Structural organization of the quiescent core region in a turbulent channel flow

The structural organization of the quiescent core region in a turbulent channel flow was explored using direct numerical simulation data at Reτ = 930. The quiescent core region is the uniform momentum zone located at the center of the channel, and contains the highest momentum with a low level of turbulence. The boundary of the quiescent core region can be identified from the probability density function of the streamwise modal velocity. The streamwise velocity changes abruptly near the boundary of the core region. The abrupt jump leads the increase of the velocity gradient, which is similar to the vorticity thickness of the laminar superlayer at the turbulent/non-turbulent interface. The strong shear induced from the abrupt change is originated from the vortical structure lying on the boundary of the core region. The spanwise population densities of the prograde and retrograde vortices have a local maximum near the boundary of the core region. The prograde vortex dominantly contributes to the total mean shear near the core boundary and the contribution to the total mean shear rapidly decreases within the core region. The prograde and retrograde vortices form a counter-rotating vortex pair at the boundary of the core region associated with the nibbling mechanism. The boundary of the core region contains large-scale concave and convex features. The concave (convex) core interface is organized by the negative-u (positive-u) regions which induce the ejections (sweeps) around the core boundary.