On the wake region of high-Reynolds-number turbulent boundary layers subject to adverse pressure gradients

TL;DR

This study experimentally investigates how moderate adverse pressure gradients influence the turbulent boundary layer's wake region at high Reynolds numbers, revealing increased energetic motions and vortex activity.

Contribution

It uniquely characterizes the wake-region turbulence and spanwise vortex dynamics under adverse pressure gradients using combined hot-wire and PIV measurements.

Findings

APG increases large-scale energetic motions in the wake region.

Spanwise vorticity and vortex populations significantly increase under APG.

Identified key vortices using swirling-strength thresholds for future analysis.

Abstract

The effect of a moderate adverse pressure gradient (APG) on the structure of a high-Reynolds-number turbulent boundary layer (TBL) was investigated experimentally using complementary multi-point measurements. Unlike many previous studies, the present work focuses on the wake region and aims to characterise the turbulent motions that are energised by local APG conditions. Simultaneous two-point hot-wire measurements of the streamwise velocity were used to estimate the linear coherence spectrum (LCS), quantifying the wall-normal coherence between a wake-region reference point and the rest of the TBL. LCS-based decomposition of the spectral energy and variance showed that motions coherent with the wake reference account for a significant part of the APG-induced increase at large time scales, but not all of the enhanced energy. The remaining increase is associated with relatively smaller-scale motions that are not correlated with the selected wake location. High-spatial-resolution snapshot PIV measurements were then used to examine this broader range of energetic motions, which are associated with spanwise vortices in the wake region. Spanwise vorticity statistics were evaluated over 0.2 < z/{\delta} < 0.4, where the largest APG-induced change in spectral energy was observed. Under APG, both the mean and variance of spanwise vorticity increased significantly in this region, while swirling-strength distributions confirmed a relative increase in both the population and magnitude of spanwise vortices. Finally, dynamically significant clockwise rotating spanwise vortices were identified using different swirling-strength thresholds. Higher thresholds produced conditionally averaged velocity fields that best captured the key wake-region dynamics, motivating their use for vortex-based conditional averaging in future analyses.