Characterisation of the Self-Similar Adverse Pressure Gradient Turbulent Boundary Layer Near Separation Using High-Spatial-Resolution PIV

This study presents a detailed experimental investigation of a self-similar turbulent boundary layer (TBL) under adverse pressure gradient (APG) conditions at the verge of separation. Experiments were conducted in a 4.5-metre-long wind tunnel, where a custom-contoured roof profile was designed to impose a strong yet relaxing pressure gradient. A nearly constant Clauser pressure gradient parameter (β) and low skin-friction coefficient (Cf ≈ 4.8 × 10⁻⁴) were maintained to promote self-similarity. High-spatial-resolution 2C–2D particle image velocimetry (PIV) was performed using a dual-camera sCMOS system, achieving sufficient spatial resolution to resolve near-wall structures and the outer region simultaneously. Mean velocity, Reynolds stresses, and similarity coefficients exhibited minimal streamwise variation over a 3.3δ domain, consistent with theoretical criteria for self-similarity. Inner-scaled velocity and stress profiles collapsed only in the viscous sublayer and buffer region, while outer-scaled profiles confirmed similarity across the entire domain. A power-law scaling of Ue ∼ δ₁⁻⁰·²³ was observed, in agreement with prior APG-TBL studies near separation. The resulting dataset offers a high-fidelity benchmark for validating turbulence models and deepening the understanding of wall-bounded flows subjected to strong adverse pressure gradients.