# Large Eddy Simulation of Film Cooling with Bulk Flow Pulsation: Comparative Study of LES and RANS
## 1 Introduction  
The idealized Brayton cycle indicates that the efficiency of gas turbines could be increased by increasing the turbine inlet temperature [1]. However, the blade surface temperature should be maintained below a tolerable limit to avoid excessive thermal stress on the blade. Film cooling is a widely used cooling method for turbine blades. The coolant bled from the compressor is injected onto the blade surface through small holes on the surface, and the resulting coolant film protects the surface from the hot mainstream by reducing the surface temperature. Multiple variables affect the performance of film cooling, and they are often studied by performing numerical analysis to save cost and time. In film cooling flow fields, turbulence is generated, and it can be modeled by conducting Reynolds-averaged Navier-Stokes simulations (RANSs), large eddy simulations (LESs), or by using a hybrid approach such as detached eddy simulation. It is known that the LES approach predicts the mixing between the mainstream and the coolant better than RANS turbulence models, even though its computation time is considerably longer [2; 3; 4]. Multiple numerical studies have been conducted to understand the film cooling physics under a steady main flow. [PERSON] and [PERSON] (2000) used RAMPANT code [5] to simulate film cooling on a flat plate [6]. They used the standard k-\(\epsilon\) of Launder and Spalding for a 3-D unstructured mesh. They found that the model could not well predict coolant reattachment in the narrow field near the hole at high blowing ratios. In addition, the RANS results overpredicted the centerline effectivenessand showed less lateral coolant spreading on the wall. [PERSON] and [PERSON] (2003) performed an LES for film cooling [7]. They used a dynamic mixed model as the subgrid scale model and applied the velocity profiles obtained using the RANS approach to the delivery tube inlet. Their LES results yielded better predictions of the adiabatic film cooling effectiveness than the RANS results because LES was able to predict the coherent structures generated due to film cooling. [PERSON] and [PERSON] (2007) investigated the effects of turbulence in the freestream on film cooling by conducting LES [8]. They found that LES was able to capture the counter-rotating vortex pair (CRVP) and that a fully turbulent jet decreased the film cooling effectiveness by increasing mixing with the main flow. [PERSON] et al. (2007) showed that a ramp placed upstream of the hole increased the film cooling effectiveness by inducing interaction between the main flow and the jet further away from the wall, resulting in the formation of a weaker horseshoe vortex [9]. In their simulations, the realizable k-\(\varepsilon\) model was used. [PERSON], [PERSON], and [PERSON] (2011) studied the effects of ratios of the delivery tube length to hole diameter (L/D), momentum ratio, and grid resolution on the effectiveness of adiabatic film cooling by using the realizable k-\(\varepsilon\) model [10]. They found that grid refinement around the trailing edge of the hole was effective for improving the effectiveness of film cooling at high momentum ratios. In addition, they stated that the adiabatic film cooling effectiveness decreased when the jet had high momentum and the L/D ratio was small because of higher jet lift-off. LES does not guarantee the best CFD (Computational Fluid Dynamics) results. [PERSON], [PERSON], and [PERSON] (2020) predicted the course of barium sulfate precipitation in jet reactors by CFD simulations and compared the results with the experimental data [11]. They performed the simulations using both LES and RANS models, showing that the results were similar and did not differ from each other significantly even though the LES approach required high computational cost. However, according to most numerical studies on film cooling, the LES approach is superior to the RANS method in predicting film cooling performance under the steady flow condition.