Free vibrations of rotating pre-twisted blades including geometrically nonlinear pre-stressed analysis

Abstract

The steady-state equilibrium deformations (SSEDs) caused by centrifugal force field in rotating blades are not necessarily perturbed disturbances. These deformations can be considered as large amplitude deformations, especially for high values of the rotating speed. Accordingly, in the current study, geometrically nonlinear terms are included in the static analysis under centrifugal forces (SACF) to accurately model the stiffening/softening effects in the vibrations of rotating pre-twisted blades. To this end, by developing a shell model based on first-order shear deformation theory (FSDT), nonlinear and linear integral boundary value problems (IBVPs) governing the SSEDs and vibrations of the blade are obtained, respectively. Multi-mode discretization of these IBVPs is carried out by the spectral Chebyshev technique. The discretization of the nonlinear IBVP results in nonlinear algebraic equations. By solving these equations, nonlinear pre-stressed analysis (NPA) is performed to achieve the SACF. Then, the free vibrations of the rotating pre-twisted blade about the determined equilibrium position is investigated. The numerical results show that the natural frequencies obtained in the presence of the nonlinear terms are extremely lower than those of the linear pre-stressed analysis.