Models

Model types

The results of foxes runs depend on a number of model choices by the user:

  • Rotor models: Evaluate the flow field at the rotor and compute ambient rotor equivalent quantities.

  • Turbine types: Define rotor diameter and hub height, and provide thrust coefficient and power yield depending on rotor equivalent quantities.

  • Wake frames: Determine the path of wake propagation and the local coordinate system that follows the centreline of the wake.

  • Wake models: Compute wake deltas for flow quantities in the wake of a turbine.

  • Partial wakes: Compute rotor disc averages of wake effects, i.e., the partial wakes models calculate the rotor effective wake deltas.

  • Turbine models: Each wind turbine within the wind farm can have individual turbine model choices. For each state and turbine, those compute data from currently existing data.

  • Point models: Calculate point-based data during the evaluation of algo.calc_points(), or as a modification of ambient states., like those from the ambient input states.

  • Vertical profiles: Analytical vertical profiles transform uniform ambient states into height dependent inflow.

All concrete models are stored in the so-called ModelBook object under a name string (or a name string template that is provided by a so-called model factory), see this example.

Rotor models

Rotor models evaluate the flow field at the rotor and compute ambient rotor equivalent quantities, for example the rotor averaged background wind speed. The list of available models and also rotor point visualizations can be found in this overview.

Turbine types

Turbine type models define rotor diameter and hub height, and provide thrust coefficient and power yield depending on rotor equivalent quantities. They are either chosen from the provided static data by their model name (e.g. DTU10MW), or added to the model book.

For example, a turbine type of a 5 MW turbine based on a csv file with columns ws for wind speed, P for power and ct for thrust coefficients can be added as

mbook = foxes.ModelBook()
mbook.turbine_types["turbine1"] = foxes.models.turbine_types.PCtFile(
    "turbine1.csv",
    col_ws="ws",
    col_P="P",
    col_ct="ct",
    D=100.5,
    H=120,
    P_nominal=5000,
    P_unit="kW",
)

If the file name follows the convention

name-<power>MW-D<rotor diameter>-H<hub height>.csv

where d replaces the dot for digits, then the above could be reduced to

mbook.turbine_types["turbine1"] = foxes.models.turbine_types.PCtFile(
    "turbine1-5MW-D100d5-H120.csv",
    col_ws="ws",
    col_P="P",
    col_ct="ct",
)

Turbine type models can also be based on other input data, e.g. cp instead of power, or other input files. The list of available turbine type classes can be found here in the API.

Wake frames

Wake frames determine the path of wake propagation, for example parallel to the wind direction at the rotor, or along a streamline, and the local coordinate system that follows the centreline of the wake.

Wake frames also determine the downwind order of the turbines, so chosing straight wakes for cases with spatially heterogeneous background flow can cause wrong results in multiple ways.

The wake coordinates are defined as follows:

  • The origin is at the rotor centre,

  • the x coordinate folows the centreline path of the wake,

  • the z coordinate starts pointing upwards at the rotor, then follows the centreline orthogonally,

  • the y coordinate closes the right-handed coordinate frame, i.e., it follows from the cross product of z with x.

The available wake frame classes are listed here in the API. The default model book contains many pre-defined wake frames, for example:

  • rotor_wd: Straight wakes, following the wind direction measured at the centre of the wake causing rotor.

  • yawed, yawed_k<k>: Wake bending due to yaw misalignment of the rotor, as represented by the YAWM variable. See this example.

  • streamlines_<step>: Streamline (or streaklines) following steady-state wakes, for a virtual time step of step seconds. See this example.

  • timelines, timelines_<dt>: Dynamic flow following wakes for spatially homogeneous wind data, optionally with time step dt, e.g. dt=10s or dt=1min, or other values with one of those two units. See this example.

  • seq_dyn_wakes, seq_dyn_wakes_<dt>: Sequential state evaluation (caution: slow, no state chunking), optionally with time step dt, e.g. dt=10s or dt=1min, or other values with one of those two units. See this example.

Wake models

Wake models compute wake deltas for flow quantities in the wake. Wind speed deficits and turbulence intensity deltas are often computed by two separate wake models, but could also stem from a single model.

The wake model classes can be found here in the API. They are organized into three sub-packages, according to their purpose and target variables:

  • wind: Wind deficit models, computing negative deltas for the wind speed variable WS,

  • ti: Positive wake deltas acting on the variable TI, modelling the turbulence increase within the wake region,

  • induction: Individual turbine induction models acting as wind speed deltas, which, in combination, model wind farm blockage effects.

Note that wind and ti wake models affect downstream turbines, while induction models mainly affect upstream and stream-orthogonal turbines. During calulations, a list of wake models is expected, so in principle, a wind deficit model, a TI wake model and a turbine induction model can be combined. If an induction model is included in the list of model selections, the Iterative algorithm has to be applied.

All wake model classes are implemented according to their mathematical nature, i.e., if applicable, they are derived from one of the following types:

  • DistSlicedWakeModel: The wake delta depends on the wake frame coordinate x differently than on (y, z), e.g., the x dependency can be factorized.

  • AxisymmetricWakeModel: Dist-sliced wake with axial symmetry, i.e., the wake can be described by x and a radial wake frame coordinate r.

  • GaussianWakeModel: Axisymmetric wake that follows a Gaussian function, where the standard deviation sigma(x) depends on x only.

  • TopHatWakeModel: Axisymmetric wake that is independent of r within the top-hat shape, and zero outside.

The reasoning behind this is that the partial wakes models can then build upon the underlying shape of the wake.

Wake superposition is part of the responsibility of the wake model. Most models expect a choice of the underlying wake superposition model in their constructor, in terms of their respective name in the model book. Examples are ws_linear for linear wind deficit superposition, or ti_quadratic for quadratic TI wake increase superposition.

The list of wake model name templates in the default model book is long, but that is mainly due to variations of various constructor argument choices. Typical examples are

The wake growth parameter k follows the convention in the above name templates that the dot after the zero is to be skipped, e.g., “004” represents the value 0.04. The superposition parameter is for example linear for the choice ws_linear or ti_linear, depending if the wake model targets wind speed or TI (cf. the model book example).

There are also model name templates in the default model book for the above models that do not specify the k parameter, e.g. Jensen_<superposition> for the Jensen model. In that case the k will be searched in the list of farm variables, which means that the values have to be provided by some other model. Typically this task is done by a kTI turbine model, cf. Section Turbine models below, but also other turbine models (or an optimization) could address this variable.

Partial wakes

Partial wakes models compute rotor disc averages of wake effects, i.e., the partial wakes models calculate the rotor effective wake deltas.

Some of the partial wakes models make use of the mathematical structure of the associated wake model:

  • PartialCentre: Only evaluate wakes at rotor centres. This is fast, but not accurate.

  • RotorPoints: Evaluate the wake model at exactly the rotor points, then take the average of the combined result. For large number of rotor points this is accurate, but potentially slow.

  • PartialTopHat: Compute the overlap of the wake circle with the rotor disc. This is mathematically exact and fast, but limited to wakes with top-hat shapes.

  • PartialAxiwake: Compute the numerical integral of axi-symmetric wakes with the rotor disc. This needs less evaluation points than grid-type wake averaging.

  • PartialSegregated: Abstract base class for segregated wake averaging, which means adding the averaged wake to the averaged background result (in contrast to RotorPoints).

  • PartialGrid: Segregated partial wakes evaluated at points of a grid-type rotor (which is usually not equal to the selected rotor model).

In the default model book, concrete instances of the above partial wakes models can be found under the names

  • centre: The centre point model,

  • rotor_points: The rotor points model,

  • top_hat: The top-hat model,

  • axiwake<n>: The axiwake model, with n representing the number of steps for the discretization of the integral over each downstream rotor,

  • grid<n2>: The grid model with n2 representing the number of points in a regular square grid.

Partial wakes are now chosen when costructing the algorithm object. There are several ways of specifying partial wakes model choices for the selected wake models:

  • by a dictionary, which maps wake model names to model choices (or default choices, if not found),

  • or by a list, where the mapping to the wake models is in order of appearance,

  • or by a string, in which case all models are either mapped to the given model, or, if that fails with TypeError, to their defaults,

  • or by None, which means all models are mapped to the default choice.

A verification of the different partial wakes models is carried out in this example: Partial wakes verification All types approach the correct rotor average for high point counts, but with different efficiency.

Turbine models

Each wind turbine within the wind farm can have individual turbine model choices. For each state and turbine, those compute data from currently existing data.

The list of available turbine model classes can be found here in the API. For example:

  • kTI: Computes the wake expansion coefficient k as a linear function of TI: k = kb + kTI * TI. All models that do not specify k explicitly (i.e, k=None in the constructor), will then use this result when computing wake deltas.

  • SetFarmVars: Set any farm variable to any state-turbine data array, or sub-array (nan values are ignored), either initially (pre_rotor=True) or after the wake calculation.

  • PowerMask: Curtail or boost the turbine by re-setting the maximal power of the turbine, see this example.

  • SectorManagement: Modify farm variables if wind speed and/or wind direction values are within certain ranges, see this example.

  • YAW2YAWM and YAWM2YAW: Compute absolute yaw angles from yaw misalignment, and vice-versa.

  • Calculator: Apply any user-written function that calculates values of farm variables.

  • LookupTable: Use a lookup-table for the computation of farm variables.

Point models

Calculate point-based data during the evaluation of algo.calc_points(), or as a modification of ambient states.

Point models can be added to ambient states objects, simply by the + operation.

The list of available point models can be found here in the API. For example:

  • WakeDeltas: Subtract backgrounds from waked results.

  • TKE2TI: Compute TI from turbulent kinetic energy data, as for example provided by mesoscale simulations.

Vertical profiles

Analytical vertical profiles transform uniform ambient states into height dependent inflow.

The list of available vertical profiles can be found here in the API. they can be added to uniform ambient states as in the following example, here for a Monin-Obukhof dependent log-profile:

states = foxes.input.states.StatesTable(
    data_source="abl_states_6000.csv.gz",
    output_vars=[FV.WS, FV.WD, FV.TI, FV.RHO, FV.MOL],
    var2col={FV.WS: "ws", FV.WD: "wd", FV.TI: "ti", FV.MOL: "mol"},
    fixed_vars={FV.RHO: 1.225, FV.Z0: 0.05, FV.H: 100.0},
    profiles={FV.WS: "ABLLogWsProfile"},
)

Notice the required variable FV.H, denoting the reference height of the provided wind data, as well as roughness length FV.Z0 and Monin-Obukhof length FV.MOL.