Experimental Investigation of In-Hole Evolutionary Flow Behavior in a 777-Shaped Jet in Crossflow Via Refractive Index Matching-Based TR-PIV Technique

Abstract The 777-shaped jet in crossflow (JICF) has been widely used in film cooling applications. To elucidate in-hole flow behavior and its evolution, time-resolved particle image velocimetry and refractive index matching were used to experimentally investigate the in-hole flow characteristics of the 777-shaped hole (35 deg of inclination angle) at velocity ratios (VRs) of 0.4, 0.8, and 1.2, with a density ratio of 1.0. Two in-hole low-speed zones were identified: at the trailing edge of the cylindrical section and within the diffuser. The region near the cylindrical section low-speed zone exhibited intensified vorticity, turbulent kinetic energy, and turbulent dissipation rate, attributed to clustered vortex shedding near the hole inlet. The complex formation of in-hole vorticity was further examined via spectral proper orthogonal decomposition. At VR ≤ 0.8, only a single Kelvin–Helmholtz (K–H) structure was observed at the trailing edge of the hole inlet. To verify the analysis at high VRs, an additional case (VR = 2.0) was considered. At VR ≥ 1.2, three K–H structures with negative vorticity were identified: at the trailing edge of the hole inlet, at the beginning of the diffuser, and within the in-hole region beneath the leading edge of the orifice. Overall, the findings of this study can enhance the experimental reference database on the in-hole flow characteristics of the 777-shaped hole and inform the design of shaped JICF structures for film cooling in gas turbines.