Experimental study on uniform momentum zones variations induced by spanwise vortex migration in the turbulent boundary layer

The migration trajectories of spanwise vortex structures were investigated to reveal their variation patterns and distribution characteristics, explicitly focusing on their correlation with the characteristic quantities of uniform momentum zones (UMZs). This approach is crucial for deepening the understanding of wall turbulence. Using a three-camera array particle image velocimetry experiment, a dataset of the stream-normal flow field was acquired over a region extending 8δ. The space-time cross correlation algorithm was then employed to extract the migration trajectories of spanwise vortex structures. The slope probability density function was applied to classify these migration trajectories, revealing the spatiotemporal topologies of vortices with different migration tendencies. The analysis also explored the relationship between instantaneous changes in the number of UMZs and vortex migration patterns. The results demonstrate that vortex structure migration can be categorized into downward, upward, and straight-line. As the initial height increases, the trajectory distribution becomes more divergent, while higher Reynolds numbers cause the trajectories to converge and exhibit a preference for upward migration. Different migration modes exhibit significant distinctions: downward migration, influenced by the sweep of the upstream vortex head, resulted in reduced vortex size; upward migration, driven by the ejection between the downstream vortex legs, promoted growth and full development of the vortex; straight-line migration reflects stable and steady evolution. The migration of vortex structures directly affects the number of UMZs by altering the heights of the turbulent/non-turbulent interface and UMZ interfaces. Downward migration increases the number of UMZs, while upward migration decreases it. Structural changes in vortex packets caused by migration are the fundamental drivers of variations in the number of UMZs.