# Large Eddy Simulation of Film Cooling with Bulk Flow Pulsation: Comparative Study of LES and RANS
## 1 Introduction
However, an unsteady main flow could be induced in the flow fields of film cooling due to various reasons, such as potential flow interactions, shockwaves, passing wakes, or freestream turbulence in the combustion chamber [12]. Understanding the effects of such an unsteady main flow on film cooling performance is crucial for improving the design of turbine components. However, the experimental and numerical studies on the effects of an unsteady main flow on film cooling are scarce. [PERSON], [PERSON], and [PERSON] (2000) experimentally investigated the effects of jet pulsing on film cooling performance [13]. They showed that, at higher pulsation frequencies of coolant injection, the effectiveness of film cooling decreased. They added that the best film cooling performance was obtained at M = 0.5. [PERSON] and [PERSON] (2009) numerically studied the effect of jet pulsation on film cooling and found that the effectiveness of film cooling could be controlled and improved through jet pulsation [14]. They reported that frequency, duty cycle, and blowing ratio are the three parameters that affected the film cooling performance. Even when the instability pattern in the main flow might be extremely complex, the oscillations could be approximated in the sinusoidal form because the instability pattern would be more similar to a sinusoidal waveform compared to a simple pulse. Among the causes of the aforementioned unsteadiness of the main flow, potential flow interaction was recognized as important, and experimental studies pertaining to the effects of bulk flow pulsation on film cooling have been published. [PERSON], [PERSON], and [PERSON] (1998) discussed the effects of pulsations in the main flow on film cooling at the frequencies of 2, 16, and 32 Hz [12]. They reported that, when the pulsation frequencies were increased at the average blowing ratio of M = 0.5 and short L/D, the film cooling effectiveness decreased and the heat transfer coefficients increased. [PERSON], [PERSON], and [PERSON] (2001) reported that the flow structures of film cooling have phase-averaged and time-averaged velocity profiles and Reynolds stresses at 0, 2, 16, and 32 Hz for the average blowing ratio of M = 0.5 [15]. They showed that, when the pulsation frequencies were increased, the effect of increasing the frequency on the flow structures became stronger. In this study, numerical (LES) simulations of film cooling from a cylindrical hole on a flat plate were conducted by applying bulk flow pulsations of 32 Hz. The LES results were compared to the experimental data and the RANS results to verify the improvement offered by the LES approach relative to the RANS approach in terms of predicting film cooling performance under unsteady flow conditions. First, we checked how LES and RANS separately predict changes in film cooling performance under pulsation of the main flow. It was found which turbulence model for RANS best predicts the film cooling performance. Second, we examined how LES and RANS predict the behavior of the injectant by comparing the time-averaged and phase-averaged temperature fields with the experimental results. There have not been any studies on the behavior of the coolant by comparing the phase-averaged temperature fields obtained by experiment, LES, and RANS. The comparison is presented in this paper. Next, by comparing the flow field according to the phase, we found that certain flow structures did not appear in unsteady RANS (URANS) but did appear in LES. Finally, we compared the changes in turbulence intensity due to pulsation, followed by a discussion about the turbulence intensity characteristics by phase.