Assessment of hybrid RANS/LES models for the prediction of the flow and scour around a wall-mounted cylinder

In many environmental flow situations, a solid body emerges from a sediment bed. This may occur in natural systems, for example when a tree-root emerges from the bed, or around a man-made structure such as a bridge pier or a wind turbine foundation. When this situation occurs, various flow-structure interactions such as the horseshoe vortex or vortex shedding lead to the scour process, namely the erosion pattern around the solid obstacle. In this context, accurate numerical prediction of the flow-structure interactions is of primary importance. In this contribution, three-dimensional hydrodynamic simulations are performed using the singlephase incompressible flow solver pimpleFOAM (OpenFOAMv2206plus). Two hybrid RANS-LES turbulence models available in OpenFOAM are tested: the non-zonal k - !SST-SAS model and the zonal k - !SST-IDDES model. As a first benchmark, a plane channel flow configuration at large bulk Reynolds number (Reb =1.9x104) is investigated to evaluate the mesh requirements and to determine a set of acceptable numerical parameters. These simulations show that while the k-!SST-IDDES model allows to simulate some features of the fluctuating motions with fewer grid requirements than LES, the k-! SST-SAS model behaves as a RANS model. Concerning the wall-mounted cylinder flow at high Reynolds number (Reb =1.8 x 105), the conclusions are quite different, the k - !SST-SAS model switches to LES in the region of interest namely the horseshoe-vortex region and the wake thanks to its non-zonal property. The k - !SST-SAS model provides better predictions than the k - !SST-IDDES model in terms of amplitude of the bimodal oscillation and vortex merging processes in the horseshoe vortex at lower CPU cost than LES. After this benchmarking and assessment of the hybrid RANS-LES model, a new k - !2006-SAS turbulence model has been developed for the two-phase flow solver sedFOAM. The model is then applied to a scour configuration around a wall-mounted cylinder and successfully represents the upstream scour depth. The simulations improve the resolution of the complex horseshoe-vortex dynamics which is directly influencing the morphological pattern of the scour process compared with RANS two-phase flow simulations. It also demonstrates interesting mesh convergence properties that RANS does not achieve.