The paper introduces a three-dimensional numerical model that solves the Reynolds Averaged Navier-Stokes (RANS) equations on a curvilinear grid system using a novel nested grid approach. The main benefit of the model is the possibility to model locally complex hydraulic features in large rivers like the flow field at hydraulic structures. The entire study domain in such a case can be discretized with a coarser resolution, whereas a much finer resolution can be applied to a defined zone of the obstructions, where a detailed description of the flow field is needed. The model is tested on a laboratory experiment carried out at the Georgia Institute of Technology, where the flow field around a single and two double circular cylinders in a flatbed flume was studied. Simulated flow velocity, turbulent kinetic energy and bed shear stress distributions are in good agreement with measurements. However, deviations downstream of the piers indicate the limitation of the steady state description of the flow in the unstable wake zone. Nevertheless, the nested grid approach presented herein is a promising step towards the modeling of the local scouring phenomenon due to the relatively low computational demand.
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