Experiments on flow characteristics of inclined jet in crossflow through RIM-based TR-PIV technique

Abstract

Flow characteristics of inclined jet in crossflow (JICF), especially in-hole fields, remain insufficiently explored in experimental observations of near-wall properties and turbulence statistics, due to strong optical distortions and blind spots caused by refractive indices mismatch at solid–fluid interfaces. This study represents the first application of the refractive index matching (RIM) technique in JICF research, enabling in-hole measurements through time-resolved particle image velocimetry (TR-PIV). Experiments were conducted on a round hole at four velocity ratios (VR = 0.4, 0.8, 1.2, and 1.5). Focusing on the in-hole and near-exit mean flow field, this study identified a low-speed separation zone on the downstream side near the hole inlet and a jet acceleration zone on the upstream side near the hole exit. Near the hole inlet, vortex is formed due to the high-speed shear effects on upstream sidewall. Within the low-speed zone, flow characteristics were associated with strong vorticity, high turbulent kinetic energy, and low Reynolds stress components. In contrast, turbulence in the jet acceleration zone is higher for two intermediate VRs, which depends on the momentum balance between jet and mainstream. At VR = 0.4, large-scale vortex structure was formed inside the hole. The mainstream blockage led to a counter vortex in the leading edge of hole exit, which caused strong oscillation and contributed to hairpin vortex downstream. As VR increased, more complex axial vortical structures were observed, and dominant frequencies were converted. At VR = 1.5, the high-speed jet was more stable to show more regular vortical structures inside the hole and induced shear vortices with strong K-H instabilities in external field. By clarifying in-hole flow characteristics and establishing correlations with external JICF behaviors, this study aims to enrich the data of experimental benchmark for in-hole JICF validation and provides insights for optimizing film cooling strategies.