The quiescent core of turbulent channel flow

The identification of uniform momentum zones in wall-turbulence, introduced by Adrian, Meinhart & Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1-54) has been applied to turbulent channel flow, revealing a large 'core' region having high and uniform velocity magnitude. Examination of the core reveals that it is a region of relatively weak turbulence levels. For channel flow in the range Re-tau = 1000-4000, it was found that the 'core' is identifiable by regions bounded by the continuous isocontour lines of the streamwise velocity at 0.95U(CL) (95% of the centreline velocity). A detailed investigation into the properties of the core has revealed it has a large-scale oscillation which is predominantly anti-symmetric with respect to the channel centreline as it moves through the channel, and there is a distinct jump in turbulence statistics as the core boundary is crossed. It is concluded that the edge of the core demarcates a shear layer of relatively intense vorticity such that the interior of the core contains weakly varying, very low-level turbulence (relative to the flow closer to the wall). Although channel flows are generally referred to as 'fully turbulent', these findings suggest there exists a relatively large and 'quiescent' core region with a boundary qualitatively similar to the turbulent/non-turbulent interface of boundary layers, jets and wakes.