Experimental study on the turbulent/non-turbulent interface in the separated and reattaching flows induced by an asymmetrically oscillating fence

The separated and reattaching flow induced by an asymmetrically oscillating fence in a laminar boundary layer is investigated using time-resolved particle image velocimetry, with a focus on the evolution of shedding vortices, the characteristics of the turbulent/non-turbulent interface (TNTI), and the entrainment process. Fast-rising and slow-rising asymmetric oscillation modes are compared against sinusoidal oscillations, all at the resonant frequency. The results reveal that asymmetric oscillations effectively modulate vortex dynamics within individual cycles: The fast-rising oscillation enhances the core swirling strength, while the slow-rising mode generates larger-scale vortices and introduces a phase lag in their development. However, these asymmetric oscillations have negligible effects on the time-averaged statistical characteristics, such as the recirculation zone size, the vorticity thickness, and the turbulent kinetic energy distribution. The influence on the geometric properties of the TNTI, such as its height distribution, is also limited, primarily confined to the region near the fence. As for dynamic properties of the TNTI, the asymmetric oscillations accelerate the decay of the TNTI thickness, suggesting a premature breakdown of the primary shedding vortices. Despite this, the dimensionless TNTI thickness converges to a similar value after flow reattachment across all actuated cases. Regarding entrainment, the asymmetric oscillations alter the streamwise evolution of the engulfment flux near the reattachment point. Additionally, they moderately enhance the local entrainment velocity and flux associated with the nibbling mechanism within the recirculation zone, while reducing these parameters in the post-reattachment region. Nevertheless, the relative contributions of the nibbling and engulfment mechanisms to the total entrainment remain largely unchanged.