DETERMINATION OF FLUTTER SPEED OF NONLINEAR WINGS

Date

2023-8-3

Author

Kösterit, Güneş

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Flutter is a phenomenon that occurs in wings or plate-like structures as a result of aerodynamical forces when a certain flow speed, i.e., flutter speed, is reached. Flutter results in severe vibrations, eventually leading to wing fatigue failure. Many solutions are suggested against the flutter phenomena. Wings or plate-like structures under the effect of flowing air may contain nonlinearities due to connections or materials used. This paper studies the effect of different structural nonlinear elements on the flutter speed using a 2D wing model. Structural nonlinearities are connected to rotational DOF. The structural nonlinearities that are considered in this work are softening cubic stiffness, piecewise linear stiffness and freeplay(gap) nonlinearity. These nonlinearities are chosen as these are the most common nonlinearities in the practice. Moreover, using the nonlinear von Karman strain theory, critical flutter speed determination and post-flutter LCO analysed for square, parallelogram, and trapezoidal flat wing models. The results are validated with commercial finite element solutions and previously presented results in the literature. Aerodynamic lift and moment acting on the airfoil are obtained using Theodorsen’s unsteady aerodynamics, which only applies to subsonic flow. For supersonic case studies, piston theory is used to model aerodynamic forces. The work in the literature uses numerical time-marching solutions to differential equations when nonlinearities are present in the system. Time marching solutions with coupled CFD solvers can also be used; however, these solvers are very costly in computational expense. In this study, Newton’s method approach is utilized to obtain the flutter speed in the frequency domain by using Describing Function Method (DFM) and Harmonic Balance Method. The nonlinear eigenvalue problem obtained is solved with Newton’ method without time marching methods. Post flutter LCO results obtained for these nonlinearities are presented and compared.

Subject Keywords

Aeroelastic Flutter, Frequency Domain, Harmonic Balance Method, Nonlinear Wings, von Karman Nonlinearity

URI

https://hdl.handle.net/11511/105207

Collections

Graduate School of Natural and Applied Sciences, Thesis