Nuclear reactor designs such as PWR and BWR are susceptible to vibrations induced on the nuclear fuel rods due to fast flowing coolants around the rods. The non-linear behaviour of flexible components have always been a challenge to compute especially when dealing with strongly coupled fluid–structure interaction cases as found in the reactors. Simulating such a behaviour involves a two-way coupling of a well resolved turbulent flow Computational Fluid Dynamics (CFD) solver to a Computational Solid Mechanics (CSM) solver. The use of a high fidelity CFD solver to resolve turbulent flows in an FSI (Fluid-Structure Interaction) simulation is computationally expensive ergo is not practical in for industrial purposes. To address this issue, a different approach is discussed in this article to simulate turbulence induced vibrations through the use of U-RANS models. The method is based on computing the modeled turbulent pressure and velocity fluctuations from values obtained by solving U-RANS (Unsteady-Reynolds Averaged Navier Stokes) equations. The calculated turbulent fluctuating field is combined with mean values to compute an instantaneous turbulent pressure field to apply an external pressure and shear force on the structure and vice-verse until convergence is achieved. This method can be used to accurately estimate the behaviour of a flexible structure in a turbulent flow. The article provides a detailed explanation of the model followed by validation with three numerical test cases. The first case involves a CFD simulation where results from the pressure fluctuation model (PFM) is compared to a benchmark DNS (Direct Numerical Simulation) of a turbulent channel flow with friction Reynolds number, Re_τ=640. Later the PFM is applied to a 2-dimensional strongly-coupled FSI simulation with a flexible steel flap in turbulent water flow to study the feasibility and stability of PFM applied to an FSI problem. Finally, the PFM is used to simulate an experimental case of a brass rod excited by turbulent water performed by Chen and Wambsganns (1972). The results show that the PFM is capable of simulating turbulence induced vibration (TIV) with low-fidelity U-RANS models.

Turbulence Induced Vibrations (TIV) or Turbulence Buffeting in a nuclear reactor is an undesirable side effect of achieving high coolant flow rates. The turbulent pressure field of these flows force the structure to vibrate within the setup. High amplitude vibrations of the fuel rods can lead to structural damage within the reactor and create a safety hazard. TIV is a difficult phenomenon to predict and limited experimental data is available. Therefore, the most pragmatic method of estimating behavior of structures is through computational approach. Fluid-structure interaction problems like these are tackled by partitioned coupling of CFD (Computational Fluid Dynamics) and CSM (Computational Solid Mechanics) solvers. To simulate TIV, it is necessary to capture the turbulent eddies by resolving the fluid flow field up to the smallest scales. Use of high-fidelity CFD solvers such as LES is an ideal approach. However, this modeling approach is very expensive when an FSI simulation is considered. Low-fidelity models such as URANS are able to simulate only the large fluid structures and are inadequate to model the fluctuation at the smaller scales, resulting in an inaccurate approach for TIV. This article proposes the use of Presure Fluctuation Model for FSI simulations to estimate the effects of pressure fluctuations (p⁰) which trigger TIV. In the proposed model, standard URANS (Unsteady Reynolds Averaged Navier-Stokes) models are complemented with this model to calculate p⁰. The calculated p⁰ is summed to the mean pressure (p) to acts as a source of external excitation for the structure in an FSI simulation. The proposed model has an advantage over high fidelity models in terms of reduced computational costs and accurate estimation of TIV. The method is validated against a DNS of a channel flow and the results have shown to be in good agreement. Afterwards, the model is applied to two coupled FSI test cases. The results conclude that the pressure fluctuation model is capable of simulating TIV without requiring an external perturbation to trigger these vibrations.