This paper focus on the geometrical modification of the unsteady pressure to reduce aerodynamic flutter excitation. Flutter occurs when the blade structure is absorbing energy from the surrounding fluid, which leads to hazardous amplification of vibrations. The unsteady pressure is caused by the motion of the vibrating blades and is responsible for local stability. Especially for free-standing blades, where most exciting aerodynamic work transfer is found at the upper tip sections, a geometrical redesign is expected to beneficially influence stability. Two approaches are pursued in this work.
This first approach is based on flow physics considerations and analytical models. The unsteady pressure field is decomposed into three physical mechanisms or effects and each effect investigated. One of these effects is the the contract-and expansion of the channelled regions of the blades for which an analytical model is derived for the limiting case of a reduced frequency of zero. Another effect discussed are the excitation by shocks and gasdynamic effects in transonic flows.
The second approach is used to validate the conclusions made in the theoretical part by numerical optimizing the geometry of a representative turbine blade. As optimization targets the aerodynamic damping and loss behaviour are used. The steady blade loading is fixed by constraining the outlet flow angle and velocity. Selected optimized designs are picked and compared with each other in terms of local excitation, aerodynamics and robustness with respect to the boundary conditions.
Based on these observations some recommendations are made for an improved turbine design.