Experimental investigation of pressure-gradient-induced turbulent separation bubbles

Abstract

Pressure-gradient-induced turbulent separation bubbles (PGI-TSBs) are investigated using time-resolved particle image velocimetry (TR-PIV) and stereoscopic PIV on multiple planes, synchronized with surface pressure measurements. Spectral analysis reveals two dominant features: a broadband low-frequency “breathing” mode and a medium-frequency peak linked to convective shear layer shedding. Spectral proper orthogonal decomposition (SPOD) identifies the low-frequency mode as a global, quasi-periodic expansion and contraction of the separated region, quantitatively capturing large fluctuations in bubble length. Furthermore, the spectral analysis modal method (SAMM) reveals that this motion is phase-locked with global pressure fluctuations and exhibits a pronounced spanwise structure correlated with sidewall vortex dynamics. This study contributes the first application of SAMM to a TSB, providing a high-resolution experimental benchmark for the low-frequency (\(\textrm{St}_{L_{\textrm{sep}}} \approx 0.01\)) breathing mode, often unresolved in Direct Numerical Simulations. Crucially, the results support the interpretation that the TSB acts as a selective amplifier of upstream perturbations that excite a weakly damped global mode, manifesting physically as a three-dimensional spanwise standing wavelike structure intrinsically coupled to sidewall vortex dynamics.