The unsteady loads in a tube bundle are studied at moderate and high Reynolds number by means of URANS and hybrid (DDES) modelling. The onset of fluid-elastic instability is analysed for different structural parameters, Scruton number and reduced velocity. The simulations have been carried out with the code NSMB (Navier-Stokes Multi Block) by using turbulence modelling methods URANS and DDES (Delayed Detached Eddy Simulation). The CEA-DIVA configuration is considered for the cylinders array for an inter-tube Reynolds number 60, 000. The study is carried out for a configuration of (4 × 5) cylinders in static conditions as well as for the vertical free motion of one of the central cylinders in one DOF (Degree Of Freedom).The inter-tube Reynolds number is 60, 000. It is found that this cylinder spontaneously displays an oscillatory motion which first corresponds to Vortex Induced Vibration (VIV), associated to a lock-in mechanism for low values of the reduced velocity and secondly develops Movement Induced Vibration, MIV for higher values of the reduced velocity. The variation of the cylinder's oscillations frequency, of the unsteady loads and the structure's displacement are studied as a function of the reduced velocity for low and high values of the Scruton number. The increase of the phase-lag between the forces and the * Corresponding author. displacement is predicted and discussed for different Scruton number values and reduced velocities.

The prediction of fluid-elastic instabilities developing in a tube bundle is of major importance in the design of modern heat exchangers for nuclear reactor cooling to prevent accidents associated with fluid-elastic instabilities leading to flutter, material fatigue, shocks between beams and damage of the solid walls. The fluid-elastic instabilities in tube arrays appear in the laminar regime and persist in the high Reynolds number turbulent flow. They are governed by non-linear interaction between fluid-elastic instability and turbulence. It is of major importance for the design to accurately predict the unsteady loads under turbulent flow and the amplification of the fluid-elastic instabilities. This work focuses on the numerical simulation of turbulent flow in tube bundle and prediction of fluid-elastic instabilities.