Numerical comparison of saw-tooth plasma actuators for film cooling flow control over a flat plate

Large eddy simulations of an inclined round jet issuing into a crossflow with and without saw-tooth plasma actuator (STPA) are performed to further enhance film cooling efficiency. A phenomenological plasma model is used to solve the electrohydrodynamic force generated by the STPA, which is installed downstream the cooling hole with two arrangement-types. Results show that the STPA is not always advantageous to the film cooling. The height of jet trajectory and strength of counter rotating vortex pair (CRVP) become larger when the root of the STPA is located at the centerline, yielding a lower film cooling efficiency. Conversely, there are three sources responsible for the film cooling efficiency enhancement when the tip of the STPA is located at the centerline. The first is that the spanwise-periodic momentum injection effect created by the STPA induces coolant outward along the wall. The second is that the anti-CRVP created by the STPA promotes the spanwise-coverage of the coolant. The third is that the downward force and streamwise momentum injection effect generated by the STPA weaken the lift-off of the CRVP and help the coolant to extend farther downstream. Moreover, the absences of hairpin vortices are observed and the elongated streaks downstream the cooling hole reduce in size, and the space auto correlation coefficients and kinetic-energy spectra of velocity fluctuations statistically confirm these phenomena in the physical and spectral spaces. Consequently, the coherent structures produce less turbulent mixing between the coolant and crossflow, and this is approximately quantified by the variations of the mixing surface area.