Upstream-propagating potential disturbances in the F109 turbofan engine inlet flow

An experimental investigation of the unsteady-velocity field upstream of the single-stage axial fan in a F109 turbofan engine was performed. Phase-locked, three-component velocity measurements were collected at multiple axial, radial, and azimuthal locations directly forward of the fan, out to 1.0 fan-blade chords upstream. The measured velocity data were ensemble averaged and resolved into mean and unsteady components, where the unsteady components were further decomposed into elements of amplitude, frequency, and phase for the primary and first-harmonic frequencies. Analysis of the primary-frequency phase information demonstrated the measured unsteady velocities to be potential in nature, generated by the fan, and to propagate upstream at acoustic speeds into the engine inlet flow. Furthermore, the axial component of the unsteady velocity data was found to exhibit peak-to-peak fluctuations of nearly 20% of the mean-axial velocity very near the fan, whereas the azimuthal (swirl) component exhibited fluctuations reaching nearly 50% of the mean-axial velocity at the same measurement locations, A computational simulation of the velocity field upstream of the F109 fan demonstrated good quantitative agreement with the measured unsteady velocity amplitudes.