# Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation

###### Abstract

A variable-coefficient forced Korteweg-de Vries equation with spacial inhomogeneity is investigated in this paper. Under constraints, this equation is transformed into its bilinear form, and multi-soliton solutions are derived. Effects of spacial inhomogeneity for soliton velocity, width and background are discussed. Nonlinear tunneling for this equation is presented, where the soliton amplitude can be amplified or compressed. Our results might be useful for the relevant problems in fluids and plasmas.

PACS numbers: 05.45.Yv, 47.35.Fg, 02.30.Jr

Keywords: Variable-coefficient forced KdV equation; Nonlinear tunneling; Spacial inhomogeneity; Bilinear method

I. Introduction

In this paper, we will investigate the following variable-coefficient forced KdV equation [1, 2, 3, 6, 9, 4, 5, 7, 8, 10] with the aid of symbolic computation [11, 12, 13],

(1) |

where is a function of the scaled spacial coordinate and temporal coordinate , , , and are the analytic functions of , and is the analytic function of and . These variable coefficients respectively represent the nonlinear, dispersive, line-damping, dissipative and external-force effects, which are caused by the inhomogeneities of media and boundaries [1, 2, 3, 6, 9, 4, 5, 7, 8, 10]. Here, we assume that the spacial inhomogeneity is linear and take the following form

(2) |

When coefficients are taken with different cases, Eq. (1) has been seen to describe nonlinear waves in a fluid-filled tube [1, 2, 3], weakly nonlinear waves in the water of variable depth [4, 5], trapped quasi-one-dimensional Bose-Einstein condensates [6], internal gravity waves in lakes with changing cross sections [7], the formation of a trailing shelf behind a slowly-varying solitary wave [8], dynamics of a circular rod composed of a general compressible hyperelastic material with the variable cross-sections and material density [9], and atmospheric and oceanic dynamical systems [10].

If the the spacial inhomogeneity is ignored, i.e., , Eq. (1) has been transformed into its several KdV-typed ones with simpler forms [14, 15, 16, 17], and has also been solved directly via the bilinear method [18]. The effects of the dispersive, line-damping, dissipative, and external-force terms on the solitonic velocity, amplitude and background have been discussed [18] with the characteristic-line method [19, 20]; Besides, Wronskian form are derived based on the given bilinear expression [21].

However, since the spacial inhomogeneity in external-force term brings into more difficulties in solving, to our knowledge, the multi-soliton solutions for Eq. (1) in the explicit bilinear forms have not been constructed directly, and the effects of spacial inhomogeneity on solitonic propagation and interaction have not been discussed.

In addition, nonlinear tunneling for the nonautonomous nonlinear Schrödinger equations has attracted attention in recent years [22, 23, 24, 25]. The concept of the nonlinear tunneling effect comes from the wave equations steming from the nonlinear dispersion relation, which has shown that the soliton can pass lossless through the barrier/well under special conditions which depend on the ratio between the amplitude of the soliton and the height of the barrier/well [22, 23, 24, 25]. In this paper, we will apply such concept to Eq. (1), a KdV-typed equation. In section II, a dependent variable transformation and two constraints will be proposed, Eq. (1) will be transformed into its bilinear form, and the multi-soliton solutions in the explicit forms will be constructed. In section III, we will show that different from Ref. [18], the nonlinear coefficient can also affect the soliton width and amplitude for the existence of spacial inhomogeneity in the forced term. In section IV, we will discuss nonlinear barrier/well of Eq. (1). Finally, Section V will present the conclusions.

II. Soliton solutions

Through the dependent variable transformation

(3) |

and the coefficient constraints,

(4) |

(5) |

where

(6) |

(7) |

is a function of and , and are constants, and denotes the derivative with respect to , Eq. (1) can be transformed into the following bilinear form,

(8) |

where is the bilinear derivative operator [26, 27] defined by

(9) |

and

(10) |

Note that the independence of is transformed to that of through constraint (5).

We expand in the power series of a parameter as

(11) |

Substituting Expansion (11) into Eq. (8) and collecting the coefficients of the same power of , through the standard process of the Hirota bilinear method, we can derive the -soliton-like solutions for Eq. (1), which can be denoted as

(12) |

with

(13) |

(14) |

(15) |

(16) |

where and are arbitrary real constants, is a summation over all possible combinations of , and means a summation over all possible pairs chosen from the set , with the condition that [27].

Specially, one soliton solution can be expressed as

(17) |

and two soliton solution can be expressed as

(18) |

III. Spacial inhomogeneity

The coefficients , , , and have the similar influences on the soliton velocity, amplitude and background, which have been discussed in Refs. [18, 21]. Thus, we will mainly discuss the influence of the spacial inhomogeneity in the forced term.

As shown in Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation, the soliton width broadens, amplitude
increases, and position of soliton raises. In
expression (14), and occur
simultaneously, so nonlinear coefficient can affect the
soliton width and amplitude.

()
()

Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation presents a case that the soliton velocity, amplitude
and background are periodic. Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation corresponds to the
periodic two soliton solution.

()
()

()
()

IV. Nonlinear tunneling

Nonlinear tunneling has been discussed for the nonlinear Schrödinger equation [22, 23, 24, 25]. Hereby, we will investigate the nonlinear tunneling for the KdV equation.

Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation shows the one soliton through well with
, while Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation shows
the one soliton through barrier with . In Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation, the soliton passes through
multiple well or barrier with . Thereinto,
and denote the height of the barrier/well, and
denote the position, and denotes the
separation distance of the barrier/well.

()
()

()
()

()
()

Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation presents the one soliton through well with periodic
background and characteristic line, and Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation corresponds
to the two soliton cases of Fig. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation.

()
()

()
()

V. Conclusions

In this paper, Eq. (1), a variable-coefficient model with spacial inhomogeneity in fluids [1, 2, 3, 6, 9, 4, 5, 7, 8, 10], is investigated with symbolic computation. Under coefficient constraints (4) and (5), Eq. (1) is transformed into its bilinear form, and the multi-soliton solutions are constructed. The function corresponds to spacial inhomogeneity, and the nonlinear coefficient can also affect the soliton width and amplitude for the existence of , as shown in Figs. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation- Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation.

Nonlinear tunneling for Eq. (1) is a special state of amplitude, so it can be regarded as a kind of variable coefficient effects. With taken as , nonlinear tunneling in Figs. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation- Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation is illustrated, where denotes the height of the barrier/well, denotes the position, and denotes the separation distance of the barrier/well. Finally, Figs. Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation and Spacial inhomogeneity and nonlinear tunneling for the forced KdV equation display the combination of nonlinear tunneling and variable coefficient effects.

Acknowledgements

We express our sincere thanks to all the members of our discussion group for their valuable comments. This work has been supported by the National Natural Science Foundation of China under Grant No. 11302014, and by the Fundamental Research Funds for the Central Universities under Grant Nos. 50100002013105026 and 50100002015105032 (Beijing University of Aeronautics and Astronautics).

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