Feature-based analysis of a turbulent boundary layer under spanwise wall oscillation

Spanwise wall oscillations alter the organization of low-speed streaks and ejections in turbulent boundary layers, eventually leading to skin friction drag reduction. Such flow regimes are represented by pointwise statistics or spatial correlation. This work attempts quantify systematic distortions dominant structures feature-analysis, intended overcome dispersion observed correlation functions. Furthermore, data from tomographic particle image velocimetry employed clarify mechanism that inhibits hairpin auto-generation, as described Kempaiah et al. [“3-dimensional based evaluation skin-friction reduction spanwise oscillation,” Phys. Fluids 32(8), 085111 (2020)]. Based on instantaneous distribution Reynolds stresses, a specific template is defined for ejections. Events corresponding this collected parametrized with their occurrence, geometrical properties (length orientation), dynamics (intensity). The approach compared most practiced statistical analysis explain significance features extracted detection algorithm relation mechanism. Data comparing stationary oscillating drag-reducing regime (A + osc = 100, T 100) investigated near-wall region (y < 100). Ejections systematically exhibit positive pitch, supporting hypothesis only rear region, close wall, affected motion. A side-tilt elongated ejection events past phase maximum oscillation velocity, which hypothesized inhibit auto-generation. latter indicates dependence regime. results also indicate reduced approximately 10% 15%, respectively, further consolidating rapid lateral distortion being responsible different organizations region.