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 freestream Mach number of 2.46 and zero angle-of-attack. High-frequency pressure measurements were recorded with both radially and circumferentially positioned transducers along the base for the blunt afterbody, while only radially positioned transducers were used for the boattailed afterbody. Measurements for the blunt afterbody showed root-mean-square (RMS) pressure fluctuations on the order of 5% of the average base pressure. Power spectral density (PSD) estimates displayed a distinct change with increasing radial position for the blunt and boattailed afterbody and showed a majority of the pressure fluctuation energy to be contained at low frequencies, i.e., less than 3 kHz. PSD estimates at the two outer-most radial locations for the blunt afterbody revealed that a significant portion of the pressure fluctuation energy is contained in a peak centered near 850Hz (St=0.094). The RMS pressure fluctuations on the boattailed afterbody were on the order of 4.5% of the mean base pressure. PSD analysis of the boattailed afterbody measurements illustrated a qualitatively similar radial variation as for the blunt afterbody. However, the PSDs at the two inner-most locations contained smaller peaks while those at the two outermost locations contained a more defined and larger peak centered near 800Hz (St=0.089). Simultaneous dual-transducer measurements on both afterbodies provided data for coherence and cross-correlation estimates. The two afterbodies demonstrated similar qualitative results for the simultaneous radial dualtransducer measurements. The two outer-most radial locations on both afterbodies are subjected to similar pressure histories and thus display large values of coherence at low frequencies. The pressure history at the center position is not well correlated to those at the two outer-most locations, resulting in low values of coherence. The second inner-most location appears to be in a transition region as it demonstrated modest coherence with both the center location and the two outer-most radial locations.