Accurate prediction of hydrodynamic forces on tandem bluff bodies at high Reynolds numbers is of interest in many fields of offshore engineering. The most commonly used turbulence modeling strategy for studying these flows is unsteady Reynolds-averaged Navier-Stokes methods (URANS) due to its speed. However, the accuracy of URANS results are problem dependent and usually poor for bluff bodies flow separation predictions. To overcome this deficiency, two different modeling methods have been considered: (i) large eddy simulation (LES) and (ii) non-linear URANS. LES are accurate and computationally feasible for low to moderate Reynolds number flows. However, the cost of LES makes it infeasible at high Reynolds numbers. On the other hand, non-linear URANS methods are fast like URANS, and its accuracy is comparable to LES for certain flows. It is usually not known in advance if the simulations using non-linear methods are accurate. Hybrid models have been proposed in the literature as an alternative to existing methods. They employ a URANS model in the near-body region and LES in the near and far wake regions. Simulations performed using hybrid models are computationally cheaper than LES and more accurate than URANS. Most hybrid models developed in the literature employ linear URANS models. The use of non-linear URANS models in the hybrid context has not received significant attention. In this study, we propose the use of a hybrid model based on a non-linear URANS model. Flow past tandem cylinders, with different spacing ratio, at sub-critical Reynolds number regime, is chosen as the test case. Simulations are also performed using URANS and linear hybrid models for comparison. It is shown that the non-linear hybrid models provides the best agreement to measurement data in the literature. Non-linear URANS models will be shown to provide acceptable prediction of hydrodynamic forces. The models are finally used to predict the current load on a generic multi-column floater.

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