Abstract

The objective of this study is to clarify the effects of the film-cooling flow pulsation and the differences between the Strouhal number ratios of 1.0 and √2. The surface-averaged film cooling effectiveness for the Strouhal number ratios of 1.0 and √2 had decreased and increased, respectively, in comparison with the steady cooling flow in the authors' previous large eddy simulations. Subsequently, clarification on the possible reasons for these changes was sought. Measurements of the instantaneous velocity fields over the smooth cutback surface at two different pulsation frequencies were performed using two-dimensional three-component particle tracking velocimetry (2D3C-PTV). Notably, the power spectrum density of the wall-normal velocity fluctuations showed that the strongest peaks appeared at the pulsation frequencies, and the peak value for the Strouhal number ratio of 1.0 was much higher than those for the steady cooling flow and the Strouhal number ratio of √2. When the absolute Reynolds shear stresses integrated for the mixing-layer region were compared, those for the Strouhal number ratios of 1.0 and √2 were found to be higher and lower, respectively, than those for the steady cooling flow. Remarkably, the suppression of the turbulent mixing for the Strouhal number ratio √2 was caused by the suppressed development of the large-scale alternating vortices shed from the lip edge by imposing the cooling-flow pulsation at the frequency nonresonant with the vortex shedding frequency of the steady cooling flow.

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