Large-scale and very-large-scale motions in a moderate-Reynolds-number turbulent boundary layer over a flat plate towed in a tank

An experimental work is conducted to investigate large-scale motions (LSMs) and very-large-scale motions (VLSMs) of a turbulent boundary layer generated by a flat plate towed in a tank. Reynolds number based on friction velocity and boundary layer thickness covers a range of 1200 <= Re-tau <= 3100. The streamwise-wall-normal plane and the streamwise-spanwise plane are separately measured using time-resolved stereoscopic particle image velocimetry (TR-SPIV) and time-resolved planar PIV (TR-PIV), respectively. The large-scale coherent structures in the two orthogonal planes are identified by the clustering method. Based on spatiotemporal evolutions of instantaneous flow fields, it is observed that low- and high-speed coherent structures collide and lean against each other, generating a strong shear layer between them. The low-speed coherent structures move away from the wall, while the high-speed coherent structures travel toward the wall near the interaction regions during the shearing, which may lead to merging and splitting of the coherent structures. The coherent structures grow in streamwise scales by self-stretching and coalescence with adjacent coherent structures. The properties of the LSMs and VLSMs, including scales, inclination angles, meandering behaviors, and contributions of high- or low-speed VLSMs to skin friction, are quantitatively examined. Furthermore, the dynamic characteristics of the LSMs and VLSMs are classified into merging, splitting, growing, and shortening, with occurrence frequencies independent of Reynolds number. Effects of shearing on the merging and splitting events are discussed. Results from this work suggest that the primary formation mechanism of the VLSMs is the self-growing of LSMs vigorously stretching along the streamwise direction.