The time signature of the turbulent/non-turbulent interface over a turbulent boundary layer

The turbulent/non-turbulent interface (TNTI) between a turbulent boundary layer and an approximately laminar free-stream is investigated with time-resolved planar particle image velocimetry (PIV). The turbulent boundary layer ($Re_\tau \approx 4000$), was formed on the floor of a water channel and was captured by a PIV system composed of a Phantom v641 4 mega-pixel camera and a Litron LDY 304 Laser. Images were acquired at 800~Hz, which was sufficient to resolve the motions of the TNTI in time. The instantaneous TNTI can be located by thresholding the velocity field based on physical arguments. A threshold of $0.95U_\infty$ can be selected based on identifying the limits of a uniform momentum zone encompassing the free-stream. The time-resolved data set allows for instantaneous tracking of the TNTI topology and detection of its convection velocity, and thus provides novel insight into the TNTI. Preliminary findings based on a sample of the data set suggest the interface is convected at a velocity between $0.6U_\infty$ and $0.7U_\infty$. The final study will include analysis of the full data set of over 400,000 time-resolved images and a more accurate estimate of the convection velocity of the interface.