Composite pretwisted rotating thin walled beams (TWB) can be used as the structural model for composite helicopter and wind turbine blades for the study of aeroelastic response of the blades. In the present study, semi-analytical solution is performed for the free vibration analysis of uniform and asymmetric composite pretwisted rotating TWB. The approximation of the Green-Lagrange strain tensor is adopted to derive the strain field of the system. The Euler–Lagrange governing equations of the dynamic system and the related boundary conditions are derived via Hamilton’s principle. In order to solve the governing set of equations, the Extended Galerkin’s Method (EGM) is employed. For this purpose, the structural variables are separated in space and time and the assumed mode shapes are defined to satisfy the essential boundary conditions. For the purpose of validating the TWB model developed, the commercial finite element analysis tool, MSC Nastran is used to compare the results of modal analysis obtained by the present structural model with the finite element solution. With the results obtained in this paper, it is aimed to ascertain the effect of various coupling in circumferentially asymmetric stiffness (CAS) and circumferentially uniform stiffness CUS configurations, pretwist, angular velocity and fibre orientation, on the natural frequencies and the mode shapes of the rotating thin-walled composite beams. The results are expected to propose better predictions of the vibrational behavior of thin walled structures in general, and in the design of rotor blades of turbomachinery, rotorcraft and wind turbine systems, in particular.

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In this study, the aeroelastic stability and response of an aircraft swept composite wing in subsonic compressible flow are investigated. The composite wing was modeled as an anisotropic thin-walled composite beam with the circumferentially asymmetric stiffness structural configuration to establish proper coupling between bending and torsion. Also, the structural model consists of a number of nonclassical effects, such as transverse shear, material anisotropy, warping inhibition, nonuniform torsional model, and rotary inertia. The finite state form of the unsteady aerodynamic loads have been modeled based on the indicial aerodynamic theory and strip theory in the subsonic compressible flow. Novel Mach dependent exponential approximations of the indicial aerodynamic functions have been developed. The extended Galerkin’s method was used to construct the mass, stiffness, and damping matrices of the nonconservative aeroelastic system. Eigen analysis of the system was performed to obtain the aeroelastic instability (divergence and flutter) boundaries. Also, solving the equations of motion in the time domain leads to the aeroelastic response of wing in different flight speeds. The obtained results are compared with the available results in the literature, which reveals an excellent agreement. The numerical results obtained in this article seek to clarify the effects of geometrical and material couplings and flight Mach number on the aeroelastic instability and response of composite wings in subsonic compressible flow.

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.

Composite pretwisted tapered rotating Thin-Walled Beams (TWB) can be used as a load-carrying structural part of composite helicopter, wind turbine and turbomachinery blades. In the present study, the variable stiffness concept with curvilinear fiber path is used to achieve improved structural statics and dynamics performance of uniform and asymmetric composite thin-walled rotating beams. A parametric study is performed to investigate the effect of different fiber paths on the structural performance metrics including frequency spacing, coupling factor, and critical buckling load. For this purpose, The Euler–Lagrange governing equations of the dynamic system are derived via Hamilton's principle. In order to solve the governing set of equations, the Extended Galerkin`s Method (EGM) is employed. For the purpose of validating the TWB model with curvilinear fibers, commercial finite element analysis tools, ABAQUS is used. I believe that the results presented here are likely to provide valuable information to the engineers involved in the design of advanced turbomachinery rotating blades using a variable stiffness concept with curvilinear fiber placement.