Coupled fluid–structure simulations of a cantilever rod in water turbulent axial flow with different CFD approaches

Fluid–structure numerical simulations of an experimental campaign by Cioncolini et al. of a cantilever rod in water axial flow were performed. The experimental configuration aims at representing a nuclear fuel rod, in terms of hydraulic diameter. Water velocity profiles and structure vibrations were measured experimentally. Two of the experimental tests were simulated numerically, one at Re=1.5$\cdot$10⁴ and one at Re=1.9$\cdot$10⁴. Different CFD approaches were tested, using code_Saturne: a wall-resolved two-equation linear viscosity model (k-$\omega$-SST), two wall-modeled Reynolds stress models (SSG and LRR), a wall-resolved Reynolds stress model (EBRSM) and a wall-resolved hybrid URANS/LES model (DDES). The structure was simulated through a one-dimensional finite element Euler–Bernoulli beam model. A 2-way coupling was implemented between the two solvers, with an Arbitrary Lagrangian Eulerian approach. Unexpectedly, wall-modeled Reynolds-stress models were found to calculate higher amplitudes of vibration than the higher-resolution EBRSM and DDES. The frequency domain analysis allowed to identify high energy flow velocity and flow-induced force harmonics at relatively low frequency calculated by LRR and SSG, not present in the EBRSM and DDES results, which explain the numerical results in terms of vibration response. This specific behavior of LRR and SSG seems to be linked to the wall function boundary condition. LRR and SSG calculate a rms amplitude of vibration close to the experiments, whereas EBRSM and DDES underestimate them by a factor of 2.5. A hypothetical small permanent deformation (4% of the hydraulic diameter) of the rod was simulated and found to increase the calculated vibration amplitudes by a factor of 2. 1-way coupling was also tested to assess the influence of damping and added mass on the results.