A hybrid CFD and semi-analytical model for streamwise fluidelastic instability of parallel triangular tube bundles

<bold>Background</bold>Fluidelastic instability (FEI) of heat transfer tube bundles under cross-flow conditions represents a critical safety concern in steam generator design, as it may result in tube wear and rupture. While most existing studies have focused on FEI in the lift direction, relatively limited attention has been paid to the drag direction. However, recent findings suggest that drag-induced FEI also poses a substantial risk.<bold>Purpose</bold>This study aims to investigate drag FEI using a hybrid computational fluid dynamics (CFD) and semi-analytical model and determine its critical velocity threshold.<bold>Methods</bold>Firstly, a 2D CFD model of a single flexible tube in a rigid parallel-triangular array was established. Based on the assumption of one-dimensional linear flow in this semi analytical model, the expression of fluid elastic force in the drag direction was derived. Then, the Fluent code was employed to conduct transient simulations under cross-flow on a parallel triangular bundle with a pitch to diameter ratio of 1.5 using to capture flow field evolution, and flow channel images were post-processed using MATLAB software to extract time delay and decay functions, as well as the flow channel boundary and flow tube area. Finally, these functions were embedded into a semi-analytical time-domain model to analyze fluidelastic response.<bold>Results</bold>The predicted critical velocity shows strong agreement with experimental results, confirming the model's accuracy and applicability in drag-direction FEI prediction.<bold>Conclusions</bold>These findings of this study provide valuable insights for the design of heat transfer tube bundles in steam generators, flow-induced vibration analysis, and fretting wear assessment.