Backflow structures in turbulent pipe flows at low to moderate Reynolds numbers

The statistical characteristics and the evolution of backflow structures are investigated in wall-bounded flows at Reynolds numbers up to $Re_{\tau }=1000$ . is caused by joining large-scale high- low-speed vicinity wall formed tail tip structure. distribution density percentage area region on both increase with number. have an average lifespan 8 units which found be slightly longer pipe than channel, they convected downstream velocities buffer approximately 10 units, similar Cardesa et al. ( J. Fluid Mech. , vol. 880, 2019, R3). occasionally split merge, can form detached from wall. Evidence shows that split, merged wall-detached near-wall structures. average, larger more spanwise-elongated due spanwise shearing streaks. A structure when trailing end its carrier ‘sits’ high-speed tend become wall-attached approaching undergoing a life cycle normal growth decay elongation because continuously pressed down driven persistent vortical region.