The current work presents the analysis of six different blockage correction methods for axial-flow turbines from the literature and compares them to a method proposed by the authors using blade-resolved CFD simulations. The analysis of the different methods is performed by comparing the corrected blocked-flow thrust and power results of a tidal turbine to a quasi free-flow condition, obtaining a quantitative measure of the error for every method at different blockages and tip-speed ratios. This study shows that not every available method for blockage correction remains valid for the range of turbine thrusts that may occur in blocked flows. Of the seven methods analysed, only three were found to provide a reliable estimate of the power and thrust across the range of blockage and tip-speed ratios analysed in this paper, including the one proposed by the authors. Of the remaining correction methods, those that could be used as a first approximation for low-blockage conditions are identified. Keywords Axial flow • Hydrokinetic • Turbine • Blockage • Correction • Tidal energy List of symbols σ Blade solidity ratio N Number of blades c Local chord length r Local radial coordinate R Blade radius λ Tip-speed ratio ω Rotational speed U Undisturbed axial flow velocity C P Power coefficient P Turbine power Q Turbine torque C T Thrust coefficient T Turbine thrust ρ Fluid density A Turbine swept area U 1 Velocity at the turbine plane B Federico Zilic de Arcos
The distribution of the angles of attack over the span of a rotor blade, together with blade element theory, provides a useful framework to understand forces, performance and other fluid dynamic phenomena of axial‐flow rotors. However, the angle of attack is not straightforward to define for a three‐dimensional rotor, where the flow is perturbed by the blade circulation, shed vorticity and wake development.This paper evaluates six methods to extract the angles of attack from blade‐resolved CFD simulations of axial‐flow turbines. Simulations of two different rotors are presented: a low solidity rotor designed for wind and a higher solidity rotor designed for tidal stream energy conversion. Of the analysed methods, five were obtained from the literature and are tested in terms of their internal parameters. The remaining method is named the streamtube analysis method (SAM) and is presented as an improvement on analysis methods that azimuthally average the flow data on the rotor plane, referred to as azimuthal averaging techniques (AATs). The SAM method accounts for the expansion of the streamtubes in flow‐field velocity sampling and exhibits improved convergence on the internal parameters compared with AAT.The six methods are benchmarked in terms of the angles of attack, axial induction factors and the local lift and drag coefficients, identifying that most perform well and converge with each other despite the different underlying assumptions or modelling approaches. However, given the limitations and inherent dependency on internal parameters, the line averaging and SAM are suggested for general flow analysis application.
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