Abstract
Tidal devices in rectilinear flows can utilise bidirectional turbines without the need for an active yawing system. This may improve device reliability by removing active components. However, in this configuration, turbines will experience the ‘foundation shadow’ effect, as for half of the tidal cycle, they operate in the downstream wake of their own foundation. Additionally, asymmetry between flood and ebb tides is common, causing misalignment between the incoming flow and the turbine rotational axis. This work uses a sliding-mesh unsteady RANS Computational Fluid Dynamics (CFD) model, validated against scale-model and full-scale data, to investigate the severity and implications of the foundation shadow, whilst incorporating flood and ebb asymmetry effects to analyse performance in misaligned flow. The simulations use the HydroWing multi-rotor device and the Tocardo two-bladed Horizontal Axis Tidal Turbine (HATT) as case studies. The use of fairings to streamline the foundation and reduce flow separation is analysed, alleviating the most severe shadowing effects. Results find that for aligned flow, foundation shadowing can induce thrust fluctuations that cause a ≈4.6% reduction in mean Power Coefficient (CP), increasing to ≈12.4% when exposed to a yawed inflow. Changing the design by adding a bidirectional fairing to the main support improves the foundation’s hydrodynamics, and in turn, reduces the shadowing effect severity to ≈2.2% for aligned flow and ≈9.7% when misaligned. The structural feasibility of using such a design in a yawed inflow is discussed, and results suggest that turbine yaw misalignment CP losses are more significant than shadowing at higher angles. Our results demonstrate that by designing for the shadowing effect, the mean CP impact and magnitude of blade thrust fluctuations can be significantly reduced. For sites with rectilinear tidal cycles, this would enable bidirectional turbines to be used instead of relying on yawing mechanisms.