Direct numerical simulation of supersonic turbulent boundary layers subject to a micro-suction duct

Wall suction via micro ducts is one promising approach of turbulence control in practical engineering and involves a series of fundamental issues that urgently need to be resolved. Presently, direct numerical simulation (DNS) is employed to study the supersonic turbulent boundary layers subjected to a micro-suction duct, with the aim of unveiling the inner-outer coupling of micro-flows. The analyses are made by exploiting the parametric DNS datasets based on tuning the back pressure prescribed at the duct exit. Numerical results demonstrate that the micro-duct, although with size one-order of magnitude smaller than boundary layer thickness, can actually cause considerable three-dimensional modification to the passing turbulence. The local mean flow highlights the existence of a leading ellipsoid-shaped vortex weakly connected to a pair of counter-rotating vortex (CRV) legs, situated downstream the suction orifice. Above the suction orifice, interestingly the peaks of turbulence intensity residing the buffer layer are moderately depressed, while the fluctuations underneath are notably energized, with the appearance of crossover trend between two positions. In the downstream, examining the relative differences of Reynolds normal stresses reveals the coexisting regions wherein the streamwise component is suppressed while the wall-normal and spanwise components are amplified. This is the evidence that CRV legs tend to enforce the energy redistribution from the streamwise direction toward the transverse counterparts. Detailed temporal spectra further point to that the suction induces a new kind of low-frequency unsteadiness, with the dominant mode of pressure fluctuations shifting toward lower temporal frequencies.