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Kessentini S.,University of Sfax | Choura S.,Micro Electro Thermal Systems Research Unit | Najar F.,Applied Mechanics Research Laboratory | Franchek M.A.,University of Houston
JVC/Journal of Vibration and Control | Year: 2010

In this paper, we develop a mathematical model of a horizontal axis wind turbine (HAWT) with flexible tower and blades. The model describes the flapping flexures of the tower and blades, and takes into account the nacelle pitch angle and structural damping. The eigenvalue problem is solved both analytically and numerically using the differential quadrature method (DQM). The closed-form and numerical solutions are compared, and the precision of the DQM-estimated solution with a low number of grid points is concluded. Next, we examine the effects of pitch angle and blade orientation on the natural frequencies and mode shapes of the wind turbine. We find that these parameters do not incur apparent alteration of the natural frequencies. Then, we examine the linear dynamics of the wind turbine subjected to persistent excitations applied to the tower. We investigate the effects of the pitch angle and blade orientation on the linear vibrations of the wind turbine. We demonstrate that the time response of the coupled system remain nearly unaffected. We show that small vibrations of the tower induce important blade deflections, and thus, the dynamic tower- blade coupling cannot be considered insignificant. © 2010 SAGE Publications Los Angeles, London, New Delhi, Singapore. Source


Samaali H.,Micro Electro Thermal Systems Research Unit | Najar F.,Applied Mechanics and Systems Research Laboratory | Choura S.,Micro Electro Thermal Systems Research Unit | Nayfeh A.H.,Virginia Polytechnic Institute and State University | Masmoudi M.,Micro Electro Thermal Systems Research Unit
Nonlinear Dynamics | Year: 2011

In this paper, we propose the design of an ohmic contact RF microswitch with low voltage actuation, where the upper and lower microplates are displaceable. We develop a mathematical model for the RF microswitch made up of two electrostatically actuated microplates; each microplate is attached to the end of a microcantilever. We assume that the microbeams are flexible and that the microplates are rigid. The electrostatic force applied between the two microplates is a nonlinear function of the displacements and applied voltage. We formulate and solve the static and eigenvalue problems associated with the proposed microsystem. We also examine the dynamic behavior of the microswitch by calculating the limit-cycle solutions. We discretize the equations of motion by considering the first few dominant modes in the microsystem dynamics. We show that only two modes are sufficient to accurately simulate the response of the microsystem under DC and harmonic AC voltages. We demonstrate that the resulting static pull-in voltage and switching time are reduced by 30 and 45%, respectively, as compared to those of a single microbeam-microplate RF-microswitch. Finally, we investigate the global stability of the microswitch using different excitation conditions. © 2010 Springer Science+Business Media B.V. Source

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