Experimental and Computational Fluid Dynamics Based Determination of Flutter Limits in Supersonic Space Turbines

Turbines operating at high pressure in high velocity flow are susceptible to flutter. As reduced frequencies become sufficiently low, negative aerodynamic damping will be found in some modes. Ensuring that the total system damping is positive over the entire turbine operating envelope for all modes is of utmost importance in any design since flutter in a turbine often causes blade failures. This is in contrast to the normal engineering approach, which is to require a positive aerodynamic damping. A unique test campaign with a 1.5 stage supersonic space turbine has been performed. The turbine was operated at simulated running conditions over a large operating envelope in order to map out flutter limits. During the test, flutter was intentionally triggered at seven different operating conditions. Unique data have been obtained during the test that supports validation of design tools and enables better understanding of flutter in this type of turbine. Based on the data the flutter boundary for the turbine could be established. Using computational fluid dynamics (CFD) tools flutter was predicted at all operating points where the flutter limit was crossed. Both in predictions and as evidenced in test the two nodal diameter backward traveling mode was the most unstable. In addition to this predicted values of aerodynamic damping at flutter agreed well with damping estimated from measured amplitude growth.