Pure Mathematics
Vol. 11  No. 04 ( 2021 ), Article ID: 41507 , 8 pages
10.12677/PM.2021.114055

广义BBM-KdV方程的一个守恒C-N差分格式

何丽,王希,胡劲松*

西华大学理学院,四川 成都

收稿日期:2021年3月2日;录用日期:2021年4月2日;发布日期:2021年4月12日

摘要

在进行非线性扩散波的研究时,BBM-KdV方程因能描述大量的物理现象如浅水波和离子波等而占有重要的地位,其数值研究少有涉及。本文研究了一类带有齐次边界条件的广义BBM-KdV方程的初边值问题,提出了一个具有二阶理论精度的两层非线性有限差分格式,合理模拟了问题本身的一个守恒量,并给出差分格式的先验估计,讨论其差分解的存在唯一性,并用离散泛函分析方法证明该格式的收敛性和无条件稳定性,最后通过数值模拟验证了该数值方法的可靠性。

关键词

广义BBM-KdV方程,差分格式,守恒,收敛性,稳定性

A Conservative C-N Difference Scheme for the Generalized BBM-KdV Equation

Li He, Xi Wang, Jinsong Hu*

School of Science, Xihua University, Chengdu Sichuan

Received: Mar. 2nd, 2021; accepted: Apr. 2nd, 2021; published: Apr. 12th, 2021

ABSTRACT

In the study of nonlinear diffusion waves, the BBM-KdV equation occupies an important position because it can describe a large number of physical phenomena such as shallow water waves and ion waves, and its numerical research is rarely involved. This paper studies the initial-boundary value problem of a generalized BBM-KdV equation with homogeneous boundary conditions, and proposes a two-level nonlinear finite difference scheme with second-order theoretical accuracy, which reasonably simulates a conserved quantity of the problem itself. A priori estimation of the difference scheme is given, and the existence and uniqueness of the difference decomposition is discussed. Discrete functional analysis is used to prove the convergence and unconditional stability of the scheme. Finally, the reliability of the numerical method is verified by numerical simulation.

Keywords:Generalized BBM-KdV Equation, Difference Scheme, Conservation, Convergence, Stability

Copyright © 2021 by author(s) and Hans Publishers Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).

http://creativecommons.org/licenses/by/4.0/

1. 引言

本文考虑广义BBM-KdV方程

u t u x x t + β u x x x + u x + γ u p u x = 0 ( x , t ) ( x L , x R ) × ( 0 , T ] (1)

的如下初边值问题:

u ( x , 0 ) = u 0 ( x ) x [ x L , x R ] (2)

u ( x L , t ) = u ( x R , t ) = 0 u x ( x L , t ) = u x ( x R , t ) = 0 t [ 0 , T ] (3)

其中, u 0 ( x ) 是一个已知的初值函数, β , γ 为正实数, p 1 为整数。问题(1)~(3)具有如下守恒律 [1]:

E ( t ) = u L 2 + u x L 2 = E ( 0 ) (4)

其中 E ( 0 ) 为与初始条件有关的常数。

p = 1 时,方程(1)即为通常的BBM-KdV方程 [1]

u t u x x t + β u x x x + u x + γ u u x = 0 (5)

作为BBM方程 [2] [3] [4] 和KdV方程 [5] 推广,BBM-KdV方程(1)或(5)在进行非线性扩散波的研究时非常重要。BBM方程和KdV方程已引起了广泛地研究 [6] - [17],而对于BBM-KdV方程的研究甚少,仅有文献 [18] [19] 通过数值模拟方法证实了BBM-KdV方程的解存在性,并讨论了其边界条件的物理意义,文献 [1] 进一步对BBM-KdV方程(5)给出了两个二阶精度的数值求解算法。

本文考虑更一般的情形,对广义BBM-KdV方程的初边值问题(1)~(3)进行数值求解研究,并进行数值模拟验证。

2. 数值格式和守恒性

剖分区域 [ x L , x R ] × [ 0 , T ] ,设 τ 为时间步长, t n = n τ 0 n N N = [ T τ ] x j = x L + j h 0 j J h = x R x L J 为空间步长;记 u j n = u ( x j , t n ) U j n u ( x j , t n ) Z h 0 = { U = ( U j ) | U 1 = U 0 = U J = U J + 1 = 0 , j = 1 , 0 , 1 , , J , J + 1 } 。规定C为与时间步长和空间步长均无关的常数,且 C > 0 。并定义以下符号:

( U j n ) x = U j + 1 n U j n h ( U j n ) x ¯ = U j n U j 1 n h ( U j n ) x ^ = U j + 1 n U j 1 n 2 h ( U j n ) t = U j n + 1 U j n τ

U j n + 1 2 = U j n + 1 + U j n 2 U n , V n = h j = 1 J 1 U j n V j n U n 2 = U n , U n U n = max 1 j J 1 | U j n |

由于 u p u x = 1 p + 2 [ u p u x + ( u p + 1 ) x ] ,于是在数值离散时,对问题(1)~(3)提出如下两层C-N有限差分数值求解格式:

( U j n ) t ( U j n ) x x ¯ t + β ( U j n + 1 2 ) x x ¯ x ^ + ( U j n + 1 2 ) x ^ + φ ( U j n + 1 2 ) = 0 j = 1 , , J 1 n = 1 , , N 1 (6)

U j 0 = u 0 ( x j ) j = 0 , , J (7)

U n Z h 0 ( U 0 n ) x ^ = ( U j n ) x ^ = 0 n = 0 , , N (8)

其中: φ ( U j n + 1 2 ) = γ p + 2 { ( U j n + 1 2 ) p ( U j n + 1 2 ) x ^ + [ ( U j n + 1 2 ) p + 1 ] x ^ }

C-N差分格式(6)~(8)对不变量(4)有如下的数值模拟结果:

定理1 若定义离散能量 E n = U n 2 + U x n 2 ,则C-N差分格式(6)~(8)关于 E n 是守恒,即

E n = E n 1 = = E 0 (9)

其中, n = 1 , 2 , , N 1

证明:以向量 2 U n + 1 2 对(6)式取内积,由(8)式和离散分部求和公式 [20],可以得到

U n t 2 + U x n t 2 + 2 β U x x ¯ x ^ n + 1 2 , U n + 1 2 + 2 U x ^ n + 1 2 , U n + 1 2 + 2 φ ( U n + 1 2 ) , U n + 1 2 = 0 (10)

U x x ¯ x ^ n + 1 2 , U n + 1 2 = 0 U x ^ n + 1 2 , U n + 1 2 = 0 (11)

φ ( U n + 1 2 ) , U n + 1 2 = γ h p + 2 j = 1 J 1 { ( U j n + 1 2 ) p ( U j n + 1 2 ) x ^ + [ ( U j n + 1 2 ) p + 1 ] x ^ } U j n + 1 2 = γ h p + 2 j = 1 J 1 ( U j n + 1 2 ) p + 1 ( U j n + 1 2 ) x ^ + γ h p + 2 j = 1 J 1 [ ( U j n + 1 2 ) p + 1 ] x ^ U j n + 1 2 = γ h p + 2 j = 1 J 1 ( U j n + 1 2 ) p + 1 ( U j n + 1 2 ) x ^ γ h p + 2 j = 1 J 1 ( U j n + 1 2 ) p + 1 ( U j n + 1 2 ) x ^ = 0 (12)

于是将(11)式和(12)式带入(10)式,有

U n t 2 + U x n t 2 = 0

然后将上式两端乘以 τ ,再对时间层n递推即可得(9)式成立。

3. 差分格式的可解性

以下我们用Brouwer不动点定理 [21] 和数学归纳法来证明C-N差分格式(6)~(8)的数值解是存在的,即

定理2 存在 U n 满足数值差分格式(6)~(8),其中 U n Z h 0 ,且 1 n N

证明:由(7)式知,显然 U 0 是C-N差分格式(6)~(8)的解;现假设 U n 是,其中 n N 1 ;下面归纳证明存在 U n + 1 满足C-N差分格式(6)~(8)。

定义g是 Z h 0 上一个算子,且满足如下条件:

g ( V ) = 2 ( V U n ) 2 ( V U n ) x x ¯ + τ V x ^ + β τ V x x ¯ x ^ + τ φ ( V ) (13)

由(11)式和(12)式可知

V x x ¯ x ^ , V = 0 V x ^ , V = 0 φ ( V ) , V = 0

于是,以向量V对(13)式取内积,得

g ( V ) , V = 2 V 2 2 U n , V + 2 V x 2 2 U x n , V x 2 V 2 2 U n V + 2 V x 2 2 U x n V x 2 V 2 ( U n 2 + V 2 ) + 2 V x 2 ( U x n 2 + V x 2 ) V 2 ( U n 2 + U x n 2 ) + V x 2 V 2 ( U n 2 + U x n 2 )

显然,如果取 V 2 = U n 2 + U x n 2 + 1 ,则有 g ( V ) , V > 0 。从而由Brouwer不动点定理 [21] 可知,必存在 V * Z h 0 ,满足 g ( V * ) = 0 ,即 U n + 1 = 2 V * U n 满足C-N差分格式(6)~(8)。从而由归纳假设知,C-N差分格式(6)~(8)的数值解是存在的。

4. 收敛性、稳定性与数值解的唯一性

我们将C-N差分格式(6)~(8)的截断误差定义如下:

r j n = ( u j n ) t ( u j n ) x x ¯ t + β ( u j n + 1 2 ) x x ¯ x ^ + ( u j n + 1 2 ) x ^ + φ ( u j n + 1 2 ) (14)

且由Taylor公式可知, r j n = O ( τ 2 + h 2 )

引理1初边值问题(1)~(3)的连续解满足如下估计:

u L 2 C u x L 2 C u L C

证明:由不变量(4)式可得:

u L 2 C u x L 2 C

再根据Sobolev不等式即有, u L C

定理3 设 u 0 H 2 [ x L , x R ] ,则C-N差分格式(6)~(8)的数值解满足以下估计:

U n C U x n C U n C ( n = 1 , 2 , , N )。

证明:由(9)式(即定理1)可得

U n C U x n C

再根据离散Sobolev不等式 [20] 即有: U n C

由定理3可知,C-N差分格式(6)~(8)的数值解 U n 以范数 关于初始值绝对稳定。

以下我们用能量方法来证明C-N差分格式(6)~(8)的数值解的收敛性,即

定理4 C-N差分格式(6)-(8)的数值解以范数 收敛到原初边值问题(1)~(3)的连续解,并且收敛阶为 O ( τ 2 + h 2 )

证明:记 e j n = u j n U j n ,用(6)式去减(14)式,可得

r j n = ( e j n ) t ( e j n ) x x ¯ t + β ( e j n + 1 2 ) x x ¯ x ^ + ( e j n + 1 2 ) x ^ + φ ( u j n + 1 2 ) φ ( U j n + 1 2 ) (15)

以向量 2 e n + 1 2 对(15)式两端取内积,并由离散分部求和公式 [20],有

e n t 2 + e x n t 2 = 2 β e x x ¯ x ^ n + 1 2 , e n + 1 2 2 e x ^ n + 1 2 , e n + 1 2 2 φ ( u n + 1 2 ) φ ( U n + 1 2 ) , e n + 1 2 + r n , 2 e n + 1 2 (16)

由(11)式,有

e x ^ n + 1 2 , e n + 1 2 = 0 e x x ¯ x ^ n + 1 2 , e n + 1 2 = 0 (17)

根据引理1和定理3,利用离散Cauchy-Schwarz不等式,有

φ ( u n + 1 2 ) φ ( U n + 1 2 ) , e n + 1 2 = γ h p + 2 j = 1 J 1 [ ( u j n + 1 2 ) p ( u j n + 1 2 ) x ^ ( U j n + 1 2 ) p ( U j n + 1 2 ) x ^ ] e j n + 1 2 γ h p + 2 j = 1 J 1 { [ ( u j n + 1 2 ) p + 1 ] x ^ [ ( U j n + 1 2 ) p + 1 ] x ^ } e j n + 1 2 = γ h p + 2 j = 1 J 1 [ ( u j n + 1 2 ) p ( e j n + 1 2 ) x ^ + e j n + 1 2 ( U j n + 1 2 ) x ^ k = 0 p 1 ( u j n + 1 2 ) p 1 k ( U j n + 1 2 ) k ] e j n + 1 2 + γ h p + 2 j = 1 J 1 [ e j n + 1 2 k = 0 p ( u j n + 1 2 ) p k ( U j n + 1 2 ) k ] ( e j n + 1 2 ) x ^ C ( e x n + 1 2 + e x n 2 + e n + 1 2 + e n 2 ) (18)

r n , 2 e n + 1 2 r n 2 + 1 2 ( e n + 1 2 + e n 2 ) (19)

将(17)~(19)三式一起代入(16)式,令 B n = e n 2 + e x n 2 ,然后整理得

B n + 1 B n τ r n 2 + C τ ( B n + 1 + B n ) (20)

将(20)式从0到 N 1 递推并求和,即得

B N B 0 + C τ n = 0 N 1 r n 2 + C τ n = 0 N B n

τ n = 0 N 1 r n 2 N τ max 0 n N 1 r n 2 T O ( τ 2 + h 2 ) 2 B 0 = O ( τ 2 + h 2 ) 2 ,于是

B N O ( τ 2 + h 2 ) 2 + C τ n = 0 N 1 B n

从而由离散Gronwall不等式 [20],有

e N O ( τ 2 + h 2 ) e x N O ( τ 2 + h 2 )

最后再根据由离散Sobolev不等式 [20],即得

e N O ( τ 2 + h 2 )

用类似定理的证明方法,有:

定理5 C-N差分格式(6)~(8)的数值解唯一。

5. 算例验证

为了验证本文数值方法的可靠性,考虑 p = 3 p = 5 两种情形分别进行数值实验。固定参数 β = 1 γ = 0.25 ,其中,当 p = 3 时,广义BBM-KdV方程(1)的孤波解为:

u ( x , t ) = 10 1 3 sech 2 3 ( 1 2 x 5 8 t )

p = 5 时,广义BBM-KdV方程(1)的孤波解为:

u ( x , t ) = 7 1 5 sech 2 5 ( 1 2 x 13 24 t )

取初值函数 u 0 ( x ) = u ( x , 0 ) 进行数值求解计算,固定 x L = 40 x R = 60 T = 20 。对时间步长 τ 和空间步长h的取不同值时,将数值解和孤波解在不同时刻误差的 l 模列于表1;将C-N差分格式对不变量(4)的数值模拟能量 E n 在不同时刻的数据列于表2

Table 1. l ∞ -errors of C-N differential scheme at different moments

表1. C-N差分格式在不同时刻的 l -误差

Table 2. Partial data of C-N difference scheme for analog E n for invariant (4)

表2. C-N差分格式对不变量(4)的模拟 E n 的部分数据

6. 结论

本文对一类带有齐次边界条件的广义BBM-KdV方程的初边值问题(1)~(3)进行了数值方法研究,提出了一个两层非线性差分格式(6)~(8),该格式是无条件稳定的。从表1可以看出,本文的格式明显具有二阶精度;从表2可以看出,数值式格式对原问题的守恒性质(4)也进行了合理有效地模拟。所以本文数值求解方法是可靠的。

基金项目

国家自然科学基金项目(11701481);四川应用基础研究项目(2019JY0387)。

文章引用

何 丽,王 希,胡劲松. 广义BBM-KdV方程的一个守恒C-N差分格式
A Conservative C-N Difference Scheme for the Generalized BBM-KdV Equation[J]. 理论数学, 2021, 11(04): 428-435. https://doi.org/10.12677/PM.2021.114055

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