Validation of Aeroelastic Actuator Line for Wind Turbine Modelling in Complex Flows

Abstract

The actuator line method is a widely used technique to model wind turbines in computational fluid dynamics, as it significantly reduces the required computational expense in comparison to simulations using geometrically resolved blades. Actuator line coupled to an aeroelastic solver enables not only the study of detailed wake dynamics but also aeroelastic loads, flexible blade deformation and how this interacts with the flow. Validating aeroelastic actuator line predictions of blade loading, deflection and turbine wakes in complex inflow scenarios is particularly relevant for modern turbine designs and wind farm studies involving realistic inflows, wind shear or yaw misalignment. This work first implements a vortex-based smearing correction in an aeroelastic coupled actuator line, and performs a grid resolution and smearing parameter study which demonstrates significant improvement in the blade loading and in the numerical dependencies of predicted thrust and power output. A validation is then performed using a 2.3 MW turbine with R = 40 m radius, comparing against blade resolved fluid-structure interaction simulations and full-scale measurement data, in both laminar and turbulent inflows including both high shear and high yaw misalignment. For an axisymmetric laminar inflow case, the agreement between blade resolved and actuator line simulations is excellent, with prediction of integrated quantities within 0.2%. In more complex flow cases, good agreement is seen in overall trends but the actuator line predicts lower blade loading and flapwise deflection, leading to underpredictions of thrust by between 5.3% and 8.4%. The discrepancies seen can be attributed to differences in wake flow, induction, the reliance of the actuator line on the provided airfoil data and the force application into the computational domain. Comparing the wake between coupled actuator line and blade resolved simulations for turbulent flow cases also shows good agreement in wake deficit and redirection, even under high yaw conditions. Overall, this work validates the implementation of the vortex-based smearing correction and demonstrates the ability of the actuator line to closely match blade loading and deflection predictions of blade resolved simulations in complex flows, at a significantly lower computational cost.

Cross-code verification of non-neutral ABL and single wind turbine wake modelling in LES

“…Overall, agreement in mean power to ≤ 7.2% and mean thrust to ≤ 6.4% between two different research group frameworks is significantly closer than in large benchmark studies [13], particularly when comparing across a range of non-neutral ABL inflows. The percentage differences are similar to that achieved using EllipSys3D and identical sheared inflow when comparing AL to a fully resolved rotor [38].…”

“…Therefore, Wake Conference 2023 Journal of Physics: Conference Series 2505 (2023) 012009 differences in normalised wake flow can be primarily attributed to the grid resolution. Other grid studies in EllipSys3D using laminar [11] or uniform turbulent inflow [38] have shown convergence of C T to within 1% for a stiff AD at R/(dxdydz) 1/3 ≥ 8, or for a flexible AL at R/(dxdydz) 1/3 ≥ 16 (both using uniform grids). The grid study results here, for a CNBL inflow, give a difference in mean C T between R/(dxdydz) 1/3 = 8 and 16 of only 0.2%, and so is consistent with previous findings in EllipSys3D.…”

Impact of Turbulent Time Scales on Wake Recovery and Operation

“…Wind turbines are modelled using the actuator line (AL) method [17], which is fully coupled to the aeroelastic BEM-based tool Flex5 [18]. EllipSys3D extracts the velocity components at the actuator positions and passes them to Flex5, which calculates the blade loads and deflections using tabulated aerofoil data and the vortex-based smearing correction [19] [20] which is implemented in the blade load computation [21]. The loads and deformed blade positions are passed back to EllipSys3D, and used to update the actuator positions and the magnitude of the applied body forces, which are smeared on to the computational grid using a Gaussian kernel.…”

“…The entire domain is discretised using 288 x 128 x 128 mesh points, resulting in a discretisation of 8 cells per radius inside the refined region. This is a relatively low resolution for an actuator line simulation, but judged to be acceptable due to the computational constraints and the fact that the vortex-based smearing correction implemented into the coupled AL reduces the difference in mean thrust coefficient between 8 and 64 cells per radius to 3% in a turbulent simulation [21]. Furthermore, the main objective here is to have the same relative thrust force for different flow scenarios, and not that the actual C T is converged through grid resolution.…”

Impact of the rotor blades elasticity on the loads and wake of the large IEA 15-MW wind turbine

“…More recently, OpenFAST was also coupled to Nalu-Wind and PALM [4,5] to study the NREL-5 MW reference wind turbine. Hodgson et al also performed validation of flexible actuator methods against high fidelity blade-resolved simulations, highlighting the ability of the ALM to accurately capture aeroelastic effect on rotors up to 5 MW [6,7,8]. Della Posta et al also considered a two-way coupling between a structural solver and an ALM, including the tower, for unsteady aerodynamics of the NREL-5 MW rotor [9,10].…”