Large Eddy Simulations of a rotating propeller in an equilibrium scour hole

Propeller-induced scour hole is one of the most fascinating present-day topics. First laboratory tests back years ago were in still water (SW) simulating harbour conditions, whereas less attention was paid to the influence of a flow impacting the propeller, which is a typical condition in navigable channels. In this work, two laboratory tests, carried out in the Laboratorio “Grandi Modelli Idraulici”, Università della Calabria, in a rectangular cross-section tilting flume, are presented. In both tests a propeller rotated close to an erodible bed compound by uniform sand, since a local scour hole at the equilibrium state was achieved. The former test was carried out in the canonical SW condition as a reference for the latter test, which presented a flow impacting the rotating propeller. Once the equilibrium scoured bathymetry was reached, punctual velocity measurements were acquired with the Acoustic Doppler Velocimetry. Finally, the flume bed was slowly drained by bottom sinks and scanned using a high-resolution photogrammetry technique. Punctual velocity measurements were used to validate the highly complex flow of the propeller jet in both SW and running flow impacting the propeller. The numerical simulation (NS) of the flow field was performed by means of the filtered unsteady Navier-Stokes equations for incompressible fluids in three dimensions and conservative form. In the framework of the Large Eddy Simulation (LES) approach, the scales smaller than the grid size were accounted in the Sub-Grid Scale (SGS) tensor through the Wall-Adapting Local Eddy (WALE) viscosity model (Nicoud & Ducros, 1999). The LES governing equations were discretized by means of the Finite Volume Method (FVM). The NS were performed using the pimpleDyMFoam solver, embedded in the OpenFOAM C++ libraries. The pimpleDyMFoam is designed to solve the incompressible, turbulent flow on a moving mesh using the Arbitrary Mesh Interface (AMI) method, handling mesh rotating movements (Aguerre et al., 2017). This solver uses the pressure implicit method for the pressure-linked equations algorithm (Jasak, 1999). The NS aims at the velocity field comparison between the propeller jet joint to the channel flow and the SW conditions. Moreover, low and high-order moments were computed to highlight the difference generated by the impact of the upstream flow.