Last modified: 2018-06-25

#### Abstract

Tube arrays subject to cross-flow can develop fluidelastic-instability. For increasing velocities damping of the system progresively reduces to a point in which it becomes negative. This point represents the critical velocity. Traditional stability maps intrinsecally consider that this value only depends on mass-damping parameter. Howerver experimental data as well as other studies indicate that pitch ratio has a non-neglegible effect on critital velocity. The aim of the present study is to evaluate the specific dependence of the critical velocity on pitch ratio.

A CFD methodology for the critical velocity prediction is proposed. Series simulations using unseteady RANS with dynamic mesh for increasing velocities below the stability threshold are carried out in free-motion conditions. Free decay tube response allow net damping to be calculated and hence the damping-velocity curves can be obtained. The zero damping point is thus determined. The rationale for this approach is twofold. Firstly, as RANS not suffer turbulent buffeting it is deterministic, and so very little amplitudes are possible, without the need for ensemble averaging. Small amplitudes facilitate the moving mesh approach. Secondly, as amplitudes are small, non-linear fluid damping and stiffness conponents that can lead to limit cycle oscillations in post-stable regime are negligible as there are cubic in form. The resulting, dependence of FEI on pitch ratio at different mass-damping parameters in stability maps is reduced to a simple algebraic expresion.