Control of the coherent structure dynamics of a film cooling flow by plasma aerodynamic actuation

Abstract

A numerical study of film cooling flow from a cylindrical hole with and without plasma aerodynamic actuation is conducted using large eddy simulation. A phenomenological plasma model is employed to solve the electrohydrodynamic body force generated by DBD plasma actuator. The main attention is focused on the unsteady behaviors of coherent structures, which determine the film cooling efficiency. The time-averaged flow field parameters are analyzed, and results show that the plasma aerodynamic actuation suppresses the formation of downstream separation spiral node vortices by injecting energy into the boundary layer so as to improve the film cooling efficiency. Moreover, the spatial-temporal evolution processes of coherent structures are presented and discussed, which show that the hairpin vortices decrease in size and number, and stay closer to the wall because of the downward force and energy injection effect generated by the plasma aerodynamic actuation, then break into small-scale streaks in a short downstream distance. Subsequently, discussions about the space correlations of velocity fluctuations coupled with kinetic-energy spectra are made using the discrete Fourier transform, and results show that the space correlation coefficients become larger and periodically oscillate in near filed region, indicating that hairpin vortices are distributed more orderly, and they occurs irregular oscillations around zero in a shorter downstream distance confirming that hairpin vortices happen to breakup earlier. Furthermore, owing to the plasma aerodynamic actuation, the energetic wavenumbers are increased and their corresponding values of the kinetic-energy spectra are decreased, signifying that the hairpin vortices reduce in size and lose their strength in spectral space. Besides, the plasma aerodynamic actuation significantly decreases the surface area of the mixing interface inferring that the entrainment of the crossflow by hairpin vortices is suppressed.