1 Introduction and main results
In 1965, Eringen [Reference Eringen11] first introduced the micropolar equations in order to model micropolar fluids. Micropolar fluids are fluids with microstructure. They belong to a class of fluids with nonsymmetric stress tensor (called polar fluids) and include, as a special case, the classical fluids modeled by the Navier–Stokes equations (see, e.g., [Reference Cowin4, Reference Erdogan10, Reference Eringen12, Reference Lukaszewicz21]). The system of the micropolar equations is a significant generalization of the Navier–Stokes equations covering many more phenomena such as fluids consisting of particles suspended in a viscous medium (see, e.g., [Reference Lukaszewicz21, Reference Popel, Regirer and Usick23, Reference Stokes24]). The micropolar equations have been extensively studied and applied by many engineers and physicists.
The 3D micropolar equations can be stated as
Here, $\mathbf {u}=\mathbf {u}(x,t)\in \mathbb {R}^3$ denotes the fluid velocity, $\mathbf {w}=\mathbf {w}(x,t)\in \mathbb {R}^3$ the field of microrotation representing the angular velocity of the rotation of the fluid particles, $\pi (x,t)$ the scalar pressure, and the positive parameter $\nu $ denotes the kinematic viscosity, $\kappa $ the microrotation viscosity, and $\gamma , \mu $ the angular viscosities.
The micropolar equations are not only important in physics, but also mathematically significant. The well-posedness problem on the micropolar and closely related equations, such as the magneto-micropolar equations, have been extensively investigated (see, e.g., [Reference Chen and Miao2, Reference Dong and Chen6, Reference Ferreira and Precioso13, Reference Galdi and Rionero15, Reference Lukaszewicz19–Reference Lukaszewicz21, Reference Yamaguchi29, Reference Yamazaki30, Reference Yuan33, Reference Yuan34]). For the initial boundary-value problem, Galdi and Rionero [Reference Galdi and Rionero15] obtained the weak solution. Lukaszewicz [Reference Lukaszewicz19] established the global existence of weak solutions with sufficiently regular initial data. The existence and uniqueness of strong solutions to the micropolar equations either local for large data or global for small data are considered in [Reference Chen and Miao2, Reference Ferreira and Villamizar-Roa14, Reference Lukaszewicz20, Reference Yamaguchi29] and the references therein. However, whether or not the smooth solutions of micropolar equations (1.1) can develop finite-time singularities remains open. Generally speaking, the global regularity problem for the micropolar equations is easier than that for the corresponding incompressible magnetohydrodynamic equations and harder than that for the corresponding incompressible Boussinesq equations. The global existence of weak solutions and strong solutions with initial data small for 3D micropolar equations were obtained in [Reference Galdi and Rionero15, Reference Lukaszewicz21].
In the 2D case, the global well-posedness problem on the 2D micropolar equations with full dissipation can be obtained similarly as that for the 2D Navier–Stokes equations (see, e.g., [Reference Constantin and Foias3, Reference Doering and Gibbon5, Reference Dong, Li and Wu7, Reference Majda and Bertozzi22, Reference Temam26]). Recently, a lot of works are focused on the 2D micropolar equations with partial dissipation (see, e.g., [Reference Dong, Wu, Xu and Ye8, Reference Dong and Zhang9, Reference Xue28]). We will apologize for not addressing exhaustive reference in this paper.
When $\mathbf {w}=0$ and $\kappa =0$ , the system (1.1) is reduced to the 3D incompressible Navier–Stokes equations.
Whether or not the classical solutions of the 3D incompressible Navier–Stokes equations (1.2) can develop finite-time singularities remains an outstanding open problem. The Millennium prize problem is supercritical in the sense that the standard Laplacian dissipation in (1.2) may not provide sufficient regularization. Some works (see, e.g., [Reference Katz and Pavlovic16, Reference Lions18]) proved that the following generalized Navier–Stokes equations
has a unique global-in-time solution with $\alpha \geq \frac 54$ and any smooth initial data $\mathbf {u}_0$ which has finite energy. The following reference [Reference Wu27] is also relevant on the generalized Navier–Stokes equations. It gives a very simple proof on the global well-posedness for $\alpha \geq \frac 54$ . Here, the fractional Laplacian operator $(-\Delta )^\alpha $ is defined via the Fourier transform
However, some scholars devoted to consider whether or not the global existence and regularity can be constructed for any $\alpha <\frac 54$ . Tao [Reference Tao25] obtained the global regularity for the system which just replace the operator $(-\Delta )^\alpha $ by $\frac {(-\Delta )^{\frac 54}}{\sqrt {\log (2-\Delta )}}$ in (1.3). Replacing the operator $\sqrt {\log (2-\Delta )}$ by $\log (2-\Delta )$ , Barbato, Morandin, and Romito [Reference Barbato, Morandin and Romito1] improved those result. All of these results imply that it is extremely difficult to reduce $\alpha $ lower than $\frac 54$ . For the system (1.3), $\alpha =\frac 54$ may be thought as the critical index of the natural energy functional. More precisely, if we assume
and inserting $\mathbf {u}_\lambda (x,t)=\lambda ^{2\alpha -1}\mathbf {u}(\lambda x,\ \lambda ^{2\alpha }t)$ into the above equation to obtain
and the natural energy functional is invariant just when $\alpha =\frac 54$ .
Very recently, Yang, Jiu, and Wu [Reference Yang, Jiu and Wu31] studied the global regularity problem on 3D Navier–Stokes equations with partial hyperdissipation. They obtained the global existence and uniqueness of strong solutions.
In this paper, we consider the 3D incompressible micropolar equations with hyperdissipation as follows:
Here, $\mathbf {u}=(u_1,\ u_2,\ u_3)$ denotes the velocity field and $\mathbf {w}=(\text {w}_1,\ \text {w}_2,\ \text {w}_3)$ the microrotation field. $\varepsilon _{ijk}, (i,j,k)\in \{1,2,3 \}$ is Levi-Civita alternating tensor defined as follows:
Here , $\Lambda _k^\alpha $ with $\alpha>0$ and $k=1,2,3$ denote the directional fractional operators defined via the Fourier transform
where $\xi =(\xi _1,\ \xi _2,\ \xi _3)$ and $\Lambda =(-\Delta )^{\frac 12}$ denotes the Zygmund operator.
The main results of this paper are stated as follows.
Theorem 1.1 Assume $(\mathbf {u}_0,\ \mathbf {w}_0)\in H^1(\mathbb {R}^3)$ . Then, system ( 1.4 ) has a global strong solution $(\mathbf {u},\ \mathbf {w})$ satisfying
The bound of $(\mathbf {u},\ \mathbf {w})$ in ( 1.6 ) is uniform in time.
Remark 1.1 Due to the symmetric, one can easily check the similar results as Theorem 1.1 holds for the cases that if $(u_1,\ \text {w}_1)$ are only lack of the hyperdissipation in the $x_2$ direction, $(u_2,\ \text {w}_2)$ are only lack of the hyperdissipation in the $x_3$ direction and $(u_3,\ \text {w}_3)$ are only lack of the hyperdissipation in the $x_1$ direction or $(u_1,\ \text {w}_1)$ are only lack of the hyperdissipation in the $x_3$ direction, $(u_2,\ \text {w}_2)$ are only lack of the hyperdissipation in the $x_1$ direction and $(u_3,\ \text {w}_3)$ are only lack of the hyperdissipation in the $x_2$ direction.
The rest of this paper is arranged as follows: Some notation and preliminaries will be given in Section 2. In Section 3, we will prove our main results. The proof of Theorem 1.1 will be divided into three stages. First, we will show the $L^2$ -bound of $(\mathbf {u},\ \mathbf {w})$ . Second, we will obtain the $L^2$ -bound of $(\nabla \mathbf {u},\ \nabla \mathbf {w})$ , and then we establish the global a priori bound for $(\mathbf {u},\ \mathbf {w})$ in $H^1$ . This section is the main parts of the proof of Theorem 1.1. There are a lot of triple product terms bounded by using divergence-free condition, Sobolev’s inequalities, Minkowski’s inequality, and so forth. Finally, we will prove the uniqueness.
2 Notation and preliminaries
For simplicity, some notations will be introduced before we prove our main results, which are used throughout this paper. We denote
and
We denote the one-dimensional $L^2$ -norm with respect to $x_i$ by $\|f\|_{L_{x_i}^{2}} (i=1,2,3)$ and $\|f\|_{L_{x_ix_j}^{2}} (i,j\in \{1,2,3\})$ denote the two-dimensional $L^2$ -norm with respect to $x_i$ and $x_j$ . In addition, we denote
The following lemma is Minkowski’s inequality (see, e.g., [Reference Lieb and Loss17]), which will be useful.
Lemma 2.1 Assume that $f=f(x,y)$ with $(x,y)\in (\mathbb {R}^m\times \mathbb {R}^n)$ is a measurable function. Let $1\leq q\leq p\leq \infty $ . Then,
The next lemma is the Sobolev embedding inequality, which will be used frequently in this paper (see [Reference Yang, Jiu and Wu32]).
Lemma 2.2 Assume that $2\leq p\leq \infty $ and $s>d\left(\frac 12-\frac 1p\right)$ . Then, there exists a constant $C=C(d, p, s)$ such that, for any d-dimensional functions $f\in H^s(\mathbb {R}^d)$ ,
In particular, when $p\neq \infty $ , ( 2.2 ) also holds for $s=d\left(\frac 12-\frac 1p\right)$ .
The following is the Hölder-type inequality, which will be useful as well.
Lemma 2.3 Assume that $f_1,\ f_2\geq 0$ and $f_1,\ f_2\in L^p$ . Assume that $s_1,\ s_2\in [0,1]$ and $s_1+s_2=1$ . Assume that $1\leq p\leq \infty $ , then
3 Global regularity for the strong solution to the 3D incompressible micropolar fluid flows
In this section, we will prove Theorem 1.1. Theorem 1.1 is proved through three stages. The first step is to establish the $L^2$ -estimate of $(\mathbf {u},\mathbf {w})$ . Second, we will obtain the $H^1$ -bound for $(\mathbf {u},\mathbf {w})$ . Finally, we will achieve the uniqueness of $(\mathbf {u},\mathbf {w})$ .
Step 1. Global $L^2$ -bound.
Proposition 3.1 Suppose that $(\mathbf {u}_0,\ \mathbf {w}_0)\in H^1(\mathbb {R}^3)$ . Then, system ( 1.4 ) has a global solution $(\mathbf {u},\mathbf {w})$ satisfying
for any $T>0$ , where $C>0$ is a constant, depending on $\|\mathbf {u}_0,\mathbf {w}_0\|_2^2$ .
Proof Multiplying equations (1.4) $_1$ , (1.4) $_2$ , (1.4) $_3$ , (1.4) $_4$ , (1.4) $_5$ , and (1.4) $_6$ by $u_1,\ u_2,\ u_3,\ \text {w}_1,\ \text {w}_2$ , and $\text {w}_3$ , respectively, and taking the $L^2$ -inner product, integrating by parts, using the divergence-free condition $\nabla \cdot \mathbf {u}=0$ and adding them together, yield that
The right-hand side of (3.2) can be estimated as
To begin with the term $I_1$ , applying Lemmas 2.2 and 2.3, we obtain
Similarly,
and
Inserting the above inequalities (3.3)–(3.5) into (3.2), choosing $\varepsilon $ small enough, and integrating from $0$ to $T>0$ yield the desired estimate (3.1).
Step 2. Global $H^1$ -bound.
The goal of this section is to establish the global $L^2$ -estimate of $(\nabla \mathbf {u},\ \nabla \mathbf {w})$ . The process of this section is more complex.
Proposition 3.2 Suppose that $(\mathbf {u}_0,\ \mathbf {w}_0)\in H^1(\mathbb {R}^3)$ . Then, system ( 1.4 ) has a global solution $(\mathbf {u},\mathbf {w})$ satisfying
for any $T>0$ , where $C>0$ is a constant, depending on $\|\mathbf {u}_0,\mathbf {w}_0\|_{H^1(\mathbb {R}^3)}^2$ .
Proof In order to obtain the global $H^1$ -bound, we apply the operator $\nabla $ to system (1.4), and taking the inner product by the resulting equations with $\nabla \mathbf {u}$ and $\nabla \mathbf {w}$ , respectively, integrating by parts, we obtain
Due to the divergence-free condition $\nabla \cdot \mathbf {u}=0$ , integrating by parts, we can rewrite $J_1$ and $J_2$ as follows:
and
To start with the term $J_1$ , similar to [Reference Yang, Jiu and Wu32], we consider the first nine terms in $J_1$ denoted by $J_{11}$ . The remaining terms in $J_1$ can be handled similar to $J_{11}$ .
For the term $J_{111}$ , integrating by parts and applying Lemmas 2.2 and 2.3, using the divergence-free condition $\nabla \cdot \mathbf {u}=0$ , one has
Similarly,
Using Lemmas 2.1 and 2.3, we can bound the term $J_{1121}$ as follows:
and
Combining the above two estimates, we obtain
Next, we will bound the term $J_{113}$ , and integrating by parts, we find that
Similar to $J_{1121}$ , one has
Applying the similar method to $J_{1122}$ , we have
Therefore,
Integrating by parts and invoking the divergence-free condition $\nabla \cdot \mathbf {u}=0$ , one has
To bound the term $J_{116}$ , integrating by parts, one yields that
Applying the similar method to $J_{1131}$ , one can easily find that
Moreover,
Furthermore,
Due to the divergence-free condition $\nabla \cdot \mathbf {u}=0$ , we will rewrite the last three terms as follows:
Integrating by parts, the term $J_{117}$ can be bounded as
Similarly,
Inserting the bounds (3.9)–(3.15) into equation (3.8), we obtain
Next, we will bound the second nine terms of $J_1$ denoted by $J_{12}$ . We write it explicitly,
Most terms in (3.17) can be bounded similarly as $J_{11}$ , we just estimate one of the most difficult terms, such as $J_{125}$ , integrating by parts, yields
Furthermore, one can easily check that
Similarly,
Next, we will bound the term $J_2$ . Similar to $J_1$ , we consider the first nine terms in $J_2$ denoted by $J_{21}$ , firstly.
To start with term $J_{211}$ , we rewrite it as follows:
Applying Lemmas 2.2 and 2.3, we can bound the term $J_{2111}$ as follows:
Integrating by parts, by Lemmas 2.1 and 2.2, $J_{2112}$ can be bounded as
Similarly, one can easily check that $J_{2113}$ satisfies
Inserting the estimates (3.23)–(3.25) into (3.22), we obtain
Next, we will bound $J_{212}$ . Integrating by parts, using Lemmas 2.1 and 2.2, we have
Employing Lemmas 2.1 and 2.2 yields
For the term $J_{2122}$ , we rewrite it as follows:
Now, we will estimate $J_{21221}$ , and one can easily find that
Similar to $J_{2121}$ , one can estimate $J_{21222}$ as follows:
Similarly,
Combining the inequalities (3.28) and (3.30)–(3.32) with (3.27), one has
For the term $J_{213}$ , using the similar method to $J_{212}$ , one can easily check that
Next, we will estimate the term $J_{214}$ . Integrating by parts, one has
First, we can bound the second term $J_{2142}$ as
Now, we return to estimate the term $J_{2141}$ . Similar to $J_{2122}$ , we rewrite it as
Applying Lemmas 2.1–2.3, we have
Similarly,
and
Furthermore, inserting the inequalities (3.38)–(3.40) into (3.37), we obtain
Integrating by parts, the term $J_{215}$ can be estimated as
Similarly,
Employing the similar method to $J_{214}$ , we can bound the term $J_{217}$ as
Furthermore, we will estimate the second term $J_{2172}$ first as follows:
Now, we return to estimate the term $J_{2171}$ . Similar to $J_{2122}$ , we rewrite it as
Applying Lemmas 2.1–2.3, we have
Similarly,
and
Furthermore, inserting the inequalities (3.47)–(3.49) and (3.45) into (3.44), we obtain
Integrating by parts, one can dominate $J_{218}$ as
Finally, we will estimate the term $J_{219}$ , and applying the divergence-free condition $\nabla \cdot \mathbf {u}=0$ and integrating by parts, we obtain
Combining the estimates (3.26), (3.33), (3.34), (3.41)–(3.43), and (3.50)–(3.52), we obtain
The remaining terms in $J_2$ can be handled similar to $J_{21}$ . We omit it here. Next, we will estimate the term $J_3$ :
To start with $J_{31}$ , we write it explicitly:
Similarly, one can easily check that
Furthermore,
Inserting the estimates of $J_{31}$ , $J_{32}$ , and $J_{33}$ into (3.31), we obtain
Combining the estimates (3.16), (3.19), (3.20), (3.53), and (3.55), choosing $\varepsilon $ small enough, we have
Applying Gronwall’s inequality, integrating from $0$ to T, we complete the proof of Proposition 3.2.
Step 3. Uniqueness
This section we will prove the uniqueness.
Proposition 3.3 Assume that the initial $(\mathbf {u}_0, \mathbf {w}_0)$ satisfies the conditions stated in Theorem 1.1. Suppose that $(\mathbf {u}^{(1)},\ \mathbf {w}^{(1)})$ and $(\mathbf {u}^{(2)},\ \mathbf {w}^{(2)})$ are two solutions to the system ( 1.4 ), then $(\mathbf {u}^{(1)},\ \mathbf {w}^{(1)})=(\mathbf {u}^{(2)},\ \mathbf {w}^{(2)})$ .
Proof Denoting by $\pi ^{(1)}$ and $\pi ^{(2)}$ the associated pressures, then the differences
satisfy
Multiplying equations (3.57) $_1$ , (3.57) $_2$ , (3.57) $_3$ , (3.57) $_4$ , (3.57) $_5$ , and (3.57) $_6$ with $\tilde {u}_1$ , $\tilde {u}_2$ , $\tilde {u}_3$ , $\tilde {\text {w}}_1$ , $\tilde {\text {w}}_2$ , and $\tilde {\text {w}}_3$ , respectively, integrating by parts, summing the results together, yields
To start with $K_1$ , we write it explicitly:
Applying the divergence-free condition $\nabla \cdot \mathbf {\tilde {u}}=0$ , by Lemmas 2.1–2.3, we can bound $K_{11}$ as follows:
Using the similar method to $K_{11}$ , the term $K_{12}$ can be estimated as
Similarly,
Furthermore,
For the term $K_{15}$ , applying the similar method to $K_{11}$ , we find that
Similar to $K_{12}$ , we can easily bound the terms $K_{16}$ , $K_{17}$ , and $K_{18}$ as
and
Finally, we will estimate the term $K_{19}$ . Using the similar method to $K_{11}$ , we obtain
Inserting the above bounds (3.37)–(3.45) into equation (3.36) yields
Next, we will estimate $K_2$ , and we write it in terms of components:
To start with $K_{21}$ , we rewrite it as follows:
Using Lemmas 2.1–2.3, we can estimate $K_{211}$ as
Similarly,
and
Combining the estimates of $K_{211}$ – $ K_{213}$ , we obtain
Similar to $K_{12}$ , $K_{22}$ can be bounded as
Furthermore,
The other terms in $K_2$ can be bounded as $K_{21}$ – $ K_{23}$ . We omit it here. Furthermore, one can easily check that
Finally, we will bound the term $K_3$ . Applying the similar methods to (3.3)–(3.5), one can easily check that
Inserting the estimates of (3.69), (3.73), and (3.74) into (3.58), and choosing $\varepsilon $ small enough, we have
Applying Gronwall’s inequality and the previous estimates, we obtain $(\tilde {\mathbf {u}},\ \tilde {\mathbf {w}})\equiv 0$ . Then, we complete the proof of Proposition 3.3. Furthermore, combining Propositions 3.1–3.3, we complete the proof of Theorem 1.1.
Acknowledgment
The author would like to thank the editors for the excellent handling of our manuscript and to express our thanks to the anonymous reviewers for the constructive and valuable suggestions to update our manuscript.