Gap spacing effects on shape and vortex dynamics of tandem ice cylinders in a cross-flow

The physics of a solid object melting under the influence of external flows governs many natural and engineered systems. For example, the shape of icebergs drifting in the ocean affect the iceberg's relative motion, wake dynamics and melt rates. Despite their vital role in ocean systems, melt rates of icebergs are frequently under-predicted and require further study on how impinging flow conditions affect melting. Previous work has found that the presence of a meltwater wake from an ice object leads to varying local melt rates. Vortex shedding has been observed to affect melt rates in numerical studies, but the relationship between ice shape dynamics and vortex dynamics has not yet been studied. Specifically, groups of melting objects such as iceberg clusters are hypothesized to experience collective effects on the shape and vortex dynamics based on their proximity to one another.

In this work, we study how the initial spacing of two tandem (in-line) ice cylinders affects the interaction of dynamic shape changes and vortex dynamics. Experiments are performed in a closed-loop water channel for initial gap ratios (g*=L/D₀) ranging from g*=1-6 and initial Reynolds number of Re=UD₀/ν=250. Flow structures are measured using laser-induced fluorescence and particle image velocimetry (PIV). Experiments are repeated in a towing channel that is designed to measure shape change dynamics via surface area measurements. We find that the shapes dynamics of the upstream and downstream cylinders, denoted UC and DC respectively, strongly depend on gap ratio and are associated with time- and gap-dependent vortex dynamics. For g*>1, vortices periodically form in the gap between the cylinders and reattach to the DC. Beyond a critical gap ratio, each cylinder forms an independent vortex wake and the shape change dynamics of UC and DC are no longer coupled. Vortex parameters such as shedding modes, Strouhal number, wake width and circulation strength are also compared across gap ratios for each cylinder. Lastly, local shape dynamics are correlated to local vortex dynamics. We find that melt rates are highest for impinging flows, and in the presence of vortex formation, melt rates are influenced by the adjacent flow recirculation region.