Eulerian and Lagrangian analysis of pulsatile jet vortices in a closed-circuit platform

Unidirectional pulsatile jets generated under continuous cyclic operation in closed-circuit mock circulatory platforms differ from open-circuit jets and synthetic-jet forcing in that backflow is suppressed by check valves and the suction phase is constrained, producing a finite net discharge each cycle while exhibiting a near-zero flow rate interval during part of the cycle. Here, we analyze how stroke ratio, forcing frequency, and working-fluid viscosity, together with the associated Reynolds-number variation, jointly shape near-field vortex dynamics and transport in such a system. Time-resolved particle image velocimetry was performed in the axial–radial plane of a submerged circular nozzle for stroke ratios L0/D0=2, 4, 6, and 8 and actuation frequencies of 60–100 BPM, using water and a 60 wt. % glycerol–water mixture. The velocity and vorticity fields were analyzed using time/phase averaging and snapshot proper orthogonal decomposition (POD) of in-plane velocity fluctuations, and Lagrangian transport boundaries were extracted from forward- and backward-time finite-time Lyapunov exponent (FTLE) fields. The near-field statistics show that the clearest working-fluid-dependent differences occur at low stroke ratio, where the evolution of shear-layer vorticity and downstream centerline metrics exhibit a stronger frequency dependence consistent with pulse-to-pulse interactions. POD reveals that the spatial organization of the dominant fluctuation modes depends on both working-fluid viscosity and stroke ratio. FTLE further reveals working-fluid and stroke-ratio-dependent differences in the formation and topology of transport boundaries, as transport ridges reorganize during successive-pulse interactions and provide connectivity information not readily inferred from vorticity alone. These results provide a combined Eulerian–Lagrangian characterization of continuously driven, closed-circuit unidirectional pulsatile jets relevant to mock circulatory applications.