Prediction of the onset of supersonic inlet buzz

Abstract

A computational fluid dynamics (CFD) simulation was conducted about the onset of supersonic inlet buzz. Then, a novel mathematical model for the onset was proposed with the aid of the CFD results. The simulation was conducted using a common approach in industrial applications, in which unsteady Reynolds-averaged Navier-Stokes equations with millions of computational grids were used. The numerical results were validated with the experimental data obtained from a wind tunnel test, in which the flow around an external-compression inlet model at a Mach number of 2.0 was tested. The inlet model consists of a wedge for the generation of an oblique shock wave, a subsonic diffuser with a rectangular cross-section for deceleration of the captured flow, and a flow plug for constriction of the exit of the diffuser. Both the numerical and experimental results show that the onset of buzz correlated well with the position of the shock wave triple point (STP), which means that the onset is closely related to the mass flux through the diffuser. The critical position of the STP for the occurrence of buzz was well predicted by this simulation. Therefore, the balance of the mass in the rear part of the diffuser was modeled. The modeling indicates that this system can be represented using a delay differential equation containing a delayed negative feedback due to the pressure waves propagating in the diffuser. The presence of this feedback mechanism is the origin of the self-sustained oscillation of the buzz. The analysis shows that the onset and frequency of the inlet buzz are well predicted using the derived equation. In general, the onset of buzz is characterized using three parameters: the sensitivities of mass flow rates into/out of the part to its mass and the time lag of changes between the inflow rate and the mass.