Identification and characterization of a starting vortex from initiation within a confined tube

Heat transfer in laminar flows is currently a limiting factor in many engineering applications, particularly at the micro-scale. Dominated by molecular diffusion, laminar flow is an inefficient mechanism for mass and hence energy transport. However, the introduction of large-scale coherent structures, specifically axially propagating toroidal vortices, can significantly enhance cross-stream transport. This study investigates the formation, propagation, and spatiotemporal evolution of a starting vortex generated by a pulsatile jet in both unconfined and confined environments. High-resolution Particle Image Velocimetry (PIV) was combined with a multi-criteria Combinatorial Vortex Detection (CVD) algorithm to objectively characterize the vortices and evaluate their topological development. Unconfined reference experiments demonstrate that the jet produces a highly symmetric toroidal vortex undergoing a distinct two-stage maturation process: an active ejection phase characterized by rapid core growth and circulation increase, followed by a passive viscous propagation phase where the vortex steadily advances to a downstream position of x = 150 mm. Conversely, experiments within a confined transparent tube reveal that wall proximity induces intense localized shear stresses. This interaction severely arrests downstream propagation, stagnating the surviving vortex core at approximately x = 65 mm, and can cause the coherent structure to undergo asymmetric fragmentation and breakdown. Ultimately, the CVD algorithm successfully isolated true rotational cores from adjacent wall-bounded shear layers, establishing a robust, validated framework for ongoing high-fidelity investigations into complex, wallbounded flow dynamics.