Tomo-TRPIV Measurement of Coherent Structure in Logarithmic Sublayer of Turbulent Boundary Layer

Experimental investigation by tomographic time-resolved particle image velocimetry (Tomo-TRPIV) was performed for three-dimensional three-component (3D-3C) velocity field of the turbulent boundary layer over the smooth plate. Multi-scale local averaged velocity structure functions were proposed to represent the multi-scale structures in turbulence. The ejection events and sweep events of coherent structures were detected by the zero-crossing points in longitudinal direction of the local-averaged velocity structure functions of the streamwise velocity. Canonical features of the detected ejection events and sweep events were obtained by the phase alignment average of the instantaneous local velocity fields, velocity gradient field, vorticity field and velocity deformation rate field, which were all presented as antisymmetric quadrupole structures. Especially, the streamwise vorticities have antisymmetric wall-normal multi-layer quadrupole structures along the streamwise, spanwise and wall-normal directions, which indicates that the coherent structures at each wall-normal layer are closely connected. The wall-normal multi-layer quadrupole structures lead to an intense exchange of momentum, mass and energy, and maintain the evolution and development of the coherent structures.