Classical flutter analysis of composite wind turbine blades including compressibility

Abstract

For wind turbine blades with the increased slenderness ratio, flutter instability may occur at lower wind and rotational speeds. For long blades, at the flutter condition, relative velocities at blade sections away from the hub center are usually in the subsonic compressible range. In this study, for the first time for composite wind turbine blades, a frequency domain classical flutter analysis methodology has been presented including the compressibility effect only for the outboard blade sections, which are in the compressible flow regime exceeding Mach 0.3. Flutter analyses have been performed for the baseline blade designed for the 5-MW wind turbine of NREL. Beamblade model has been generated by making analogy with the structural model of the prewisted rotating thin-walled beam (TWB) and variational asymptotic beam section (VABS) method has been utilized for the calculation of the sectional properties of the blade. To investigate the compressibility effect on the flutter characteristics of the blade, frequency and time domain aeroelastic analyses have been conducted by utilizing unsteady aerodynamics via incompressible and compressible indicial functions. This study shows that with use of compressible indicial functions, the effect of compressibility can be taken into account effectively in the frequency domain aeroelastic stability analysis of long blades whose outboard sections are inevitably in the compressible flow regime at the onset of flutter.

Improvement of structural characteristics of composite thin-walled beams using variable stiffness concept via curvilinear fiber placement

“…where M and K are the structural mass and stiffness matrices, respectively, and they are given in the previous studies of the authors. 10,27 Equation ( 17) can be solved for the eigenvalues and the corresponding eigenvectors accordingly.…”

“…where the distribution of inertia terms, mass per unit length ðb 1 Þ, and mass moment of inertia about the x, y axes ðb 4 , b 5 Þ along the axis of the TWB are defined in the author's previous works. 10,27 Utilizing the strain displacement relations defined by equations ( 3) and (4) and taking the integral along the wall thickness and along the contour of the cross section of the TWB, the strain energy due to the deformation of the blade caused by the external forces can be expressed as…”

“…where a ij and b i are given in. 10,27 The definition of the nonvanishing stiffness terms is shown in Table 1.…”

Flutter behavior of highly flexible blades for two- and three-bladed wind turbines

“…They concluded that there was a reasonable similarity between flutter predictions for a full turbine versus analysis of an isolated blade uncoupled from the tower. More recently, Farsadi and Kayran (2021) developed a method to account for compressibility effects on the wind turbine blades, but the resulting flutter speeds were comparable to the classical flutter analysis methods developed by Hansen (2007).…”

An intuitive representation and analysis of multi‐rotor wind turbine whirling modes

“…This makes rotor blades more exposed to manufacturing errors which potentially can lead to a higher risk of laminate failure 3–5 . Thirdly, as the flexibility increases with the blade length, so does the risk of aeroelastic instabilities such as classical flutter 6–10 or vortex‐induced vibrations 11,12 . Lastly, increasing the rotor blades makes it more costly and complicated to do testing, and as the natural frequencies decrease, the time for fatigue evaluation of a rotor blade increases.…”