Coupled Bending-Twist Vibration of a Horizontal Axis Wind Turbine Blade Accounting for Tower Shadow Under Turbulent Flow

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Abstract

Due to global warming effect humanity is forced to seek cleaner forms of energy. One of these are renewable energies, which are inexhaustible and increasingly competitive. More specifically wind energy is of highly importance due to its benefits. In order to achieve ultimate wind energy exploitation, the design of wind turbines should be perfected. This rises a lot of challenges for engineering companies, which conduct research on more economically efficient and structurally reliable structures.


However, due to difficulties in modeling, industry turns into simpler ways of simulating the conditions and loads under which a wind turbine functions. This project aims at providing a simple yet precise model, which takes into account the aerodynamic interaction of a horizontal axis wind turbine blade. The blade is modeled using an Euler-Bernoulli beam and the cross section is considered to be symmetric, taking into account the coupled bending- twist vibration of the blade. The aerodynamic interaction depends on the relative wind velocity, which includes the vibrational velocity of the blade.


The main objective of this project is the tower shadow effect on the structural behavior of the blade. The tower is modeled using a mass attached to a stiffness spring, which are connected to the blade. The chosen mass and stiffness were tuned to the first bending mode of the tower so as to obtain its first fore-aft natural frequency . The tower effect on the natural frequencies of the blade is not as expected. Hence a simpler 2 DOF system was constructed to assess this behavior. By doing so, it is observed that the tower induces an extra vibration to the blade tip response which is also observed in the first mode of the system, which is a rigid body mode with initial displacement due to the existence of the spring.

The wind profile used was Kaimal Spectrum and the aerodynamic loading was simulated using instantaneous aerodynamics.The tower shadow velocity profile was created for two cases, an upwind and a downwind wind turbine. For the upwind case, the velocity profile was derived from the stream function around a cylindrical tower and for the downwind case Powles model was utilized. The study revealed that the downwind tower shadow has a bigger effect on the blade response. However, it should be noted that the turbulence close to the downwind edge of the tower has not been studied extensively in literature and as a result is has been roughly represented here.To better understand and model this effect more experiments should be conducted in the future.