# Existence of Weak Solutions for a Nonlinear Elliptic System

- Ming Fang
^{1}Email author and - Robert P Gilbert
^{2}

**2009**:708389

https://doi.org/10.1155/2009/708389

© M. Fang and R.P. Gilbert 2009

**Received: **3 April 2009

**Accepted: **31 July 2009

**Published: **26 August 2009

## Abstract

## Keywords

## 1. Introduction

Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. When the material is in contact with the mold wall surface, one has three choices: (i) no slip (which implies that the material sticks to the surface) (ii) partial slip, and (iii) complete slip [1–5]. Navier [6] in 1827 first proposed a partial slip condition for rough surfaces, relating the tangential velocity to the local tangential shear stress

where indicates the amount of slip. When , (1.1) reduces to the no-slip boundary condition. A nonzero implies partial slip. As , the solid surface tends to full slip.

There is a full description of the injection molding process in [3] and in our paper [7]. The formulation of this process as an elliptic system is given here in after.

Problem 1.

Here we assume that is a bounded domain in with a boundary. We assume also that , , , , and are given functions, while is a given positive constant related to the power law index; is the pressure of the flow, and is the temperature. The leading order term of the PDE (1.3) is derived from a nonlinear slip condition of Navier type. Similar derivations based on the Navier slip condition occur elsewhere, for example, [8, 9], [10, equation ( )] .

The mathematical model for this system was established in [7]. Some related papers, both rigorous and formal, are [3, 11–13]. In [11, 13], existence results in no-slip surface, , are obtained, while in [3, 7], Navier's slip conditions, and , are investigated, and numerical, existence, uniqueness, and regularity results are given. Although the physical models are two dimensional, we shall carry out our proofs in the case of dimension.

In Section 2, we introduce some notations and lemmas needed in later sections. In Section 3, we investigate the existence, uniqueness, stability, and continuity of solution to the nonlinear equation (1.3). In Section 4, we study the existence of weak solutions to Problem 1.

Using Rothe's method of time discretization and an existence result for Problem 1, one can establish existence of week solutions to the following *time-dependent* problem.

Problem 2.

The proof is only a slight modification of the proofs given in [11, 13] and is omitted here.

## 2. Notations and Preliminaries

### 2.1. Notations

In this paper, for let and denote the usual Sobolev space equipped with the standard norm. Let

where . The conjugate exponent of is

We assume that the boundary values and for Problem 1 can be extended to functions defined on such that

We further assume that there exist positive numbers and such that

Finally, we assume that for , a.e. in indicates

For the convenience of exposition, we assume that

Next, we recall some previous results which will be needed in the rest of the paper.

### 2.2. Preliminaries

An important inequality (e.g., see [11, page 550] ) in the study of -Laplacian is as follows:

where and are certain constants.

To establish coercivity condition, we will use the following inequality:

Using the Sobolev Embedding Theorem and Hölder's Inequality, we can derive the following results (for more details, see [11, Lemma 3.4] and [13, Lemma 4.2]).

Lemma 2.1.

The following statements hold

The existence proof will use the following general result of monotone operators [14, Corollary III.1.8, page 87] and [15, Proposition 17.2].

Proposition 2.2.

The uniqueness proof is based on a supersolution argument (similar definition can be found in [15, Chapter 3]).

Definition 2.3.

## 3. A Dirichlet Boundary Value Problem

We study the following Dirichlet boundary value problem:

Definition 3.1.

Theorem 3.2.

Assume that conditions (2.1)–(2.6) are satisfied. Then there exists a unique weak solution to the Dirichlet boundary value problem (3.1) in the sense of Definition 3.1. In addition, the solution satisfies the following properties.

where is a constant independent of and ;

The idea behind the existence proof is related to [15, 16]. We will first consider the following Obstacle Problem.

Problem 3.

Lemma 3.3.

If is nonempty, then there is a unique solution p to the Problem 3 in .

Proof of Lemma 3.3.

Our proof will use Proposition 2.2.

It follows from the proof in [15, Proposition 17.2] that is a closed convex set.

Here we used Assumption (2.6), that is, . Therefore we have whenever . Moreover, it follows from inequality (2.7) that is monotone.

Inequality (2.8) is used to arrive at the last step. This implies that is coercive on .

weakly in . Hence is weakly continuous on . We may apply Proposition 2.2 to obtain the existence of .

Our uniqueness proof is inspired by [15, Lemmas , , and Theorem ]. Since does not satisfy condition (3.4) of operator in [15], we need to prove the following lemma, which is equivalent to [15, Lemma 3.11]. Then uniqueness can follow immediately from [15, Lemma 3.22].

Lemma 3.4.

Proof.

and the lemma follows.

Similar to [15, Corollary 17.3, page 335], one can also obtain the following Corollary.

Corollary 3.5.

Let be bounded and . There is a weak solution to (3.1) in the sense of Definition 3.1.

Proof of Theorem 3.2.

for all . If we take in above equation, from inequality (2.7), we have the following.

is applied to the last inequality.

Poincaré's inequality implies that a.e. We complete the uniqueness proof.

and (3.3) follows immediately from (2.3) and (2.6).

Denote the right-hand side by . Similar to arguments in the uniqueness proof, we arrive at the folloing:

Theorem 3.2 is proved.

## 4. Nonlinear Elliptic Dirichlet System

Definition 4.1.

Theorem 4.2.

Assume that (2.1)–(2.6) hold. Then there exists a weak solution to Problem 1 in the sense of Definition 4.1.

We shall bound the critical growth, , on the right-hand side of (4.2).

Lemma 4.3.

Proof.

After some straightforward computations this yields exactly (4.5).

According to Sobolev's imbedding theorems, the integrability of depends on . We estimate II in three different cases.

The estimate of the first term used Hölder inequality and Sobolev's imbedding theorems. The argument of the second estimate is similar to that of I.

Recall that . Similar to II, we estimate IV in three different cases.

Proof of Theorem 4.2.

We notice that (4.2) is the same as [11, equation ( )]. Therefore, arguments after [11, equation ( )] can be used to complete the proof of Theorem 4.2.

## Declarations

### Acknowledgments

The project is partially supported by NSF/STARS Grant (NSF-0207971) and Research Initiation Awards at the Norfolk State University. The second author's work has been supported in part by NSF Grants OISE-0438765 and DMS-0920850. The project is also partially supported by a grant at Fudan University.

## Authors’ Affiliations

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