Frequency Effect on Properties of Turbulent/Non-turbulent Interface in Separated and Reattaching Flows Past an Oscillating Fence

The turbulent/non-turbulent interface (TNTI) in the separated and reattaching flows induced by an oscillating fence is experimentally study using particle image velocimetry in this paper. The oscillation frequency of fence is varied spanning subcritical, transitional, critical and supercritical flow regimes, respectively. In the subcritical case, the flow pattern and the TNTI properties are very similar to that in the static case, except in the vicinity of the fence where the actuated low-frequency flapping of the separated shear layer causes larger TNTI height fluctuations. The strongest conditional averaged vorticity is observed in the critical case. However, the faster transition to turbulence is induced by the higher oscillation frequency rather than the critical one. Higher oscillation frequency results in the slower decrease and less fluctuations in the TNTI height normalized by the boundary thickness. Additionally, the greatest TNTI height fluctuation in the recirculation zone is observed in the transitional case, where the vortex shedding frequency is locked to the oscillation frequency resulting in larger shedding vortices.