Modern multi-megawatt wind turbines have large flexible blades that often exhibit significant vibration when exposed to turbulent wind. The present study proposes longitudinal stiffening of blades to address this issue using tendon made of shape memory alloy. A reduced order model of the combined system is developed that incorporates blade stiffening. The nonlinear material behavior of the stiffener is modeled by combining the principle of thermodynamics with the constitutive model as proposed in the literature. Thus, super-elastic effects are utilized in active mode (i.e. using Jule heating with the help of current flow) to apply actuation force that opposes blade deformation. Three-dimensional wind field passing through the rotor plane is simulated using TurbSim package freely available from National Renewable Energy Laboratory, USA. Aerodynamic loads are computed using modified Blade Element Momentum theory and wave loads are simulated using Morison's equation where wave time histories are simulated from the JONSWAP spectrum. Using these loads as input, the response of a benchmark wind turbine is simulated to show the performance of the proposed control strategy. Numerical results presented in this paper clearly demonstrate the efficiency and advantage of the SMA based stiffener to improve the vibration characteristics of the large turbine blades.
Summary
The aim of this study is to reduce the deformation of large horizontal axis wind turbine blades using shape memory alloy (SMA)‐based centrifugal stiffening. A discrete model considering dominant modes of the tower, drive train and blades is developed in this study to demonstrate the performance of the proposed stiffening strategy. Here, super‐elastic behaviour of SMA is characterized by Graesser‐Cozzarelli model. Aerodynamic loads acting on the blades are evaluated using blade element momentum theory. The response is simulated using aerodynamic damping, which is estimated in each mode of vibration. Numerical results presented in this paper clearly show the significance of the proposed SMA‐based stiffening to reduce blade vibration. Sensitivity analysis is also carried out to demonstrate the performance envelop of the proposed stiffening strategy over the operational range of the benchmark 5‐MW wind turbine. The study clearly highlights the performance enhancement in terms of deformation in two orthogonal directions and design in terms of longitudinal stress that ultimately improve the serviceability of the blade.
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