NREL has recently put considerable effort into improving the overall modularity of its FAST wind turbine aero-hydro-servo-elastic tool to (1) improve the ability to read, implement, and maintain source code; (2) increase module sharing and shared code development across the wind community; (3) improve numerical performance and robustness; and (4) greatly enhance flexibility and expandability to enable further developments of functionality without the need to recode established modules. The new FAST modularization framework supports module-independent inputs, outputs, states, and parameters; states in continuous-time, discrete-time, and in constraint form; loose and tight coupling; independent time and spatial discretizations; time marching, operating-point determination, and linearization; data encapsulation; dynamic allocation; and save/retrieve capability. This paper explains the features of the new FAST modularization framework, as well as the concepts and mathematical background needed to understand and apply it correctly. It is envisioned that the new modularization framework will transform FAST into a powerful, robust, and flexible wind turbine modeling tool with a large number of developers and a range of modeling fidelities across the aerodynamic, hydrodynamic, servo-dynamic, and structural-dynamic components.

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New Modularization Framework for the FAST Wind Turbine CAE Tool: Preprint

This paper presents a numerical implementation of the geometrically exact beam theory based on the Legendre-spectral-finite-element (LSFE) method. The displacement-based geometrically exact beam theory is presented, and the special treatment of three-dimensional rotation parameters is reviewed. An LSFE is a high-order finite element with nodes located at the Gauss–Legendre–Lobatto points. These elements can be an order of magnitude more computationally efficient than low-order finite elements for a given accuracy level. The new module, BeamDyn, is implemented in the FAST modularization framework for dynamic simulation of highly flexible composite-material wind turbine blades within the FAST aeroelastic engineering model. The framework allows for fully interactive simulations of turbine blades in operating conditions. Numerical examples are provided to validate BeamDyn and examine the LSFE performance as well as the coupling algorithm in the FAST modularization framework. BeamDyn can also be used as a stand-alone high-fidelity beam tool. Copyright © 2017 John Wiley & Sons, Ltd.

BeamDyn: a high-fidelity wind turbine blade solver in the FAST modular framework

https://iopscience.iop.org/article/10.1088/1742-6596/1452/1/012071/pdf

ExaWind: A multifidelity modeling and simulation environment for wind energy

Modern offshore wind turbines are susceptible to blade deformation because of their increased size and the recent trend of installing these turbines on floating platforms in deep sea. In this paper, an aeroelastic analysis tool for floating offshore wind turbines is presented by coupling a high‐fidelity computational fluid dynamics (CFD) solver with a general purpose multibody dynamics code, which is capable of modelling flexible bodies based on the nonlinear beam theory. With the tool developed, we demonstrated its applications to the NREL 5 MW offshore wind turbine with aeroelastic blades. The impacts of blade flexibility and platform‐induced surge motion on wind turbine aerodynamics and structural responses are studied and illustrated by the CFD results of the flow field, force, and wake structure. Results are compared with data obtained from the engineering tool FAST v8.

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Aeroelastic analysis of a floating offshore wind turbine in platform-induced surge motion using a fully coupled CFD-MBD method

Over the past few years, the FAST wind turbine simulation tool has undergone a major restructuring. FAST is now, at its core, an algorithm and software framework for coupling time-dependent multi-physics modules relevant to computer-aided engineering (CAE) of wind turbines. Each module, which represents one or more turbine components or physics control volumes, is constituted by a mathematical model composed of time-dependent constraint and/or differential equations that are typically nonlinear. Under this new modular form, modules can interact through matching or non-matching spatial meshes and can be time advanced with different time steps and different time integrators. Sharing of data between modules is accomplished with a predictor-corrector approach, which allows for either implicit or explicit time integration within each module. This new modularity positions FAST as a backbone for coupling both high-fidelity and engineering-level wind turbine physics models. In this paper, we describe new features of the FAST modular framework. In particular, we describe a new mixed-time-step algorithm, sparse-matrix storage, a direct solver for sparse linear systems, and interpolation of rotation fields in space for mesh mapping and in time for time advancement. We also show several numerical examples that demonstrate the performance and flexibility of the FAST framework, and we use those results to provide modeling guidance to users.

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FAST Modular Framework for Wind Turbine Simulation: New Algorithms and Numerical Examples

/pdf/beamdyn-a-high-fidelity-wind-turbine-blade-solver-in-the-b1o7ozeayy.pdf

BeamDyn: A High-Fidelity Wind Turbine Blade Solver in the FAST Modular Framework

This paper presents a numerical implementation and examination of new wind turbine blade finite element model based on Geometrically Exact Beam Theory (GEBT) and a high-order spectral finite element method. The displacement-based GEBT is presented, which includes the coupling effects that exist in composite structures and geometric nonlinearity. Legendre spectral finite elements (LSFEs) are high-order finite elements with nodes located at the Gauss-Legendre-Lobatto points. LSFEs can be an order of magnitude more efficient that low-order finite elements for a given accuracy level. Interpolation of the three-dimensional rotation, a major technical barrier in large-deformation simulation, is discussed in the context of LSFEs. It is shown, by numerical example, that the high-order LSFEs, where weak forms are evaluated with nodal quadrature, do not suffer from a drawback that exists in low-order finite elements where the tangent-stiffness matrix is calculated at the Gauss points. Finally, the new LSFE code is implemented in the new FAST Modularization Framework for dynamic simulation of highly flexible composite-material wind turbine blades. The framework allows for fully interactive simulations of turbine blades in operating conditions. Numerical examples showing validation and LSFE performance will be provided in the final paper.

Nonlinear Legendre Spectral Finite Elements for Wind Turbine Blade Dynamics: Preprint

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BeamDyn: A High-Fidelity Wind Turbine Blade Solver in the FAST Modular Framework: Preprint

Abstract. Throughout wind energy development, there has been a push to increase wind turbine size due to the substantial economic benefits. However, increasing turbine size presents several challenges, both physically and computationally. Modeling large, highly flexible wind turbines requires highly accurate models to capture the complicated aerodynamic response due to large deflections and nonstraight blade geometries. Additionally, development of floating offshore wind turbines requires modeling techniques that can predict large rotor and tower motion. Free vortex wake (FVW) methods model such complex physics while remaining computationally tractable to perform the many simulations necessary for the turbine design process. Recently, a FVW model—cOnvecting LAgrangian Filaments (OLAF)—was added to the National Renewable Energy Laboratory engineering tool OpenFAST to allow for the aerodynamic modeling of highly flexible turbines along with the aerohydro- servo-elastic response capabilities of OpenFAST. In this work, FVW and low-fidelity blade-element momentum (BEM) structural results are compared to high-fidelity simulation results for a highly-flexibly downwind turbine for varying TI, shear exponent, and yaw misalignment conditions. Through these comparisons, it was found that for all considered quantities of interest, SOWFA, OLAF, and BEM results compare well for steady inflow conditions with no yaw misalignment. For OLAF results, this strong agreement was consistent for all yaw misalignment values. The BEM results, however, deviated significantly more from SOWFA results with increasing absolute yaw misalignment. Differences between OLAF and BEM results were dominated by yaw misalignment angle, with varying shear exponent and TI leading to more subtle differences. Overall, OLAF results were more consistent than BEM results when compared to SOWFA results under challenging inflow conditions.


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Comparison of Free Vortex Wake and BEM Structural Results Against Large Eddy Simulations Results for Highly Flexible Turbines Under Challenging Inflow Conditions

Nick Johnson

Papers

BeamDyn: a high-fidelity wind turbine blade solver in the FAST modular framework

BeamDyn: A High-Fidelity Wind Turbine Blade Solver in the FAST Modular Framework

Nonlinear Legendre Spectral Finite Elements for Wind Turbine Blade Dynamics: Preprint

BeamDyn: A High-Fidelity Wind Turbine Blade Solver in the FAST Modular Framework: Preprint

Comparison of Free Vortex Wake and BEM Structural Results Against Large Eddy Simulations Results for Highly Flexible Turbines Under Challenging Inflow Conditions