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WAKEBENCH:
Benchmarking Wind Farm Flow Models

A Task to produce best practice guidelines for wind farm flow modeling from model intercomparison benchmarks
In this IEA Wind Task, participants will verify, validate, and quantify the uncertainties of the most widely used models.
A set of validation test cases will be used to benchmark models with increasing levels of complexity. These inter-comparison case studies will produce information to discuss model evaluation strategies that combine field and laboratory measurements in order to identify and quantify best practices for using these models under a range of conditions, both onshore and offshore, from flat to very complex terrain. This benchmarking is an important step to identify the best models and to highlight areas for improvement.

Objectives

The stated objectives of this task are:

•  To make an inventory of state-of-the-art models for the simulation of wind and wakes for site assessment applications: inputs, model equations, outputs, etc
•  To define procedures for the definition of test cases for validation purposes of wind and wake models: requirements on measurement data, filtering processes, metrics, etc
•  To identify the most critical aspects of the modeling chain by quantifying the associated uncertainties: boundary conditions, turbulence model, stability, etc
•  To define the range of applicability of the models under investigation: site conditions, wind regimes, wind farm size, etc
•  To reach consensus on best practice guidelines for the verification and validation of wind and wake models

Background
The standard model for wind resource assessment has been the Wind Atlas Analysis and Application Program (WAsP). The model, based on a linearization of the Navier Stokes equations originally introduced by Jackson and Hunt (1975), is reliable when used in neutral atmospheric conditions over mild terrain, with gentle slopes to ensure fully attached flows. However, because it is simple to use and well known, WAsP has been used in situations out of its range of applicability.

The alternative to linear models, such as WAsP, is to retain the non-linearity of the Navier Stokes equations and simulate both momentum and turbulence with computational fluid dynamics (CFD) models adapted to atmospheric flows. Using CFD in operational wind resource assessment is less than 10 years old and a large variety of commercial and research models are in the market.

Wake modeling for wind turbines originated in the 1980’s with work by Ainslie (1988). These algebraic models, which are still widely used for wind farm layout today, are based on simple momentum and fluid dynamic similarity theories or simplified solutions to the Navier Stokes equations. The problem with these models is that they lack many of the required physical processes needed to predict wind turbine wake behavior. This results in unpredicted wake losses by 10% in many operational wind farms.

The turbine models embedded in atmospheric models vary in complexity and in scales of calculations. The simplest is a drag element that extracts momentum and injects turbulence over a few simulation grid points. Such models often draw upon mesoscale models with larger domains to determine macro influences of large wind facilities. The next level of complexity is blade element momentum-based models that calculate blade forces and the wake influence using a global momentum balance. The forces in these models are then distributed around a disk and the influence of axial and rotational momentum is then propagated into the wake. Such a model can also be coupled to a wake meandering model that predicts the unsteady oscillation of the wake as it moves downstream. As turbine models get more complicated, the details of the blade aerodynamics become more prevalent. Recent calculations of multiple turbine interactions have used actuator line methods, where the blades are treated as airfoils distributed along rotating lines. Various other inviscid calculations of blade aerodynamics can also be used, including panel methods and boundary element methods that directly calculate the blade forces instead of using airfoil lookup tables.

These models require the verification, validation, and uncertainty quantification (VV&UQ) process that is fundamental in the development of any engineering model. This process allows a comprehensive transition from experience and test-based design to simulation-based design, producing more efficient and cost-effective design solutions. VV&UQ procedures have not been applied in wind resource assessment due to the inherent complexity of the system to model. The main difficulties are threefold: first, the domain size requires large wind tunnels and computer clusters, second, the wind conditions are the result of the interaction of a wide range of spatial and temporal scales, and third, the simulation of open flow fields produces ill-defined boundary conditions.

A clever strategy for VV&UQ that combines field and laboratory measurements will be developed in this IEA Wind Task. To this end, a set of validation test cases will be selected for benchmarking of models with increasing levels of complexity. Some test cases are readily available from the literature and some others will come from experimental facilities of the partners of the project or from industrial sites. These inter-comparison case studies will produce enough background information for the discussion of the VV&UQ strategies.

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