Classification of unstart flow in a two-dimensional hypersonic inlet

In this paper, attempts have been made to the classification of hard unstart flow in a two-dimensional hypersonic inlet based on the characteristics of mass flow capture ratio vs freestream Mach number and the corresponding flow field structures obtained by numerical simulations. It is found that hard unstart can be broadly classified into four categories: 1) cowl angle unstart (CAU), 2) upper separation unstart (USU), 3) throat compression unstart (TCU), and 4) throat expansion unstart (TEU). As the internal contraction ratio (ICR) increases, the mass flow capture ratio for CAU remains constant because the disturbance from throat is blocked by a fully supersonic region. USU can be regarded approximately as a special form of CAU with the influence from the separation bubble at upper wall. TEU is equivalent to introducing throat expansion wave effect to CAU by reducing ICR. TCU is dominated by throat high pressure with a large subsonic region at bottom wall as a medium for the pressure signal propagation. Different unstart categories exhibit distinct geometric sensitivities, with a particularly pronounced difference observed between CAU and TCU. Under large cowl angle, it is found that two novel flow regimes, namely the degenerated TCU and the quasi CAU, may arise due to system bifurcation. Finally, based on appropriate assumptions, empirical formulas for predicting the boundaries between different unstart categories are constructed. The prediction agrees well with the simulation results.