Turbulent/Non-Turbulent Interface Detection in High Reynolds Number APG Turbulent Boundary Layers

Abstract: This study investigates the turbulent/non-turbulent interface (TNTI) in high Reynolds number (Re?) adverse-pressure-gradient (APG) turbulent boundary layers (TBL) using well-resolved two-dimensional particle image velocimetry (PIV). The experimental setup employs four high-magnification cameras in a tower configuration, capturing the entire TBL with a viscous-scaled resolution of approximately 11 x 11 in the streamwise and wall-normal directions. The first- and second-order velocity statistics for zero-pressure-gradient (ZPG) and moderately-strong APG TBLs are validated against previously published hotwire measurements conducted under matched flow conditions (Marusic et al., 2024). The local kinetic energy (LKE) method, previously proposed by Chauhan et al. (2014), is adapted and modified to detect TNTI in a high Re? APG TBL for the first time. The LKE threshold to segregate turbulent and non-turbulent regions is estimated based on a joint probability density function following Yang et al. (2020) and is found to be higher for APG compared to ZPG TBLs. Sensitivity analysis for different LKE thresholds reveals quasi-non-gradient behavior around the chosen threshold for both ZPG and APG cases. Overall, the findings indicate a decrease in the average TNTI height as a fraction of the boundary layer thickness with increasing APG strength. However, more comprehensive methods are necessary to compare TNTI height quantitatively, as the chosen threshold can still affect TNTI height despite the low sensitivity Keywords: Experimental fluid mechanics, turbulent boundary layer, adverse pressure gradient, turbulent/non turbulent interface, PIV