Aeroacoustic Coupling Effect During the Ascent of Space Transportation Systems

After the failure of maiden flight 157 of Ariane 5 Evolution Cryotechnique type A (ECA), the inquiry board responsible for the investigation reported as one of the probable causes the "non-exhaustive definition of the loads to which the Vulcain 2 engine is subjected during flight." As a result, many research activities were set up with the objective to isolate the driving mechanisms. Driving mechanisms such as pumping, flapping, or swinging were found to excite oscillations in the base region. Despite this finding, the question of why the pressure fluctuations and base flow excitations are especially pronounced in the subsonic to transonic flow regime remained unclear. This question is addressed in the study at hand. Results from the literature are combined with recent wind-tunnel measurements on the base model of a space transportation system. The measurement data were acquired by means of particle image velocimetry, pressure transducers, and high-speed schlieren imaging. The results suggest that an aeroacoustic coupling takes place between a jet noise generation mechanism called screeching and the near-wake dynamics. Due to the nature of both effects, they are likely to reach a resonating frequency during the ascent. The resonant frequency appears to increase unfavorable fluctuations in the base region, which are held responsible for the amplified unsteady loads. A schematic model concept is proposed, which describes the underlying governing mechanism. It provides an explanation for question why buffeting effects are especially amplified in the high subsonic ambient flow regime.