Time-series analysis of supersonic base-pressure fluctuations

An investigation into the dynamic nature of supersonic base-pressure fluctuations was conducted. Two axisymmetric models, a blunt and a boattailed afterbody, were studied at a freestrearn Mach number of 2.46 and zero angle of attack. High-frequency pressure measurements were recorded with both radially and circumferentially positioned transducers. Blunt afterbody measurements showed rms pressure fluctuations on the order of 5% of the average base pressure. Power spectral density (PSD) estimates at the two outermost radial locations for the blunt afterbody revealed that a significant portion of the pressure fluctuation energy is contained in a peak centered near 850 Hz (Sr = 0.094). RMS pressure fluctuations on the boattailed afterbody were on the order of 4.3% of the average base pressure. The PSD estimates at the two outermost locations contained a more defined and larger peak centered near 800 Hz (Sr = 0.089) as compared to the blunt base. The two outermost radial locations. on both afterbodies are subjected to similar pressure histories and, hence, display large values of coherence at low frequencies. The pressure history at the center position is not well correlated to those at the two outermost locations, resulting in low values of coherence. Possible mechanisms for the substantial pressure fluctuation energy contained at low frequencies are pulsing or flapping of the recirculation region, whereas higher-frequency energy could be attributed to large-scale turbulent structures in the shear layer.