On the spatial and temporal development of reverse flow- and high wall-shear stress events in ZPG-TBL

The 3D Lagrangian particle tracking method Shake-The-Box (STB) and, subsequently, the data assimilation (DA) methods FlowFit3 and a novel PINN implementation have been applied to measure time-series of flow fields in a narrow wall-attached parallel volume of a zero-pressure-gradient (ZPG) turbulent boundary layer (TBL) at various Reynolds numbers. The measurement volume has an extension of ~60 mm along flow direction, up to 15 mm in spanwise- and 2 mm in wall-normal direction. The STB experiments have been conducted in the 1m-WT facility of DLR in Göttingen at relatively high particle image densities (~0.07 ppp) using µm-sized DEHS particles and high-repetition pulse lasers at up to 38.1 kHz. More details are given in. Almost all features of the flow inside the viscous sub-, buffer- and lower logarith-mic- layer in space and time at Reynolds numbers up to Reθ = 5,455 have been resolved. The gained data consists of long time-series of 3D velocity, acceleration and pressure fields given by FlowFit3 (see Figure 1, 5 and 6) and short time series of high-resolution PINN data assimilation (see Figure 7 and 8), as well as a huge amount of long Lagrangian particle trajectories. Selecting the viscous sublayer part of the measurement volume provides time-series of the 2D distribu-tions of both components of the instantaneous wall-shear stresses tau_u,w. Extreme events of high positive- and of rare negative wall-shear stresses are selected to understand the multi-scale-interaction of their spatial and temporal development and the small scale local topologies of vor-tical- and streaky structures responsible for such intermittent events in the near-wall region.