Simultaneous time-resolved PIV measurements of two orthogonal planes in a flat-plate turbulent boundary layer at moderate Reynolds numbers

An experimental study is conducted to investigate the spatiotemporal evolution of the turbulent boundary layer (TBL) over a flat plate towed in a large tank. The friction Reynolds number (Re_τ) covers the moderate-Re_τ range of 1200 ≤ Re_τ ≤ 3100. While time-resolved stereoscopic particle image velocimetry (TR-SPIV) is used to measure the streamwise-wall-normal plane, time-resolved planar PIV (TR-PIV) is employed to simultaneously measure the wall-parallel plane. Large-scale coherent structures such as large-scale motions (LSMs), very large-scale motions (VLSMs) and “footprints” of hairpin vortices in the two orthogonal planes are captured by the simultaneous TR-SPIV and TR-PIV measurements, enabling investigations of their spatiotemporal dynamics and interactions from a three-dimensional perspective. The spatial characteristics of the LSMs and VLSMs, which are elongated and inclined in the streamwise direction and meandering in the spanwise direction, are observed. Owing to the discrepancy in the convection velocities of different coherent structures, low- and high-speed LSMs (or VLSMs) interact and stretch vigorously, generating strong shear and leading to merging or splitting. Furthermore, conditionally averaged flow fields based on events of strong shear reveal that low-speed coherent structures move away from the wall to the edge of the TBL, whereas high-speed coherent structures entrain towards the wall. Finally, two-point correlations of different flow variables are made to examine the spatial coherence of these large-scale coherent structures. The outer-scaled length and width as well as the angle of inclination of the correlation contours remain unchanged within the Reynolds number range considered in this work; however, the length and width scales vary with the wall-normal height.