Investigation of unsteady separation using novel back-illuminated temperature sensitive paint setup and near wall Lagrangian particle tracking

This study presents an experimental investigation into the detection and analysis of unsteady flow separation on a cylinder in cross-flow at a subcritical Reynolds number of ReD = 1.9×105. The methodology combines a novel modelintegrated, back-illuminated temperature-sensitive paint (TSP) system with simultaneous near-wall Lagrangian particle tracking and external volumetric three-dimensional particle tracking velocimetry (3D-PTV). Although traditional Eulerian criteria (such as vanishing wall shear stress) are widely applied, they exhibit significant physical limitations in unsteady flow regimes. Consequently, time-resolved spatial and Lagrangian analyses provide a more robust, frameindependent approach to characterizing fluid separation. To achieve this, a specialized transparent experimental setup was developed, internally housing all optical components and utilizing a transparent conductive heating layer. This inside-out configuration permits continuous surface thermography without obstructing the external flow field. The acquired optical data is mathematically decomposed using Robust Principal Component Analysis (RPCA) to effectively isolate the near-wall tracer kinematics from the underlying low-rank thermodynamic footprint. Furthermore, Spectral Proper Orthogonal Decomposition (SPOD) is applied to identify the dominant spectral frequencies governing the separation dynamics. By successfully fusing the time-resolved volumetric particle tracks with the transient TSP surface fields, this study provides unambiguous experimental evidence of near-wall reverse flow and discrete Lagrangian fluid ejection events. The established methodology forms the requisite foundation for future extractions of instantaneous wall-shear-stress fields, ultimately aiming to experimentally validate the Lagrangian Repulsion Rate separation criterion.