Two-phase cross-flow exists in many shell-and-tube heat exchangers. Flow-induced vibration excitation forces can cause tube motion that will result in long-term fretting wear or fatigue. Studies on the subject, providing results on turbulence-induced displacement, fluid-elastic instabilities, and flow patterns have already been performed. It has been shown that the flow configuration plays an important role in the vibrations excitation mechanism. Previous studies showed the existence of unexpected quasi-periodic forces acting on a tube bundle subjected to two-phase cross-flow. The present work aims to understand the physical origin of these forces. A simple numerical model was developed to simulate two-phase cross-flow acting on a tube bundle. This model considers a continuous liquid potential flow across a tube bundle, with virtual bubbles being introduced in the flow. Three kinds of forces act on the bubbles: buoyancy forces, drag forces, and impact forces. These forces take place between two bubbles, or between a bubble and a cylinder. Two bubbles may coalesce if they hit each other, and conversely a bubble may split into two bubbles if the shear flow is strong enough. These local considerations on bubbles have global consequences on the flow configuration. Preliminary results show similarities between the numerical flow configuration and the experiments.

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