Pancake-shaped turbulence with strong near-wall compression in separation-induced transition

This study identifies pancake-shaped turbulence in the near-wall region within a laminar separation bubble over the National Advisory Committee for Aeronautics (NACA) 0012 airfoil at low Reynolds numbers, which occurs only in moderate-to-high subsonic regimes and a limited low-Reynolds-number range. This turbulence is dominated by wall-normal limiting compression within the viscous sublayer and part of the buffer layer. It exhibits a limiting symmetric, pancake-shaped structure with strong compressive characteristics, distinct from the conventional separation-induced transition and the near-wall quasi-two-component turbulence. As the separated flow approaches the wall, the wall-normal Reynolds stress is compressed, and the spanwise and streamwise Reynolds stresses reach nearly equal magnitudes, thereby forming the near-wall pancake-shaped turbulence. This phenomenon causes a pronounced intensification of spanwise turbulence within the viscous sublayer and part of the buffer layer (n+ < 15). It is also observed in the flow field of a low-Reynolds-number compressor blade, suggesting its potential universality across different physical scenarios and indicating that it may be a common feature in high-speed subsonic flows within a limited low-Reynolds-number regime. Investigating its characteristics and underlying physical mechanisms provides new insights into the transition processes of laminar separation bubbles.