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Global classical solutions for 3D compressible Navier-Stokesequa...
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含真空和粘性系数依赖于密度的3-D可压Navier-Stokes方程组古典解存在性,刘生全,张剑文,本文研究了粘性系数依赖于密度的三维可压Navier-Stokes方程组的Cauchy问题。如果初值满足小能量条件,作者证明了全局古典解的存在性,所得
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˖ڍመڙጲ
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含真空和粘性系数依赖于密度的 3-D 可压
Navier-Stokes 方程组古典解存在性
刘生全,张剑文,赵俊宁
厦门大学数学科学学院,厦门 361005
摘要:本文研究了粘性系数依赖于密度的三维可压 Navier-Stokes 方程组的 Cauchy 问题。如
果初值满足小能量条件,作者证明了全局古典解的存在性,所得到结果包含初始密度有真空的
情况。并且本文还得到了解的大时间行为。
关键词:可压 Navier-Stokes 方程组;全局古典解;粘性系数依赖密度;真空
中图分类号: 0175;
Global classical solutions for 3D compressible
Navier-Stokes equations with vacuum and a
density-dependent viscosity coefficient
LIU Sheng-Quan, ZHANG Jian-Wen, ZHAO Jun-Ning
School of Mathematical Sciences, Xiamen University, Xiamen 361005, China
Abstract: In this paper, we prove the global existence of classical solutions to the three
dimensional (3D) compressible Navier-Stokes equations with a density-dependent viscosity
coefficient (λ = λ(ρ)) provided the initial data is of small energy. This in particular implies
that the solutions may have large oscillations and contain vacuum states. As a result of the
uniform estimates, the large-time behavior of the solution is also studied.
Key words: compressible Navier-Stokes equations; global classical solutions;
density-dependent viscosity; vacuum
基金项目: This work was supported by a grant from the Ph.D. Programs Foundation of Ministry of Education of China
(200803840016)
作者简介: Liu Sheng-Quan(1983-),male,Doctor,major research direction:Paritial differential equations. Zhang
Jian-Wen(1977-),male,Associate Professor,major research direction:Paritial differential equations. Correspondence
author:Zhao Jun-Ning(1945-),male,Professor,major research direction:Paritial differential equations.
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˖ڍመڙጲ
http://www.paper.edu.cn
0 Introduction
In this paper, we consider the following 3D compressible Navier-Stokes equations with a
density-dependent viscosity coefficient:
ρ
t
+ div(ρu) = 0, x ∈ R
3
, t > 0,
(ρu)
t
+ div(ρu ⊗u) + ∇P(ρ) = µ∆u + ∇((µ + λ(ρ))divu),
(1)
where the unknowns ρ, u = (u
1
, u
2
, u
3
), and P (ρ) = Aρ
γ
with A > 0, γ > 1 are the fluid
density, velocity, and pressure, respectively. The dynamic viscosity coefficient µ > 0 is a
positive constant, however, the second viscosity coefficient λ = λ(ρ) is a function of ρ.
We look for the global classical solutions, (ρ(x, t), u(x, t)), to the Cauchy problem of (1)
with the initial data
ρ(x, 0) = ρ
0
(x), u(x, 0) = u
0
(x), x ∈ R
3
, (2)
and the far field behavior
ρ(x, t) → ˜ρ ≥ 0, u(x, t) → 0 as |x| → ∞, t > 0, (3)
where ˜ρ ≥ 0 is a fixed nonnegative constant.
In the last decades, there have been a huge number of studies on the compressible Navier-
Stokes equations with constant viscosity coefficients. Since the work of Kazhikov and Shelukhin
(cf. [1]), the one-dimensional compressible Navier-Stokes equations have been extensively stud-
ied by many people and significant progress has been made on the mathematical topics. For
the multi-dimensional Navier-Stokes equations with constant viscosity coefficients, when the
initial density is strictly away from vacuum, the local-in-time solvability to various initial and
boundary value problems for the compressible Navier-Stokes equations was proved by Nash [2],
Solonnikov [3], and Matsumura et al [4, 5]. When the initial vacuum is allowed, i.e., the initial
density need not be positive and may vanish in open sets, the local existence of strong/classical
solutions was shown in [6, 7]. It is worth mentioning that the result in [8] is so far the unique
theory about the global existence and uniqueness of large solutions of multi-dimensional com-
pressible Navier-Stokes equations. One of the most important breakthrough is the work of Lions
[9], who first proved the global existence of weak solutions (the so-called “finite energy weak
solutions”) to the initial/inital-boundary value problem of (1) with generally large initial data
when the initial energy is finite and the adiabatic exponent γ is suitably large (i.e. γ > 3/2).
We also refer to [10] for the global existence of weak solutions with large symmetric data.
A lot of attention has also been paid to the compressible Navier-Stokes equations with
density-dependent viscosity coefficients due to its physical importance in ocean physics and
its close connection to the shallow water equations. In the one-dimensional case, there is an
extensive literature on the density-dependent viscosity system, see, for example, [11, 12, 13,
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14, 15] and among others. For the multi-dimensional case, Bresch et al (cf. [16]) studied the
global weak solutions for the multi-dimensional equations with the Korteweg stress tensor or
the additional quadratic friction term provided the following algebraic relation between µ(ρ)
and λ(ρ) holds,
λ(ρ) = 2(ρµ
′
(ρ) − µ(ρ)). (4)
Mellet and Vasseur [17] proved that the Korteweg or friction terms are unnecessary in the proof
of compactness for the entropy weak solution. Guo-Jiu-Xin [18] considered the global spherically
symmetric solutions to the 3D compressible Navier-Stokes equations with density-dependent
viscosity coefficients satisfying (4) in a particular form.
Recently, many efforts have been made to improve the global well-posedness theory of
the multi-dimensional compressible Navier-Stokes equations. The global existence of classical
solutions to the 3D compressible Navier-Stokes equations with a density-dependent viscosity
coefficient λ(ρ) was studied by Zhang [19] when the initial data and solution are small in
energy-norm. It should be noted that in both [20] and [19], the density is technically required
to be strictly away from vacuum, which, as emphasized in [6, 7, 22, 23, 24, 25], is one of the
major difficulties in the mathematical study of compressible Navier-Stokes equations. More
recently, Huang-Li-Xin [21] considered the 3D isentropic compressible Navier-Stokes equations
with constant viscosity coefficients, and established the first existence result of classical solutions
which may have large oscillations and can contain vacuum states.
As that in [19], in this paper we focus our interesting on the compressible Navier-Stokes
equations (1) with a density-dependent viscosity coefficient λ(ρ). Indeed, as aforementioned, the
system with density-dependent viscosity coefficient was firstly studied by Vaigant and Kazhikov
in 1995. However, the proof in [8] is based on the particular form of the constitutive relations
and the fact that the problem is posed in two space dimensions, and thus, the method therein
cannot be adopted to deal with the 3D case. Moreover, the authors technically required in [8]
that the initial density is strictly away from vacuum. The global weak solutions, which are of
small energy but may contain vacuum states, to the 2D equations of (1) with µ = Const. and
λ = λ(ρ) were studied by Zhang and Fang [26].
For 1 < r ≤ ∞ and k ∈ Z
+
, we shall use the following simplified notations for the standard
homogeneous and inhomogeneous Sobolev spaces:
L
r
= L
r
(R
3
), D
k,r
=
u ∈ L
1
loc
(R
3
) : ∥∇
k
u∥
L
r
< ∞
, ∥u∥
D
k,r
= ∥∇
k
u∥
L
r
,
W
k,r
= L
r
∩ D
k,r
, H
k
= W
k,2
, D
k
= D
k,2
, D
1
= {u ∈ L
6
: ∥∇u∥
L
2
< ∞}.
The initial energy of the compressible flows is defined as
E
0
=
1
2
ρ
0
|u
0
|
2
+ G(ρ
0
)
dx, (5)
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where G , G(ρ) is the potential energy density defined by
G(ρ) = ρ
ρ
˜ρ
P ( s) − P (˜ρ)
s
2
ds (6)
with ˜ρ ≥ 0 being the far field density in (3).
Then our main result in this paper reads as follows.
Theorem 1. Let µ > 0 be a positive constant and λ(ρ) be a smooth function of the density ρ
satisfying
λ(ρ) ∈ C
3
(R
+
), 0 ≤ λ(ρ) < ∞ for all ρ ∈ [0, ∞). (7)
Assume that for given numbers M > 0 (not necessarily small) and ¯ρ ≥ ˜ρ + 1, the initial data
(ρ
0
, u
0
) satisfy
0 ≤ inf ρ
0
≤ sup ρ
0
≤ ¯ρ, (ρ
0
− ˜ρ, P (ρ
0
) − P (˜ρ)) ∈ H
3
,
ρ
0
|u
0
|
2
+ G(ρ
0
) ∈ L
1
, u
0
∈ D
1
∩ D
3
, ∥∇u
0
∥
2
L
2
≤ M,
(8)
and that the the following compatibility condition
−µ△u
0
− ∇((µ + λ(ρ
0
))divu
0
) + ∇P (ρ
0
) = ρ
0
g, (9)
holds for some g ∈ D
1
and
√
ρ
0
g ∈ L
2
. Then there exists a positive constant ε > 0, depending
on µ, a, γ, ˜ρ, ¯ρ and M , such that if
E
0
≤ ε, (10)
the Cauchy problem (1)-(3) has a unique global classical solution (ρ, u) satisfying
0 ≤ ρ ≤ 2¯ρ, (x, t) ∈ R
3
× (0, T ), (11)
(ρ − ˜ρ, P (ρ) − P (˜ρ)) ∈ C([0, T ]; H
3
),
u ∈ C([0, T ]; D
1
∩ D
3
) ∩ L
2
(0, T ; D
4
) ∩ L
∞
(τ, T ; D
4
),
u
t
∈ L
∞
(0, T ; D
1
) ∩ L
2
(0, T ; D
2
) ∩ L
∞
(τ, T ; D
2
) ∩ H
1
(τ, T ; D
1
),
√
ρu
t
∈ L
∞
(0, T ; L
2
),
(12)
for any 0 < τ < T < ∞.
Furthermore, the following large-time behavior
lim
t→∞
(|ρ − ˜ρ|
q
+ ρ
1/2
|u|
4
+ |∇u|
2
)dx = 0, (13)
holds for q ∈ (2, ∞) if ˜ρ > 0 and for q ∈ (γ, ∞) if ˜ρ = 0.
Theorem 1 will be proved by combining the local existence result and the global a priori
estimates. The key issue in the proof of global estimates is to derive both the time-independent
upper bound for the density and the time-depending higher norm estimates of the smooth
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