Formation and evolution of shear layers in a developing turbulent boundary layer

The evolution and formation mechanism of large-scale shear layers in a turbulent boundary layer are investigated using time-resolved PIV datasets of a developing turbulent boundary layer from inception at the trip up to a friction Reynolds number of Reτ =3000. The spatially developing boundary layer is formed on a 5m long flat plate towed through a water tank. In this frame of reference, evolving large-scale features with convection velocities close to the freestream appear nominally stationary within the field of view, enabling us to track the development of these features. An analysis of the instantaneous convection velocity associated with low- and high-speed motions reveals that there are differences in the trajectory and local convection velocity between these large-scale motions at a given wall-height in the outer region. As these motions travel at different speeds, a sequence of instantaneous streamwise velocity fluctuation fields shows that strong streamwise velocity gradients appear along the interfaces where low- and high-speed regions interact. To further investigate how these regions are associated with the formation of shear layers in a turbulent boundary layer, we compute conditional averages of streamwise velocity fluctuation based on strong shear layers. The results suggest that the difference in convection velocities between low- and high-speed regions can cause these regions to come together, forming internal shear layers in the outer layer. In addition, a sequence of instantaneous velocity fluctuation fields exhibit signs of shear layer roll-up following the formation of these shear layers, leading to the development of a large-scale slowly overturning motion. Based on these findings, we discuss a conceptual scenario which describes dynamic interactions of these shear layers and their associated large-scale coherent motions.