Determination of aerodynamic damping at high reduced frequencies

Abstract: Forced response which is blade vibration due to an external excitation can lead to blade failure. The estimation of the level of vibration is dependent on the determination of aerodynamic damping. This thesisinvestigates the determination of aerodynamic damping at high reduced frequencies in turbomachines. The aerodynamic damping was calculated by a linearized Navier-Stokes flow solver with exact 3D non-reflecting boundary conditions. The method was validated using the two-dimensional test cases (Standard Configuration 5 and 8). Thereafter, two 3D profiles were also investigated: an aeroelastic turbine rig (AETR) which is a subsonic turbine case, and a virtual integrated compressor (VINK) which is a transonic compressor case. In AETR case, the first bending mode with reduced frequency 2.0 was studied. The 3D acoustic modes were calculated and the rate of decay was plotted as a function of nodal diameter and radial order. This plot identified six acoustic resonant points which included two points corresponding to the first radial order. The six resonance points correspond to six peaks in the damping curve. In VINK case, the fifth mode (1854 Hz, reduced frequency 3.1) was investigated. Acoustic resonance was predicted to occur for the first and second radial orders at the inlet. It was concluded that the higher order resonance points are influencing the damping curve. There were some inconsistencies in the results and grid convergence was not achieved. These inconsistencies were due to the difficulty in calculating the acoustic modes at the transonic inlet with an impinging shock.