Oscillatory Behaviors of a Hypersonic Inlet with Trips

A hypersonic inlet should be operated in a started mode for the efficient operation of an air-breathing propulsion system [1]. Various factors, either in design or in usage, may cause the hypersonic inlet to unstart, such as a large internal contraction ratio, low operating Mach number, delay of boundary layer transition, and high back pressure in the combustor, etc. When unstarted, a typical type of oscillatory flow (known as buzz) is often observed [2–5]. Violent shock oscillation, prominent pressure fluctuation, and substantial supersonic airflow spillage can occur in the oscillation flow [2, 3, 5], which are not only harmful to the engine performance but also highly detrimental to structural safety and flight control. Therefore the prevention of the oscillatory flow is important to hypersonic flights, and such a tough task is a complicated engineering problem that is intensively related to complex fluid mechanism [6–8]. As an important method to enhance boundary layer transition and help-to-start capabilities, trips are widely applied to hypersonic inlets design and practical flights [9–11]. The tripping mechanism requires the formation of streamwise vorticity on a scale within the boundary layer; hence the vortexes in trip wakes would act as that of micro vortex generators [12] to some extent. In Valdivia’s experiments [8], vortex generators were fixed on the side walls of a two-dimensional inlet/isolator model to reduce the movement scales of isolator shocks. However how such trips affect inlet oscillatory flow was overlooked. To the best of the authors’ knowledge, effects of trips have hardly been considered in the previous researches of hypersonic inlet oscillation. Consequently the present work is to experimentally investigate the effects of trips on the oscillatory behaviors of a hypersonic inlet, in which an axisymmetric inlet model was chosen to avoid the sidewall complexity.