Effect of mass injection on secondary instability of hypersonic boundary layer over a blunt cone

In low environmental disturbances, secondary mechanisms play a crucial role in flow instability and transition. Over the years, researchers have shown the existence of secondary waves prior to nonlinear breakdown and turbulence. In high-speed flows, the vehicles are subjected to extreme thermal loads, and usually, an ablative heat shield is used for protection. The ablative materials eject gaseous products upon heating, significantly affecting flow stability. Mass injection through the surface in the boundary layer loosely replicates the ablation process. Its effect on stability and transition has been explored earlier using experiments, numerical simulation, and Linear Stability Theory (LST). However, the effect of the surface injection on secondary instability, the most viable path to transition, remains uncertain. The present work studies the secondary instability of a hypersonic boundary over a 7° half-angle blunt cone in the presence of mass injection through the surface of the boundary layer. Mean flow over the cone is solved using a high-order shock fitting direct numerical simulation code. Primary instability is studied using the LST, and secondary instability is studied using the secondary instability theory based on the Floquet model. Computations are carried out for different injection rates, and it is found that instability increases with mass injection rate. The mass injection has increased the primary and secondary growth rates. Fundamental modes are more dominant than subharmonic and detuned modes at higher primary wave amplitudes. The mass injection has increased the primary and secondary N-factor, and transitioning behavior is observed at the maximum injection rate.