In-plane fluidelastic instability is a dynamic phenomenon requiring fluid coupling of at least two degrees-of-freedom, in this case, at least two flexible tubes. Due to the nature of the mechanism causing streamwise fluidelastic instability, a purely experimental or an unsteady determination would require intensive experimental effort. As a compromise between experimental effort and prediction accuracy, the quasi-steady model is used in the current study. In the present work, previously measured quasi-steady and unsteady forces are used to estimate the time delay first between the displacement of an oscillating tube and the forces generated on itself then the time delay between the displacement of a central oscillating tube and the forces induced on the adjacent tubes. The estimated time delays are then used together with drag coefficients and derivatives to predict the in-plane fluidelastic instability in a rotated triangular tube array of P/D = 1.5 subjected to two-phase flow. The results closely replicate dynamic test results and confirm the predominance of the stiffness controlled mechanism and the potential of the quasi-steady model in accurately predicting streamwise fluidelastic instability in arrays subjected to two-phase flows.

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