Experimental study on the characteristics of near-wall flow and instantaneous wall shear stress in front of wall-mounted cylinders

Characteristics of near-wall flow and instantaneous wall shear stress in front of wall-mounted cylinders are crucial for understanding the mechanism and correctly predicting the depth of local scour at piers. Here, the near-wall flow in front of circular and square cylinders mounted vertically in an open channel with Reynolds numbers from 9800 to 16,300 was measured via high-resolution particle tracking velocimetry (PTV). The instantaneous wall shear stress was obtained after determining the viscous sublayer thickness. The viscous sublayer, where the time-averaged streamwise velocity varies linearly along the wall-normal direction, is located below the downward flow, i.e., the turbulent horseshoe vortex, corner vortex, and jet-like reverse flow along the wall. Its thickness in the region below and away from the turbulent horseshoe vortices is thicker and thinner than that of uniform open channel flows, respectively. The instantaneous wall shear stress in the region dominated by the turbulent horseshoe vortex system exhibits a bimodal feature. The time-averaged wall shear stress is significantly amplified, with maximum magnification factors of approximately 2.5 and 4.0 in the flows upstream of the circular and square cylinders, respectively. Moreover, the corresponding turbulence intensities of the wall shear stress influenced by the turbulent horseshoe vortex in these two flows are approximately 2.0 and 5.5. The results indicate that the fluctuation of wall shear stress is crucial for correctly predicting the local scour at in-water cylinder structures such as bridge piers. However, the characteristics of wall shear stress are significantly different from those of turbulent open channel flows.