High-spatial-resolution PIV measurements of a self-similar adverse pressure gradient turbulent boundary layer at the verge of separation

This study presents high-spatial-resolution two-component, two-dimensional particle image velocimetry (PIV) measurements of a self-similar adverse pressure gradient turbulent boundary layer (SS-APG-TBL) at the verge of separation. The experiments were performed in the LTRAC APG-TBL wind tunnel, where the flexible roof was adjusted to impose the pressure distribution required for self-similar development within the test section. The APG-TBL had a Reynolds number based on the boundary-layer thickness of R e δ = 7 . 9 × 1 0 4 , and the Clauser pressure-gradient parameter β = 18 . 8 was maintained throughout the measurement domain. A dual-camera 2C-2D PIV system was employed to capture both large-field and near-wall velocity fields, enabling accurate measurement of the flow adjacent to the wall. Calibration procedures, including lens-distortion correction and camera-overlap alignment, were performed to ensure sub-pixel accuracy in the velocity fields. The probability density and cumulative distribution functions of the instantaneous streamwise velocity adjacent to the wall are used to assess the wall-shear condition and demonstrate that the boundary layer is at the verge of separation. The probability density distribution peaks near zero and exhibits positive skewness, indicating a near-zero mean with intermittent high-velocity events. The results confirm a low-skin-friction state consistent with incipient separation, with nearly constant wall-shear stress across the measurement stations. Furthermore, the mean-velocity defect and Reynolds stresses collapse when expressed in self-similar variables, providing strong experimental evidence of a self-similar APG-TBL at moderate Reynolds number. The present database establishes an experimental benchmark, extending the study of SS-APG-TBLs beyond direct numerical simulation and to higher Reynolds numbers, and offers a new reference for turbulence modelling and theoretical analyses of self-similar adverse pressure gradient turbulent boundary layers. • Self-similar APG-TBL at the verge of separation created with a contoured tunnel roof. • Dual-camera 2C-2D PIV combined high spatial resolution with a large field of view. • Near-zero wall shear stress confirmed by near-wall velocity PDF. • Mean-defect and Reynolds stresses collapse under self-similar scaling. • Experimental database at β ≈ 20 . 3 and Re δ ≈ 7 . 9 × 1 0 4 for SS-APG-TBL studies.