Aerodynamic Damping Measurements in a Transonic Compressor

A method has been developed and demonstrated for the direct measurement of aerodynamic damping in a transonic compressor. The method is based on the inverse solution of the structural dynamic equations of motion of the blade-disk system. The equations are solved inversely to determine the forces acting on the system. If the structural dynamic equations are transformed to multiblade or modal coordinates, the damping can be measured for blade-disk modes, and related to a reduced frequency and interblade phase angle. This method of damping determination was demonstrated using a specially instrumented version of the MIT Transonic Compressor run in the MIT Blowdown Compressor Test Facility. No regions of aeroelastic instability were found. In runs at the operating point, the rotor was aerodynamically excited by a controlled two-per-revolution, fixed, upstream disturbance. The disturbance was sharply terminated midway through the test. Analysis of the data in terms of multiblade modes led to a direct measurement of aerodynamic damping for three interblade phase angles. Comparison between experimental damping values and theoretical values calculated using a weak shock two dimensional analysis show reasonable agreement.