Space-Time Correlation of Large-Scale Structures in a Turbulent Boundary Layer

Taylor's hypothesis is often used to project temporal data into the spatial domain and has been used in the past to show the presence of large-scale structures ($>10\delta$) in the log and lower wake region of the turbulent boundary layer (TBL). To investigate the spatial and temporal evolution of such large-scale structures, the present study employs time-resolved Particle Image Velocimetry (PIV) in several streamwise-spanwise planes in the log-layer of a TBL ($Re_\theta=2,000$). In order to capture the full extent of these structures, four high-speed, high-resolution PIV systems are combined to span a region of approximately $3\delta \times 12\delta$ and a continuous time sequences of $\approx 50\delta/U$. Such data sets are currently unavailable from previous experimental investigations and reveal the existence of long and very long ($>8\delta$) low- and high-speed structures. Two-point space-time correlations are employed to examine the temporal extent and meandering nature of these structures with respect to their size and spacing in the log-layer. Furthermore, the validity of Taylor's hypothesis is tested for such long projection distances.