The flow field generated by the interaction between a zero-net-mass-flux (synthetic) jet and a boundary layer crossflow is studied in the present work. The investigation consists in Direct Numerical Simulations (DNS) of the resulting unsteady field, for various boundary layer Reynolds numbers Reg^ and jet reduced frequencies F+. In particular, the actuation frequency is chosen in order to match the unstable (or less stable) frequency of the incoming crossflow. The analysis is focused on the near-field behaviour of the flow, i.e. next to the jet exit orifice, and in the downstream region. It has been found that the presence of the jet deeply modifies the boundary layer transition. Indeed, the instantaneous flow field is characterized by the presence of unsteady, hairpin-like vortical structures, similar to the ones generated by localized roughness elements. Moreover, streaky structures can be detected in the time-averaged flow field. The overall behaviour of the flow field (investigated via time-averaged boundary layer integral parameters) allows to state that early transition can be achieved by using SJ devices. This work is part of a wider project which aims at realizing a reduced-order modeling of a piezo-driven synthetic jet in crossflow, based on simulations and lumped element modeling of the actuator.
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