Analysis of vortex populations in turbulent boundary layers based on tomographic PIV

Vortex populations in the logarithmic region of turbulent boundary layers were investigated using results from tomographic PIV. The experiments were carried out in a water channel facility with $\delta \approx $ 125 mm and Re$_{\tau }\approx $ 2500 (Re$_{\theta }\approx $ 6200). Measurement volumes were about 90 x 80 x 9mm$^{3}$ (1650 x 1470 x 130 viscous units) spanning a wall-normal range from z$^{+}$ = 150 to 280. Four 2K x 2K cameras were mounted above the channel and aimed at the measurement volume with tilt angle about 30 degrees to the wall normal direction. The magnification was 0.07 mm/pixel. Correlations were performed on 48 x 48 x 48 voxel volumes with 75{\%} overlap yielding a vector spacing of 17 x 17 x 17 viscous units. Swirl strength and swirl direction were used to identify and characterize vortices in terms of orientation, circulation, size, and convection velocity. The results showed that swirl direction was a better indicator than vorticity of eddy orientation. Eddy circulation was found to increase approximately quadratically with eddy radius. The advantages and limitations of tomographic PIV vs. dual plane PIV will be discussed.