Scalings of small-scale shear layers in temporally developing turbulent boundary layers

Small-scale shear layers in temporally developing turbulent boundary layers are investigated using direct numerical simulations at momentum-thickness Reynolds numbers from 2000 to 13 000. Flow statistics are evaluated in a local coordinate system aligned with each shear layer identified by the shear intensity. Conditional mean fields show that shear layers are embedded in a biaxial strain. Near the wall, these conditional fields further reveal two distinct regions of intense rigid-body rotation beneath the shear layer, leading to momentum transfer through a mechanism consistent with counter-rotating vortex pairs. Two distinct scalings are found for the shear layers. In the logarithmic and outer layers, the mean shear-related velocity jump normal upper Delta u 3 Δ u 3 $\Delta u_3$ and its width delta 3 δ 3 $\delta _3$ scale with the Kolmogorov velocity u Subscript eta upper T u η T $u_{\eta T}$ and length eta Subscript upper T η T $\eta _T$ evaluated in the turbulent region. In the buffer layer, delta 3 δ 3 $\delta _3$ scales with the viscous length delta Subscript nu Baseline equals nu divided by u Subscript tau δ ν = ν / u τ $\delta _\nu =\nu /u_\tau$ ( nu ν $\nu$ is the kinematic viscosity and u Subscript tau