Conditionally Averaging of Pressure Data from HART II

Abstract

Content of this report is the analysis of the HART II (Higher Harmonic Control [=HHC] Aeroacoustic Rotor Test II) blade pressure data with respect to the conditional averaging of the force coefficient normal to the chord center line, C_nM², for the three operational conditions Base Line (BL) without HHC, Minimum Noise (MN) and Minimum Vibration (MV, both with HHC). C_nM² is obtained by numerical integration of the chord wise pressure distribution. Up to now simple (or ensemble) averaging of the instantaneous C_nM² (or pressure values of each sensor) versus time were used, but without consideration of differences caused by fluctuations in rota-tional speed and varying vortex locations. For the low frequency content these fluctuations do not affect the averaged time history, but high frequency effects like aerodynamic loading due to blade-vortex interaction can be significantly biased by simple averaging in both peak-to-peak magnitude as well as in the time (or azimuthal) width of the event. This happens even in the case of identical individual time histories in each revolution, just with a scatter in its azimuthal location.