Tomographic particle image velocimetry measurements of synthetic jet in turbulent boundary layer

We experimentally investigate the 3-D flow characteristics caused by synthetic jet in the turbulent boundary layer (TBL), with the aim of analyzing the differences and similarities of hairpin vortices generated by jet of different hole diameters. For flow fields with hole diameters of 3 mm, 4 mm, 5 mm, the 2D time-resolved particle image velocimetry (TR-PIV) is used for preliminary experiment to determine the generation region of these hairpin vortices, and then the three-dimensional instantaneous snapshots of the region are obtained by tomographic PIV (Tomo-PIV). The statistical average results show that the downstream velocity deficit area is positively correlated with the hole diameter, and the drag reduction effect looks better with small hole diameter. The phase average extracts the three-dimensional morphology of the hairpin vortices produced by synthetic jet, and its distribution tends to be dense with the hole diameter, which is related to the velocity deficit. The two-point cross-correlation coefficient represents the scale of the coherent structure, and the three component scales of these hairpin vortices are smaller with large hole diameter, which is due to insufficient space for development. The flow fields are divided into high-energy and low-energy by proper orthogonal decomposition (POD). It is found that the increase of hole diameter can transfer the generated hairpin vortices from low-energy to high-energy, showing that the strength of high-energy hairpin vortices is positively correlated with the hole diameter.