Experimental investigation of vibrating compressor blades in the presence of a periodic unsteady inlet flow
Final Report Abstract
An experimental investigation of oscillating compressor blades in the presence of a periodic unsteady inlet flow was carried out at the test facility of aeroelastic investigations at the chair of Aero Engines at TU Berlin. The measurement section of the test facility contains a linear compressor cascade with elven blades. Nine of the blades can be forced by stepper motors to oscillate in a traveling wave setup. The project studied the effect of the phase between the unsteady inlet flow and the oscillating compressor cascade. To realize a periodic unsteady inlet flow, a mechanical device was designed and realized that looks similar to a cascade setup. Nine symmetrical blades were forced to oscillate by one common stepper motor, the design is similar to a camshaft known from automotive engineering. The phase angle between adjacent blades can be adjusted to generate a periodic unsteady inlet flow. The blades of the mechanical device featured the same phase shift between adjacent blades as the blades of the cascade. The baseline case with the lowest negative damping was chosen to study the effect of the phase between aerodynamics of the oscillating cascade and inlet flow. The phase was varied in steps of φ = −180 : 45 : 180 degrees. The aerodynamic damping increases for φ = −45, 0, 45, 90 degrees compared to the baseline case and decreased for the remaining interblade phase angle. These results were not in line with the computational results where periodic boundary conditions were applied, further investigations revealed that the sidewalls have a huge effect on the unsteady aerodynamics, shifting the damping curves relatively by approx. 45 degrees at the three central blades of the cascade. Thus periodicity was not given for the unsteady quantities when the compressor cascade was oscillating in a traveling wave setup and a periodic unsteady inlet flow was present. Thus the applicability of the superposition principle is limited in this setup. An outlook for this study is to investigate further the phase shift of the damping curve with additional measurement techniques at the sidewalls and blades.
