Unsteady analysis of adiabatic film cooling effectiveness behind circular, shaped, and sand-dune-inspired film cooling holes: Measurement using fast-response pressure-sensitive paint

The unsteady adiabatic effectiveness behind three types of holes was measured with fast-response pressure-sensitive paint and a high-speed camera, i.e., a circular hole, a shaped hole and the sand-dune-inspired hole. During the experiment, coolant fluid (CO2) was discharged from a single injection hole at an inclination angle of 35°. The blowing ratio (M) was varied from 0.40 to 1.40. The unsteady behavior of effectiveness was quantified clearly in terms of standard deviations (SDs), spatial correlations, and dynamic mode decomposition. In contrast to the circular hole, the coolant film injected from shaped and Barchan dune-shaped injection compound (BDSIC) holes remained attached to the surface. No separation appeared in any configuration. Especially for the BDSIC concept, significantly greater effectiveness, but a lower SD, was found behind the dune than in the other two configurations. Using two-point spatial correlation, the prominent signatures (i.e., a counter-rotating vortex pair and horseshoe-like vortices) buried in the cross-flow jet were determined, which were responsible for the enhanced SDs in the circular and shaped holes. As for the BDSIC configuration, large-scale coherent structures (i.e., such as circulations, anti-counter-rotating vortex pairs, and strong shear) were identified from the measured effectiveness at blowing ratios (M) of 0.40 and 0.90. Based on dynamic mode decomposition analysis, the corresponding dynamic modes were extracted from the instantaneous effectiveness fields. Although the circular hole featured dominant frequencies (St) of 0.026 and 0.008 at blowing ratios (M) of 0.40 and 0.90, respectively, the BDSIC configuration demonstrated a constant shedding frequency (St) of 0.009. This paper represents the first effort to use fast-response PSP sampling at a high frame rate to quantify the unsteady behavior of adiabatic effectiveness over a flat plate.