Boundary layer transition induced by low-speed synthetic jets

The effect of low-speed, circular synthetic jets (SJ) on the turbulent transition of a laminar boundary layer (BL) is studied through Direct Numerical Simulations (DNS). The SJ capability in fixating the streamwise location of transition onset is analyzed in terms of its operation parameters (reduced frequency $F^+$ and momentum coefficient $C_\mu$). The effect of free-stream turbulence (FST) on the near-wall vortical structures generated by the synthetic jet is analyzed as well, to mimic the actual operation of the control system. Velocity spectra, phase portraits and dynamic mode decomposition (DMD) allow to investigate flow unsteadiness and transition to a chaotic state. In most of the investigated cases, SJs successfully promote transition, as the result of varicose-symmetric hairpin-like vortices generated at the jet exit. In particular, it is found that increasing the momentum coefficient always reduces the size of the laminar region; a non-monotonic behaviour of the laminar fetch is noted as the reduced frequency is increased, suggesting the existence of an optimal frequency value. Combination of FST and SJ actuation results in spanwise-asymmetric vortical structures, with little difference in the location of the transition onset as compared to the previous case. The present analysis can be used to gather information on the practical implementation of low-speed SJ actuators as active turbulators.