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Cahn-Hilliard算子分裂方法是稳定的_The operator-splitting method for Cahn-H
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Cahn-Hilliard算子分裂方法是稳定的_The operator-splitting method for Cahn-Hilliard is stable.pdf
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arXiv:2107.01418v1 [math.NA] 3 Jul 2021
The operator-splitting method for Cahn-Hilliard
is stable
Dong Li
∗
Chaoyu Quan
†
Abstract
We prove energy stability of a standard operator-splitting method for
the Cahn-Hilliard equation. We establish uniform bound of Sobolev norms
of the numer ical solution and convergence of the splitting approximation.
This is the fir st unconditional energy stability result for the operator-
splitting method for the Cahn -Hilliard equation. Our analysis can be
extended to many other models.
1 Introduction
We consider numerical solutions of the Cahn-Hilliard ([2]) equation:
(
∂
t
u = ∆(−ν∆u + f(u)), (t, x) ∈ (0, ∞) × Ω,
u
t=0
= u
0
,
(1.1)
where u = u(t, x) is a real-valued function corresponding to the concentration
difference in a binary system. The parameter ν > 0 is usually called t he mobility
coefficient which is taken to a constant here for simplicity. The nonlinear term
f(u) is derived from a standard do uble well potential, namely:
f(u) = u
3
− u = F
′
(u), F (u) =
1
4
(u
2
− 1)
2
.
Due to this specific choice of equal-well double potential, the minima of the
potential are located at u = ±1 which corresponds different phases or states. The
∗
Department of Ma thematics, the Hong Kong University of Science & Technology, Clear
Water Bay, Hong Kong. Email: mpdongli@gmail.com.
†
SUSTech International Center for Mathematics, Southern University of Science and Tech-
nology, Shenzhen, China. Email: quancy@sustech.edu.cn.
1
length scale of the transitional region is usual proportional to
√
ν. In this note
we take the spatial domain Ω in (1.1) as the two-dimensional 2π-periodic torus
T
2
= R
2
/2πZ
2
= [−π, π]
2
. Our analysis extends to other physical dimensions
d ≤ 3 but we choose the prototypical case d = 2 to simplify t he presentation. For
simplicity we consider mean zero initial data, that is
Z
T
2
u
0
(x)dx = 0. (1.2)
For smooth solutions, there is the mass conservation law
d
dt
M(t) =
d
dt
Z
Ω
u(t, x)dx ≡ 0. (1.3)
It follows that u(t, ·) has zero mean for all t > 0. For the class of mean-zero
functions with suitable regularity, one can employ the operator |∇|
s
for s < 0 as
the Fourier multiplier |k|
s
· 1
k6=0
. The system (1.1) naturally arises as a gra dient
flow of a Ginzburg-Landau type energy functional E(u) in H
−1
, namely
∂
t
u = −
δE
δu
H
−1
= ∆(
δE
δu
), (1.4)
where
δE
δu
H
−1
,
δE
δu
denote the standard variational derivatives in H
−1
and L
2
respectively, and
E(u) =
Z
Ω
1
2
ν|∇u|
2
+ F (u)
dx =
Z
Ω
1
2
ν|∇u|
2
+
1
4
(u
2
− 1)
2
dx. (1.5)
For smooth solutions, the fundamental energy conservation law takes the form
d
dt
E(u(t)) + k|∇|
−1
∂
t
uk
2
2
=
d
dt
E(u(t)) +
Z
Ω
|∇(−ν∆u + f(u))|
2
dx = 0. (1.6)
It follows that
E(u(t)) ≤ E(u(s)), ∀t ≥ s; (1.7a)
k∇u(t)k
2
≤
r
2
ν
E(u(t)) ≤
r
2
ν
E(u
0
), ∀t > 0. (1.7b)
In particular, one obtains a prio r i
˙
H
1
-norm control of the solution for all t > 0.
Since the scaling-critical space for CH is L
2
in 2D, the global wellposedness and
regularity for H
1
-initial data follows easily.
For τ > 0, we let S
L
(τ) = e
−τ ν∆
2
be the exact solution operator to t he linear
equation:
∂
t
u = −ν∆
2
u. (1.8)
2
We define S
N
(τ) : w → u as the solution operator to the f ollowing problem:
u − w
τ
= ∆(w
3
− w). (1.9)
In yet other words,
u = S
N
(τ)w = w + τ∆(w
3
− w). (1.10)
This is one of the simplest discretization on the timer interval [0, τ] for the exact
problem
(
∂
t
u = ∆(u
3
− u), t > 0;
u
t=0
= w.
(1.11)
By using the operator-splitting, the solution of the original equation from time t
to time t + τ is approximated as
u(t + τ, x) ≈
S
L
(τ)S
N
(τ)u
(t, x). (1.12)
The main purpose of this note is establish stability of the above operator-
splitting algorithm. Prior to our work, there were very few rigor ous results on the
analysis of the operator-splitting type algorithms for the Cahn-Hilliard equation
and similar models. In [24], Weng, Zhai and Feng considered a viscous Cahn-
Hilliard model of the for m
(1 − α)∂
t
u = ∆(−ǫ
2
∆u + f(u) + α∂
t
u), (1.13)
where 0 < α < 1. They considered a fast explicit Strang splitting and estab-
lished stability and converg ence under t he assumption that A = k∇u
num
k
2
∞
,
B = ku
num
k
2
∞
are bounded, and satisfy a technical condition 6A + 8 − 24B > 0
(see Theorem 1 on pp. 7 of [24]), where u
num
denotes the numerical solution. In
[23], Gidey and Reddy considered a convective Cahn-Hillia r d model of the form
∂
t
u − γ∇ · h(u) + ǫ
2
∆
2
u = ∆(f(u)), (1.14)
where h(u) =
1
2
(u
2
, u
2
). They considered operato r -splitting of (1.14) into hyper-
bolic part, nonlinear diffusion part and diffusion part respectively, and obtained
various conditional results concerning certain weak solutions. In [22], Cheng,
Kurganov, Qu and Tang considered the Strang splitting for the Cahn-Hilliard
equation and molecular beam epitaxy type models. Some conditional results
were given in [22] but rigorous analysis of energy stability has remained open.
The purpose of this note is to establish a new theoretical framework for the rig-
orous analysis of energy stability and higher-order Sobolev-norm stability for the
operator- splitting method applied to these difficult equations. Our first result
establishes unifor m Sobolev control of the numerical solution for a ll time.
3
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