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# Remarks on the regularity criteria for the 3D MHD equations in the multiplier spaces

Zujin Zhang1*, Xiqin Ouyang2, Dingxing Zhong1 and Shulin Qiu1

Author Affiliations

1 School of Mathematics and Computer Sciences, Gannan Normal University, Ganzhou, Jiangxi, 341000, P.R. China

2 College of Mathematics, Gannan Institute of Education, Ganzhou, Jiangxi, 341000, P.R. China

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Boundary Value Problems 2013, 2013:270  doi:10.1186/1687-2770-2013-270

 Received: 27 September 2013 Accepted: 22 November 2013 Published: 12 December 2013

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this paper, we consider the regularity criteria for the 3D MHD equations. It is proved that if

3 ( u + b ) L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 ) ,

or

3 ( u b ) L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 ) ,

then the solution actually is smooth. This extends the previous results given by Guo and Gala (Anal. Appl. 10:373-380, 2013), Gala (Math. Methods Appl. Sci. 33:1496-1503, 2010).

MSC: 35B65, 35Q35, 76D03.

##### Keywords:
MHD equations; regularity criteria; regularity of solutions

### 1 Introduction

In this paper, we consider the following three-dimensional (3D) magnetohydrodynamic (MHD) equations:

{ u t + ( u ) u ( b ) b u + π = 0 , b t + ( u ) b ( b ) u b = 0 , u = 0 , b = 0 , u ( 0 ) = u 0 , b ( 0 ) = b 0 , (1)

where u = ( u 1 , u 2 , u 3 ) is the fluid velocity field, b = ( b 1 , b 2 , b 3 ) is the magnetic field, π is a scalar pressure, and ( u 0 , b 0 ) are the prescribed initial data satisfying u 0 = b 0 = 0 in the distributional sense. Physically, (1) governs the dynamics of the velocity and magnetic fields in electrically conducting fluids such as plasmas, liquid metals, and salt water. Moreover, (1)1 reflects the conservation of momentum, (1)2 is the induction equation, and (1)3 specifies the conservation of mass.

Besides its physical applications, MHD system (1) is also mathematically significant. Duvaut and Lions [1] constructed a global weak solution to (1) for initial data with finite energy. However, the issue of regularity and uniqueness of such a given weak solution remains a challenging open problem. Many sufficient conditions (see, e.g., [2-16] and the references therein) were derived to guarantee the regularity of the weak solution. Among these results, we are interested in regularity criteria involving only partial components of the velocity u, the magnetic field b, the pressure gradient ∇π, etc.

In [8], Jia and Zhou used an intricate decomposition technique and delicate inequalities to obtain the following regularity criterion:

3 π L p ( 0 , T ; L q ( R 3 ) ) , 2 p + 3 q = 2 , 3 q < , (2)

that is, if (2) holds, then the solution of (1) is smooth. Applying a more subtle decomposition technique (see [[13], Remark 3]), Zhang, Li, and Yu [13] could be able to prove smoothness condition (2) in the case 3 / 2 q 3 . Noticeably, Zhang [17] treated the range 3 2 q in a unified approach.

As far as one directional derivative of the velocity field is concerned, Cao and Wu [2] proved the following regularity criterion:

3 u L p ( 0 , T ; L q ( R 3 ) ) , 2 p + 3 q = 1 , 3 q ;

Jia and Zhou [10] showed that if

3 u L p ( 0 , T ; L q ( R 3 ) ) , 2 p + 3 q = 3 4 + 3 2 q , q > 2 ,

then the solution is regular. These results were improved by Zhang [18] to be

3 u L p ( 0 , T ; L q ( R 3 ) ) , 2 p + 3 q = 1 + 1 q , q > 2 .

We would like to give another contribution in this direction and prove that if

3 ( u + b ) L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 ) ,

or

3 ( u b ) L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 ) ,

then the solution actually is smooth. Here, X ˙ r is the multiplier spaces, which is strictly larger than L 3 r ( R 3 ) (see Section 2 for details).

Notice that our result extends

3 u L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 )

which is in [19] for the Navier-Stokes equations. At this moment, for MHD equations (1), we could not be able to add the regularity condition on one directional derivative of the velocity field only, since we need to convert (1) into a symmetric form. We also improve the result

( u + b ) L 2 2 r ( 0 , T ; X ˙ r ) ( 0 r 1 )

of [20] in the sense that we need only one directional derivative of u + b or u b to ensure the smoothness of the solution.

Before stating the precise result, let us recall the weak formulation of MHD equations (1).

Definition 1 Let ( u 0 , b 0 ) L 2 ( R 3 ) with u 0 = b 0 = 0 , and T > 0 . A measurable R 3 -valued pair ( u , b ) is called a weak solution to (1) with initial data ( u 0 , b 0 ) provided that the following three conditions hold:

(1) u L ( 0 , T ; L 2 ( R 3 ) ) L 2 ( 0 , T ; H 1 ( R 3 ) ) , b L ( 0 , T ; L 2 ( R 3 ) ) L 2 ( 0 , T ; H 1 ( R 3 ) ) ;

(2) (1)1,2,3,4 are satisfied in the distributional sense;

(3) the energy inequality

( u , b ) L 2 2 ( t ) + 2 0 t ( u , b ) L 2 2 d s ( u 0 , b 0 ) L 2 2 .

Now, our main results read as follows.

Theorem 2Let ( u 0 , b 0 ) H 1 ( R 3 ) with u 0 = b 0 = 0 , and T > 0 . Assume that ( u , b ) is a given weak solution pair of MHD system (1) with initial data ( u 0 , b 0 ) on ( 0 , T ) . If

3 ( u + b ) L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 ) ,

or

3 ( u b ) L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 ) ,

then the solution is smooth on ( 0 , T ) .

The proof of Theorem 2 will be given in Section 3. In Section 2, we introduce the multiplier spaces X ˙ r and recall their fine properties.

### 2 Preliminaries

In this section, we recall the definition and fine properties of the multiplier spaces X ˙ r (see [21,22] for example).

Definition 3 For 0 r < 3 / 2 , the homogeneous space X ˙ r is defined as the space of f L l o c 2 ( R 3 ) such that

f X ˙ r sup g H ˙ r 1 f g L 2 < ,

where H ˙ r ( R 3 ) is the space of distributions u such that

u H ˙ r ( ) r 2 u L 2 < .

We have the following scaling properties:

f ( + x 0 ) X ˙ r = f X ˙ r , x 0 R 3 , f ( λ ) X ˙ r = 1 λ r f X ˙ r , λ > 0 .

When r = 0 , we have

X ˙ 0 BMO ,

where BMO is the homogeneous space of bounded mean oscillations associated with the semi-norm

f BMO = sup x R 3 , r > 0 1 | B r ( x ) | B r ( x ) | f ( x ) 1 B r ( y ) B r ( y ) f ( z ) d z | d y .

Furthermore, we have the following strict imbeddings:

L 3 r ( R 3 ) ⊆̷ X ˙ r ( R 3 ) ,

which could be justified simply as

f g L 2 f L 3 r g L 6 3 2 r ( Hölder inequality ) C f L 3 r g H ˙ r ( Sobolev imbeddings ) C f L 3 r ( g H ˙ r ( R 3 )  with  g H ˙ r 1 ) .

### 3 Proof of Theorem 2

In this section, we shall prove Theorem 2. As we will see in the proof below, we need only to consider the case that

3 ( u + b ) L 2 1 r ( 0 , T ; X ˙ r ) ( 0 r 1 ) . (3)

First, let us convert (1) into a symmetric form. Writing

ω ± = u ± b ,

we find by adding and subtracting (1)1 with (1)2,

{ ω t + + ( ω ) ω + ω + + π = 0 , ω t + ( ω + ) ω ω + π = 0 , ω + = ω = 0 , ω + ( 0 ) = ω 0 + u 0 + b 0 , ω ( 0 ) = ω 0 u 0 b 0 . (4)

The rest of the proof is organized as follows. In the first step, we dominate 3 ω ± ( t ) L 2 and the time integral of 3 ω ± ( t ) L 2 , while the second step is devoted to controlling ω ± ( t ) L 2 .

Step 1. 3 ω ± L 2 estimates.

Taking the inner product of (4)1 with 3 3 ω + , (4)2 with 3 3 ω in L 2 ( R 3 ) respectively, adding them together, and noticing that ω ± = 0 , we obtain

1 2 d d t 3 ω ± L 2 2 + 3 ω ± L 2 2 = R 3 [ ( ω ) ω + ] 3 3 ω + d x R 3 [ ( ω + ) ω ] 3 3 ω d x = R 3 [ ( 3 ω ) ω + ] 3 ω + d x + R 3 [ ( 3 ω + ) ω ] 3 ω d x I . (5)

We now estimate I as

I 3 ω + 3 ω L 2 ω ± L 2 ( Hölder inequality ) 3 ω + X ˙ r 3 ω H ˙ r ω ± L 2 ( definition of multiplier spaces ) 3 ω + X ˙ r 3 ω L 2 1 r 3 ω L 2 r ω ± L 2 ( interpolation inequalities ) C 3 ω + X ˙ r 2 2 r ω ± L 2 2 2 r 3 ω L 2 2 ( 1 r ) 2 r + 1 2 3 ω ± L 2 2 ( Young inequality ) C [ 3 ω + X ˙ r 2 1 r + ω ± L 2 2 ] [ 3 ω L 2 2 + 1 ] + 1 2 3 ω ± L 2 2 ( Young inequality ) . (6)

Plugging (6) into (5), invoking the Gronwall inequality, and noticing (3), we deduce

sup 0 t T 3 ω ± ( t ) L 2 C < , and 0 T 3 ω ± ( t ) L 2 2 d t C < . (7)

Step 2. ω ± L 2 estimates.

Taking the inner product of (4)1 with ω + , (4)2 with ω in L 2 ( R 3 ) respectively, adding them together, and noticing that ω ± = 0 , we obtain

1 2 d d t ω ± L 2 2 + ω ± L 2 2 = R 3 [ ( ω ) ω + ] ω + d x R 3 [ ( ω + ) ω ] ω d x = R 3 [ ( ω ) ω + ] ω + d x + R 3 [ ( ω + ) ω ] ω d x J . (8)

Utilizing interpolation inequalities and the following multiplicative Gagliardo-Nireberg inequalities

f L 6 C 1 f L 2 1 3 2 f L 2 1 3 3 f L 2 1 3 , (9)

it follows that

J ω ± L 3 3 ω ± L 2 3 2 ω ± L 6 3 2 ( interpolation inequalities ) C ω ± L 2 3 2 ( 1 , 2 ) ω ± L 2 3 ω ± L 2 1 2 ( by  (9) ) C ω ± L 2 3 3 ω ± L 2 + 1 2 ω ± L 2 2 ( Young inequality ) C [ 3 ω ± L 2 2 + ω ± L 2 2 ] ω ± L 2 2 + 1 2 ω ± L 2 2 ( Young inequality ) . (10)

Putting (10) into (8), utilizing the Gronwall inequality, and noticing (7), we deduce

sup 0 t T ω ± ( t ) L 2 C < , and 0 T ω ± ( t ) L 2 2 d t C < ,

and consequently,

u = 1 2 ( ω + + ω ) L ( 0 , T ; L 6 ( R 3 ) ) .

The classical Serrin-type regularity criterion [6] then concludes the proof of Theorem 2.

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

We declare that all authors collaborated and dedicated the same amount of time in order to perform this article.

### Acknowledgements

This work was partially supported by the Youth Natural Science Foundation of Jiangxi Province (20132BAB211007), the Science Foundation of Jiangxi Provincial Department of Education (GJJ13658, GJJ13659) and the National Natural Science Foundation of China (11326138, 11361004). The authors thank the referees for their careful reading and suggestions which led to an improvement of the work.

### References

1. Duvaut, G, Lions, JL: Inéquations en thermoélasticité et magnétohydrodynamique. Arch. Ration. Mech. Anal.. 46, 241–279 (1972)

2. Cao, CS, Wu, JH: Two regularity criteria for the 3D MHD equations. J. Differ. Equ.. 248, 2263–2274 (2010). Publisher Full Text

3. Chen, QL, Miao, CX, Zhang, ZF: On the regularity criterion of weak solutions for the 3D viscous magneto-hydrodynamics equations. Commun. Math. Phys.. 284, 919–930 (2008). Publisher Full Text

4. Duan, HL: On regularity criteria in terms of pressure for the 3D viscous MHD equations. Appl. Anal.. 91, 947–952 (2012). Publisher Full Text

5. Fan, JS, Jiang, S, Nakamura, G, Zhou, Y: Logarithmically improved regularity criteria for the Navier-Stokes and MHD equations. J. Math. Fluid Mech.. 13, 557–571 (2011). Publisher Full Text

6. He, C, Xin, ZP: On the regularity of weak solutions to the magnetohydrodynamic equations. J. Differ. Equ.. 213, 235–254 (2005). Publisher Full Text

7. Ji, E, Lee, J: Some regularity criteria for the 3D incompressible magnetohydrodynamics. J. Math. Anal. Appl.. 369, 317–322 (2010). Publisher Full Text

8. Jia, XJ, Zhou, Y: A new regularity criterion for the 3D incompressible MHD equations in terms of one component of the gradient of pressure. J. Math. Anal. Appl.. 396, 345–350 (2012). Publisher Full Text

9. Jia, XJ, Zhou, Y: Regularity criteria for the 3D MHD equations involving partial components. Nonlinear Anal., Real World Appl.. 13, 410–418 (2012). Publisher Full Text

10. Jia, XJ, Zhou, Y: Regularity criteria for the 3D MHD equations via partial derivatives. Kinet. Relat. Models. 5, 505–516 (2012)

11. Ni, LD, Guo, ZG, Zhou, Y: Some new regularity criteria for the 3D MHD equations. J. Math. Anal. Appl.. 396, 108–118 (2012). Publisher Full Text

12. Zhang, ZJ: Remarks on the regularity criteria for generalized MHD equations. J. Math. Anal. Appl.. 375, 799–802 (2011). Publisher Full Text

13. Zhang, ZJ, Li, P, Yu, GH: Regularity criteria for the 3D MHD equations via one directional derivative of the pressure. J. Math. Anal. Appl.. 401, 66–71 (2013). Publisher Full Text

14. Zhou, Y: Remarks on regularities for the 3D MHD equations. Discrete Contin. Dyn. Syst.. 12, 881–886 (2005)

15. Zhou, Y, Fan, JS: Logarithmically improved regularity criteria for the 3D viscous MHD equations. Forum Math.. 24, 691–708 (2012)

16. Zhou, Y, Gala, S: Regularity criterion for the solutions to the 3D MHD equations in the multiplier space. Z. Angew. Math. Phys.. 61, 193–199 (2011)

17. Zhang, ZJ: Remarks on the regularity criteria for the 3D MHD equations in terms of the gradient of the pressure (submitted)

18. Zhang, ZJ: Mageto-micropolar fluid equations with regularity in one direction (submitted)

19. Guo, ZG, Gala, S: A regularity criterion for the Navier-Stokes equations in terms of one directional derivative of the velocity field. Anal. Appl.. 10, 373–380 (2013)

20. Gala, S: Extension criterion on regularity for weak solutions to the 3D MHD equations. Math. Methods Appl. Sci.. 33, 1496–1503 (2010)

21. Gala, S, Lemarié-Rieusset, PG: Multipliers between Sobolev spaces and fractional differentiation. J. Math. Anal. Appl.. 322, 1030–1054 (2006). Publisher Full Text

22. Lemarié-Rieusset, PG: Recent Developments in the Navier-Stokes Problem, Chapman & Hall, London (2002)