Effects of the micro-blowing on the wall heat flux of the turbulent boundary layer at Mach 6

This study investigates micro-blowing effects on wall heat flux reduction in a Mach 6.0 turbulent boundary layer by direct numerical simulations, with three blowing strengths and a baseline case. Decomposition of the mean heat flux indicates that the reduction is predominantly attributable to convection and turbulent heat transport. The contribution of convection is associated with boundary-layer thickening, in a manner analogous to the friction-reduction mechanism, whereas the contribution of turbulent transport to the heat flux reduction is enhanced by blowing, which removes heated fluid from the near-wall region via advection. Meanwhile, micro-blowing also increases near-wall Reynolds stresses, whose work partially offsets the reduction. For fluctuations, a strong correlation with dilatation is found, with correlation coefficients rising from 0.49 (baseline) to 0.59 (strongest blowing). This stems from amplified wall-normal velocity fluctuations, which strengthen genuine compressibility and thereby enhance heat flux fluctuations. Premultiplied spectra reveal energy transfer from longitudinal streaks to traveling wave-packet structures, which dominate heat-flux generation. Conditional averaging of extreme positive events indicates that the associated coherent structures consist of counter-rotating vortices, alternating velocity streaks, and expansion/compression structures around the extreme-heat-flux location.