Direct numerical simulation of streak-based control and path modulation of K-type boundary-layer transition at Ma = 0.2

Boundary-layer transition has critical implications for aerodynamic performance, and K-type transition is a typical natural transition mode that requires targeted control. To address the research gaps in the coordinated regulation of control streak positions and amplitudes, as well as the insufficient exploration of transition path intervention, this study employs direct numerical simulation to investigate the control of K-type transition in an adiabatic flat-plate boundary layer at Mach number Ma = 0.2. Control streaks are arranged at three positions (upstream of, coincident with, and downstream of the blowing–suction strip) with amplitudes ranging from 0.01 to 0.10. Results show a concerted regulation mechanism: streak position determines the advancement or delay of transition onset, while amplitude dictates the magnitude of the regulation effect. Specifically, upstream streaks exhibit amplitude dependence (small amplitudes promote transition, medium amplitudes delay or suppress it, and large amplitudes suppress it); coincident streaks uniformly inhibit modal growth without triggering transition; downstream streaks only fully suppress transition at large amplitudes, whereas transition onset advances with increasing amplitude under other conditions. Notably, streaks can intervene in transition paths: some cases exhibit an atypical K-type transition with stage-dependent modal dominance, and others deviate from the K-type pathway and display characteristics similar to O-type transition (dominated by spanwise axisymmetric modes and presenting slender spanwise axisymmetric vortex streak structures). This study clarifies the coordinated regulation mechanism of K-type transition via control streak positioning and amplitude adjustment and provides a theoretical basis for the optimization of transition control technologies in low-Mach-number flat-plate boundary-layer flows.