Abstract
This study aims to clarify the effects of installing teardrop-shaped dimples on the cutback surface in film cooling at the airfoil trailing edge, focusing on the mixing between the main and cooling flows. In a previous heat transfer experiment, the authors found that installing teardrop-shaped dimples on the cutback surface increased both the surface-averaged film cooling effectiveness and the Nusselt number by 11% and 58%, respectively. To investigate the factors contributing to these improvements, instantaneous velocity fields over both smooth and dimpled cutback surfaces were measured using two-dimensional three-component particle tracking velocimetry (2D3C-PTV). The results showed that for the teardrop-shaped dimple case, the peak value of the power spectrum density of fluctuating velocity in the cutback-surface-normal direction behind the lip decreased by 86% compared to the smooth case. Additionally, the integral time scale in the cutback-surface-normal direction decreased by approximately 70%. These findings suggest that dimple installation suppressed the development of alternating large-scale vortex shedding behind the lip. Consequently, the area-integrated Reynolds shear stress in the cutback-normal direction, indicating the strength of turbulent transport, was lower for the dimpled cases than that for the smooth case in the mixing layer. Conversely, the dimpled cases exhibited higher values of the area-integrated Reynolds shear stress near the wall. These suppression and enhancement effects of turbulent transport due to dimple installation could explain the experimentally observed improvements in film cooling effectiveness and Nusselt number ratio.
