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Table of Contents

Generating Turbulent Wind Fields

To run a simulation that uses a 3-dimensional turbulent wind field, a suitable turbulence field must first be generated. When generated, these turbulence fields are saved to disc in a file so that they can be used for future simulations.

Turbulence fields contain time histories of wind speed variations at a number of points on a rectangular grid. The time history at each point will have spectral characteristics conforming to one of the available turbulence models. The time histories at any two points in the rotor plane will be correlated with each other in accordance with an appropriate model of the lateral and vertical coherence characteristics of atmospheric turbulence. Linear or cubic interpolation is used between grid points, as well as between successive time points.

To generate a new turbulence field, first define the turbulence characteristics and then generate the turbulence field as described below. The turbulence models supported by Bladed are the Kaimal and Mann turbulence models. Further details about the accuracy of the turbulent flow generations are given in the verification report on turbulent flow generation in Bladed.

Defining Turbulence Characteristics

Click the Wind icon on the toolbar and select Define turbulence. Then define the following:

  • Number of points along Y: the number of grid points horizontally.

  • Number of points along Z: the number of grid points vertically.

  • Length of Y: the width of the turbulent wind field - this should be at least the rotor diameter, or twice the greatest distance from the tower centreline to the blade tip to allow for operation at non-zero yaw angles, or the maximum width of the part of the structure immersed in the turbulence.

  • Length of Z: the height of the turbulent wind field - this should be at least the rotor diameter, or hub height plus radius if the turbulence is required all the way down the tower, or the maximum height of the part of the structure immersed in the turbulence.

  • Duration of wind file: this should be sufficient for the length of simulation required. The 'frozen' turbulence field moves past the turbine at the specified hub height mean wind speed or the surface velocity of the sub-surface current. The product of the duration and the wind speed gives the length of the turbulent wind field which will be generated. See 3D turbulent wind for details.

  • Frequency along X: the resolution to be used for the time histories. This is used to calculate the distance between successive points, which move past the turbine at the appropriate wind speed.

  • Mean wind or flow speed: this should correspond to the hub height mean wind or flow speed to be used for the simulations. In fact the turbulence characteristics are used in dimensionless form which means that they only depend weakly on wind speed. This means that the mean wind speed used here to generate the turbulence need not be the same as the wind speed used in a subsequent simulation which uses the turbulence field, since the turbulence will be re-scaled accordingly. However, for strict conformance to the specified turbulence characteristics, generate a different turbulent wind field, using the correct mean wind speed, for each mean wind speed to be used in the simulations.

  • Turbulence seed: this is a seed for the random number generator, and should be an integer between 0 and 999. Change the seed to generate a different turbulence field with the same statistical characteristics.

  • Spectrum type: select Kaimal or Mann, and click Define\... to enter the detailed turbulence characteristics, as follows:

    • Kaimal model: three models are available:

      • 1-component model: enter the lateral, vertical and longitudinal length scales of the longitudinal component of turbulence.

      • IEC-2 model: enter the scale parameter (\(\Lambda_1\) as defined in IEC 61400-1).

      • General model: enter the longitudinal length scales for the three components of turbulence, the coherence scale parameter \(L_c\), and the coherence decay constant \(H\). Note that this model encompasses the IEC 61400-1 model by setting \(^xL_u = 8.1\Lambda_1\), \(^xL_v = 2.7\Lambda_1\), \(^xL_w > = 0.66\Lambda_1\), and in the case of Edition 2 \(L_c = 3.5\Lambda_1\), and \(H = 8.8\), while for Edition 3 \(L_c = 8.1\Lambda_1\), and \(H = 12\). For turbine hub height \(z > 60 \bunit{m}\), the Edition 3 model yields \(L_c = 340.2\) and \(H=12\) such that \(^xL_u = 340.2\), \(^xL_v = 113.4\), and \(^xL_w =27.72\).

    • Mann model: enter the following parameters:

      • Shear parameter (gamma): Zero gives isotropic turbulence; the recommended value according to IEC 61400-1 is 3.9, which produces a spectrum similar to a Kaimal spectrum.
      • Scale length (L): The scale length is given by \(L = 0.8 \Lambda_{1}\) (as defined in IEC 61400-1). The recommended value for IEC edition 3 is 33.6m when the turbine hub height \(z > 60 \bunit{m}\).
      • FFT points: The number of points used for the FFT in the lateral and vertical directions. This must be a power of 2 and 128 points is recommended. Cubic interpolation is used the find the wind at the requested grid points.
      • Max. lateral/vertical wavelength: This should be appreciably greater than the rotor diameter to avoid periodicity in the lateral and vertical directions. A value of at least four times the scale length is recommended.

      To start from an existing turbulent wind field definition, click Import details... to select an existing turbulent wind file. The parameters defining the turbulence in that file will then be loaded.

Turbulence Evolution

By default the turbulence file is generated as Frozen turbulence. Taylors frozen turbulence hypothesis means that turbulent eddies passed a fix point advect entirely as a function of the mean wind speed. In other words, the turbulent wind field is not altered in any way just merely transported.

However, when using LIDAR it may be useful to include Turbulence Evolution such that eddies are not just advected downwind but instead can evolve. If the Allow turbulence evolution option is enabled then a wind file is generated that includes additional data suitable for the calculation of evolving turbulence. The user will need to generate two wind files and specify both in time domain calculations to include evolution in a Turbulent Time Domain calculation.

Advanced Options

Under Advanced options there is an option to generate turbulence that matches a measured wind time history.

Generate turbulence to match a measured wind time history at one grid point: this option is available only with the Kaimal turbulence model. It generates a wind file in which the wind speed time history at a specified point in the rotor plane exactly matches a given, perhaps measured, time history. The coherence function associated with the chosen turbulence model is used to generate appropriate time histories at all the other points in the plane, but the single-point spectrum associated with the chosen turbulence model is not used, since the measured data defines this. The measured data may include more than one component of turbulence: any unmeasured components should be provided as zeros, and the program will then synthesise these components entirely, using the single-point spectrum of the chosen turbulence model. The following information must be supplied:

  • a file of measured data, with one column for each component specified in the chosen turbulence model (using zeros for any components for which measured data is not available).

  • The mean wind speed for the measured data.

  • The sampling frequency of the measured data.

  • The grid point at which the simulated turbulence should match the measured data.

The duration of the measured dataset must be at least as long as the duration of the wind file which is to be created. The sampling frequency should of course be sufficient to capture the wind speed variations in sufficient detail, but need not match the frequency of points specified for the final turbulence file: the measured data is simply re-sampled as required.

Run Execution

Having defined the turbulence characteristics as above, click the appropriate button to generate the turbulence field either Now or In Batch. Alternatively, click the Calculations icon on the toolbar to bring up the Calculations screen (if it is not already present). Select Wind or sea turbulence, and then click Run now or Run in Batch. The turbulent wind will be stored with a specific Bladed turbulent wind file formatting and is written as the .wnd file.

Last updated 15-11-2024