Investigation of Coherent Motions in a Flat-Plate Turbulent Boundary Layer with Adverse Pressure Gradient

Convection velocities of small- and large-scale patterns in a turbulent boundary layer obtained via Direct Numerical Simulation are computed by correlating finite-size interrogation windows between consecutive time steps, which is common practice in Particle-Image Velocimetry. A particular region in the logarithmic layer and the lower defect layer of the mean flow is identified where different patterns belonging to different flow variables are convected downstream by the local fluid material. The corresponding structures are unique and match with the well-known high- and low-speed streaks of the streamwise velocity component. Outside of the region where these structures are observed, the convection of local disturbance patterns exhibits wave character, i.e., they travel faster than the mean flow close to the wall and slower in the outer defect layer and at the edge of the boundary layer. This further confirms our interpretation of the presence of dominant material-bound coherent structures in the intersection zone of the log layer with the defect layer.