r Ghanbari - Department of Applied Mathematics, Faculty of Mathematical Science, Ferdowsi Univer-sity of Mashhad, Mashhad, Iran.
a Heydari - Department of Applied Mathematics, Payame Noor University, Mashhad, Iran.
s Nezhadhosein - Department of Applied Mathematics, Payame Noor University, Tehran, Iran.

In this paper, a two-phase algorithm, namely IVNS, is proposed for solving nonlinear optimal control problems. In each phase of the algorithm, we use a variable neighborhood search (VNS), which performs a uniform distribution in the shaking step and the successive quadratic programming, as the local search step. In the first phase, VNS starts with a completely random initial solution of control input values. To increase the accuracy of the solution obtained from the phase 1, some new time nodes are added and the values of the new control inputs are estimated by spline interpolation. Next, in the second phase, VNS restarts by the solution constructed by the phase 1. The proposed algorithm is implemented on more than 20 well-known benchmarks and real world problems, then the results are compared with some recently proposed algorithms. The numerical results show that IVNS can find the best solution on 84% of test problems. Also, to compare the IVNS with a common VNS (when the number of time nodes is same in both phases), a computational study is done. This study shows that IVNS needs less computational time with respect to common VNS, when the quality of solutions are not different signifcantly.

Nonlinear optimal control problem; Variable neighborhood search; Successive quadratic programming