INVESTIGATION ON THE DISTRIBUTION OF UNIFORM MOMENTUM ZONES WITH VARIABLE-INTERVAL SPATIAL AVERAGE DETECTION

The coherent structures in the turbulent boundary layer (TBL) are of great importance for the generation of Reynolds stress and turbulent kinetic energy (TKE). It is essential for the effective control of the TBL to investigate the evolution of the coherent structures. Uniform momentum zones (UMZs) are typical large-scale coherent structures in the TBL, and the relationship between their spatial distribution and other coherent structures is still lack of comprehensive understanding and experimental evidence. In this work, the large-scale structures are detected through variable-interval spatial average (VISA) based on both positive and passive streamwise velocity gradients with particle image velocimetry (PIV) results, which have a large field of view and are achieved in the streamwise-wall-normal plane of a flat-plate TBL in the water tunnel. Subsequently, the influence of the structures on the spatial distribution of UMZs is studied. It is observed that the number of UMZs increases near the large-scale structures detected by VISA with both positive and passive streamwise velocity gradients due to the strengthened hairpin heads, while the number is always larger for the detection with positive streamwise velocity gradient. Furthermore, the UMZ number near the VISA-detected low-speed coherent structures is always larger than that around the high-speed region, which is independent on the streamwise velocity gradient during the VISA detection.