A turbulent/non-turbulent interface based method to identify the boundary layer edge in compressible turbulent flat plate flow

The boundary layer, a critical region in fluid dynamics, requires precise identification to study internal flow phenomena and establish physical scaling laws. While the classical time-averaged streamwise velocity profile method has been widely adopted to determine the boundary layer edge, it suffers from limitations, such as inapplicability to intermittent flows, high spatial resolution requirements, and subjective threshold selection. In this study, direct numerical simulation of a Mach 2.9 supersonic flat-plate turbulent flow is employed to analyze the existed boundary layer identification methods. Tri-zonal distribution characteristics are identified in the high-order moments of streamwise velocity. The formation of high-order moment extremum regions and the Reynolds stress boundary layer edge are attributed to the cumulative effects of low-speed large-scale motions near the boundary layer edge. Notably, non-zero skewness values are observed in proximity to the turbulent/non-turbulent interface (TNTI), resulting in reduced reliability of skewness-based criteria for the boundary layer edge determination. Building on these findings, a novel time-averaged TNTI boundary layer edge determination methodology is developed for enstrophy, turbulent kinetic energy, and momentum defect fields to provide a new framework for boundary layer edge determination in complex flow. Threshold selection subjectivity is reduced via optimized criteria, with the Reynolds stress distribution being well distinguished.