An improved spectral homotopy analysis method for solving boundary layer problems

Boundary layer flow problems have wide applications in fluid mechanics. In this article, we propose an improved spectral-homotopy analysis method (ISHAM) for solving general boundary layer problems. Three boundary layer problems are considered and solved in this study using the novel technique. The first problem considered is the classical two-point nonlinear boundary value Blasius problem which models viscous fluid flow over a semi-infinite flat plate. Although solutions for this problem had been obtained as far back as 1908 by Blasius [1], the problem is still of great interest to many researchers as can be seen from the several recent studies [2–5].

The second problem considered in this article is the third-order nonlinear Falkner-Skan equation. The Falkner-Skan boundary layer equation has been studied by several researchers from as early as 1931 [6]. More recent studies of the solutions of the The Falkner-Skan equation include those of Harries et al. [7], Pade [8] and Pantokratoras [9]. The third problem considered is magnetohy-drodynamic (MHD) boundary layer flow. Such boundary layer problems arise in the study of the flow of electrically conducting fluids such as liquid metal. Owing to its many applications such as power generators, flow meters, and the cooling of reactors, MHD flow has been studied by many researchers, for example [10, 11].

Owing to the nonlinearity of equations that describe most engineering and science phenomena, many authors traditionally resort to numerical methods such as finite difference methods [12], Runge-Kutta methods [13], finite element methods [14] and spectral methods [4] to solve the governing equations. However, in recent years, several analytical or semi-analytical methods have been proposed and used to find solutions to most nonlinear equations. These methods include the Adomian decomposition method [15–17], differential transform method [18], variational iteration method [19], homotopy analysis method (HAM) [20–23], and the spectral-homotopy analysis (SHAM) (see Motsa et al. [24, 25]) which sought to remove some of the perceived limitations of the HAM. More recently, successive linearization method [26–28], has been used successfully to solve nonlinear equations that govern the flow of fluids in bounded domains.

In this article, boundary layer equations are solved using the ISHAM. The ISHAM is a modified version of the SHAM [24, 25]. One strength of the SHAM is that it removes restrictions of the HAM such as the requirement for the solution to conform to the so-called rule of solution expression and the rule of coefficient ergodicity. Also, the SHAM inherits the strengths of the HAM, for example, it does not depend on the existence of a small parameter in the equation to be solved, it avoids discretization, and the solution obtained is in terms of an auxiliary parameter ħ which can conveniently be chosen to determine the convergence rate of the solution.

We consider the general nonlinear third-order boundary value problem

(2.1)

subject to the boundary conditions

(2.2)

where c _i , b _j (i = 1, ..., 4 j = 1, 2, 3) are constants.

Equation 2.1 can be solved easily using methods such as the HAM and the SHAM. In each of these methods, an initial approximation f₀(η) is sought, which satisfies the boundary conditions. The speed of convergence of the method depends on whether f₀(η) is a good approximation of f (η) or not. The approach proposed here seeks to find an optimal initial approximation f₀ that would lead to faster convergence of the method to the true solution. We thus first seek to improve the initial approximation that is used later in the SHAM to solve the governing nonlinear equation.

We assume that the solution f(η) may be expanded as an infinite sum:

(2.3)

where f _i 's are unknown functions whose solutions are obtained using the SHAM at the i th iteration and f _n , (n ≥ 1) are known from previous iterations. The algorithm starts with the initial approximation f₀(η) which is chosen to satisfy the boundary conditions (2.2). An appropriate initial guess is

(2.4)

Substituting (2.3) in the governing equation (2.1-2.2) gives

(2.5)

subject to the boundary conditions

(2.6)

where the coefficient parameters a_k,i-1, (k = 1, ..., 3) and r_i-1are defined as

(2.7)

(2.8)

Starting from the initial approximation (2.4), the subsequent solutions f _i (i ≥ 1) are obtained by recursively solving Equation 2.5 using the SHAM, [24, 25]. To find the solutions of Equation 2.5, we begin by defining the following linear operator:

(2.9)

where q ∈ 0[1] is the embedding parameter, and F _i (η; q) is an unknown function.

The zeroth-order deformation equation is given by

(2.10)

where ħ is the non-zero convergence controlling auxiliary parameter and is a nonlinear operator given by

(2.11)

Differentiating (2.10) m times with respect to q and then setting q = 0, and finally dividing the resulting equations by m! yield the m th-order deformation equations:

(2.12)

subject to the boundary conditions

(2.13)

where

(2.14)

The initial approximation f_i,0that is used in the higher-order equations (2.12) is obtained on solving the linear part of Equation 2.5 which is given by

(2.15)

subject to the boundary conditions:

(2.16)

Since the coefficient parameters and the right-hand side of Equation 2.15 for i = 1, 2, 3, ... are known (from previous iterations), the equation can easily be solved using numerical methods such as finite differences, finite elements, Runge-Kutta-based shooting methods or collocation methods. In this article, Equation 2.15 are solved using the Chebyshev spectral collocation method. The method (see, for example, [29–31]), is based on the Chebyshev polynomials defined on the interval [-1, 1] by

(2.17)

To implement the method, the physical region [0, ∞) is transformed into the region [-1, 1] using the domain truncation technique whereby the problem is solved in the interval [0, L] instead of [0, ∞). This leads to the mapping

(2.18)

where L is the scaling parameter used to invoke the boundary condition at infinity. We use the popular Gauss-Lobatto collocation points [29, 31] to define the Chebyshev nodes in [-1, 1], namely:

(2.19)

where N is the number of collocation points. The variable f_i,0is approximated by the interpolating polynomial in terms of its values at each of the collocation points by employing the truncated Chebyshev series of the form:

(2.20)

where T _k is the k th Chebyshev polynomial. Derivatives of the variables at the collocation points may be represented by

(2.21)

where s is the order of differentiation and , with being the Chebyshev spectral differentiation matrix (see, for example [29, 31]) whose entries are defined as

(2.22)

Substituting Equations 2.20-2.21 in 2.15-2.16 gives

(2.23)

subject to

(2.24)

where

(2.25)

(2.26)

(2.27)

In the above definitions, T stands for transpose and a_k,i-1(k = 1, 2, 3) denotes a diagonal matrix of size (N + 1) × (N + 1). The boundary condition f _i (ξ _N ) = 0 is implemented by deleting last row and last column of A_i-1, and deleting the last rows of F_i,0and R_i-1. The derivative boundary conditions in (2.24) are then imposed on the resulting first row and last row of A_i-1and setting the first and last rows of F_i,0and R_i-1to be zero. The solutions for f_i.0(ξ) are then obtained from soloving

(2.28)

In a similar manner, applying the Chebyshev spectral transformation on the higher order deformation equations (2.12)-(2.13) gives

(2.29)

subject to the boundary conditions

(2.30)

where A_i-1and R_i-1, are as defined in (2.25) and (2.27), respectively, and

(2.31)

(2.32)

To implement the boundary condition f_i,m(ξ _N ) = 0, we delete the last rows of P_i,m-1and R_i-1and delete the last row and the last column of A_i-1in (2.29). The other boundary conditions in (2.30) are imposed on the first and the last rows of the modified A_i-1matrix on the left side of the equal sign in (2.29). The first and the last rows of the modified A_i-1matrix on the right side of the equal sign in (2.29) are then set to be zero. This results in the following recursive formula for m ≥ 1:

(2.33)

where Ã_i-1is the modified matrix A_i-1after incorporating the boundary conditions (2.30). Thus, starting from the initial approximation, which is obtained from (2.28), higher-order approximations f_i,m(ξ) for m ≥ 1, can be obtained through the recursive formula (2.33).

The solutions for f _i are then generated using the solutions for f_{i, m}as follows:

(2.34)

The [i, m] approximate solution for f (η) is then obtained by substituting f _i (obtained from 2.34) in equation 2.3.

In this article, we have proposed an ISHAM for solving general nonlinear differential equations. This novel technique was compared against both numerical approximations and the MATLAB bvp4c routine for solving Falkner-Skan and MHD boundary layer problems. The results demonstrate the relatively more rapid convergence of the ISHAM, and they show that the ISHAM is highly accurate.