Wall-shear stress patterns of coherent structures in turbulent duct flow

The wall-shear stress distribution in turbulent duct flow has been assessed using the micro-pillar shear-stress sensor MPS(3). The spatial resolution of the sensor line is 10.8 l(+) (viscous units) and the total field of view of 120 l(+) along the spanwise direction allows to capture characteristic dimensions of the wall-shear stress distribution at sufficiently high resolution. The results show the coexistence of low-shear and high-shear regions representing 'footprints' of near-wall coherent structures. The regions of low shear resemble long meandering bands locally interrupted by areas of higher shear stress. Conditional averages of the flow field indicate the existence of nearly streamwise counter-rotating vortices aligned in the streamwise direction. The results further show periods of very strong spanwise wall-shear stress to be related to the occurrence of high streamwise shear regions and momentum transfer towards the wall. These events go along with a spanwise oscillation and a meandering of the low-shear regions.