## Nonlinear Waves and Differential Equations in Applied Mathematics and Physics

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Kai Fan, Cunlong Zhou, "Mechanical Solving a Few Fractional Partial Differential Equations and Discussing the Effects of the Fractional Order", *Advances in Mathematical Physics*, vol. 2020, Article ID 3758353, 17 pages, 2020. https://doi.org/10.1155/2020/3758353

# Mechanical Solving a Few Fractional Partial Differential Equations and Discussing the Effects of the Fractional Order

**Academic Editor:**Zhi-Yuan Sun

#### Abstract

With the help of Maple, the precise traveling wave solutions of three fractal-order model equations related to water waves, including hyperbolic solutions, trigonometric solutions, and rational solutions, are obtained by using function expansion method. An isolated wave solution is selected from the solution of each nonlinear dispersive wave model equation, and the influence of fractional order change on these isolated wave solutions is discussed. The results show that the fractional derivatives can modulate the waveform, local periodicity, and structure of the isolated solutions of the three model equations. We also point out the construction rules of the auxiliary equations of the extended ()-expansion method. In the “The Explanation and Discussion” section, a more generalized auxiliary equation is used to further emphasize the rules, which has certain reference value for the construction of the new auxiliary equations. The solutions of fractional-order nonlinear partial differential equations can be enriched by selecting other solvable equations as auxiliary equations.

#### 1. Introduction

Because of many phenomena, integer-order differential equations cannot be well described, which makes fractional nonlinear differential equations have research significance. As an effective mathematical modeling tool, it is widely used in the mathematical modeling of nonlinear phenomena in biology, physics, signal processing, control theory, system recognition, and other scientific fields [1–4]. In order to better understand the mechanism behind the phenomena described by nonlinear fractional partial differential equations, it is necessary to obtain the exact solution, which also provides a reference for the accuracy and stability of the numerical solution. With the rapid development of computer algebraic system-based nonlinear sciences like Mathematica or Maple, divers’ effective methods have been pulled out to acquire precise solutions to nonlinear fractional-order partial differential equations, such as the fractional first integral method [5, 6], the fractional simplest equation method [7, 8], the improved fractional subequation method [9], the Kudryashov method [10], the fractional subequation method [11, 12], the generalised Kudryashov method [13], the fractional exp-function method [14–19], the sech-tanh function expansion method [20, 21], the fractional ()-expansion method [22–29], the generalized Sinh-Gorden expansion method [30], the fractional functional variable method [31], the rational ()-expansion method [32], the modified Khater method [33–36], and the fractional modified trial equation method [37, 38]. Many of these methods are constructed by fractional complex transform [39, 40] and use of the solutions of some solvable differential equations. However, there is no one way to solve all kinds of nonlinear problems, and for the same nonlinear differential equation, different methods will give you different forms of solutions. There are many articles about solving different equations by different methods, but the effect of fractional order on the solution is rarely discussed.

The first model equation we want to solve is the fractional-order Boussinesq equation in space and time, which is suitable for studying the propagation of water in heterogeneous porous media [41].for the case of [42]:where represents displacement. , , and are constant coefficients. and are fractional derivatives. When , equation (1) curtails to the Boussinesq equation of the form

Equation (3) was first derived by Boussinesq when he studied the propagation of nonviscous shallow water waves [43–45]. Darvishi et al. obtained solitary wave solutions of some equations similar to Boussinesq in literature [46]. Combined with fractional complex transformation, we obtain multiple traveling wave solutions of equation (2) using extended ()-expansion method and show the effect of fractional order parameters on the waveform of an isolated wave solution of these solutions.

The second model equation we solved was a diffusion model describing shallow water waves (the time fractional-order Boussinesq-Burgers equation) [47].

There are several ways to solve this equation. For example, Javeed et al. solved it by the first integral method [47], and Kumar et al. solved it by the residual power series method [48].

Combined with fractional complex transformation, we obtain multiple traveling wave solutions of equation (4) using ()-expansion method and show the effect of fractional order parameters on the waveform of an isolated wave solution of these solutions.

Finally, the third model equation that we want to solve can simulate the propagation of surface water waves with a depth far less than the horizontal scale, which is the fractional coupled Boussinesq equations in space and time [49].

There are several ways to solve this system of equations. For example, Yaslan and Girgin solved it by the first integral method [49], Hosseini and Ansari obtained its solution by the modified Kudryashov method [50], and Hoseini et al. solved it by the -expansion method [51]. Combined with fractional complex transformation, we obtain multiple traveling wave solutions of equation (5) using extended ()-expansion method and show the effect of fractional order parameters on the waveform of an isolated wave solution of these solutions.

Given a function . Then, the conformable fractional derivative of of order is defined as [52]

The derivative has the following properties [53].

#### 2. The ()-Expansion Method Combined with Fractional Complex Transformation and Its Extension Method

Consider nonlinear fractional partial differential equationswhere is the unsolved function of the variables and . is a polynomial function, which consists of and its fractional derivatives.

The fractional -expansion method and extended fractional -expansion method are used to solve equation (8); the steps are listed as follows:

*Step 1. *Under the fractional complex transform,where is a constant, and it cannot be zero. When , equation (9) is the usual travelling wave variation.

In the complex fraction transformation, we getSubstituting (9) and (10) into (8), a nonlinear ordinary differential equation is formulatedwhere . If the form of equation (11) allows, we can integrate first and set the integral constant to zero, which will help simplify the following calculation.

*Step 2. *For the fractional ()-expansion method, we assume that equation (11) has a quasisolution of equation (11) of the following formFor the extended fractional ()-expansion method, we assume that equation (11) has a quasisolution of the following formwhere and are undetermined constants. In combination with the form of equation (12) or (13), the highest derivative term and the nonlinear term in equation (11) are balanced by the homogeneous equilibrium principle, and the value of the positive integer in equation (12) or (13) can be obtained. Let us say that the degree of is , and then, we can easily derive the degrees of other forms of terms as follows:Thus, the value of in equation (12) or equation (13) can be determined. The appearing in equation (12) or (13) is the solution of the second-order differential equation below.where and are undetermined constants. In addition, the derivative of () isEquation (16) reveals that we can set the ordinary differential equation (15) to the following form or some other ordinary differential equation can make equation (11) in polynomial form of () [54].where , , and are undetermined constants.

*Step 3. *Substitute equation (12) or (13) into equation (11), use ordinary differential equation (15) concerning () to combine the same power terms of (), then set the coefficients of all powers of () to zero, we get a nonlinear algebraic system of equations concerning the unknowns .

*Step 4. *We can use Maple to solve the equations obtained in the third step. By substituting the obtained results into equation (12) or (13) and using the general solutions of equation (15) in different situations, multiple exact solutions of different types of equation (8) can be obtained.

The solutions of equation (15) under different conditions are shown below.where are free constants.

When and satisfy different conditions, these results can be further written in simpler forms.The solutions of equation (17) under different conditions are shown below.

#### 3. Applications of Fractional ()-Expansion Method and Its Extended Methods

##### 3.1. Precise Solutions of the Fractional Boussinesq Equation in Space and Time with Generalised Fractional ()-Expansion Method

Equation (2) is written as follows.

Under the fractional complex transform,

Substituting (22) into (21), we convert our problem into a nonlinear ordinary differential equation.where . By integrating twice with respect to travelling wave variable factor and setting the constant from the integral to 0, you get the following equation.

Applying the homogeneous equilibrium principle to equation (24), we get . By taking to be 2 in equation (13), we get the form of the proposed solution of equation (24) as follows.

By using equation (15), from equation (25), we have

Equations (25), (26), and (27) are substituted into equation (24), and then, we can rearrange and combine equation (24) with respect to () and set the coefficients of all powers of () to be zero. The resulting nonlinear algebraic system with respect to the unknowns is as follows.

The nonlinear algebraic equations were solved by using Maple symbol computing system, and the following solutions were obtained.

*Case 1. *

*Case 2. *Substituting the values from (29) or (30) and equation (18) into (25), the exact solutions of equation (21) in different forms can be obtained under different parameter constraints.

*Case 1. *When , the exact solution of equation (21) in hyperbolic form is as follows.where . are constants that can take any number.

If , then becomeAgain, using (19), the general solutions for in simplified forms are written aswhen , and .when , and .

When , the exact solution of equation (21) in trigonometric form is as follows.where . are constants that can take any number.

In particular, if , then becomeWhen , the exact solution of equation (21) in rational form is as follows.where . are constants that can take any number.

*Case 2. *When , the exact solution of equation (21) in hyperbolic form is as follows.where . are constants that can take any number.

If , then becomeAgain, using (19), the general solutions for in simplified forms are written aswhen , and .when , and .

When , the exact solution of equation (21) in trigonometric form is as follows.where . are constants that can take any number.

In particular, if , then becomeWhen , the exact solution of equation (21) in rational form is as follows.where . are constants that can take any number.

Similarly, if the auxiliary equation (17) and its solution (20) are used in the process of solving, we should also be able to get exact solutions, which we can prove later.

##### 3.2. Precise Solutions of the Fractional Coupled Boussinesq-Burger Equation with Fractional ()-Expansion Method

Equation (4) is written as follows.

Under the fractional complex transformations,

We get the following output.

Substituting (46) and (47) into (45a) and (45b), we convert our problem into nonlinear ordinary differential equations:where . By integrating once with respect to travelling wave variable factor and setting the constant from the integral to 0, you get the following equation.

From equation (49a), we get

Surrogating equation (50) in equation (49b)

Applying the homogeneous equilibrium principle to equation (51), we get . By taking to be 1 in equation (12), we get the form of the proposed solution of equation (51) as follows.

By using equations (15) and (52), from equation (51), we have

The coefficients before all powers of () in equation (53) are set as 0, and the resulting nonlinear algebraic system with respect to the unknowns is as follows.

The symbolic computing system Maple was used to solve the nonlinear algebraic equations, and four sets of solutions were obtained.

*Case 1. *

*Case 2. *

*Case 3. *

*Case 4. *Substituting the values from (55), (56), (57), or (58) and equation (18) into (52), the exact solutions of equations (45a) and (45b) in different forms can be obtained under different parameter constraints.

*Case 1. *When , the exact solution of equations (45a) and (45b) in hyperbolic form is as follows.where , and are constants that can take any number.

If , then becomeAgain, using (19), the general solutions for in simplified forms are written aswhen , and .when , and .

When , the exact solution of equation (45a) and (45b) in trigonometric form is as follows.where , and are free constants that can take any number.

If , then become

When , the exact solution of equation (45a) and (45b) in rational form is as follows.where , and are free constants.

For Case 2, Case 3, and Case 4, we can similarly obtain the exact solutions of equations (45a) and (45b). For simplicity, they are unnecessary to repeat. If the auxiliary equation (17) and its solution (20) are used in the process of solving, we should also be able to get exact solutions, which we can prove later.

##### 3.3. Precise Solutions of the Fractional Coupled Boussinesq Equations in Space and Time with Generalised Fractional ()-Expansion Method

Equation (5) is written as follows.

Under the fractional complex transformations:where is a nonzero constant. We get the following output.

Substituting (67) and (68) into (66), we convert our problem into nonlinear ordinary differential equations