`Mathematical Problems in EngineeringVolume 2012, Article ID 693453, 10 pageshttp://dx.doi.org/10.1155/2012/693453`
Research Article

## A Note on the Solutions of the Van der Pol and Duffing Equations Using a Linearisation Method

School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa

Received 17 March 2012; Accepted 4 July 2012

Copyright © 2012 Sandile S. Motsa and Precious Sibanda. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We present a novel application of the successive linearisation method to the classical Van der Pol and Duffing oscillator equations. By recasting the governing equations as nonlinear eigenvalue problems we obtain accurate values of the frequency and amplitude. We demonstrate that the proposed method can be used to obtain the limit cycle and bifurcation diagrams of the governing equations. Comparison with exact and other results in the literature shows that the method is accurate and effective in finding solutions of nonlinear equations with oscillatory solutions, nonlinear eigenvalue problems, and other nonlinear problems with bifurcations.

#### 1. Introduction

The Van der Pol equation is where defines the displacement of the periodic solution, is the amplitude of the oscillations, and is a classical problem in nonlinear dynamics that models systems with self-sustained oscillations. The equation is often studied for its rich set of oscillatory and chaotic solutions (Atay [1]) and as prototype for testing numerical schemes since one of its attributes is that for large , the equation is stiff with solutions exhibiting oscillations on a large time scale.

Finding solutions of the Van der Pol equation using the perturbation series expansion method generally fails since the solutions have multiple time scales. Nonetheless, solutions in the form of a Taylor series and numerical solutions are well documented in the literature, for example, Buonomo [2] presented a procedure for finding the periodic solutions as a power series in powers of . Recent attempts to solve the Van der Pol equation include using, among other methods, the decomposition method, Asadi Cordshooli and Vahidi [3], the parameter expansion methods (Darvishi and Kheybari [4] Kimiaeifar et al. [5]), and the homotopy analysis method, Li et al. [6].

We also consider the so-called Duffing oscillator in space, which is given in dimensionless form as where the prime denotes differentiation with respect to the variable and is a given parameter. The bifurcation of this equation for some values of was examined using norm forms in Kahn and Zarmi [7] and the homotopy analysis method by Liao [8].

In this paper, we present a novel application of the successive linearisation method to initial value problems and boundary value problems with oscillatory behaviour. We demonstrate the application of this technique by finding solutions of the classical Van der Pol and the Duffing oscillator equations. We calculate the amplitude and frequency of the limit cycle of the Van der Pol equation and compare our results with known exact solutions and other results from literature. In the case of the Duffing oscillator we obtain the bifurcation diagrams and compare with exact solutions.

#### 2. SLM Solution of Van der Pol Equation

In this section we present the basic description of the successive linearisation method (SLM) that is used to obtain solutions of the nonlinear initial value problem (IVP), (1.1) and the boundary value problem (BVP), (1.2). However, before the governing equation (1.1) is solved it is converted into a boundary value problem by using the definition of period functions. In its basic form, the SLM [10, 11] seeks to linearize and reduce the governing nonlinear equation to a system of linear differential equations.

Following [12, 13] we introduce the transformation , where is the frequency of the oscillations. In addition we set so that the boundary conditions are independent of . The frequency and amplitude are considered to be unknown parameters which will be determined by the SLM. Under these transformations, (1.1) becomes where the primes denote differentiation with respect to . Since and are considered to be unknown parameters, additional boundary conditions are required to fully solve (2.1). The extra boundary conditions are derived by noting that the Duffing-Van der Pol equation (1.1) has translational invariance, that is, if is a solution, then so is for an arbitrary constant . The limit cycle is automatically periodic with a period . We therefore introduce the following conditions: We now seek a solution of the form: where , , are obtained iteratively by solving the linearized equations that result from substituting (2.3) in the governing equation (2.1) and using , , and as initial approximations. Substituting (2.3) in (2.1) and linearizing (neglecting nonlinear terms in , ,,   and ) yield subject to the boundary conditions where The linearized system (2.4) was solved using the Chebyshev collocation spectral method. We used the transformation to map to []. The solution space is discretized using the Chebyshev-Gauss-Lobatto collocation points which are the extrema of the th order Chebyshev polynomial We approximated the derivatives of the unknown variables at the collocation points as the matrix vector product: where , with being the Chebyshev derivative matrix and is the vector function of at the collocation points (see, for example, [14, 15]). The entries of can be computed in different ways. In this work we use the method proposed by Trefethen [15] in the cheb.m MATLAB m-file. If we denote the entries of the derivative matrix by , we can apply the spectral collocation method, with derivative matrices on the linearized equation (2.4) and the boundary conditions (2.5) to obtain the linear matrix system: where and , are the diagonal matrices corresponding to . The vectors , are of size and they correspond to . The appropriate boundary conditions are

After imposing the conditions (2.12) on (2.10) we obtain the following matrix system: The boundary conditions and appearing in (2.12) have been imposed by deleting the first and last rows of and . Two additional rows are also added to accommodate the two derivative boundary conditions. Thus, starting from the initial approximations , , and , the subsequent solutions for , ,    can be obtained by solving the matrix system (2.13).

##### 2.1. SLM Solution of the Duffing Equation

The exact solution of (1.2) is given (see [7, 8]) in terms of the relation between and as which gives the exact solution where is the complete elliptic integral of the first kind.

To obtain an approximate solution of (1.2), it is convenient to introduce the following transformations [8]: Substituting in (1.2), we have Since is unknown in (2.17), we require an additional boundary condition to be able to solve (2.17). The extra boundary condition can be obtained from (2.16) as Equations (2.17) and (2.18) form an eigenvalue problem with being the unknown eigenvalue. We now look for solutions and in the form where and are unknown functions. Substituting (2.19) in (2.17) and neglecting nonlinear terms give subject to the boundary conditions where The linearized system (2.20) was solved using the Chebyshev collocation spectral method described in the previous section. Using the spectral method and the transformation to map to [] gives with the boundary conditions where and , are the diagonal matrices corresponding to . After imposing the conditions (2.24) on (2.23) we obtain the following matrix system: Starting from suitable initial approximations, and , (2.26) can be used to find approximate and .

#### 3. Results and Discussion

In this section we present solutions of the Van der Pol equation (1.1) and Duffing oscillatory equation (1.2) obtained using the SLM solution procedure outlined in the previous sections. The initial approximations were obtained using physical considerations and the given boundary conditions. For (1.1), the initial approximation used is while for the Duffing equation (1.2) was used. In both cases the initial values of all eigenvalues were set to be 1. The number of collocations points used was and for the Van der Pol and Duffing equations, respectively. To determine the accuracy and performance of our method, the results were compared against previous results in literature and exact solutions for selected parameter values. In particular, for the Van der Pol equation, the present results were compared to the exact solution and the homotopy analysis method results reported in [9] and against the power series solution results of [2] when is small. For the Duffing equation we compared with the exact solution (2.15).

Table 1 shows a comparison of the amplitude and frequency of the Van der Pol equation obtained using the successive linearisation method compared to the exact results and recent solutions obtained using the homotopy analysis method (HAM). The parameter has been kept fixed. We note firstly that only six iterations of the linearisation method are sufficient to match the exact result to ten decimal places compared to at least fifty iterations in the case of the homotopy analysis method, Chen and Liu [9]. This shows that the rate of convergence of the present SLM approach is higher than that of the HAM.

Table 1: Comparison of the results of the frequency and amplitude by the proposed SLM approach with the exact results and the homotopy analysis method results of Chen and Liu [9] when .

Table 2 shows the effect of increasing the parameter on the accuracy and efficiency of the successive linearisation method. The solutions are compared with the power series solutions of Buonomo [2] and the homotopy analysis results of Chen and Liu [9]. For values of only two iterations of the SLM are sufficient to give accuracy of up to 6 decimal places. However, as increases, more iterations are required to achieve the same level of accuracy. We also note that the SLM requires fewer iterations to converge to the exact solutions compared to the HAM.

Table 2: Comparison of the frequency by the SLM approach with Buonomo [2] and the HAM, Chen and Liu [9].

Figure 1 shows the limit cycle generated using the SLM when compared with numerical results generated using the MATLAB ode45 solver. It can be seen from the graph that there is good agreement between the two results.

Figure 1: Limit cycle when . Solid line: 6th order SLM solution. Circles: numerical solution.

In Table 3 we give the SLM results of the unknown parameter at different orders when and . The results are compared with the HAM and homotopy-Padé results of [8]. The SLM converges much faster than both the HAM and the homotopy-Padé technique. When , convergence to 5 decimal places of accuracy is achieved after only 4 iterations in the SLM compared to 20 iterations and [6, 6] in the HAM and homotopy-Padé techniques, respectively. We also note that convergence to 8 digits of accuracy is achieved after only 5 iterations when . When , convergence to 5 decimal places is achieved after only 3 iterations in the SLM compared to [10, 10] in the homotopy-Padé. The HAM solution will not have converged to a 5-digit accurate solution after 30 iterations. This is an advantage of SLM method over other nonperturbation techniques when solving BVPs of type (1.2).

Table 3: Comparison of the SLM results and the HAM and Homotopy-Padé [8] approximate results for at different orders for for and .

Figure 2 gives the bifurcation diagram in the () plane and a further comparison between the exact solution (2.15) and the fourth-order SLM result. It can be see from the graph that there is good agreement between the two results. The two examples considered in this paper show that the SLM is sufficiently robust to handle solutions of nonlinear problems with oscillatory solutions. In Motsa and Sibanda [16], it has been shown that, with minor modifications, the SLM can be easily extended to chaotic and hyperchaotic systems of initial value problems.

Figure 2: Comparison of the exact solution (dots) with the 4th order SLM solution (line).

#### 4. Conclusion

We have presented a new application of the successive linearisation method to find solutions of oscillatory systems. The application of the method has been demonstrated by solving the Van der Pol and Duffing oscillatory equations. Using suitable transformations, the governing equations were transformed to eigenvalue boundary value problems and solved iteratively using the SLM. Comparison was made between exact analytical solutions of the governing equations and against results in the literature. Our results show that the SLM rapidly converges to the exact solutions and can be used as an efficient method for finding solutions of boundary value problems, requiring only a few iterations to give accurate solutions. The method can also be easily extended to other nonlinear oscillating systems and nonlinear problems with bifurcations.

#### References

1. F. M. Atay, “Van der Pol's oscillator under delayed feedback,” Journal of Sound and Vibration, vol. 218, no. 2, pp. 333–339, 1998.
2. A. Buonomo, “On the periodic solution of the van der Pol equation for small values of the damping parameter,” International Journal of Circuit Theory and Applications, vol. 26, no. 1, pp. 39–52, 1998.
3. Gh. Asadi Cordshooli and A. R. Vahidi, “Solutions of Duffing-van der Pol equation using decomposition method,” Journal of Physics, vol. 3, pp. 121–129, 2011.
4. M. T. Darvishi and S. Kheybari, “An approximate solution of the classical Van der Pol oscillator coupled gyroscopically to a linear oscillator using parameter-expansion method,” International Journal of Engineering and Natural Sciences, vol. 5, pp. 208–210, 2011.
5. A. Kimiaeifar, A. R. Saidi, A. R. Sohouli, and D. D. Ganji, “Analysis of modified Van der Pol's oscillator using He's parameter-expanding methods,” Current Applied Physics, vol. 10, no. 1, pp. 279–283, 2010.
6. Y. Li, B. T. Nohara, and S. Liao, “Series solutions of coupled Van der Pol equation by means of homotopy analysis method,” Journal of Mathematical Physics, vol. 51, no. 6, p. 063517, 12, 2010.
7. P. B. Kahn and Y. Zarmi, Nonlinear Dynamics: Exploration through Normal Forms, John Wiley & Sons, New York, NY, USA, 1998.
8. S. Liao, Beyond Perturbation: Introduction to Homotopy Analysis Method, vol. 2, Chapman & Hall/CRC, Boca Raton, Fla, USA, 2004.
9. Y. M. Chen and J. K. Liu, “A study of homotopy analysis method for limit cycle of van der Pol equation,” Communications in Nonlinear Science and Numerical Simulation, vol. 14, no. 5, pp. 1816–1821, 2009.
10. S. S. Motsa, “New algorithm for solving non-linear BVPs in heat transfer,” International Journal of Mod-eling, Simulation & Scientific Computing, vol. 2, no. 3, pp. 355–373, 2011.
11. S. S. Motsa and P. Sibanda, “A linearisation method for non-linear singular boundary value problems,” Computers and Mathematics with Applications, vol. 67, pp. 1197–1203, 2012.
12. S.-J. Liao, “An analytic approximate approach for free oscillations of self-excited systems,” International Journal of Non-Linear Mechanics, vol. 39, no. 2, pp. 271–280, 2004.
13. M. Turkyilmazoglu, “An optimal analytic approximate solution for the limit cycle of Duffing-van der Pol equation,” Journal of Applied Mechanics, vol. 78, no. 2, Article ID 021005, 2011.
14. C. Canuto, M. Y. Hussaini, A. Quarteroni, and T. A. Zang, Spectral Methods in Fluid Dynamics, Springer, Berlin, Germany, 1988.
15. L. N. Trefethen, Spectral Methods in MATLAB, vol. 10, SIAM, Philadelphia, Pa, USA, 2000.
16. S. S. Motsa and P. Sibanda, “A multistage linearisation approach to four-dimensional hyperchaotic system with cubic nonlinearity,” Nonlinear Dynamics. In press.