Interference effects on the wake dynamics of a horizontal circular cylinder close to a wall

This study investigates the interference caused by the presence of a bed on the wake of a circular cylinder placed horizontally in a thick boundary layer. Experiments were conducted at two Reynolds numbers of 4000 and 8000, with the cylinder-to-bed gap varying from 25% to 150% of the diameter. The experimental design adopted enabled an exploration of the flow by introducing overlapping gap ratios across the two Reynolds numbers. The time-averaged and phase-averaged statistics were analyzed, and proper orthogonal decomposition techniques were employed to assess the wake dynamics. The results from the near-bed reverse-flow intermittency analysis revealed that for the same gap ratio, lower Reynolds number cases were more prone to flow separation. The interference from the bed resulted in a slower growth of the shear layer emanating from the lower side of the cylinder compared to the upper side. Phase-average analysis revealed strong vortex interaction and vorticity cancelation at lower gap ratios, while alternate Kármán vortex shedding dominated at higher gap ratios. Qualitatively similar flow structures were observed for the same gap ratio at different Reynolds numbers, highlighting the role of gap ratio over the physical gap size in governing the wake behavior. The interaction between the bed and the lower cylinder shear layer at smaller gap ratios promoted the breakdown of the large-scale flow structures, which resulted in the increased dominance of the incoherent component of the streamwise Reynolds normal stress.