Using Turbulent Wind in Bladed Calculations

Bladed allows a user to generate three-dimensional turbulent flow fields and include this in a time-domain simulation. This article is aimed to describe available options for defining the turbulent wind in Bladed calculations.

Setup turbulence in time domain simulation

Use this option to perform a simulation in which any part of the turbine is immersed in a turbulent wind field which varies both in space and time. The turbulence is superimposed on any steady-state spatial variations defined by current profiles or wind shear, tower shadow and any upstream turbine wake. The turbulent variations at any point in the rotor disc have defined spectral characteristics, and the variations at any two points in the rotor disc are correlated by a defined coherence relationship representative of the spatial structure of real atmospheric turbulence. The turbulent wind field may have just the longitudinal component of turbulence, or it may have all three components.

Click the Wind icon on the toolbar, select Time varying wind, and choose the 3D turbulent wind option.

Then enter the following information:

Turbulent wind file name: Click here to select a file which contains an appropriate turbulent wind field: see Generating turbulent wind fields. Click Properties to display the characteristics of the turbulent wind field. Please see this guidance for information on how to generate a wind file and the input parameters that should be used.
Mean wind speed (turbulent wind only): the mean wind speed for the turbulent wind at the reference height. If this does not match the wind speed for which the turbulence characteristics were defined, the turbulence field will be scaled appropriately. However, since the dimensionless characteristics of turbulence are not quite invariant with wind speed, this scaling is not strictly valid.
Height at which speed is defined (turbulent wind only): the reference height to which the Mean wind speed applies, unless Refer wind speed to hub height has been selected. If wind shear is defined, the mean wind speed at any other height will be different.
Turbulence intensity: the standard deviation of turbulent wind speed variations as a percentage of the mean wind speed. If using a three-component wind field, specify the turbulence intensity for each component.
Wind direction (turbulent wind only): measured clockwise from North - see wind direction.
Flow inclination (turbulent wind only): for non-horizontal flows (e.g. on the side of a hill). A positive value indicates a rising wind.
Allow turbulence file to wrap around: if this is checked, the turbulence file can be recycled indefinitely by looping from the end back to the beginning.

For the Height of turbulent wind field select one of the following options. These do not affect the mean wind speed at any height, only the location of the turbulent wind fluctuations:

Centred on hub height: The centre of the turbulent wind field is placed at hub height.
Best fit for rotor and tower: If the turbulence field vertical dimension is less than the height of the turbine, the top of the turbulence field will be located at the height of the top of the rotor, so that the turbulence field extends as far down the tower as possible. If the turbulence field vertical dimension is greater than the turbine height, the turbulence field will start at ground level and will envelope the whole turbine.

Turbulent variations outside the defined turbulence field will be taken from the nearest defined point.

When generating turbulence the data is defined at a number of grid points, both in the rotor plane and in the alongwind direction. In between these points, either linear or cubic interpolation can be used to determine the turbulence. Specify one of the following options for the Interpolation scheme:

Linear: uses linear interpolation
Cubic in rotor plane only: uses cubic interpolation laterally and vertically, and linear interpolation in the along wind direction
Fully cubic: uses three-dimensional cubic interpolation.

If desired (for turbulent wind only), specify also a superimposed sinusoidal wind direction transient. This is particularly useful if only a single turbulence component is being used. Enter the following parameters:

Amplitude of direction change: the peak-to peak amplitude of the transient. Note that the specified wind direction (see above) refers to the start of the simulation. The mean wind direction will be different from this if a transient is added.
Start time of transient: the time into the simulation at which the transient starts.
Duration of transient: the duration of the transient from the time it starts to the time it finishes.
Type of transient: half or full wave: see transients.

LIDAR setup

If a LIDAR sensor is used to provide wind preview information to the external controller, the upstream wind velocities measured by the LIDAR cannot be expected to convect towards the turbine unchanged as implied by Taylor's frozen turbulence hypothesis. Select Evolving turbulence to allow the turbulence to change as it convects towards the turbine. Please see the evolving turbulence model for more detail. The following additional information should be provided:

Additional wind file name: Select a second file containing a turbulent wind field, which should be identical to the first turbulent wind field apart from the random number seed. Both files should be generated using the Evolving Turbulence option. While the turbulence reaching the turbine is still given by the first turbulent wind field, the turbulence measured by the LIDAR is calculated by combining frequency components from both wind fields depending on the upstream distance of the measurement point. Note that this can result in significantly slower simulations, especially if there are many simultaneous LIDAR measurement points and/or many points are defined for the LIDAR weighting function.
The form of the evolving turbulence model can be changed via Project Info as shown below.

MSTART EXTRA
EVOLVING_TURBULENCE 2 * 0 = None, 1 = Kristensen, 2 = Exponential
EXPONENTIAL_FACTOR 0.3 * The value to be used with Exponential model
MEND

Note

The evolving turbulence calculation will only be applied to the LIDAR sensor wind speed lookup and is not used for computing wind velocity directly on the turbine.

Turbulence models

The wind turbulence method adopted in Bladed is based on that described by Veers, 1988. The rotor plane is covered by a rectangular grid of points, and a separate time history of wind speed is generated for each of these points in such a way that each time history has the correct single-point turbulence spectral characteristics, and each pair of time histories has the correct cross-spectral or coherence characteristics.

Calculations using such a turbulent wind field will take into account the crucially important 'eddy slicing' transfer of rotor load from low frequencies to those associated with the rotational speed and its harmonics. This 'eddy slicing', associated with the rotating blades slicing through the turbulent structure of the wind, is a significant source of fatigue loading. The wind speed time histories may, in principle, be generated from any user‑specified auto-spectral density and spatial cross‑correlation characteristics.

Two turbulent models are recommended in Bladed to conform with the standards, either to use the the Kaimal or the Mann (1994 ; 1998). Both models are generally accepted as good representations of real atmospheric turbulence, and they are described in the third edition of the IEC 61400-1 standard. The verification report on turbulent flow generation in Bladed describes the comparison of Bladed generated turbulent flow against reference theoretical quantities.

Notes on use of 3D turbulence in simulations

The 3D wind field should be generated to obtain a wind file. The data contained in the wind file is described in 3D turbulent wind. The following points should be noted when using these turbulent wind fields for wind turbine simulations:

The length of the wind field, \(L_{field}\), must be sufficient for the simulation to be carried out. For a simulation of \(T\) in seconds at a mean wind speed of \(U\) in m/s, \(L_{field}\) must be at least \(UT + D\) in metres where \(D\) is the turbine diameter (the extra diameter is needed in case the turbine is yawed with respect to the mean wind direction). Alternatively there is an option to allow the turbulent flow field to wrap around at the end, starting again from the beginning. This allows an arbitrarily long simulation.
The width and height of the wind field must evidently be sufficient to envelope the whole rotor, i.e. at least equal to the rotor diameter. The flow field can also be made tall enough to encompass the entire rotor and tower.
The number of grid points in the lateral and vertical directions should be chosen to achieve adequate sampling of the wind speed variations in the rotor plane. A grid point spacing of 6 – 7 m is likely to be adequate for this. The time taken to generate the turbulence file increases with the fourth power of the number of points, so it is important not to use more points than necessary. In the wind direction, a resolution of 10 Hz is likely to be sufficient. For certification calculations, definitive guidelines should be sought from the certifying body.
If a simulation uses only a part of a turbulent time history, the mean wind speed and turbulence intensity for that part of the time history may not be the same as for the whole time history, and therefore may not match the mean wind speed and turbulence intensity which was specified for the simulation since this assumes that the whole time history will be used. Note also that the number of points along the wind direction must be a power of two for efficient calculation, since Fast Fourier Transform techniques can then be used. If it is not a power of two, then the along-wind spacing of points will automatically be decreased.
Different time histories with the same turbulence characteristics can be generated by changing the random number seed.
A sinusoidal half- or full-wave wind direction transient as described in transient wind may be superimposed on the turbulent wind field. This is intended for use with turbulent wind fields when only the longitudinal component has been generated, to ensure that some yaw error occurs during the simulation. Using all three components of turbulence should give a more realistic variation of yaw error.
The wind file is a binary file that can be obtained by a pre-processing calculation to generate three-dimensional turbulent flow fields. The output is a *.wnd binary file with the following format.