Wake behavior and entrainment motion in the far-field stage of over-tripped boundary layer turbulence

This study investigates the far-field wake behavior and entrainment motions of the over-tripped boundary layer (OT-BL) turbulence induced by a family of wall-mounted tripwires with different wall blockage rate ${\sigma }_w$. Friction Reynolds number ${\mathrm{Re}}_{\tau }$ is more than twice that of the unperturbed canonical turbulent boundary layer. Far-field streamwise development of the OT-BL is referred to as a cooperative action of wall-driven and wake-driven mechanisms. Spatial two-point correlation and intermittency factor are employed to quantify the wall-driven and wake-driven ranges. The wall driver shows a weak dependence on the tripwire configuration, whereas the wake driver is clearly related to the tripwire configuration. Along with downstream development, the wake-driven region exhibits an upward and spreading trend, which correlates positively with ${\sigma }_w$ of tripwires. The wake dynamics in turbulent region and near-interface region are revealed from the turbulent/non-turbulent interface perspective. In the upstream stage, the wake is controlled by the tripwire-excited disturbed eddies, and then, in the downstream stage, the near-interface wake dynamics are gradually similar to canonical pattern. Further, entrainment is considered based on nibbling and engulfing motions. For OT-BL, tripwire-excited disturbed eddies enhance nibbling and engulfing entrainment. Nibbling entrainment is the main entrainment mode, although engulfing process is also a significant component. Finally, the study assesses the far-field recovery trend of the OT-BLs’ wake dynamics and entrainment motion. This research provides a reference for tripwire design to simulate higher-${\mathrm{Re}}_{\tau }$ OT-BL in a finite test section. Meanwhile, we discuss the evaluation scheme on the wake turbulence characteristics of OT-BL from multiple perspectives.