Experimental investigation of the water exit of the accelerating circular cylinder

We investigate the vortical dynamics generated during the forced exit of a circular cylinder through a quiescent free surface. While previous work on exit flows has largely emphasized cavity collapse and force generation, the formation and evolution of the starting vortex beneath the body have received much less attention. Using high-speed particle image velocimetry at up to 2000 Hz, we isolate and track the primary vortex shed during exit. The body motion is driven by constant vertical acceleration from rest, such that the cylinder kinematics satisfy $v_{cyl}^2$ = $2$$α$$Δ$$y_{cyl}$, with $Δ$ $y_{cyl}$ the displacement since release. This controlled protocol allows systematic variation of the acceleration alpha while keeping the maximum crossing velocity fixed. We characterize vortex trajectories, equivalent radius, peak vorticity, and circulation, and compare across accelerations. The results show that circulation growth collapses under the impulse-based scaling $Γ/ \sqrt(α(Δy_{cyl})^3)$, independent of the imposed acceleration. A consistent transition is observed as the vortex centroid approaches the free surface, where its evolution is modified by surface interaction. The trajectories remain predominantly vertical with only weak lateral drift, robust across all accelerations. Taken together, these findings establish a systematic framework for water-exit vortex dynamics under constant acceleration and highlight scaling features that parallel the classical starting-vortex problem in entry flows.