Hindawi Publishing Corporation
Boundary Value Problems
Volume 2008, Article ID 735846, 14 pages
doi:10.1155/2008/735846
Research Article
Global Existence and Uniqueness of Strong
Solutions for the Magnetohydrodynamic Equations
Jianwen Zhang
School of Mathematical Sciences, Xiamen University, Xiamen 361005, China
Correspondence should be addressed to Jianwen Zhang, jwzhang@xmu.edu.cn
Received 21 June 2007; Accepted 5 October 2007
Recommended by Colin Rogers
This paper is concerned with an initial boundary value problem in one-dimensional magnetohy-
drodynamics. We prove the global existence, uniqueness, and stability of strong solutions for the
planar magnetohydrodynamic equations for isentropic compressible fluids in the case that vacuum
can be allowed initially.
Copyright q2008 Jianwen Zhang. This is an open access article distributed under the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
1. Introduction
Magnetohydrodynamics MHDconcerns the motion of a conducting fluid in an electromag-
netic field with a very wide range of applications. The dynamic motion of the fluids and the
magnetic field strongly interact each other, and thus, both the hydrodynamic and electrody-
namic effects have to be considered. The governing equations of the plane magnetohydrody-
namic compressible flows have the following form see, e.g., 1–5:
ρtρux0,
ρutρu2p1
2|b|2x
λuxx,
ρwtρuw−bxµwxx,
btub−wxυbxx,
ρetρeux−κθxxλu2
xµ
wx
2υ
bx
2−pux,
1.1
where ρdenotes the density of the fluid, u∈Rthe longitudinal velocity, ww1,w
2∈R2
the transverse velocity, bb1,b
2∈R2the transverse magnetic field, θthe temperature,
2 Boundary Value Problems
ppρ, θthe pressure, and eeρ, θthe internal energy; λand µare the bulk and shear
viscosity coefficients, υis the magnetic viscosity, κis the heat conductivity. Notice that the
longitudinal magnetic field is a constant which is taken to be identically one in 1.1.
The equations in 1.1describe the macroscopic behavior of the magnetohydrodynamic
flow. This is a three-dimensional magnetohydrodynamic flow which is uniform in the trans-
verse directions. There is a lot of literature on the studies of MHD by many physicists and
mathematicians because of its physical importance, complexity, rich phenomena, and mathe-
matical challenges, see 1–14and the references cited therein. We mention that, when b0,
the system 1.1reduces to the one-dimensional compressible Navier-Stokes equations for the
flows between two parallel horizontal plates see, e.g., 15.
In this paper, we focus on a simpler case of 1.1, namely, we consider the magneto-
hydrodynamic equations for isentropic compressible fluids. Thus, instead of the equations in
1.1, we will study the following equations:
ρtρux0,1.2
ρutρu2p1
2|b|2x
λuxx, 1.3
ρwtρuw−bxµwxx,1.4
btub−wxυbxx,1.5
where pRργwith γ≥1 being the adiabatic exponent and R>0 being the gas constant.
We will study the initial boundary value problem of 1.2–1.5in a bounded spatial domain
Ω0,1without loss of generalitywith the initial-boundary data:
ρ, ρu, ρw,bx, 0ρ0,m
0,n0,b0x,x∈Ω,1.6
u, w,b|x0,10,1.7
where the initial data ρ0≥0,m
0,n0,b0satisfy certain compatibility conditions as usual and
some additional assumptions below, and m0n00 whenever ρ00. Here the boundary
conditions in 1.7mean that the boundary is nonslip and impermeable.
The purpose of the present paper is to study the global existence and uniqueness of
strong solutions of problem 1.2–1.7. The important point here is that initial vacuum is
allowed; that is, the initial density ρ0may vanish in an open subset of the space-domain
Ω0,1, which evidently makes the existence and regularity questions more difficult than
the usual case that the initial density ρ0has a positive lower bound. For the latter case, one
can show the global existence of unique strong solution of this initial boundary value problem
in a similar way as that in 3,9,14. The strong solutions of the Navier-Stokes equations for
isentropic compressible fluids in the case that initial vacuum is allowed have been studied in
16,17. In this paper, we will use some ideas developed in 16,17and extend their results to
the problems 1.2–1.7. However, because of the additional nonlinear equations and the non-
linear terms induced by the magnetic field b, our problem becomes a bit more complicated.
Our main result in this paper is given by the following theorem the notations will be
defined at the end of this section.
Jianwen Zhang 3
Theorem 1.1. Assume that ρ0,m
0ρ0u0,n0ρ0w0,andb0satisfy the regularity conditions:
ρ0∈H
1,ρ
0≥0,u0,w0∈H
1
0∩H
2,b0∈H
1
0.1.8
Assume also that the following compatibility conditions hold for the initial data:
λu0xx −Rργ
01
2
b0
2x
ρ1/2
0ffor some f∈L
2Ω,1.9
µw0xx b0xρ1/2
0gfor some g∈L
2Ω.1.10
Then there exists a unique global strong solution ρ, u, w,bto the initial boundary value problem
1.2–1.7such that for all T∈0,∞,
ρ∈L
∞0,T;H1,u, w∈L
∞0,T;H1
0∩H
2,b∈L
∞0,T;H1
0,
ρt,√ρut,√ρwt∈L
∞0,T;L2,ut,wt∈L
20,T;H1,bt,bxx∈L
20,T;L2.
1.11
Remark 1.2. The compatibility conditions given by 1.9,1.10play an important role in the
proof of uniqueness of strong solutions. Similar conditions were proposed in 16–18when the
authors studied the global existence and uniqueness of solutions of the Navier-Stokes equa-
tions for isentropic compressible fluids. In fact, one also can show the global existence of weak
solutions without uniqueness if the compatibility conditions 1.9,1.10are not valid.
We will prove the global existence and uniqueness of strong solutions in Sections 3and
4, respectively, while Section 2 is devoted to the derivation of some a priori estimates.
We end this section by introducing some notations which will be used throughout the
paper. Let Wm,pΩ denote the usual Sobolev space, and Wm,2Ω HmΩ,W0,pΩ LpΩ.
For simplicity, we denote by Cthe various generic positive constants depending only on the
data and T, and use the following abbreviation:
Lq0,T;Wm,p≡L
q0,T;Wm,pΩ,Lp≡L
pΩ,·p≡·
LpΩ.1.12
2. A priori estimates
This section is devoted to the derivation of a priori estimates of ρ, u, w,b. We begin with the
observation that the total mass is conserved. Moreover, if we multiply 1.3,1.4,and1.5by
u, w,andb, respectively, and sum up the resulting equations, we have by using 1.2that
1
2ρu2
w
21
2
b
2t
1
2ρuu2
w
2u
b
2−w·bx
upx
λuuxµw·wxυb·bxx−λu2
xµ
wx
2υ
bx
2.
2.1
Integrating 1.2and 2.1over 0,t×Ω, we arrive at our first lemma.
4 Boundary Value Problems
Lemma 2.1. For any t∈0,T, one has
Ω
ρx, tdx Ω
ρ0xdx ≤C,
ΩGρ1
2ρu2
w
21
2
b
2x, tdx t
0Ωλu2
xµ
wx
2υ
bx
2x, sdx ds ≤C,
2.2
where Gρis the nonnegative function defined by
Gρ⎧
⎪
⎨
⎪
⎩
Rργ
γ−1if γ>1
Rρln ρ−ρ1if γ1.
2.3
The next lemma gives us an upper bound of the density ρx, t, which is crucial for the
proof of Theorem 1.1.
Lemma 2.2. For any x, t∈QT:Ω×0,T,ρx, t≤Cholds.
Proof. Notice that 1.3can be rewritten as
ρutλux−ρu2−p−
b
2
2x
.2.4
Set
ψx, t:t
0λux−ρu2−p−1
2
b
2x, sds x
0
m0ζdζ, 2.5
from which and 2.4, we find that ψsatisfies
ψxρu, ψtλux−ρu2−p−1
2
b
2,ψ|t0x
0
m0ζdζ. 2.6
In view of Lemma 2.1 and 2.6, we have by using Cauchy-Schwarz’s inequality that
ψx
L∞0,T;L1≤C,
Ω
ψx, tdx
≤C, 2.7
which imply
ψL∞0,T×Ω ≤
ψx
L∞0,T;L1
Ω
ψx, tdx
≤C. 2.8
Letting Dt:∂tu∂xdenote the material derivative and choosing Fexp ψ/λ,we
obtain after a straightforward calculation that
DtρF:∂tρFu∂xρF−1
λp
b
2
2ρF ≤0,2.9
which, together with 2.8, yields Lemma 2.2 immediately.
Jianwen Zhang 5
To be continued, we need the following lemma because of the effect of magnetic field b.
Lemma 2.3. The magnetic field bsatisfies the following estimates:
sup
0≤t≤T
bt
L∞
bxt
L2
bt
L20,T;L2≤C,
bxx
L20,T;L2≤C. 2.10
Proof. Multiplying 1.5by btand integrating over 0,t×Ω,wehave
1
4t
0Ω
bt
2x, sdx ds υ
2Ω
bx
2x, tdx
≤υ
2Ω
b0x
2xdx t
0Ωu2
bx
2u2
x
b
2
wx
2x, sdx ds
≤C2t
0Ω
u2
xx, sdxΩ
bx
2x, sdxds,
2.11
where we have used Cauchy-Schwarz’s inequality, Lemma 2.1, and the following inequalities:
max
x∈Ωu2·,s≤
uxs
2
L2,max
x∈Ω
b·,s
2≤
bxs
2
L2.2.12
Since uxL20,T;L2≤Cbecause of Lemma 2.1, we thus obtain the first inequality indi-
cated in this lemma from 2.11by applying Gronwall’s lemma and then Sobolev’s inequality.
To prove the second part, we multiply 1.5by bxx and integrate the resulting equation
over 0,T×Ωto deduce that
T
0Ω
bxx
2x, tdx dt
≤CT
0Ω
bt
2
wx
2u2
x
b
2u2
bx
2x, tdx dt
≤CCsup
t∈0,T
bt
2
L∞T
0
uxt
2
L2dt CT
0
uxt
2
L2
bxt
2
L2dt
≤CCsup
t∈0,T
bt
2
L∞
bxt
2
L2T
0
uxt
2
L2dt ≤C,
2.13
where we have used Cauchy-Schwarz’s inequality, Sobolev’s inequality 2.12,Lemma 2.1,and
the first part of the lemma. This completes the proof of Lemma 2.3.
Lemma 2.4. The following estimates hold for the velocity u, w:
sup
0≤t≤T
ut
L∞
uxt
L2
√ρut
L20,T;L2≤C,
sup
0≤t≤T
wt
L∞
wxt
L2
√ρwt
L20,T;L2≤C.
2.14
