Unveiling the fundamentals of two-phase axial-flow-induced vibrations of cantilever rods.

Flow-induced vibrations (FIVs) in nuclear fuel assemblies can cause fretting wear and costly unplanned reactor outages, yet fundamental mechanistic understanding and predictive modelling of FIVs in gas-liquid flows remain hindered by the lack of non-intrusive diagnostic tools. Here, we introduce a Hall-effect-based electromagnetic sensing technique that, for the first time, enables comprehensive resolution of the axial-FIV dynamics of a cantilevered rod with different tip geometries over a range of air-water flow regimes. Our experiments reveal that increasing the void fraction amplifies chaotic vibrations while suppressing periodic oscillations, a transition driven by the increased intensity of stochastic-forcing induced by gas-liquid interactions and bubble impacts. As such, a dual-regime response emerges where vibration amplitudes increase at low Reynolds numbers but decrease/plateau at high Reynolds numbers. Strikingly, beyond a critical void fraction of 0.2, amplitudes converge across Reynolds numbers, signalling two-phase stochastic force dominance. Our findings elucidate the mechanistic competition between stochastic and periodic excitations in two-phase axial-FIVs with a simplified paradigmatic configuration that provides valuable preliminary information for nuclear reactor applications. The developed novel technique provides an enabling tool for real-time, non-intrusive FIV diagnostics, with potential applications extending beyond nuclear engineering.