Experimental study of flow field around pile groups using PIV

The flow fields around different pile groups (multiple circular cylinders) mounted vertically on a flat bed were experimentally studied by using Particle Image Velocimetry (PIV). The characteristics of the horseshoe vortex (HV) in front of the pile groups were systematically investigated, including its time-averaged flow topology and instantaneous evolution, and the associated bed shear stress. The contours of the time-averaged velocity components, turbulence intensities and Reynolds shear stresses in the gap region between the piles were presented. Additionally, the flow fields on the symmetry plane of the pile groups were also described in detail. This study thoroughly discussed the influences of pile Reynolds number (Re-D) and pile spacing (G/D) on the flow characteristics around pile groups. The results show the horizontal position of the time-averaged HV being closer to the pile with increasing Re-D in the range of Re-D < 10(4). The variation of the time-averaged HV position with G/D is not monotonically increasing or decreasing. The process of the transient HV evolution mainly includes merging the weak vortex, being swallowed up by the strong vortex and breaking up due to dissipation and diffusion. The bed shear stress in front of the pile for the case of G/D = 1.5 is relatively larger compared with the cases of G/D = 1 and 2. With the increase of G/D, the reverse flow and upward flow between piles become significant. The flow on the symmetry plane of pile groups is accelerated due to the contraction of lateral piles, and the velocity profiles keep similarity for different flow conditions.