On the effects of square-fractal turbulators on the flow field generated by a synthetic jet actuator

•Experimental investigation of a synthetic jet with different inlet conditions.•Stroke lengths values covering a range from the nearly continuous to the pure vortex case are investigated.•The fractal grid shortens the outer vortex lifetime.•The vortex trajectory scales with the stroke length, regardless the inlet conditions. In this work the effects due to the introduction of a 3-iterations square fractal grid inserted at the exit section of a synthetic jet actuator are experimentally analysed and discussed. The contributions related to the secondary iterations are assessed comparing the results with those obtained inserting a single-square grid (obtained by removing the 2nd and the 3rd iteration bars). The flow field generated by an axisymmetric synthetic jet is also investigated as a reference. The synthetic jet is produced under three different dimensionless stroke lengths, L0/D=7.1,19.2 and 50.0, covering flow regimes ranging from the nearly pure vortex to the nearly continuous jet. Results are reported both in terms of time-averaged and phase-averaged statistics. Notwithstanding the different nature of the jet flow, fractal turbulators confirm the interesting feature of an extended range of turbulence production, as already documented for continuous jets. Phase-averaged results show that the vorticity field generated by the presence of the grid interacts with the vortex produced in the jet shear layer, causing its azimuthal distortion. This phenomenon can be prevented in the single-square grid case at lower L0/D values, but it is still clear when the fractal one is located at the nozzle exit section. Regardless of the inlet conditions, the outer vortex position scales with the stroke length. However, the introduction of the grid shows an effect on the lifetime of the vortex, with a faster dissipation in the fractal grid case.