PIV EXPERIMENTAL STUDY ON STATISTICAL FRACTAL CHARACTERISTICS OF UNIFORM MOMENTUM ZONES IN TURBULENT BOUNDARY LAYER

Uniform momentum zones (UMZs), as local regions with similar momentum, become a new member of turbulent coherent structures. To study statistical characteristics, variation rules, its root causes and internal relationship with other turbulent structures about UMZs are an important way to understand wall turbulence. In this paper, the streamwise and wall-normal flow field of turbulent boundary with high spatio-temporal resolution was measured by time-resolved particle image velocimetry (TRPIV) system, statistical analysis was conducted on the partition and interface position of the UMZs. The position correlation between the UMZs boundary and the turbulent structure dominated by hairpin vortices (packets) was analyzed as well. The results revealed that the statistical analysis of streamwise velocity PDF and interface thickness of UMZs showed universal fractal characteristic, not affected by TNTI interfaces and Reynolds numbers. There are more UMZs number in the turbulent intermittent zone, and less UMZs number in instantaneous flow fields with well-developed turbulent structures with rich layers. The spatial distribution law of hairpin vortex packet determines the statistical fractal characteristics of the UMZs. The UMZs boundary near the wall converges towards the turbulent structure, while the UMZs boundary crosses the spanwise vortex core in the outer region. Forward spanwise vortex caused the aggregation of UMZs boundary in the wall-normal direction, while reverse spanwise vortex produced separation of UMZs boundary in the streamwise direction.