Investigations of intermediate buzz phenomenon during streamtraced inlet restart

Intermittent buzz prolongs the process of reestablishment of the design air flow through supersonic/hypersonic intakes, called “restart,” in the event of an accidental unstart or engine stall. The present work investigates the processes that drive intermittent buzz in a self-starting axisymmetric hypersonic Busemann inlet. The restart process is triggered by rapidly decreasing the aerodynamic blockage from a high-bandwidth high-momentum counter-injected jet at the isolator exit. The short timescale of aerodynamic blockage removal mimics the high-bandwidth fuel throttling in practical scenarios. Two-dimensional time-resolved pressure fields over inlet and isolator surfaces and time-resolved external shock fields are obtained at a 10 kHz repetition rate, which is an order of magnitude higher than the intermediate buzz frequency of 600–700 Hz. Both measurements showed that the intermediate buzz extended over the entire duration of the restart process and modulated the unstart shock train motions. A detailed statistical analysis helped identify the sources and interactions that drive the intermediate buzz. The buzz oscillations are impacted by a combination of the inherent separation vortex oscillations set up in the external contraction segment of the inlet and the back pressure response in the isolator. The key driving mechanism of the buzz phenomenon appears to be the constant imbalance between the mass within the isolator and the mass passed into the inlet cowl; the latter is modulated by the variable flow blockage caused by inherent separation vortex pulsations.