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# 3-D flow of a compressible viscous micropolar fluid with spherical symmetry: a local existence theorem

Ivan Dražić1* and Nermina Mujaković2

Author Affiliations

1 Faculty of Engineering, University of Rijeka, Rijeka, Croatia

2 Department of Mathematics, University of Rijeka, Rijeka, Croatia

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Boundary Value Problems 2012, 2012:69  doi:10.1186/1687-2770-2012-69

 Received: 26 December 2011 Accepted: 12 June 2012 Published: 2 July 2012

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

We consider nonstationary 3-D flow of a compressible viscous heat-conducting micropolar fluid in the domain to be the subset of bounded with two concentric spheres that present solid thermoinsulated walls. In thermodynamical sense fluid is perfect and polytropic. Assuming that the initial density and temperature are strictly positive we will prove that for smooth enough spherically symmetric initial data there exists a spherically symmetric generalized solution locally in time.

##### Keywords:
micropolar fluid; generalized solution; spherical symmetry; weak and strong convergence

### 1 Introduction

The theory of micropolar fluids is introduced in [1] by Eringen. Various problems with different initial and boundary conditions for incompressible micropolar fluid are presented in [2], but the theory of compressible micropolar fluid is still in the beginning. N. Mujakovic in [3] developed the model for one-dimensional isotropic, viscous, compressible micropolar fluid which is in thermodynamical sense perfect and polytropic. In the same work, the local existence of the solution for homogeneous boundary conditions is proved. N. Mujakovic in [4] and in the references cited therein proved the local and global existence of inhomogeneous boundary conditions for velocity and microrotation as well as stabilization and regularity. In [5] the Cauchy problem for the described problem was also considered. In the last years we find some interesting works with different kind of problems concerning micropolar fluid, e.g., [6,7], but till now the described model of compressible micropolar fluid in three-dimensional case has not been considered.

In this work we consider the three-dimensional model with spherical symmetry. The first article in which the problem of spherical symmetry was described is [8], but for classical fluid. The spherical symmetry for classical fluid is also considered in articles [9-12].

In the setting of the field equations we use the Eulerian description.

In what follows we use the notation:

ρ - mass density

v - velocity

p - pressure

T - stress tensor

- an axial vector with the Cartesian components , where is Levi-Civita alternating tensora

,

ω - microrotation velocity

- skew tensor with Cartesian components

- microinertia density (a positive scalar field)

C - couple stress tensor

θ - absolute temperature

E - internal energy density

q - heat flux density vector

f - body force density

g - body couple density

δ - body heat density

The problem we consider here is based on local forms of the conservation laws for mass, momentum, momentum moment and energy, which are stated respectively as follows:

(1)

(2)

(3)

(4)

where denotes material derivative of a field a:

The scalar product of tensors A and B is defined by

The linear constitutive equations for stress tensor, couple stress tensor and heat flux density vector are respectively in the forms:

(5)

(6)

(7)

where

λ, μ - coefficients of viscosity,

, , , - coefficients of microviscosity,

k - heat conduction coefficient

are constants with the properties

(8)

(9)

Assuming that the fluid is perfect and polytropic, for pressure and internal energy we have the equations

(10)

(11)

where R and are positive constants.

Let , , denote the domain bounded by two concentric spheres with radii a and b. The boundary of the described domain is . We shall consider the problem (1)-(11) in the region (where is arbitrary) with the following initial conditions:

(12)

for and boundary conditions

(13)

for ; the vector ν is an exterior unit normal vector.

For simplicity we also assume that and .

The initial boundary problems for the system (1)-(13) so far have not been considered in three-dimensional case. The same and similar models in one-dimensional case were considered in [3,5,13] and [4]. In [2] the three-dimensional model was considered but for an incompressible micropolar fluid.

In this paper we prove the local existence of generalized spherically symmetric solution to the problem (1)-(13) in the domain Ω, assuming that the initial functions are also spherically symmetric. In the proof we use the Faedo-Galerkin method. We follow some ideas of [14] where this method was applied to a classical fluid (where microrotation is equal to zero) in one-dimensional case as well as the ideas from [3] and [13] where the same result as here was provided for one-dimensional case.

The paper is organized as follows. In the second section, we derive a spherically symmetric form of (1)-(4), introduce Lagrangian description, and present the main result. In the third section, we consider an approximate problem and get an approximate solution for each . In the forth section, we prove uniform a priori estimates for the approximate solutions. The proof of the main result is given in the fifth section.

### 2 Spherically symmetric form and the main result

We first derive the spherically symmetric form of (1)-(7) and (10)-(11). A spherically symmetric solution of (1)-(7) has the form:

(14)

where , , and . We assume that

(15)

where , , and are known real functions defined on , and thus we reduce system (1)-(7) and conditions (10)-(13) to the following equations for , , and of the form:

(16)

(17)

(18)

(19)

with the following initial and boundary conditions

(20)

(21)

To investigate the local existence, it is convenient to transform the system (16)-(19) to that in Lagrangian coordinates. The Eulerian coordinates are connected to the Lagrangian coordinates by the relation

(22)

where is defined by

(23)

We introduce the new function η by

(24)

Note that if for (which is assumed in Theorem 2.1 later), then there exists an inverse function . Let the constant L be defined as

(25)

From (16) we can easily get the equation

(26)

i.e.,

(27)

It is useful to introduce the next coordinate

(28)

and the following functions

Similarly as in [15], for a new coordinate we get

(29)

Taking into account (26) and (24), we obtain that the functions , , , and satisfy the system that we write omitting the primes for simplicity:

(30)

(31)

(32)

(33)

in , where is arbitrary. Now we have the following boundary and initial conditions

(34)

(35)

(36)

for ,

(37)

(38)

for . We also have

(39)

From

putting and integrating over , we get

(40)

where is a radius of the smaller boundary sphere.

We assume the inequalities

(41)

where .

Before stating the main result, we introduce the following definition.

Definition 2.1 A generalized solution of the problem (30)-(38) in the domain is a function

(42)

where

(43)

(44)

that satisfies Equations (30)-(33) a.e. in and conditions (34)-(38) in the sense of traces.

Remark 2.1 From the embedding and interpolation theorems (e.g., [16] and [17]) one can conclude that from (43) and (44) it follows:

(45)

(46)

(47)

It is easy to check that the solution (42) with properties (43)-(44) satisfies the condition for a strong solution of the described problem.

The aim of this paper is to prove the following statements.

Theorem 2.1Let the functions

(48)

(49)

satisfy conditions (41). Then there exists, , such that the problem (30)-(38) has a generalized solution in, having the property

(50)

For the functionr, it holds

(51)

(52)

Remark 2.2 Notice that the function introduced by (40) belongs to . Because of the embedding we can conclude that there exists such that

(53)

From (40) and (41) we get

(54)

(55)

where and .

The proof of Theorem 2.1 is essentially based on a careful examination of a priori estimates and a limit procedure. We first study, for each , an approximate problem and derive the a priori estimates for approximate solutions independent of n by utilizing a technique of Kazhikov [14,18] and Mujakovic [3,13] for one-dimensional case. Using the obtained a priori estimates and results of weak compactness, we extract the subsequence of approximate solutions, which, when n tends to infinity, has limit in the same weak sense on for sufficiently small , . Finally, we show that this limit is the solution to our problem.

### 3 Approximate solutions

We shall find a local generalized solution to the problem (30)-(38) as a limit of approximate solutions

(56)

obtained in what follows. First, we introduce the approximations and of the functions v and r by

(57)

(58)

where is defined by (40) and , are unknown smooth functions defined on an interval , .

Then, we can write the solution to the problem

(59)

in the similar way as in [3] and [13] in the form

(60)

Since and are sufficiently smooth functions, we can conclude that the function is continuous on the rectangle with the property . Because of aforementioned, we can conclude that there exists such , that

(61)

We also introduce the approximations and of the functions ω and θ respectively by

(62)

(63)

where and are again unknown smooth functions defined on an interval , .

Evidently, the boundary conditions

(64)

for are satisfied.

According to the Faedo-Galerkin method, we take the following approximate conditions:

(65)

(66)

(67)

for , .

Let , , and be the Fourier coefficients of the functions , , and respectively:

Let , and be

(68)

(69)

(70)

We take the initial conditions for , and in the form

(71)

(72)

(73)

Let , and be

(74)

(75)

(76)

then we have

(77)

(78)

where and are known functions. Taking into account (57), (62), (63), (74)-(78), from (65)-(67) we obtain for , the following Cauchy problem:

(79)

(80)

(81)

(82)

(83)

(84)

(85)

(86)

Here we have , for and

(87)

(88)

(89)

Notice that the functions on the right-hand side of (79)-(84) satisfy the conditions of the Cauchy-Picard theorem [19,20] and we can easily conclude that the following statements are valid.

Lemma 3.1For eachthere exists such, that the Cauchy problem (79)-(86) has a unique solution defined on. The functions, anddefined by the formulas (57), (62) and (63) belong to the class, and satisfy conditions (71)-(73).

From (77) and (78) we can also easily conclude that

(90)

(91)

Lemma 3.2There exists, , such that the functions, andsatisfy the conditions

(92)

(93)

(94)

on. The constantsm, a, , Mandare introduced by (40), (41), (53) and (55).

Proof The statements follow from (90)-(91), (41), (53) and (55). □

### 4 A priori estimates

Our purpose is to find , such that for each there exists a solution to the problem (79)-(86), defined on . It will be sufficient to find uniform (in ) a priori estimates for the solution defined through Lemmas 3.1 and 3.2.

In what follows we denote by or () a generic constant, not depending on , which may have different values at different places.

We also use the notation

Some of our considerations are very similar or identical to that of [3] or [13]. In these cases we omit proofs or details of proofs making references to corresponding pages of the articles [3] or [13].

Lemma 4.1Forit holds

(95)

Proof From (58) follows

and using Remark 2.2 we get (95) immediately. □

Lemma 4.2For, the following inequality holds:

(96)

Proof Multiplying (66) by and summing over , after integration by parts, we obtain

Integrating over , , and taking into account (72), we obtain

Using (92), we get (96). □

In what follows, we use the inequalities

(97)

(for a function f vanishing at and and with the first derivative vanishing at some point ) that satisfy the functions , and .

Lemma 4.3For, the following inequality holds:

(98)

Proof Multiplying (65) by and summing over , after integration by parts and adding to (67) for , we have

Integrating over , and using (92) we get

Taking into account (96), (71), (73), and (97) we obtain (98). □

Lemma 4.4 ([3], Lemma 5.3)

For, the following inequality holds:

(99)

Lemma 4.5For, the following inequality holds:

(100)

Proof Taking the derivative of the function with respect to x and using the estimates (92)-(94), we obtain

With the help of (97) applied to the function , the Hoelder and Young inequalities as well as (95), we get (100). □

Lemma 4.6Forit holds

(101)

where

Proof As in [3] Lemma 5.5, [14] pp.63-66 and in [13] Lemma 5.6, multiplying (65), (66) and (67) respectively by , and and taking into account (57), (62) and (63), after summation over and addition of the obtained equations, we get

(102)

where

Taking into account (92)-(94) and (95)-(100), we estimate the terms on the right-hand side of (102). For instance,

Applying the Young inequality, we get

where is arbitrary. In the analogous way, we obtain the following inequalities:

Using these inequalities with sufficiently small ε and estimates (92)-(94), from (102) we get (101). □

Lemma 4.7There exists, () such that for eachthe Cauchy problem (79)-(86) has a unique solution defined on. Moreover, the functions, , , andsatisfy the inequalities

(103)

(104)

(105)

(106)

(107)

(a, , mandMare defined by (41) and (53)-(55)).

Proof To get the estimate (103) we use an approach similar to that in [3] (Lemma 5.6) and [14] (pp.64-67). First, we introduce the function

(108)

Using Lemma 4.6, we find that the function satisfies the differential inequality

(109)

There exists a constant such that

and we can conclude that

(110)

We compare the solution of the problem (109)-(110) with the solution of the Cauchy problem

(111)

(112)

Let , be an existence interval of the solution of the problem (111)-(112). From (109)-(112) we conclude that

(113)

Let be such that . From (113) and (108) we obtain

(114)

and from (101) it follows

(115)

Integrating (101) over , and using estimates (110) and (115), we immediately get (103).

Now, using the inequalities (97) for the function v, we easily get

(116)

Using (116), we derive the following estimates:

(117)

(118)

where C and B are from (103). Assuming that

and using (117) and (118) from (58) and (60), we get (104)-(105).

Because of (57), (62) and (63), from (103) and (98), we easily get that for hold

(119)

(120)

and we can conclude that the solution of the problem (79)-(86) is defined on . □

From (119) and (120), we can easily conclude that

(121)

and from (95), (100) and (99) it follows

(122)

Lemma 4.8Letbe defined by Lemma 4.7. Then for eachthe inequalities

(123)

(124)

(125)

hold true.

Proof Multiplying (65) by , summing over and using (104)-(105), we obtain

(126)

Using (121), (122), (103), (116) and applying the Young inequality, we get

(127)

Taking into account (103) for sufficiently small from (127), we obtain

(128)

In the same way, from (66) and (67) we obtain the estimates for and respectively. The estimates (124) and (125) follow directly from (59) and (58). □

From Lemmas 4.7 and 4.8 we easily derive the following statements.

Proposition 4.1Letbe defined by Lemma 4.7. Then for the sequencethe following properties are satisfied:

(i) is bounded in, and;

(ii) is bounded in;

(iii) is bounded in, and;

(iv) , , are bounded in, and.

### 5 The proof of Theorem 2.1

In the proofs we use some well-known facts of functional analysis (e.g., [21]). Let be defined by Lemma 4.7. Theorem 2.1 is a consequence of the following lemmas.

Lemma 5.1There exists a function

(129)

and the subsequence of (for simplicity reasons denoted again as) such that

(130)

(131)

(132)

(133)

The functionrsatisfies the conditions

(134)

(135)

whereis defined by (40).

Proof The conclusions (130) and (131) follow immediately from Proposition 4.1. Let , belong to . Then we have

Using (104), (58), (116), (103) and (107), we obtain

(136)

(137)

(138)

(139)

and we can conclude that the sequences and satisfy the conditions of Arzelà-Ascoli theorem. Applying that theorem, we get the strong convergence in (132) and (133). Because of (132) and (104) we have

(140)

for each and sufficiently big . From (140) we can easily conclude that (134) is satisfied. From (132) it follows

(141)

and because of that we have (135). □

Lemma 5.2There exists a function

(142)

and the subsequence of (denoted again as) such that

(143)

(144)

(145)

The functionρsatisfies the conditions

(146)

(147)

Proof Taking into account Proposition 4.1, estimates (103)-(106) and the Arzelà-Ascoli theorem, we prove in the same way as in the previous lemma the properties (143)-(147). □

Lemma 5.3There exist functions

and the subsequence of (denoted again as) such that

(148)

(149)

(150)

(151)

The functionsv, ωandθsatisfy the conditions

(152)

(153)

(154)

Proof The conclusions (148)-(151) follow from Proposition 4.1 and embedding properties (see Remark 2.1). For verification of the boundary and initial conditions (152), (153) and (154), we use the Green formula as follows.

Let φ be a function of equal to zero in a neighborhood of , with and . Using the integration by parts we have for and v (e.g.) the following equalities:

(155)

(156)

Passing in (155) and comparing (155) and (156), we obtain

(157)

and conclude

(158)

In the similar way, we get all the remaining properties in (152)-(154). □

Lemma 5.4The functionsr, ρ, v, ω, θ, defined by Lemmas 5.1, 5.2 and 5.3 satisfy the Equations (30)-(33) a.e. in.

Proof Let be subsequence defined by Lemmas 5.1, 5.2 and 5.3. Taking into account (144), (149) and strong convergencies (132), (133), (145) and (151) we get that (30) follows immediately from (59). We can write Equation (65) in the form

(159)

for each , where denotes the space of test functions. Now we consider the convergence of one integrand. For example, we will prove the convergence

(160)

when . Taking into account

(161)

and using already mentioned convergences, we can easily conclude that (160) is satisfied. In the same way, we can derive the convergences of other integrals in (159). Analogously, we get that (32) and (33) follow from (66) and (67). □

Remark 5.1 Taking into account (150) and (58), we can easily prove that the function r defined by Lemma 5.1 has the form

(162)

where v is from Lemma 5.3.

Lemma 5.5There exists, such that the functionθdefined by Lemma 5.3 satisfies the condition

(163)

Proof Because of the inclusion (see Remark 2.1), in the same way as in [3], we conclude that for each there exists , , such that for holds

The conclusions of Theorem 2.1 are an immediate consequence of the above lemmas. □

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

The paper is the result of joint work of both authors who contributed equally to the final version of the paper. All authors read and approved the final manuscript.

### Acknowledgement

The paper was made with financial support of the scientific project Mathematical analysis of composite and thin structures (037-0693014-2765), Ministry of Science, Education and Sports, Republic of Croatia.

### End notes

1. We assume Einstein notation for summation.

### References

1. Eringen, CA: Simple microfluids. Int. J. Eng. Sci.. 2(2), 205–217 (1964). Publisher Full Text

2. Lukaszewicz, G: Micropolar Fluids: Theory and Applications, Birkhäuser, Boston (1999)

3. Mujakovic, N: One-dimensional flow of a compressible viscous micropolar fluid: a local existence theorem. Glas. Mat.. 33, 71–91 (1998)

4. Mujakovic, N: Nonhomogeneous boundary value problem for one-dimensional compressible viscous micropolar fluid model: regularity of the solution. Bound. Value Probl.. 2008, Article ID 189748 (2008)

5. Mujakovic, N: One-dimensional compressible viscous micropolar fluid model: stabilization of the solution for the Cauchy problem. Bound. Value Probl.. 2010, Article ID 796065 (2010)

6. Easwaran, C, Majumdar, S: A uniqueness theorem for compressible micropolar flows. Acta Mech.. 68, 185–191 (1987). Publisher Full Text

7. Chen, M: Blowup criterion for viscous, compressible micropolar fluids with vacuum. Nonlinear Anal., Real World Appl.. 13(2), 850–859 (2012). Publisher Full Text

8. Hoff, D: Spherically symmetric solutions of the Navier-Stokes equations for compressible, isothermal flow with large, discontinuous initial data. Indiana Univ. Math. J.. 41, 1225–1302 (1992). Publisher Full Text

9. Jiang, S: Global spherically symmetric solutions to the equations of a viscous polytropic ideal gas in an exterior domain. Commun. Math. Phys.. 178, 339–374 (1996). Publisher Full Text

10. Jiang, S, Zhang, P: On spherically symmetric solutions of the compressible isentropic Navier-Stokes equations. Commun. Math. Phys.. 215, 559–581 (2001). Publisher Full Text

11. Fujita-Yashima, H, Benabidallah, R: Equation à symétrie sphérique d’un gaz visqueux et calorifère avec la surface libre. Ann. Mat. Pura Appl.. 168, 75–117 (1995). Publisher Full Text

12. Yanagi, S: Asymptotic stability of the spherically symmetric solutions for a viscous polytropic gas in a field of external forces. Transp. Theory Stat. Phys.. 29(3-5), 333–353 (2000). Publisher Full Text

13. Mujakovic, N: Non-homogeneous boundary value problem for one-dimensional compressible viscous micropolar fluid model: a local existence theorem. Ann. Univ. Ferrara. 53, 361–379 (2007). Publisher Full Text

14. Antontsev, SN, Kazhikhov, AV, Monakhov, VN: Boundary Value Problems in Mechanics of Nonhomogeneous Fluids, North-Holland, Amsterdam (1990)

15. Weigant, V: Example of non-existence in the large for the problem of the existence of solutions of Navier-Stokes equations for compressible viscous barotropic fluids. Dokl. Akad. Nauk SSSR. 339, 155–156 (1994)

16. Lions, JL, Dautray, R: Functional and Variational Methods, Springer, Berlin (2000)

17. Lions, JL, Dautray, R: Evolution Problems I, Springer, Berlin (2000)

18. Kazhikhov, AV: To a theory of boundary value problems for equation of one-dimensional non-stationary motion of viscous heat-conductive gases. Bound. Value Probl. Hydrodyn. Equ.. 50, 327–362 (1981)

19. Arnold, VI: Ordinary Differential Equations, MIT Press, Cambridge (1978)

20. Petrowski, IG: Vorlesungen über die Theorie der gewöhnlichen Differentialgleichungen, Teubner, Leipzig (1954).

21. Brezis, H: Analyse fonctionnelle: théorie et applications, Masson, Paris (1983).