Introduction to Aeroelasticity
Modern turbomachines – gas turbines as well as jet engines – have to be reliable, efficient and environmentally compliant. The requirements for higher performance and lighter, more compact turbomachines have pushed modern turbomachinery towards higher aerodynamic loading, lower blade stiffness, closer blade row spacing and higher rotational speeds. Each of the aforementioned factors acts to increase the susceptibility of the blading to flow-induced vibrations which has a negative impact on engine availability and maintenance costs. Blade flow-induced vibrations in a turbomachine are induced by different phenomena :
- flow separation in subsonic flow regimes
- flow separation due to shock-boundary layer interaction
- blade instability due to sonic blocage regimes
- shock wave oscillations
- periodic pressure oscillations on the vibrating blade
Consequently, a reliable prediction of the aeroelastic behaviour is very important. The aeroelastic behaviour is determined by studying simultaneously the influence of the interaction of vibrational dynamics, structural elasticity and aerodynamics on the blading. The interaction of these domains and thus the definition of aeroelasticity can be depicted by the “Collar triangle” (Figure 1).
Figure 1: The Collar Triangle of Aeroelastic Forces
As Collar shows, aeroelasticity treats the interaction of inertial, aerodynamic and elastic forces. In the centre of the triangle are the main aeroelastic phenomena observed in turbomachinery: these are the flow-induced vibrations, either forced response or flutter.
In the flutter case, the vibration occurs at the blade natural frequency . Unsteady aerodynamic forces are generated by the motion of the blades and the energy is transmitted from the flow to the blading. The vibration is supported and amplified by the interaction between the vibrating blade and the unsteady pressures present in the flow.
Forced response are externally excited flow induced vibrations (whose frequency corresponds to a natural frequency of the system) which depend of the external periodic disturbance (flow or mechanical) as well as the blade motion. The energy is dissipated into the flow. Unsteady forces act on the blades and force them to vibrate.
Aeroelastic experiment at GTT:
In the Non-Rotating Annular Test Facility, various aeroelastic experiments on annular cascades are investigated :