On the wake dynamics of wall-mounted helical straked cylinders

The wake dynamics of wall-mounted circular cylinders with helical strakes are investigated using particle image velocimetry under shallow flow conditions. The helical strakes attached to the cylinder surface have a three-start configuration with a pitch equal to 10d and a height of 0.2d, where d is the cylinder diameter. The influence of strake orientation is explored by positioning the cylinder to align one of the strakes at an angle of 0 degrees or 30 degrees with the streamwise flow at the cylinder-bed junction. In the presence of the strakes, the wakes displayed elevated turbulence levels and reduced mean velocity, which influence the vortex formation length and other wake characteristics. The strakes induce asymmetric behavior across the depth of the flow resulting in the disruption of coherent vortex shedding. When aligned at 0 degrees to the oncoming flow, the strakes increase the Reynolds stresses throughout the flow depth and enhance their diffusion, which can facilitate better wake mixing and momentum exchange. When the strakes are positioned at 30 degrees, the stresses are distributed over a wider region with decreased magnitude. There is a notable increase in energy contributions near the bed for the 0 degrees case as revealed by proper orthogonal decomposition. The spatial modes exhibit a wide range of scales across the flow depth for the straked cylinders.