An experimental investigation of potential-disturbance aerodynamic forcing in the F109 turbofan engine compressor

Aerodynamic forcing experiments were performed within the single-stage axial compressor of an AlliedSignal F109 turbofan engine. Unsteady velocity was measured both forward and aft of the F109 fan at several locations, with unsteady surface pressure also measured along sixteen, transducer-instrumented stator vanes. Three fan RPM were considered, with time-resolution of the unsteady data obtained through a photoelectric sensor coupled to the fan rotation. The velocity data collected forward of the fan exhibited evidence of upstream-propagating disturbances in the engine inlet flow, where these disturbances were potential in nature, emanating from the fan, and traveling acoustically in a helical pattern. The disturbance peak-to-peak unsteady amplitudes, in the swirl direction, reached nearly 50% of the mean-axial velocity at the fan face, dropping to 2–5% at one blade chord upstream. Such large velocity fluctuations may be important in terms of component high-cycle-fatigue, particularly in closely spaced, axial compressor stages. Aft of the fan, the average unsteady velocity waveforms measured across five azimuthal locations demonstrated characteristics indicative of a strong vortical and potential disturbance interaction, where the interacting disturbances had the same forcing frequency, but different amplitudes and propagation speeds. Further reduction of the fan-aft velocity data also produced evidence of upstream-propagating disturbances. These disturbances were found to be potential in nature and emanating from the F109 stator vanes; thus creating a cumulative, unsteady aerodynamic field upstream of the stators comprised of multiple interacting disturbances. The amplitudes of the stator-induced disturbances were on the order of 20–40% of the measured, downstream-propagating vortical wake amplitudes. Finally, results from stator-vane surface-pressure measurements compared favorably in both magnitude and phase to similar results collected in previous cascade experiments. Phase information from these pressure data suggested a strong vortical and potential disturbance interaction across the vanes. Evidence of compressor vortical and potential disturbance interactions may be important; as such interactions could lead to unpredicted high-cycle-fatigue failure of compressor structural components.