Experimental Study of a High-Reynolds-Number Self-Similar Adverse Pressure Gradient Turbulent Boundary Layer at the Verge of Separation

This study presents high-spatial-resolution (HSR) two-component, two-dimensional particle image velocimetry measurements of a high-Reynolds-number self-similar adverse pressure gradient turbulent boundary layer (SS-APG-TBL) at the verge of separation. Experiments were conducted in the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) APG-TBL wind tunnel, where a flexible roof generated the required pressure distribution, maintaining β = 20.3 at Re_δ = 7.9 × 10⁴ over a stream-wise domain exceeding 5 δ₁. A dual-camera PIV system captured both outer-layer and near-wall fields, with lens-distortion correction and overlap calibration ensuring sub-pixel accuracy. Wall-shear conditions were assessed from the probability density function of near-wall velocity, which peaked near zero, confirming a low-friction state consistent with incipient separation. Mean-velocity deficit and Reynolds-stress profiles collapsed in self-similar variables, providing strong evidence of self-similar development. This dataset extends SS-APG-TBL studies to higher Reynolds numbers, offering an experimental benchmark for turbulence modelling and non-equilibrium boundary-layer theory.