Alireza Fath

Shiraz University Iran
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Semi-active vibration control of a semi-submersible offshore wind turbine using a tuned liquid multi-column damper

In this paper, a control system is proposed for the vibration suppression of a semi-submersible offshore wind turbine equipped with a tuned liquid multi-column damper (TLMCD). The TLMCD consists of three columns of liquid integrated into the superstructure of the semi-submersible platform. To improve the vibration suppression performance of the TLMCD, unlike previous works, the TLMCD is operated in a semi-active mode by introducing three flow control valves that allow liquid to transfer between columns. Since the rotor dynamics may greatly affect the platform vibrations, it has been included in the derived nonlinear dynamic model. In the proposed control loop, the closed-loop performance objectives of platform stabilization and rotor speed regulation are accomplished by two distinct controllers. For the platform stabilization purpose, three controller design methods of displacement-based ground-hook, velocity-based ground-hook, and bang–bang are investigated. Meanwhile, to achieve the rated rotor speed, two methods of the H and gain-scheduling control schemes are attempted. The closed-loop performance is investigated through numerical simulations. Realistic wind profiles along with a wave disturbance are implemented in the numerical simulations. The results show that the gain-scheduling control scheme for the rotor speed regulation and the velocity-based ground-hook method for the platform stabilization outperform other methods. Furthermore, to study the effect of the design parameters of the TLMCD on the closed-loop performance, several cases with different values of the design parameters are examined and compared.

Surface effect on the biaxial buckling and free vibration of FGM nanoplate embedded in visco-Pasternak standard linear solid-type of foundation

In this study, nonlocal elasticity theory in conjunction with Gurtin–Murdoch elasticity theory is employed to investigate biaxial buckling and free vibration behavior of nanoplate made of functionally graded material (FGM) and resting on a visco-Pasternak standard linear solid-type of the foundation. The material characteristics of simply supported FGM nanoplates are assumed to be varied continuously as a power law function of the plate thickness. Hamilton’s principle is implemented to derive the non-classical governing equations of motion and related boundary conditions, which analytically solved to obtain the explicit closed-form expression for complex natural frequencies and buckling loads. Finally, attention is focused on considering the influences of various parameters on variation of damped natural frequency and buckling load ratio such as nonlocal parameter, surface effects, geometric parameters, power law index and properties of visco-Pasternak foundation and it is clearly demonstrated that these factors highly affect on vibration and buckling behavior.

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