Evolution of coherent structures in turbulent boundary layers based on moving tomographic PIV

A moving tomographic particle image velocimetry method was designed and implemented to measure temporal evolution of velocity fields in three-dimensional volumes and to track coherent structures within a turbulent boundary layer with Re _τ ≈ 2,410. The evolution of hairpin structures and eddy packets was examined at two locations in the logarithmic region: z ⁺ = 100–300 and 300–500. Meandering, merging and breaking of long slow regions associated with packets were observed. The meandering of long slow regions at both wall-normal locations was tracked using cross-correlation between neighboring time steps. It was found that long slow regions could persist within the logarithmic region over a travel distance of 15δ corresponding to a time period 24.3δ/U _∞ (t ⁺ > 2,300) and that the packet regions could travel stably forward in the streamwise direction while maintaining fixed spanwise inclinations in the range 0°–10°.