Study of the Turbulent/Non-turbulent Interface of Zero-Pressure-Gradient Turbulent Boundary Layer Using the Uniform Momentum Zone Concept

This paper investigates the turbulent--non-turbulent interface (TNTI) in a zero-pressure-gradient turbulent boundary layer (ZPG-TBL) using a novel, threshold-free method based on the uniform momentum zone (UMZ) concept. Requiring only planar streamwise velocity data, the method is directly applicable to experimental PIV and ensures consistent TNTI detection across simulations and experiments. Its performance is demonstrated using DNS data at $Re_\tau = 1,000 - 2,000$. The TNTI height scales with the local boundary layer thickness ($\delta$), yielding an error-function-like intermittency profile and statistics consistent with prior studies. Sensitivity to streamwise domain length is minimal. Compared to TKE- and vorticity-based methods, the UMZ-TNTI partially overlaps with the TKE interface but differs significantly from the vorticity threshold, which lies farther from the wall. Conditional averages reveal sharp velocity gradients across the TNTI, consistent with mixing-layer-like dynamics. When normalized by TNTI height and velocity jump, mean velocity profiles collapse across Reynolds numbers. Reynolds stresses respond asymmetrically: $\tilde{\overline{u'u'}}$ varies most, $\tilde{\overline{v'v'}}$ moderately, and $\tilde{\overline{w'w'}}$ least. Mean and fluctuating vorticity profiles collapse well when scaled by the UMZ-TNTI vorticity scale. A localized peak in spanwise mean vorticity is observed within the TNTI, while $\tilde{\overline{\omega_x'\omega_x'}}$ decreases across it and the other components show local maxima.