if static condensation is not employed, this number further reduces to at most a value of 8, even for high $p$. Preconditioning options can be up to $p^3$ times more expensive to setup, although this difference significantly decreases for some popular preconditioners such as Incomplete LU factorization.

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