Drag reduction mechanism in a supersonic turbulent boundary layer using micro-blowing arrays

Micro-blowing technology shows significant promise for drag reduction in supersonic turbulent boundary layers. Most numerical studies neglect internal micro-hole flows, limiting insights into the underlying drag reduction mechanisms. Our study performs direct numerical simulation of a supersonic boundary layer with micro-blowing arrays, explicitly resolving the internal flow within micro-holes. Results show that micro-blowing generates bubble-like low-friction structures in the control region, achieving a total drag reduction of 17.92%. Skin friction decomposition indicates that pressure gradients and convection dominate drag variation: adverse pressure gradients reduce drag upstream and within the control region, while favorable pressure gradients and convection affect near-downstream variations. Farther downstream, sustained drag reduction arises from boundary layer thickening and consequent velocity gradient reduction, despite turbulent equilibrium reestablishment. Micro-blowing also modifies the turbulence structure. Within the control region, low-speed streaks are lifted from the wall, narrowing the spacing of outer-layer streaks. The technique reduces near-wall streamwise Reynolds stress while enhancing far-wall stress. The low-stress region propagates downstream and stabilizes near the turbulence production zone, attenuating turbulent energy transfer toward the wall. Spectral analysis corroborates these findings, demonstrating near-wall energy decreases and outer-layer energy increases in the control region. Downstream evolution exhibits low-energy bands matching the hole spanwise spacing, which gradually broaden and cascade toward smaller scales.