Filtered lifting line correction (FLLC)

To accurately represent wind turbine using the actuator line model (ALM) with OpenFAST, the user must select ALM parameters that are appropriate for the turbine of interest and for the near-turbine grid resolution.

The standard ALM has uniformly spaced points and thus requires a fine value of epsilon and a large number of points along the blade to properly model it. In recent years, a correction, called filtered lifting line correction (FLLC), has been developed [Martínez-Tossas and Meneveau, 2019]. Such a correction also includes the use of points that are not uniformly spaced, thus lifting the requirement of small values of epsilon. It is recommended that you run the ALM with the FLLC on.

To enable the ALM with OpenFAST coupling, Actuator should be added to incflo.physics, ActuatorForcing should be added to ICNS.source_terms, and Actuator.type = TurbineFastLine should be set. Next, the user should enable FLLC, choose its type, and set the option for a non-uniform point distribution:

Actuator.TurbineFastLine.fllc = 1
Actuator.TurbineFastLine.fllc_nonuniform = 1
Actuator.TurbineFastLine.fllc_type = variable_chord

When selecting the Actuator.TurbineFastLine.fllc_nonuniform as 1, a new distribution of points is calculated internally, based on the Actuator.TurbineFastLine.num_points_blade and Actuator.TurbineFastLine.fllc_epsilon_dr_ratio entries. The Actuator.TurbineFastLine.fllc_epsilon_dr_ratio value should be at least 1 with a maximum value of 3. It is recommended to use the value 3. This parameter controls the distribution of points, and its value should be selected based on the desired accuracy of the correction, according to Table 1 given in [Martinez-Tossas et al., 2024].

Actuator.TurbineFastLine.fllc_epsilon_dr_ratio = 3

Note

The entries Actuator.TurbineFastLine.num_points_blade and Actuator.TurbineFastLine.num_points_tower should match the entries NumBlNds from the AeroDyn blade file and NumTwrNds from the AeroDyn input file.

Two values of epsilon should be set for the FLLC: the regular epsilon and an epsilon for the chord. The regular epsilon should be set to twice the grid resolution. If you have a very fine grid near the turbine, e.g. 0.625 m,, then you can set epsilon to be 3 to 4 times the local grid resolution. The smaller the epsilon value, the more sensitive to the resolution the ALM will be. The epsilon_chord parameter represents the ideal epsilon value if FLLC were not to be used. However, we set both epsilon parameters for FLLC. One of the key details of the correction is that it uses the epsilon_chord on the non-uniform points, and this should be set to a target value regardless of the underlying grid resolution. The target values again depend on the desired accuracy of the correction and guidelines are also given on Table 1 of [Martinez-Tossas et al., 2024].

Actuator.TurbineFastLine.epsilon       = 10 10 10        # for a near-turbine grid of 5 m
Actuator.TurbineFastLine.epsilon_chord = 0.25 0.25 0.25  # for any grid resolution

Tip

If your near-turbine grid is not isotropic, then consider your “grid resolution” to be the largest of \(\Delta x\), \(\Delta y\), and \(\Delta z\).

Lastly, in certain cases, a numerical instability can arise from the application of the correction on the first few time steps. When a simulation begins, there is often an initial spike in some of the ALM quantities that affect the correction. Thus, in these scenarios, the user can choose a delay, in seconds, to start the correction. In cases where instabilities are observed, a 5-second delay has been shown to be sufficient:

Actuator.TurbineFastLine.fllc_start_time = 5

More details on other parameters are available in the description of each entry on the input file reference page.