Investigation of the turbulent/non-turbulent interface in separated and reattaching flows

Characteristics of the turbulent/non-turbulent interface (TNTI) and entrainment in separated and reattaching flows induced by an oscillating fence are investigated using time-resolved particle image velocimetry. Disturbed flows are classified into subcritical, transitional, critical and supercritical cases based on the ratio of the oscillation frequency to the natural vortex shedding frequency. In the recirculation zone, distinct vortices across different cases lead to significant variations in TNTI characteristics. In the subcritical case, the TNTI evolution resembles that in the stationary fence case but with intensified height fluctuations due to the undulation of separated shear layer. For other cases, the mean TNTI height increases with the oscillation frequency, while height fluctuation diminishes. The TNTI thickness varies with nearby vortices, scaling with the Taylor microscale. After the reattachment, TNTI height distributions converge into two groups: subcritical and transitional cases exhibit larger fluctuations and positively skewed probability density functions (PDFs), while critical and supercritical cases show smaller fluctuations and basically symmetric PDFs. The TNTI thickness becomes consistent across various cases, matching the adjacent small-scale vortex size. Besides, the nibbling mechanism of entrainment aligns well with the flow development. The minimum mean entrainment velocity coincides with the strongest prograde vortex while the maximum occurs at $x\approx 1.2x_{{r}}$ (where $x$ denotes the streamwise coordinate and $x_{{r}}$ is the mean reattachment position) in all cases. Engulfment is enhanced near the reattachment location by oscillations in the transitional and critical cases, but is suppressed in the supercritical cases due to the weakness of vortex structures at higher oscillation frequencies.