In this paper, we consider the one-dimensional viscous micropolar fluid model in Lagrangian coordinates:
Here is the Lagrangian space variable, the time variable and the primary dependent variable are the specific volume , the velocity , the absolute temperature and the microrotation velocity . The positive constants , and A denote the viscosity coefficient, heat conduction coefficient and microviscosity coefficient, respectively. The pressure p and the internal energy e are given by the state equations
where is the adiabatic exponent, s is the entropy of fluid, R and B are the positive constants. We impose the following initial and far field conditions:
where , , are given constants, and we assume , , as compatibility conditions.
It is known that the large-time behavior of solutions of the Cauchy problem (1) and (2) is closely related to the Riemann problem of the compressible Euler system:
with the Riemann initial data
It is well-known that the above system has three eigenvalues:
which implies that the first and third characteristic fields are genuinely nonlinear and the second field is linearly degenerate. Then it is known that the basic Riemann solutions of the problem (3)-(4) are dilation invariant solutions: shock waves, rarefaction waves, contact discontinuities, and the linear combinations of these basic waves. In particular, the contact discontinuity solution of the Riemann problems (3) and (4) takes the form  :
The viscous contact wave with corresponding to the contact discontinuity with defined in (5) and for the compressible micropolar fluid model (1) becomes smooth and behaviors as a diffusion waves due to the effect of heat conductivity. As in  , we can define the viscous contact wave as follows.
Since the pressure of the profile is expected to be almost constant, we set
Then the leading part of the energy Equation (1)3 is
Using the Equation (7), and (8), we get a nonlinear diffusion equation
which has a unique self-similar solution , because
of   . Furthermore, is a monotone function, increasing if and decreasing if . On the other hand, there exists some positive constant , such that for , satisfies
where and are two positive constants depending only on and . Once is determined, the viscous contact wave profile is defined as follows:
It is straightforward to check that satisfies
which means the nonlinear diffusion wave approximates the contact discontinuity to the Euler system (3) in norm, on any finite time interval as the heat conductivity coefficient tends to zero. And it is easy to check that the viscous contact wave satisfies the system
We are now in a position to state our main results. Let
For interval , we define a function space
We can get main result from  and it is stated as the following theorem.
Theorem 1.1. For any given , suppose that satisfies (6), Let be the viscous contact wave defined in (11) with strength . Then there exist positive constants and , such that if and the initial data satisfies
then the Cauchy problem (1)-(2) admits a unique global solution satisfying and
When the relation (6) fails, the basic theory of hyperbolic systems of conservation laws (for example, see  ) implies that for any given constant state with and , there exists a suitable neighborhood of such that for any , the Riemann problem of the Euler system (3)-(4) has a unique solution. In this paper, we only consider the stability of the superposition of the viscous contact wave and rarefaction waves. In this situation, we assume that
It is well-known  that there exists some suitably small such that for
there exists a positive constant and a unique pair of points and in satisfying
Moreover, the points and belong to the 1-rarefaction wave curve and the 3-rarefaction wave curve , respectively, where
The points and may coincide with and , respectively. The 1-rarefaction wave (respectively the 3-rarefaction wave ) connecting and (respectively and ) is the weak solution of the Riemann problem of the Euler system (3) with the following initial Riemann data
Since the rarefaction waves are not smooth enough solutions, it is convenient to construct smooth approximate ones. Motivated by  , the smooth solutions of Euler system (3), , which approximate are given by
where (respectively ) is the solution of the initial problem for the typical Burgers equation:
with and (respectively and ).
Let be the viscous contact wave constructed in (11) and (9) with replaced by , respectively. We define
Then our main result of this paper is as follows:
Theorem 1.2. For any given , suppose that (16) holds for some small . Let be as in (20) with strength . Then there exist positive constants and , such that if and the initial data satisfies
then the Cauchy problem (1)-(2) admits a unique global solution satisfying and
Now, we briefly recall some related work in this aspect and make some comments on the analysis in this paper. The nonlinear stability of some basic wave patterns has been studied by many authors. The stability toward contact waves for solutions of systems of viscous conservation laws was first studied by Xin  who proved the nonlinear stability of a weak contact discontinuity for the compressible Euler equations with uniform viscosity. Later, Liu and Xin  showed the stability of contact discontinuities for a class of general systems of nonlinear conservation laws with uniform viscosity. And this result was improved by Xin and Zeng in  . The large-time asymptotic nonlinear stability of the supposition of viscous shock waves and contact discontinuities for system of viscous conservation laws with artificial viscosity under small initial perturbations was proved by Zeng  . Some interesting results have been obtained for compressible Navier-Stokes system. The asymptotics toward the rarefaction waves for compressible Navier-Stokes system is established in     . For a free-boundary value problem, the asymptotic stability of a viscous contact wave of the one-dimensional compressible Navier-Stokes system was first proved by an elementary energy method by Huang, Matsumura, and Shi  , where the initial perturbation and the strength of the contact discontinuity are suitably small. The asymptotic stability of the linear combination wave of viscous contact wave and the rarefaction waves for the Cauchy problem of the one-dimensional compressible Navier-Stokes system was obtained by Huang, Li and Matsumura in  , and provided the strength of the combination wave is suitably small. The viscous shock profiles and viscous rarefaction waves have been shown to be asymptotically stable for quite general perturbation for the compressible Navier-Stokes system and more general systems of viscous strictly hyperbolic conservation laws     -  . There are many results have been obtained for the nonlinear stability of some basic wave patterns consisting of viscous shock waves, rarefaction waves, viscous contact discontinuities and their certain linear superpositions with small perturbation. We refer to  -  and the references therein for viscous shock waves,    for rarefaction waves,      for viscous contact discontinuities,     for the composition of a viscous contact wave and rarefaction waves. For the corresponding results with large initial perturbation, see    and the references cited therein.
The compressible micropolar fluid model has become an important area of interest for mathematicians in the last several decades. The model for compressible flow of micropolar fluid in the one-dimensional case was first studied by N. Mujaković. She considered the local-in-time existence and uniqueness  , the global existence  and regularity of solutions  to an initial-boundary value problem with homogeneous boundary conditions of the compressible one-dimensional micropolar fluid system respectively. Other results were proved in    for the corresponding non-homogeneous boundary value problems. Besides, she also analyzed large time behavior of the solutions and the stabilization of solutions to the Cauchy problem  of the one-dimensional model. There are other authors in   showed the nolinear stability of some basic waves (such as rarefaction waves and viscous contact wave etc.). The stability of composite wave for one-dimensional compressible micropolar fluid model without viscosity was studied by Zheng, Chen and Zhang  . For the three-dimensional compressible micropolar fluid model, I. Dražić and N. Mujaković in      studied the local existence, global existence, uniqueness, large time behavior and regularity of spherical symmetry solutions.
In this paper, we shall show the asymptotic stability of combination of contact discontinuity with rarefaction waves for the Cauchy problem of the one-dimensional viscous micropolar fluid model, provided the strength of the combination wave is suitably small. After stating some notations, we will reformulate the original problem and give some preliminary lemmas and a priori estimates of solutions to the Cauchy problem (22) in Section 2. Finally, section 3 completes the proof of Theorem 1.2.
Notation. Throughout this paper, several positive generic constants are denoted by C, c without confusion. For functional spaces, denotes the lth order Sobolev space with its norm
2. Reformation of the Problem and Preliminaries
Noticing that satisfies Euler system (3) and satisfies (1)1 and (8). To make it more convenient to prove Theorem 1.2, in this section, we will reformulate the problem (1), then the system (1)-(2) be rewritten as
We derive an elementary inequality concerning the heat kernel which will play an essential role later. For , we define
It is easy to check that
Then we have
Lemma 2.1 (see  ) For , suppose that satisfies
Then the following estimate holds:
For the proof Lemma 2.1, one refers to  . Next, we summarize some basic properties of the viscous contact wave .
Lemma 2.2 (see  ) Assume that for some positive constant . Then there exists two positive constants and C such that the viscous contact wave satisfies the following estimates:
Lemma 2.2 can be proved directly from Equations (10) and (11), the details are omitted here. The solution of the Cauchy problem (19) has the following properties.
Lemma 2.3 (see  ) For given , and , let Then the problem (19) has a unique smooth global solution in time satisfying the following:
ii) For any , there exists some positive constant such that for and ,
iii) If , for any
iv) If , for any ,
v) For the Riemann solution of the scalar Equation (19)1 with the Riemann initial data
We use Lemma 2.3 to investigate the properties of the smooth rarefaction waves constructed in (20) and the viscous contact wave , we divided the domain into three parts, that is, with
Then, Lemma 2.3 and (10) easily give
Lemma 2.4. (see  ) For any given , assume that satisfies (16) with Then the smooth rarefaction waves constructed in (18) and the viscous contact discontinuity wave satisfy the following:
ii) For any , there exists some positive constant such that for and ,
iii) There exists some positive constant such that for and
we have in that
and in ,
iv) For the rarefaction waves determined by (3) and (17), it holds
Finally, we give a Sobolev inequality without proof.
Lemma 2.5. (see  ) For any , we have
Since the local existence of the solution is well known (for example, see  ), to prove the global existence part of Theorems 1.2, we only have to establish the following a priori estimates.
(A priori estimate) There exist positive constants , and C, such that for and satisfying
it follows the estimate
Once Proposition 2.1 is proved, we can extend the unique local solution which can be obtained as in  , to . Estimate (32) together with the Equation (22) implies that
which as well as (32) and the Sobolev inequality easily leads to the asymptotic behavior of the solutions, that is, (21).
From now on until the end of this paper, we always assume that . Proposition 2.1 is an easy consequence of the following lemmas.
3. Energy Estimates
In this section we will drive some a priori energy estimates for the solutions to the system (1). Since Theorem 1.1 has been proved by Liu and Yin that we can see the details in  , we will give here the proof of Theorem 1.2 for brevity. We first give the following key estimate.
Lemma 3.1. For and satisfying (31) with suitably small , we have for ,
Proof. First, we use Lemma 2.4 to investigate some aspects of F and G.
Since direct calculation yields that
It follows from (20) that
thus, it derives from Lemma 2.4 that
We can treat the other terms on the righthand side of (34) in the same way to obtain
the estimate (10) and Lemma 2.4 imply
The estimates (36)-(40) give
Similar to (36), we have
It follows from (10) and Lemma 2.4 that
Thus, one derives from (34), (42)-(47) that
Now, multiplying (22)1 by , (22)2 by and (22)3 by , (22)4 by , then adding the resulting equations together, and using that
then we have
Notice that and , there exists positive constants and such that
due to Lemma 2.4 and (10). It is easy to compute
After integrating (51) on , we deduce from (30), (41), (48), (22), (56), (28) and (59) that
Lemma 3.1 thus follows directly from (60) and the following Lemma 3.2 by choosing in (61) and suitably small. ,
Lemma 3.2. For and w defined in (25), there exists some positive constant C depending on such that the following estimate holds
Proof. The proof of (61) is divided into the following two parts:
and for any ,
In fact, adding (63) to (62) and taking first then suitably small thus implies (61) easily.
Here we used the same method as in  and combined with  , then, we can complete the proof of Lemma 3.2. We omit the details for simplicity. ,
Lemma 3.3. Suppose that satisfies (31) with suitably small . Then it holds for ,
Proof. We rewrite Equation (22)2 as
where we have used the following simple fact
due to . Multiplying (65) by , using , noticing that
Using (36), we obtain by direct calculation
It follows from Lemma 2.4 that
so, we have
The Cauchy inequality leads to
The estimate (10) and Lemma 2.4 yield that
Lemma 3.3 thus follows directly from Lemma 3.1 and (61), (66)-(72) by first choosing suitably small then suitably small.
Lemma 3.4. Suppose that satisfies (31) with suitably small . Then it holds for ,
Proof. Multiplying (22)2 by , we have
Now, we need to control the term . Using (10), (37), Lemma 2.4 and Lemma 2.5, we obtain by direct calculation
It follows from (10), (36), (75) and Lemma 2.4 that
The estimate (76) yields that
The Cauchy inequality leads to
So, we have
where we have used
due to (77).
Integrating (74) over , using (78), and first choosing suitably small then suitably small, we can obtain Equation (73). This completes the proof of Lemma 3.4. ,
Lemma 3.5. Suppose that satisfies (31) with suitably small . Then it holds for ,
Proof. Multiplying Equation (22)3 by , we obtain
Now, we are devoted to controlling the term . Using (10), (44) and Lemma 2.4, we obtain by direct calculation
it follows from (43), (46) and (81) that
Using Lemma 2.2, Lemma 2.4, (31), the Hölder inequality and the Young inequality, we have
The estimate (82) and (83) yields that
The Cauchy inequality and Lemma 2.5 lead to
Then, by using the same argument as the previous, and integrating (80) over , using (84)-(86) and following Lemma 3.6, and first choosing suitably small then suitably small, one can obtain equation (79). This completes the proof of Lemma 3.5. ,
The next Lemma is devoted to controlling the term .
Lemma 3.6. Suppose that satisfies (31) with suitably small . Then it holds for ,
Proof. Multiplying (22)4 by , we have
We have by Lemma 2.5 (31), the Hölder inequality and the Young inequality that
Integrating (88) over , using (89) and (90), and first choosing suitably small then suitably small, one can obtain Equation (87). This completes the proof of Lemma 3.6. ,
Thus, we finish the proof of Proposition 2.1, and so the proof of Theorem 1.2 is completed.
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