Two-Dimensional Visualization of Turbulent Wall Shear Stress Using Micropillars

The wall shear-stress sensor MPS(3) based on flexible micropillars has been used to experimentally assess the two-dimensional wall shear-stress distribution in turbulent duct How at moderate Reynolds number. The sensor covers an area of 90 x 125 viscous length scales along the streamwise and spanwise directions, respectively. Applying Taylor's hypothesis allows a further increase in the streamwise extension. Preliminary results evidence the coexistence of low- and high-shear regions representing footprints of near-wall coherent structures. The low-shear regions resemble long meandering bands interrupted by local high-shear-stress regions. A qualitative comparison evidences the structures detected in the present study to be similar to wall shear-stress distributions reported in the literature and to structures found in higher regions of the boundary layer. These preliminary findings indicate the potential of the micropillar sensor concept to be capable of detecting the multidirectional planar wall shear-stress distribution in turbulent flows. A detailed discussion of the wall shear-stress characteristics and of the geometric properties of the streaklike structures is beyond the scope of this paper, and a more intrusive discussion needs to be done in the future.