Performance and wake prediction of a ducted tidal stream turbine in yaw misalignment using the lattice Boltzmann method

Abstract

Ducts can enhance the energy capture efficiency of tidal stream turbines (TSTs) in low-current velocity environments of deep seas. However, complex marine environments (e.g., seabed topography and waves) alter the water flow direction, causing the TST to operate in a yawed state. To address this issue, this paper employs a combination of the lattice Boltzmann method (LBM) and large eddy simulation (LES) to investigate the hydrodynamic performance and wake characteristics of short-tube ducted TSTs in yaw misalignment ranging from 0° to 45°. The study first evaluates TST performance through experiments in a water flume. Subsequently, an LBM-LES-based numerical model for TSTs is developed to calculate the power characteristics and compare them with experimental results, verifying the method’s accuracy. Finally, numerical simulations are conducted for both open and ducted TSTs under yaw conditions. The results indicate that: (1) The LBM-LES coupling method accurately captures flow field characteristics such as tip vortices and wake evolution, with simulation results showing strong agreement with experiments. This method serves as a high-precision tool for evaluating TST performance. (2) The duct improves the energy capture efficiency of the TST across the entire tip speed ratio range by accelerating the flow through the channel. It also mitigates the decline of the power coefficient and thrust coefficient in yaw misalignment. (3) The decay of hydrodynamic coefficients for open TSTs in yaw misalignment follows the cosine theory, whereas the decay for ducted TSTs is slower due to flow field modulation and requires correction using a low-exponent model. (4) The upstream side of TST blades exhibits both positive and negative pressures while the downstream side is dominated by negative pressure, with differences among blades at varying position angles. (5) The wake of the ducted TST exhibits an initial reverse deflection followed by realignment, with a smaller amplitude of wake center offset compared to the open TST. Additionally, the turbulent energy across all frequency bands is higher than that of open TSTs. The induced complex vortex system delays wake vortex dissipation and enhances flow field stability. The research results provide key parameters and yaw performance correction models for ducted TSTs in low-current velocity environments, laying a theoretical foundation for the efficient and stable operation of tidal stream energy devices in complex flow fields.