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J. Comput. Nonlinear Dynam. 2018;13(11):111001-111001-8. doi:10.1115/1.4041033.

Extensive studies have been done on the phenomenon of phase and anti-phase synchronization (APS) between one drive and one response systems. As well as, combination synchronization for chaotic and hyperchaotic systems without delay also has been investigated. Thus, this paper aims to introduce the concept of phase and anti-phase combination synchronization (PCS and APCS) between two drive and one response time delay systems, which are not studied in the literature as far as we know. The analysis of PCS and APCS are carried out using active control technique. An example is given to test the validity of the expressions of control forces to achieve the PCS and APCS of time delay systems. This example is between three different systems. When there is no control, the PCS does not occur where the phase difference is unbounded. The bounded phase difference appears when the control is applied which means that PCS is achieved. The special case which is the combination synchronization is studied as well.

Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111002-111002-8. doi:10.1115/1.4041029.

In this paper, dynamic response of a rotating shaft with geometrical nonlinearity under parametric and external excitations is investigated. Resonances, bifurcations, and stability of the response are analyzed. External excitation is due to shaft unbalance and parametric excitation is due to periodic axial force. For this purpose, combination resonances of parametric excitation and primary resonance of external force are assumed. Indeed, simultaneous effect of nonlinearity, parametric, and external excitations are investigated using analytical method. By applying the method of multiple scales, four ordinary nonlinear differential equations are obtained, which govern the slow evolution of amplitude and phase of forward and backward modes. Eigenvalues of Jacobian matrix are checked to find the stability of solutions. Both periodic and quasi-periodic motion were observed in the range of study. The influence of various parameters on the response of the system is studied. A main contribution is that the parametric excitation in the presence of nonlinearity can be used to suppress the forward synchronous vibration. Indeed, in the presence of combination parametric excitation, the energy is transferred from forward whirling mode to backward one. This property can be applied in control of rotor unbalance vibrations.

Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111003-111003-10. doi:10.1115/1.4040951.

This paper presents an efficient numerical method for solving the distributed fractional differential equations (FDEs). The suggested framework is based on a hybrid of block-pulse functions and Taylor polynomials. For the first time, the Riemann–Liouville fractional integral operator for the hybrid of block-pulse functions and Taylor polynomials has been derived directly and without any approximations. By taking into account the property of this operator, the problem under consideration is converted into a system of algebraic equations. The present method can be applied to both linear and nonlinear distributed FDEs. Easy implementation, simple operations, and accurate solutions are the essential features of the proposed hybrid functions. Illustrative examples are examined to demonstrate the performance and effectiveness of the developed approximation technique, and a comparison is made with the existing results.

Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111004-111004-10. doi:10.1115/1.4041084.

This paper propounds addressing the design of a high gain observer optimization method in order to ensure a reliable state synchronization of nonlinear perturbed chaotic systems. The salient feature of the developed approach lies in the optimization of the high gain observer by using the optimal control theory associated with a proposed numerical algorithm. Thereby, an innovative quadratic optimization criterion is proposed to calculate the required optimal value of the observer setting parameter θ, characterizing the observation gain and corresponding to the minimal value of the cost function, by achieving a compromise between the correction term of the state observer and its observation error. Moreover, the exponential stability of the high gain observer is demonstrated within the Lyapunov framework. The efficacy of the designed approach is highlighted by numerical simulation on two prominent examples of nonlinear perturbed chaotic systems.

Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111005-111005-9. doi:10.1115/1.4041083.

Stability of a dynamical system may change from stable to unstable or vice versa, with the change of some parameter of the system. This is the phenomenon of stability switches, and it has been investigated intensively in the literature for conventional time-delay systems. This paper studies the stability switches of a class of fractional-delay systems whose coefficients depend on the time delay. Two simple formulas in closed-form have been established for determining the crossing direction of the characteristic roots at a given critical point, which is one of the two key steps in the analysis of stability switches. The formulas are expressed in terms of the Jacobian determinant of two auxiliary real-valued functions that are derived directly from the characteristic function, and thus, can be easily implemented. Two examples are given to illustrate the main results and to show an important difference between the fractional-delay systems with delay-dependent coefficients and the ones with delay-free coefficients from the viewpoint of stability switches.

Topics: Stability , Delays , Switches
Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111006-111006-9. doi:10.1115/1.4041234.

The application of the proper orthogonal decomposition (POD) method to the vibration response of a cracked rotor system is investigated. The covariance matrices of the horizontal and vertical whirl amplitudes are formulated based on the numerical and experimental whirl response data for the considered cracked rotor system. Accordingly, the POD is directly applied to the obtained covariance matrices where the proper orthogonal values (POVs), and the proper orthogonal modes (POMs) are obtained for various crack depths, unbalance force vector angles, and rotational speeds. It is observed that both POVs and their corresponding POMs are highly sensitive to the appearance of the crack and the unbalance force angle direction in the neighborhoods of the critical rotational speeds. The sensitivity zones of the POVs and POMs to the crack propagation are found to be coinciding with the unstable zones found by the Floquet's theory of the considered cracked system.

Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111007-111007-8. doi:10.1115/1.4041236.

Saddle-node or period-doubling bifurcations of the near-grazing impact periodic motions have been extensively studied in the impact oscillators, but the near-grazing Neimark-Sacker bifurcations have not been discussed yet. For the first time, this paper uncovers the novel dynamic behavior of Neimark-Sacker bifurcations, which can appear in a small neighborhood of the degenerate grazing point in a two degree-of-freedom impact oscillator. The higher order discontinuity mapping technique is used to determine the degenerate grazing point. Then, shooting method is applied to obtain the one-parameter continuation of the elementary impact periodic motion near degenerate grazing point and the peculiar phenomena of Neimark-Sacker bifurcations are revealed consequently. A two-parameter continuation is presented to illustrate the relationship between the observed Neimark-Sacker bifurcations and degenerate grazing point. New features that differ from the reported situations in literature can be found. Finally, the observed Neimark-Sacker bifurcation is verified by checking the existence and stability conditions in line with the generic theory of Neimark-Sacker bifurcation. The unstable bifurcating quasi-periodic motion is numerically demonstrated on the Poincaré section.

Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111008-111008-20. doi:10.1115/1.4041239.

In this paper, a novel generalized structure-dependent semi-explicit method is presented for solving dynamical problems. Some existing algorithms with the same displacement and velocity update formulas are included as the special cases, such as three Chang algorithms. In general, the proposed method is shown to be second-order accurate and unconditionally stable for linear elastic and stiffness softening systems. The comprehensive stability and accuracy analysis, including numerical dispersion, energy dissipation, and the overshoot behavior, are carried out in order to gain insight into the numerical characteristics of the proposed method. Some numerical examples are presented to show the suitable capability and efficiency of the proposed method by comparing with other existing algorithms, including three Chang algorithms and Newmark explicit method (NEM). The unconditional stability and second-order accuracy make the novel methods take a larger time-step, and the explicitness of displacement at each time-step succeeds in avoiding nonlinear iterations for solving nonlinear stiffness systems.

Commentary by Dr. Valentin Fuster
J. Comput. Nonlinear Dynam. 2018;13(11):111009-111009-13. doi:10.1115/1.4041322.

In this paper, a novel fractional-order (FO) backstepping sliding-mode control is proposed for a class of FO nonlinear systems with mismatched disturbances. Here the matched/mismatched disturbances are estimated by an FO nonlinear disturbance observer (NDO). This FO NDO is proposed based on FO backstepping algorithm to estimate the mismatched disturbances. The stability of the closed-loop system is proved by the new extension of Lyapunov direct method for FO systems. Exponential reaching law considerably decreases the chattering and provides a high dynamic tracking performance. Finally, three simulation examples are presented to show the features and the effectiveness of the proposed method. Results show that this observer approximates the unknown mismatched disturbances successfully.

Commentary by Dr. Valentin Fuster

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