فهرست مطالب مجید شاهقلی

Nowadays, the use of wind as one of the main sources of low carbon and renewable energy is expanding rapidly all around the world. Recently, with the development of wind farms and the consequent increase in the size of wind turbines, their maintenance has become more difficult. Therefore, researchers have deeply focused on the analysis and the control of their vibration. In this study, a wind turbine blade with a type of nonlinear absorber, called highly efficient nonlinear energy sink is analyzed, and the interaction between the heavy and long blade and the nonlinear energy sink (NES), under the influence of gravity in the vertical plane and wind force varying periodically due to height dependence is examined. For this purpose, the equations of motion for the blade connected to the NES are extracted using the energy method and solved numerically. Finally, the sensitivity of the parameters affecting the performance of the NES is analyzed and the performance of the system under rotation of the standalone blade and also the rotation of the blade in the presence of the optimal NES is studied.

In this paper, free lateral vibration of exponentially functionally graded beams is studied. For these inhomogeneous axial beams, characteristic equations in closed form are derived under various boundary conditions. The frequency and characteristic equations convert to analytical classical Euler-Bernoulli beams equations with ignoring gradient index. In addition, variation of mode shapes of the structure in terms of gradient parameter is demonstrated and compared with isotropic materials. In order to comparison and validation, the obtained results were compared with other available references. The results show that the natural frequency of the beams and mode shapes of the systems are strongly dependent to end conditions and gradient index. Furthermore, for functionally graded beams, there exists a critical frequency which leads to jump phenomenon in frequency spectrum so that harmonic vibration occurs in frequencies higher than the critical value and in frequencies lower than the critical value or pseudo-frequency, non-propagating fields occur. This feature does not exist in homogeneous beams. The presented results can be used for beams with exponentially decaying width and constant thickness. Moreover, the minimum frequency of the considered systems is helpful for engineers to design optimum non-uniform and nonhomogeneous structures.

In this paper free and forced vibrations analysis of a viscoelastic nonlinear nano rotating beam by considering surface effects is investigated. Using Hamilton principle and Gurtin Murdoch theory, the equations of motion are obtained and discretized by Galerkin method. Using the multiple time scales method the equations of motion are solved. In free vibrations analysis, the analytical expressions for amplitude and phase are obtained. In forced vibrations analysis the steady state solution are obtained. The effect of surface effect, damping coefficients, dimensions of cross section area, external excitation amplitude etc. on frequency response curves are investigated. It is seen that in free vibrations, by increasing surface stress the amplitude of the system decreased, and by increasing surface density or elasticity it is increased. Also, by increasing internal and external damping coefficients free vibration amplitude is decreased. In forced vibrations, it is seen that considering surface effect the amplitude of the system is decreased and the first bifurcation point is obviously changed. By increasing internal and external damping coefficients the amplitude is decreased and the first bifurcation point occur in frequencies near the natural frequency. It is seen that for two different dimensions of cross section with same area, amplitude and the loci of the bifurcation points are changed. By increasing the amplitude of external excitation the amplitude of response is increased the bifurcation points occur in frequencies far away from natural frequency. So, considering the surface effects for free and forced vibrations analysis of the nano rotating beams is mandatory.

In this study, free vibrations and resonances analysis of a nanocomposite beam with internal damping is investigated. For this purpose the various distributions of carbon nanotubes with arbitrary average volume fractions are considered. System includes the geometry and inertia nonlinearities. With the aid of Hamilton principle the equations of motion are derived and using the Galerkin method are reduced to ordinary ones. To analyze the system the multiple scales method is utilized. In free analysis the analytical expressions for amplitude, phase and nonlinear natural frequency are obtained. Also, the effect of system parameters such as damping coefficients, kind of the carbon nanotube distribution, average volume fraction of nanotubes in them are probed. In free analysis, it is observed that by increasing the external damping the amplitude is decreased. Also, by increasing the average volume fraction, the nonlinear natural frequency is increased. In resonance analysis, by depicting the frequency response curves, it is observed that by increasing internal damping coefficient the amplitude is decreased and the loci of the bifurcations is changed. Also carbon nanotube distribution and average volume fractions of them on the solution and bifurcations have an important effect. Also, it is seen that by decreasing the external force, the amplitude of the system is decreased and bifurcations occur in higher internal damping coefficients. An isotropic beam in the highest and a nano-composite beam in the lowest values of internal damping coefficients become completely stable.

In this paper stability analysis of a nonlinear micro rotating shaft near the primary resonances by considering the modified couple stress theory and micro inertia effect is investigated. The geometric nonlinearities due to classical and non-classical theory (the modified couple stress theory) are considered. Using Hamilton principle, the nonlinear equations of motion are obtained. In order to solve the equations of motion the multiple scales method are used and an analytical expression is presented for forward and backward frequencies which can be seen the effects of modified couple stress theory and micro inertia effect. The frequency response curves, amplitude versus damping coefficient, amplitude versus total eccentricities, etc. are reported. It is seen that due to the modified couple stress theory and micro inertia effect the amplitude of the system is decreased and the loci of bifurcation points is changed. Symmetrical micro-shaft in the presence of classical theory and without micro inertia effects becomes completely stable in the least damping coefficient and asymmetrical micro-shaft in the presence of classical theory and without micro inertia effects becomes completely stable in the most damping coefficient. Symmetrical micro-shaft in the presence of modified couple stress theory and with micro inertia effects becomes completely stable in the least total eccentricity and asymmetrical micro-shaft in the presence of classical theory and without micro inertia effects becomes completely stable in the most total eccentricity. So, considering the small-scale effects due to strain and velocity gradients for analysis of the system is mandatory.