Aspect ratio effects on flow past a bed-mounted emergent cylinder

The wake characteristics of an emergent bed-mounted circular cylinder of diameter d are investigated for intermediate aspect ratios located within a fully developed turbulent boundary layer using particle image velocimetry. The Reynolds number based on flow depth (H) and bulk velocity (Um) is 24 700, and the aspect ratio (AR), defined as the ratio of H/d, is varied from 1.7 to 6.7. The impact of AR on time-averaged flow statistics including mean velocity, Reynolds stresses, flow anisotropy, and two-point correlations was analyzed in the vertical central plane and across three spanwise planes. This study builds upon the earlier work of Heidari et al. [Phys. Fluids 29(6), 065111 (2017)], which investigated an emergent, bed-mounted cylinder with AR = 11. The results reveal that the flow topology and coherent structures in the wake are influenced by AR across all measurement planes. In the low AR cases, detached reverse flow regions were observed near the bed, as bed friction interferes with the formation of the von Kármán vortex street. Proper orthogonal decomposition (POD) reveals a decrease in energy content in the near-bed plane with increasing AR consistent with enhanced small-scale activity. In contrast, the trend is reversed in the mid-depth and near-free surface planes, where turbulent kinetic energy becomes concentrated in the low-order modes as AR increases. Furthermore, for AR = 1.7, the POD modes are highly correlated throughout the flow depth, whereas higher AR cases show weaker correlations, indicating complex flow structures.