Operation of a hydrokinetic turbine over a mobile sediment bed

Abstract

Some sites for hydrokinetic turbine array projects are often characterised by erodible sedimentary areas, such as the Bristol Channel, Irish Sea, and Spanish Riaz. While it is known that turbine deployment and operation can influence sediment transport and bedform dynamics at these locations, the specific characteristics of bed motion remain insufficiently studied.

This study adopts  an in-house large-eddy simulation model to simulate a single horizontal tidal-stream turbine with mobile sediment to investigate the impact of a turbine's operation on bedform dynamics. The simulations are conducted using the DOFAS solver (Digital Offshore Farms Simulator, Ouro et al. 2019, J Fluids Struct 91:102732), which adopts the Large Eddy Simulation (LES) framework with the WALE sub-grid scale model to capture the energetic large-scale turbulent structures in the flow. The Actuator Line Method (ALM) is used to represent the three blades comprising the turbine’s rotor, while the Immersed Boundary Method (IBM) is used to model the vertical support structure of the turbine. The main simulation is fed from a precursor simulation that included the mobile bed with the goal to generate fully-developed inflow conditions. Simulations considering a flat seabed are used to validated the simulations against laboratory-scale experimental data from Stallard et al. (2015, J Fluids Struct 54:235-246), ensuring the reliability of the numerical approach. The turbine diameter is 0.27m. The depth-averaged flow velocity of 0.47 m/s and the water depth is 0.54 m. For the mobile sediment bed simulations, the sediment dynamics module from the in-house code Hydro-3D solver (Liu et al., 2019, Physics of Fluids 31, 096603) are implemented into DOFAS to account for the sediment dynamics. The sediment bed was represented using the discrete points as per the IBM and the temporal evolution of bed elevation is obtained by solving the Exner–Polya equation. The sediment median grain size (d₅₀) is 0.5mm, and grain density (ρ_s) is 2650kg/m³.

Results from these first-of-its-kind large eddy simulations will reveal the processes by which the morphology of the sediment bed evolves in response to the operation of a single tidal turbine. Specifically, we will look at the variation of the mean seabed height downstream of the turbine over its wake region but also in its outskirts. Characterisation of the different wake dynamics, e.g. recovery rate, will also be assessed.