Extracting angles of attack from blade‐resolved rotor CFD simulations

Abstract

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.

Development and validation of a lifting‐line code associated with the vortex particle method software Dorothy

“…This is all the more interesting that a significant part of the blade, mainly close to the root, experiences a separated flow for this case according to literature. 59,60 F I G U R E 9 Radial distribution of blade loads for all three Mexnext-III cases.…”

The effect of flow sampling on the robustness of the actuator line method

“…The rotation is modelled using the multiple reference frame (MRF) approach . More information about the BR simulations software, meshing, and convergence can be found in Zilic de Arcos et al 17 .…”

“…For each blade section at different radii, each tip-speed ratio simulation is used to extract a different (𝛼, 𝐶 𝐿 , 𝐶 𝐷 ) triplet. The angles of attack at each section are sampled using the line average sampling method following Zilic de Arcos et al 17 . By considering multiple tip-speed ratios which each yield a different 𝛼, 𝐶 𝐿 and 𝐶 𝐷 , variations with 𝛼 can be developed for each radial position.…”

“…Zilic de Arcos et al 17 presented an analysis of six different methods to extract angles of attack from BR CFD simulations. Two of these, the vortex sheet method 18 and the pressure fitting method 19 , use information from the pressure distribution on the blade surface and are therefore not appropriate in the context of the ALM.…”

Tidal Turbine Benchmarking Project: Stage I - Steady Flow Experiments

“…The desire for simplicity originates from modelling and interpretation issues that have been encountered in previous research involving different experimental and commercial turbines that uses multiple airfoil or hydrofoil profiles (e.g. [9], [10], [11]), as well as from the positive experiences of other research groups that have employed a single profile; [1] and [3]. It can be difficult for engineering and CFD models to accurately reproduce the exact geometries of the transitions between different foil sections, and coupled with this is the use of a compatible interpolation method for foil lift and drag data representing different sections.…”