Mathematical Problems in Engineering

Volume 2008, Article ID 394103, 13 pages

http://dx.doi.org/10.1155/2008/394103

## An Approximate Solution for Boundary Value Problems in Structural Engineering and Fluid Mechanics

^{1}Departments of Civil Engineering and Mechanical Engineering, Mazandaran University of Technology, P.O. Box 484, Babol, Iran^{2}Technical and Engineering Faculty, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran

Received 10 January 2008; Accepted 19 May 2008

Academic Editor: David Chelidze

Copyright © 2008 A. Barari et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Variational iteration method (VIM) is applied to solve linear and nonlinear boundary value problems with particular significance in structural engineering and fluid mechanics. These problems are used as mathematical models in viscoelastic and inelastic flows, deformation of beams, and plate deflection theory. Comparison is made between the exact solutions and the results of the variational iteration method (VIM). The results reveal that this method is very effective and simple, and that it yields the exact solutions. It was shown that this method can be used effectively for solving linear and nonlinear boundary value problems.

#### 1. Introduction

This paper discusses the analytical approximate solution for fourth-order equations with nonlinear boundary conditions involving third-order derivatives. The general form of the equation for a fixed positive integer , is a differential equation of order : subject to the boundary conditions where are finite constants.

It is assumed that is sufficiently differentiable and that a unique solution of (1.1) exists. Problems of this kind are commonly encountered in plate-deflection theory and in fluid mechanics for modeling viscoelastic and inelastic flows [1–3]. Usmani [1, 2] discussed sixth order methods for the linear differential equation subject to the boundary conditions , , . The method described in [1] leads to five diagonal linear systems and involves at and , while the method described in [2] leads to nine diagonal linear systems.

Ma and Silva [4] adopted iterative solutions for (1.1)
representing beams on elastic foundations. Referring to the classical beam
theory, they stated that if denotes the configuration of the deformed
beam, then the bending moment satisfies the relation where is the Young modulus of elasticity and
is the inertial moment. Considering the deformation caused by a load they deduced, from a free-body diagram, that and where denotes the shear force. For *u* representing an elastic beam of length which is clamped at its left side and resting on an elastic bearing at its right
side =1, and adding a load along its length to cause deformations
(Figure 1), Ma and Silva [4] arrived at
the following boundary value problem assuming an : the boundary conditions were taken as where and are real functions. The physical
interpretation of the boundary conditions is that is the shear force at and the second condition in (1.5) means that the
vertical force is equal to which denotes a relation, possibly nonlinear,
between the vertical force and the displacement Furthermore, since indicates that there is no bending moment at the beam is resting on the bearing .

Solving (1.3) by means of iterative procedures, Ma and Silva [4] obtained solutions and argued that the accuracy of results depends highly upon the integration method used in the iterative process.

With the rapid development of nonlinear science, many different methods were proposed to solve differential equations, including boundary value problems (BVPS). These two methods are the homotopy perturbation method (HPM) [5–7] and the variational iteration method (VIM) [8–17]. In this paper, it is aimed to apply the variational iteration method proposed by He [14] to different forms of (1.1) subject to boundary conditions of physical significance.

#### 2. Basic Idea of He’s Variational Iteration Method

To clarify the
basic ideas of He's VIM, the following differential equation is considered:
where *L* is a linear
operator, *N* is a nonlinear
operator, and is an
inhomogeneous term. According to VIM, a correction functional could be written as
follows:where is a general Lagrange multiplier which can be
identified optimally via the variational theory. The subscript indicates the th approximation and is considered as a restricted variation, that
is, .

For fourth-order boundary value problem with suitable boundary conditions, Lagrangian multiplier can be identified by substituting the problem into (2.2), upon making it stationary leads to the following: Solving the system of (2.3) yields and the variational iteration formula is obtained in the form

#### 3. The Applications of VIM Method

In this section, the VIM is applied to different forms of the fourth-order boundary value problem introduced in through (1.1).

*Example 3.1. * Consider the following linear
boundary value problem:subject to the boundary conditions
The exact
solution for this problem is According to (2.5),
the following iteration formulation is achieved: Now it is assumed
that an initial approximation has the form where , and are unknown constants to be further determined.

By the iteration formula (3.4), the
following first-order approximation may be written: Incorporating
the boundary conditions (3.2), into ,
the following coefficients can
be obtained: Therefore, the
following first-order approximate solution is derived: Comparison of
the first-order approximate solution with exact solution is tabulated in
Table 1, showing a remarkable agreement.

Similarly, the following second-order approximation is obtained: Therefore, the second-order
approximate solution may be written as
Again, the obtained
solution is of distinguishing accuracy, as indicated in
Table 2 and Figure 2.

*Example 3.2. *
Consider
the following linear boundary value problem: subject to the boundary conditions The exact
solution for this problem is According to (2.5), the iteration
formulation may be written as Now it is assumed that an initial
approximation has the form Where ,
and are unknown constants to be further determined.

By the iteration formula (3.14), the
following first-order approximation is developed: Incorporating the boundary
conditions (3.12), into ,
it can be written as Therefore, the following first-order
approximate solution is obtained: Comparison of
the first-order approximate solution with exact solution is tabulated in
Table 3,
again showing a clear agreement. Even higher accurate solutions could be
obtained without any difficulty.

Similarly, the following second-order
approximation can be written as Incorporating
the boundary conditions, (3.12), into ,
yields The following
second-order approximate solution is then achieved in the following form: The obtained solution is of evident accuracy, as shown in
Table 4 and Figure 3.

*Example 3.3. *
Consider
the following nonlinear boundary value problem: subject to the boundary conditionswhere The exact solution for this problem
is According to (2.5), the iteration
formulation is written as follows: Now it is assumed
that an initial approximation has the form where
and are unknown constants to be further determined.

By the iteration
formula (3.26), the following first-order approximation is obtained: Incorporating the boundary
conditions (3.23), into ,
results in the following values: The following
first-order approximate solution is then achieved: Comparison of
the first-order approximate solution with exact solution is tabulated in
Table 5,
which once again shows an excellent agreement.

Similarly, the
following second-order approximation may be written: Incorporating
the boundary conditions, (3.23), into ,
yields The following second-order
approximate solution is obtained: The obtained solution is once again of remarkable accuracy, as shown in
Table 6 and Figure 4.

#### 4. Conclusion

This study showed that the variational iteration method is remarkably effective for solving boundary value problems. A fourth-order differential equation with particular engineering applications was solved using the VIM in order to prove its effectiveness. Different forms of the equation having boundary conditions of physical significance were considered. Comparison between the approximate and exact solutions showed that one iteration is enough to reach the exact solution. Therefore the VIM is able to solve partial differential equations using a minimum calculation process. This method is a very promoting method, which promises to find wide applications in engineering problems.

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