126 research outputs found
High-efficiency and positivity-preserving stabilized SAV methods for gradient flows
The scalar auxiliary variable (SAV)-type methods are very popular techniques
for solving various nonlinear dissipative systems. Compared to the
semi-implicit method, the baseline SAV method can keep a modified energy
dissipation law but doubles the computational cost. The general SAV approach
does not add additional computation but needs to solve a semi-implicit solution
in advance, which may potentially compromise the accuracy and stability. In
this paper, we construct a novel first- and second-order unconditional energy
stable and positivity-preserving stabilized SAV (PS-SAV) schemes for and
gradient flows. The constructed schemes can reduce nearly half
computational cost of the baseline SAV method and preserve its accuracy and
stability simultaneously. Meanwhile, the introduced auxiliary variable is
always positive while the baseline SAV cannot guarantee this
positivity-preserving property. Unconditionally energy dissipation laws are
derived for the proposed numerical schemes. We also establish a rigorous error
analysis of the first-order scheme for the Allen-Cahn type equation in
norm. In addition we propose an energy
optimization technique to optimize the modified energy close to the original
energy. Several interesting numerical examples are presented to demonstrate the
accuracy and effectiveness of the proposed methods
A convergent SAV scheme for Cahn--Hilliard equations with dynamic boundary conditions
The Cahn-Hilliard equation is one of the most common models to describe phase
separation processes in mixtures of two materials. For a better description of
short-range interactions between the material and the boundary, various dynamic
boundary conditions for this equation have been proposed. Recently, a family of
models using Cahn-Hilliard-type equations on the boundary of the domain to
describe adsorption processes was analysed (cf. Knopf, Lam, Liu, Metzger,
ESAIM: Math. Model. Numer. Anal., 2021). This family of models includes the
case of instantaneous adsorption processes studied by Goldstein, Miranville,
and Schimperna (Physica D, 2011) as well as the case of vanishing adsorption
rates which was investigated by Liu and Wu (Arch. Ration. Mech. Anal., 2019).
In this paper, we are interested in the numerical treatment of these models and
propose an unconditionally stable, linear, fully discrete finite element scheme
based on the scalar auxiliary variable approach. Furthermore, we establish the
convergence of discrete solutions towards suitable weak solutions of the
original model. Thereby, when passing to the limit, we are able to remove the
auxiliary variables introduced in the discrete setting completely. Finally, we
present simulations based on the proposed linear scheme and compare them to
results obtained using a stable, non-linear scheme to underline the
practicality of our scheme
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