Large eddy simulation of unsteady turbulent flow structures and film-cooling effectiveness in a laidback fan-shaped hole

Abstract

Large eddy simulation (LES) is used to predict the turbulent flow structures and film-cooling characteristics of the unsteady jet/cross-flow interactions in a fan-shaped hole. The coolant hole is located on a flat plate surface with a 35 degree angle with respect to the main flow stream at a constant film density ratio DR=2 and two different blowing ratios M=1 and 2. The computational results under the different blowing ratios are validated by measurements data for the laterally-averaged and time-averaged film-cooling effectiveness. In addition, two different hybrid RANS/LES turbulence models, SAS (scale-adaptive simulation) and DES (detached-eddy simulation), are employed to show the influence of the turbulence model on the film-cooling effectiveness. The results reveal that the LES model is the only turbulence model that can calculate the film-cooling performance with an acceptable accuracy as compared to the experimental results. Comprehensive analyses are performed on the time-averaged and instantaneous turbulent flows within the cooling hole and mainstream channel, and the results show that the cooling jet flow structures and forest of hairpin vortices on the flat plate are significantly changed due to the blowing ratio. It is also observed that a pulsating behavior exists within the coolant hole, which is convected toward the hole exit. The interaction between the cooling hole flow and channel mainstream flow leads to unsteadiness at the interface mixing region (Kelvin-Helmholtz instability). In addition, velocity disturbances with various convective velocities are observed in the time-series flow fields at different blowing ratios. The evaluation of pressure fluctuation signals in the time and frequency domains show that the dominant frequency of the fan-shaped hole is shifted by changing the blowing ratio, and increasing the blowing ratio leads to the generation of more periodic vortical structures as well as improved film-cooling performance.