Over the last 3 years, several Institutes of the German Aerospace Center (DLR) investigated the possible gains of a counter rotating fan arrangement manufactured from CFRP designed with an automated optimization tool chain. While counter rotating fans promise aerodynamic efficiency improvements, they might suffer from aerodynamic exitation phenomena as well. The wakes, potential fields and shocks on the blade suction sides might cause blade vibrations leading to high cycle fatigue. Therefore, numerical investigations into aerodynamic excitation are necessary to estimate the amplitude of induced vibrations. At the Institute of Aeroelasticity, a time-linearized loosely coupled approach was used to determine the aerodynamic forcing of the blade rows of this counter rotating fan arrangement.
A finite element model consisting of shell elements was created for the blades in order to be able to model the CFRP material properties. Subsequently, nonlinear finite element load calculations (inertia and blade surface pressure) with a modal analysis in the last step were performed to generate a Campbell diagram of the rotor blades. Critical operating points were identified from the Campbell diagram. Nonlinear steady CFD simulations of these operating points were performed. Based on these calculations, time-linearized unsteady simulations at the crititcal inter-blade phase angle were performed with forced blade motion to determine the aerodynamic damping. Similarly, time-linearized unsteady simulations were performed with gust boundary conditions to determine the aerodynamic forcing. The results of aerodynamic damping and aerodynamic forcing simulations were combined to yield the predicted forced response amplitude of the eigenmode shape that is going to be excited at the respective critical operating point.
As a last step, a nonlinear finite element displacement simulation is conducted to determine the static and dynamic stresses and strains during a forced response vibration. These static and dynamic stresses and strains are then compared to the material properties of the CFRP material to determine if the blades will keep their structural integrity over time. The results of these calculations are presented and discussed.