Linear stability of axisymmetric hypersonic flow over tangent-ogive cylinders

Insights into high-speed transition over canonical tangent-ogive forebodies (TOF) representative of hypersonic vehicles is obtained with a generalized formulation of the nonlinear spatial eigenvalue problem. The linear stability predictions identify amplification characteristics of first, second, and entropy-layer modes. Mode synchronization and phase-speed variations are quantified as a function of nose bluntness of the TOF. For sharp TOF, second mode instabilities dominate the growth rates, with progressively lower frequencies appearing further away from the leading edge, and exhibiting longer streamwise extents of amplification. The upper limit of the N-factor envelope is defined by lower-frequency second modes. Higher nose-bluntness attenuates the second modes, but gradients associated with strong shock curvature result in additional instability modes in the entropy layer. Complementary nonlinear simulations predict changes in transition mechanisms from secondary instabilities of roller-dominated second-mode structures in sharp TOF, to streak-dominated breakdown in blunter TOF, as in Klebanoff modes.