The relation between shearing motions and the turbulent/non-turbulent interface in a turbulent planar jet

The relation between shearing motions and the turbulent/non-turbulent interfacial (TNTI) layer is studied with direct numerical simulation of a temporally evolving planar jet. Small-scale shear layers are detected with the triple decomposition of the velocity gradient tensor, which is decomposed into shear, rotation, and elongation tensors. The shear layers are found in the turbulent sublayer more frequently than in the turbulent core region although they hardly appear in the viscous superlayer. The shear layers undergo a biaxial strain with stretching in the shear vorticity direction and compression in the interface normal direction. This compressive strain is related to the non-turbulent fluid, which is relatively advected toward the shear layer. The shear layer thickness in the TNTI layer is well predicted by Burgers vortex layer. The velocity jump of the shear layer is about seven times the Kolmogorov velocity both in the turbulent core region and the TNTI layer. However, the layer thickness normalized by the Kolmogorov scale is about 6 in the turbulent core region and decreases in the TNTI layer, where consequently, the shear Reynolds number becomes small. The shear layers have significant contributions to the enstrophy production in the turbulent sublayer and the viscous enstrophy-diffusion toward the viscous superlayer. The shear layer and the outer edge of the TNTI layer have a curvature radius of about 50 times the Kolmogorov scale. The alignment between the shear layer orientation and the interface normal direction confirms that the shear layers near the interface are mostly parallel to the TNTI layer.