Excitation mechanisms and vibration response of a horizontal elbow in gas-liquid two-phase flow

Flow-induced vibration caused by gas-liquid two-phase flow in elbow piping systems is a critical concern in energy and process industries, as it can lead to structural fatigue, noise, and operational instability. In this study, gas-liquid two-phase flow in a horizontal downward elbow was simulated using the Volume of Fluid (VOF) method combined with the SST k-ω turbulence model, and the corresponding vibration responses were analyzed through a fluid-structure interaction (FSI) approach. The research examined flow pattern transitions, excitation force fluctuations, and the resulting vibration response and modal switching as different gas-liquid patterns pass through horizontal elbow. The results show that interfacial inertia and momentum differences at the elbow intensify secondary flow and drive downstream pattern transitions. Momentum flux fluctuations caused by interfacial oscillations and slug switching at the bend form the main excitation source, and both excitation amplitude and dominant frequency increasing with superficial velocity. Flow pattern strongly shapes the spatiotemporal vibration characteristics: under stratified-wavy flow, the response is first-mode dominated, with a decaying in-plane peak at the center of the horizontal span, whereas slug flow excites coupled first and third modes and produces sustained, periodic in-plane and out-of-plane vibrations. Overall, the study highlights the crucial roles of flow pattern, superficial velocity and natural modes in governing excitation forces and vibration responses in internal two-phase flow.