Bubbles and drops in the vicinity of turbulent/non-turbulent interface in turbulent boundary layers

The properties of low-vorticity pockets within the turbulent region (named as bubbles) and high-vorticity pockets within the potential flow region (named as drops) in the vicinity of turbulent/non-turbulent (T/NT) interface in turbulent boundary layers are first experimentally investigated using two-dimensional time-resolved particle image velocimetry. The T/NT interface is detected with a turbulent kinetic energy (TKE) criterion. The bubbles and drops which enlarge the T/NT interface area will promote the entrainment process. A detailed study revealed that the T/NT interface above bubbles is generally higher than the mean interface position, whereas the T/NT interface below drops is lower. Due to different sizes of bubbles and drops, though the temporal probability of bubbles is much larger than drops, the spatial probability is similar. Moreover, the conditional mean velocity distribution and the convection speed of the bubbles and drops are first obtained, especially in experiments. The streamwise velocity of fluids in bubbles at different Reynolds number is larger than surrounding turbulent flow, while in drops, the streamwise velocity is smaller than surrounding free-stream. The conditional mean streamwise velocity difference can be well scaled with respective friction velocity. Besides, bubbles and drops will convect downstream with the velocity of surrounding fluids.