Multiple solutions of problems in fluid mechanics by predictor optimal homotopy asymptotic method A. K. Alomari1 , N. Ratib Anakira2,∗, and I. Hashim2,3 1 Department of Mathematics, Faculty of Science, Yarmouk University, 211-63 Irbid, Jordan 2 Centre for Modelling and Data Analysis, School of Mathematical Sciences, Universiti Kebangsaan Malaysia, 43600 Bangi Selangor, Malaysia 3 Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589, Saudi Arabia October 10, 2014 Abstract A new algorithm based on the standard optimal homotopy asymptotic method namely the predictor optimal homotopy asymptotic method (POHAM) is proposed to predict the multiplicity of the solutions of nonlinear differential equations with boundary conditions. This approach is successfully implemented to obtain multiple solutions of a mixed convection flow model in a vertical channel and a nonlinear model arising in heat transfer. A new technique for obtaining the initial guess to accelerate the convergence of the series solution is presented. Keywords: Multiple solutions, Nonlinear differential equations, Optimal homotopy asymptotic method, Predictor optimal homotopy asymptotic method ∗ Corresponding author. Email: alanaghreh [email protected], Tel: 0060189189664 1 1 Introduction The study of the existence of multiple solutions for nonlinear differential equations has drawn a lot of attention in the last few years [1, 2]. Multiple solution for nonlinear boundary value problems via homotopy analysis method was investigated by several papers for example, Liao [3, 4], Li and Liao [5], Xu and Liao [6]. Moreover, Abbasbandy and Shivanian [7] presented a kind of analytical method namely predictor homotopy analysis method (PHAM) to predict the multiplicity of the solutions of nonlinear differential equations with boundary conditions. Also, Alomari et al. [8] used the PHAM to find the multiple solutions of fractional boundary value problem (FBVP). In the last few years, optimal homotopy asymptotic method (OHAM) has been introduced and developed by Marinca and Herisanu [9, 10, 11, 12], and has been applied successfully to many strongly nonlinear problems [13, 14, 15, 16]. Iqbal et al. [17] obtained OHAM solutions to linear and nonlinear Klein-Gordan equations, Anakira et al. [18] used OHAM for solving singular two-point boundary value problems and Alomari et al. [19] employed OHAM to obtain approximate solution of nonlinear system of boundary value problems arising in fluid flow problems. Moreover, Idrees et al. [20, 21] and Mabood et al. [22, 23] have applied OHAM effectively to different higher-order boundary values problems. The main aim of this paper is to present a type of analytical method namely the predictor optimal homotopy asymptotic method (POHAM) to predict the multiplicity of the solutions of the nonlinear differential equations with boundary conditions by using one auxiliary linear operator, one auxiliary function and one initial guess. This paper is composed as follows: In Section 2, the structure of POHAM is formulated for finding multiple solutions of nonlinear differential equations. In Section 3, we present two numerical examples and finally, we give the conclusion of this study in Section 4. 2 The predictor optimal homotopy asymptotic method (POHAM) Consider the following differential equation: L(y(x)) + g(x) + N (y(x)) = 0, x∈Ω (1) with boundary conditions: ( ∂y B y, ∂η ) = 0, 2 x ∈ Γ, (2) where L is the linear operator, N is the linear or nonlinear operator, g(x) is a known function, B is a boundary operator and Γ is the boundary of the domain Ω. The key step of POHAM depends on the fact that the boundary-value problem (1) and (2) should be transcribed to an equivalent problem so that the boundary condition (2) involves an unknown parameter, the so-called prescribed parameter δ, and is decomposed into B ′ ( ∂y y, δ, ∂η ) x ∈ Γ, = 0, y(α) = β, (3) where y(α) = β is the forcing condition that is resulted from the original condition (2) and B ′ is the remaining boundary operator that contains the prescribed parameter δ. As it will be noticed, the parameter δ with the help of convergence-controller parameter Ci ’s will play a significant role to realize the multiplicity of solutions. Now, we construct the POHAM on equations (1) and (3). By using a homotopy map h(v(x, δ; q); q) : Ω×[0, 1] → ℜ which satisfies (1 − q)[L(v(x, δ; q)) − y0 (x; δ)] = H(q)[L(v(x, δ; q)) + g(x) + N (v(x, δ; q))], ) ( ∂v(x, δ; q) ′ = 0, B v(x, δ; q), ∂x (4) where x ∈ ℜ, q ∈ [0, 1] is an embedding parameter, H(q) is a nonzero auxiliary function for q ̸= 0, H(0) = 0 and v(x, δ; q) is an unknown function. Obviously, when q = 0 and q = 1, it holds that v(x, δ; 0) = y0 (x; δ) and v(x, δ; 1) = y(x) respectively. Thus, as q varies from 0 to 1, the solution v(x, δ; q) approaches from y0 (x; δ) to y(x), where y0 (x; δ) is the initial guess that satisfies the linear operator and the boundary conditions B ′ ( dy0 y0 , dx ) = 0. (5) Next, we choose the auxiliary function H(q) in the form H(q) = qC1 + q 2 C2 + q 3 C3 + · · · , (6) where C1 , C2 , C3 , . . . are constants which will be determined later. To obtain an approximate solution, we expand v(x, δ; q) in Taylor’s series about q in the following manner, v(x, δ; q) = y0 (x, δ) + ∞ ∑ yk (x, δ)q k . (7) k=1 Substituting (7) into (4) and equating the coefficient of like powers of p, we obtain the → following linear equations, by defining the vector − y = {y0 (x), y1 (x), . . . y (x)}. The k k first- and second-order problems are given as L(y1 (x, δ)) + g(x) = C1 N0 (y0 (x, δ)), 3 B ′ ( dy1 y1 , dx ) = 0, (8) and L(y2 (x, δ)) − L(y1 (x, δ)) = C2 N0 (y0 (x, δ)) + C1 [L (y1 (x) + N1 (y0 (x, δ) , y1 (x, δ))] , ( ) dy2 ′ B y2 , = 0. (9) dx The general governing equations for yk (x, δ) are L(yk (x, δ)) − L(yk−1 (x, δ)) = Ck N0 (y0 (x, δ)) + k−1 ∑ Ci [L(yk−i (x, δ)) i=1 ( B ′ yk , dyk dx − +Nk−i (→ y k−1 (x, δ))], ) = 0, (10) where k = 2, 3, . . . and − Nm (→ y m−1 , x, δ) = ∑ n 1 ∂ m−1 N [ ∞ n=0 yn (x, δ)q ] . (m − 1)! ∂q m−1 q=0 (11) The convergence of the series (7) depends upon the auxiliary convergent control parameters C1 , C2 , C3 , . . .. If it is convergent at q = 1, then ∞ ∑ v(x, Ci ) = y0 (x, δ) + yk (x, C1 , C2 , . . . , Ck ). (12) k=1 Thus the mth-order solution is given by y˜(x, δ, C1 , C2 , C3 , . . . , Cm ) = y0 (x, δ) + m ∑ yi (x, δ, C1 , C2 , . . . , Ci ). (13) i=1 Substituting (12) into (1) yields the following residual R (x, δ, C1 , C2 , C3 , . . . , Cm ) = L(˜ y (x, δ, C1 , C2 , C3 , . . . , Cm )) + g (x) +N (˜ y (x, δ, C1 , C2 , C3 , . . . , Cm )). (14) If R = 0, then y˜ will be the exact solution. Generally, such a case will not arise for nonlinear problems, but we can minimize the functional ∫ b J(C1 , C2 , C3 , . . . , Cm ) = R2 (x, δ, C1 , C2 , C3 , . . . , Cm )dx, (15) a where a and b are two values, depending on the problem. The unknown convergent control parameters Ci (i = 1, 2, 3, . . . , m) and the so-called prescribed parameter δ can be identified from the conditions ∂J ∂J ∂J = = ··· = = 0, ∂C1 ∂C2 ∂Cm y˜(α) = β. (16) With those known convergent control parameters, the approximate solution (of order m) is determined. Note that the force condition plays a significant role to determine the multiple solutions. 4 3 Applications In this Section, we will present two examples to demonstrate the effectiveness and high precision of present method. 3.1 Example 1 The first example comes from a mixed convection flow problem in a vertical channel studied by Barletta et al. [25]. The two walls are assumed to be isothermal. Furthermore, the effect of viscous dissipation is taken into account and the Boussinesq approximation is adopted. By employing appropriate dimensionless quantities, the governing equations were reduced to the following non-linear fourth-order ordinary differential equation for the dimensionless velocity field (cf. [25, 7]) E d4 y = 4 dx 16 ( dy dx )2 , (17) subject to the boundary conditions ∫ y(1) = y ′ (0) = y ′′′ (0) = 0, 1 y(x)dx = 1, (18) 0 where y is the dimensionless velocity, x is the transversal coordinate and E is a constant. In the case E = 0, Eqs. (17) and (18) are easily solved and admit a unique solution 3 y(x) = (1 − x2 ). 2 (19) It was shown by perturbation method and a numerical method that Eqs. (17) and (18) admit multiple solutions to any given E in the interval (−∞, 0) ∪ (0, Emax ) where Emax = 228.128 (cf. [24, 25]). To find out the multiple solutions, we consider Eqs. (17) and (18) and suppose that y ′′ (0) = δ, where δ is a prescribed parameter that plays an important role in recognizing the multiplicity of solutions, so that the problem becomes d4 y E = 4 dx 16 ( dy dx )2 , (20) y ′′ (0) = δ, (21) subject to the boundary conditions y(1) = y ′ (0) = y ′′′ (0) = 0, with additional force condition ∫ 1 y(x)dx = 1. 0 5 (22) According to POHAM, we chose the linear and nonlinear operator as follows, respectively, L[v(x, δ, q)] = ∂ 4 v(x, δ, q) , ∂x4 N [v(x, δ, q)] = E 16 ( ∂v(x, δ, q) ∂x )2 . (23) Under the rule of the solution expression and according to conditions (21) it is easy to choose δ y0 (x, δ) = (x2 − 1), 2 (24) as initial guess. The first-order problem obtained from Eq. (8) as follow (4) −Ex2 δ 2 C1 , 16 y1′ (0) = y1′′′ (0) = y1 (1) = y1′′ (0) = 0. y1 (x, δ) = (25) From Eq. (9) the second-order problem is defined as Ex2 δ 2 C2 ExδC1 ′ − y1 (x, δ) + y1′′′′ (x, δ) + C1 y1′′′′ (x, δ), 16 8 y2′ (0) = y2′′′ (0) = y2 (1) = y2′′ (0) = 0. y2′′′′ (x, δ) = − (26) By using Eq. (10) for k = 3 and k = 4, the third- and fourth-order problems are given, respectively, as follows 1 1 Ex2 δ 2 C3 − ExδC2 y1′ (x, δ) 16 8 1 1 ′ 2 − EC1 (y1 ) (x, δ) − ExδC1 y2′ (x, δ) 16 8 −C2 y1′′′′ (x, δ) − C1 y2′′′′ (x, δ), y3′′′′ (x, δ) = y2′′′′ (x, δ) − y3′ (0) = y3′′′ (0) = y3 (1) = y3′′ (0) = 0. (27) and Ex2 δ 2 C4 16 ExδC3 ′ EC2 ′ 2 ExδC2 ′ − y1 (x) − (y ) (x, δ) − y2 (x, δ) 8 16 1 16 ExδC1 ′ EC1 ′ − y (x, δ)y2′ (x, δ) − y3 (x, δ) + C3 y1′′′′ (x, δ), 8 1 8 y4′ (0) = y4′′′ (0) = y4 (1) = y4′′ (0) = 0. y4′′′′ (x, δ) = C2 y2′′′′ (x, δ) + y3′′′′ (x, δ) + C1 y3′′′′ (x, δ) − (28) By substituting the solution of Eqs. (25)–(28) into Eq. (13), yields the fourth-order approximate solutions (m = 4) for Eqs. (20) and (21): y˜(x, δ) = y0 (x, δ) + y1 (x, δ) + y2 (x, δ) + y3 (x, δ) + y4 (x, δ). (29) By substituting the fourth-order approximate solution (29) into Eq. (14) yields the residual error and the functional J, respectively: R˜ y (x, δ) = y˜′′′′ (x, δ) − 6 E ′ (˜ y (x, δ))2 . 16 (30) Table 1: Values of Ci ’s corresponding to the prescribed parameter δ for Example 1. E = −20 E = 20 δ1 = −3.09773 δ2 = −161.726 δ1 = −2.92300 δ2 = 170.039 C1 −1.63557 × 10−15 −0.738509 −1.07453 −0.438751 C2 5.54432 × 10−15 −0.695483 0.005063 −0.592496 C3 10−15 −0.617731 −0.000767 0.259233 −1.14066 −0.836292 −0.088768 −0.518427 1.05665 × C4 ∫ J(C1 , C2 , C3 , C4 ) = 1 R2 (x, δ, C1 , C2 , C3 , C4 )dx, (31) 0 with the additional force condition ∫ 1 y(x)dx = 1. (32) 0 Now, to be specific, we consider two case consisting of E = 20 and E = −20, with a prescribed parameter δ as a function of the convergence controller parameter. The values of the convergent control parameters Ci ’s are obtained based on the values of the prescribed parameter δ as presented in Table 1. We plot the multiple (dual) solutions in Fig. 1. It is worth to mention here that Fig. 1(a) indicating existence of two solutions for E = 20 so that, u′′ (0) = −3.08411 for the first branch of the solution and u′′ (0) = −161.726 for the second branch of the solution. The same procedure was done for case E = −20. As we see in Fig. 1(b). It is obviously that the results being obtained by using fourth terms POHAM approximate solution is to an extent identical compared with the 25th terms PHAM approximate solution [7]. This means that the POHAM solution reveals very good agreement with PHAM solution in few terms which prove the validity and the efficiency of our procedure in solving strongly nonlinear problems. 3.2 Example 2 Our second example comes from the heat conduction problem for a fin with heat transfer coefficient varying as a power-law function of temperature considered by Chang [27]. The rate of heat transfer on a solid surface can be enhanced by fins. In the mathematical modelling, Chang [27] considered a straight fin of finite length and uniform cross-section area. The fin surface is exposed to a prescribed ambient temperature. The dimensionless equation for the one-dimensional steady state heat conduction is given as (cf. [27, 7]), d2 y − N 2 y n+1 = 0, dx2 7 (33) (a) 1.0 ∆ = -3.08411 yH0L = 1.51086 y4 HxL y4 H0L 0.5 0.0 ∆ = -161.726 -0.5 yH0L = 14.248 -1.0 0.0 0.2 0.4 0.6 0.8 1.0 x (b) 1.0 ∆ = -2.92300 yH0L = 1.49072 y4 H0L y4 HxL 0.5 0.0 ∆ = 170.039 yH0L = -16.1591 -0.5 -1.0 0.0 0.2 0.4 0.6 0.8 1.0 x Figure 1: Multiple solutions of Eqs. (20) and (21) for (a) E = 20 and (b) E = −20. 8 subject to the boundary conditions dy (0) = 0, dx y(1) = 1, (34) where N is the convective-conductive parameter. It was shown that the problem (33) and (34), when −4 ≤ n ≤ −2, either admit multiple solutions or does not admit any solution for a given convective-conductive parameter N [26]. In particular, suppose that N= 2 5 and n = −4 then the Eq. (33) is converted into 4 = 0, 25 (35) y(0) = δ, (36) y ′′ (x)y 3 (x) − subject to the boundary conditions y ′ (0) = 0, where δ is the temperature of the fin tip and is determined by the rule of multiplicity of the solutions. With the additional force condition y(1) = 1. (37) The exact multiple (dual) solutions of this problem is given by 5 √ 2 λ y (x) − λ2 = x, 2 (38) where y(0) = λ is represented by two values: λ = 0.4472135954 for the first branch of the solution and λ = 0.8944271909 for the second branch of the solution. The linear operator can be defined as L[v(x, δ, q)] = ∂ 2 v(x, δ, q) . ∂x2 (39) It is straightforward to choose y0 (x) = δ (40) as the initial guess. The first four order problems are as follows: First-order problem given by Eq. (8): 4C1 , 25 y1 (0) = 0, y1′ (0) = 0. y1′′ (x, δ) = − (41) Second-order problem given by Eq. (9): y2′′ (x, δ) = y1′′ (x, δ) + δ 3 C1 y1′′ (x, δ) − y2 (0) = 0, y2′ (0) = 0. 9 4C2 , 25 (42) Third-order problem obtained from Eq. (10) for k = 3: y3′′ (x, δ) = δ 3 C2 y1′′ (x, δ) + 3δ 2 C1 y1 (x, δ)y1′′ (x, δ) 4C3 +y2′′ (x, δ) + δ 3 C1 y2′′ (x, δ) − , 25 y3 (0) = 0, y3′ (0) = 0. (43) Fourth-order problem obtained from Eq. (10) for k = 4 y4′′ (x, δ) = δ 3 C3 y1′′ (x, δ) + 3δ 2 C2 y1 (x, δ)y1′′ (x, δ) +3δC1 (y1 (x, δ))2 y1′′ (x, δ) +3δ 2 C1 y2 (x, δ)y1′′ (x, δ) +δ 3 C2 y2′′ (x, δ) + 3δ 2 C1 y1 (x, δ)y2′′ (x, δ) 4C4 +y3′′ (x, δ) + δ 3 C1 y3′′ (x, δ) − , 25 y4 (0) = 0, y4′ (0) = 0. (44) The fourth-order POHAM approximate analytical solution obtained by substituting the solutions of Eqs. (41), (42), (43), and (44) into Eq. (13) for m = 4 y˜(x, δ) = y0 (x, δ) + y1 (x, δ) + y2 (x, δ) + y3 (x, δ) + y4 (x, δ). (45) By using Eq. (45) into Eq. (14), yields the residual error and the functional J, respectively: 4 R˜ y (x, δ) = y˜′′ (x, δ)˜ y 3 (x, δ) − , 25 ∫ b J(C1 , C2 , C3 , C4 ) = (46) R2 (x, δ, C1 , C2 , C3 , C4 )dx, (47) a with the additional force condition y(1) = 1. (48) The optimal values of the convergent control parameters Ci ’s obtained corresponding to the values of the prescribed parameter δ1 = 0.44721 and δ2 = 0.89442 are as shown in Table 2. Obviously, multiple (dual) solutions exist and are plotted in Fig. 2. It wroth to mention her that Fig. 2 indicating existence of two solutions, θ(0) = 0.44721 for the first branch of the solution and θ(0) = 0.89442 for the second branch of the solution. We notice that POHAM does not yield good agreement with the exact solutions for both cases shown in Fig. 2. To improve the accuracy we can use a new initial guess. We use the standard homotopy analysis method (HAM) for two terms and employ the Least Squares method to define the optimal values of ¯h = 1 . δ3 This yields the following new initial guess y0 (x, δ) = δ + 2x2 2x4 − . 25δ 3 625δ 7 10 (49) Table 2: The values of Ci ’s corresponding to the prescribed parameter δ for Example 2. 1.0 δ1 = 0.44721 δ2 = 0.89442 C1 −1.89861 −0.794786 C2 −11.7257 −0.486958 C3 −84.2776 14.4166 C4 133.886 −12.3558 Exact solution POHAM solution 0.9 ∆ = 0.89442 yHxL 0.8 0.7 ∆ = 0.44721 0.6 0.5 0.0 0.2 0.4 0.6 0.8 1.0 x Figure 2: Comparison between the fourth-order POHAM approximate solutions and the exact multiple (dual) solutions (38) for Example 2. 11 Table 3: The values of Ci ’s corresponding to the new initial guess and the prescribed parameter δ for Example 2. 1.0 δ1 = 0.44721 δ2 = 0.89442 C1 −2.53856 −0.828156 C2 2.24311 −0.402523 C3 −1.8665 0.157102 Exact solution POHAM solution 0.9 ∆ = 0.89442 yHxL 0.8 ∆ = 0.44721 0.7 0.6 0.5 0.0 0.2 0.4 0.6 0.8 1.0 x Figure 3: Comparison between the third-order POHAM approximate solutions and the exact multiple (38) solutions for Example 2. With this new initial guess and taking only three terms in POHAM series solution we obtaine δ1 = 0.44721 and δ2 = 0.89442 as shown in Table 3. Fiq. 3 demonstrate the good accuracy of the POHAM multiple solutions. In this regards, it is very important to notify that this results which is obtained by using three terms POHAM approximate solution compared with 40th terms PHAM approximate solution [7]. This implies that this method could be a useful and effective in solving nonlinear differential equations and it can be converge to the exact solution in few terms. 4 conclusions In this work, a new algorithm called the predictor optimal homotopy asymptotic method (POHAM) is employed to find approximate solutions of nonlinear differential equations. POHAM has been shown to be a reliable method for obtaining multiple (dual) solutions of nonlinear differential equations which arise in fluid mechanics. General framework for the multiple solutions is given without any need to perturbation methods, special discretization or transformation. The validity and applicability of this procedure is in12 dependent whether there exists a small parameter in the governing equations or not. 5 Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper. References [1] G. Zhang, and J. Sun, “Multiple positive solutions of singular second-order m-point boundary value problems,” Journal of Mathematical Analysis and Application, vol. 317, pp. 442–447, 2006. [2] H. Feng, W. Ge, and M. Jiang, “Multiple positive solutions for m-point boundaryvalue problems with a one-dimensional p-Laplacian,” Nonlinear Analysis: Theory, Methods & Applications, vol. 68, pp. 2269–2279, 2008. [3] S. J. Liao, “Finding multiple solutions of nonlinear problems by means of the homotopy analysis method,” Journal of Hydrodynamics, vol. 18, pp. 54–56, 2006. [4] S. J. 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Chang, “A decomposition solution for fins with temperature dependent surface heat flux,” International Journal of Heat and Mass Transfer, vol. 48, pp. 1819–1824, 2005. 15 List of Tables 1 Values of Ci ’s corresponding to the prescribed parameter δ for Example 1. 2 The values of Ci ’s corresponding to the prescribed parameter δ for Example 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 7 11 The values of Ci ’s corresponding to the new initial guess and the prescribed parameter δ for Example 2. . . . . . . . . . . . . . . . . . . . . . . . . . . 12 List of Figures 1 Multiple solutions of Eqs. (20) and (21) for (a) E = 20 and (b) E = −20. 2 Comparison between the fourth-order POHAM approximate solutions and the exact multiple (dual) solutions (38) for Example 2. . . . . . . . . . . . 3 8 11 Comparison between the third-order POHAM approximate solutions and the exact multiple (38) solutions for Example 2. . . . . . . . . . . . . . . . 16 12
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