Effects of leading-edge receptivity on subharmonic resonance transition in a subsonic flat-plate boundary layer

The influence of leading-edge receptivity on subharmonic resonance transition is investigated using a linearized harmonic Navier–Stokes method together with direct numerical simulation. External disturbances, such as freestream acoustic waves, are converted into instability modes within the boundary layer through a receptivity process, generating a fundamental mode that comprises not only the Tollmien–Schlichting (TS) wave but also other coexisting modes. However, the majority of prior transition studies have relied on controlled scenarios (CSs) in which an isolated TS wave is only considered, thereby neglecting these coexisting modes. The present study explicitly compares such a controlled scenario against receptivity-informed scenarios in which the receptivity-generated fundamental mode, incorporating all coexisting modes, is considered. The results demonstrate that the coexisting modes, while remaining linearly decoupled from the TS wave, can induce additional growth of the subharmonic mode and accelerate turbulent breakdown. However, it is also shown that the TS wave remains the critical factor governing subharmonic resonance, as a sufficiently weak TS wave fails to lead to breakdown even in the presence of the coexisting modes. These findings suggest that the multimode composition of the receptivity-generated disturbance needs to be taken into account for accurate transition simulations.