Environmental Condition - Wind

Articles on this page:

• Turbulent wind files
• Generating turbulence to match a measured time history
• Cup anemometer definition
• Mann Turbulence model; Mann vs Kaimal comparison
• 3D wind file too small
• Getting wind speed at arbitrary location and time
• How is rotor-averaged wind speed calculated in Bladed?
• How to overcome the wind file grid size limit
• Calculation of the rotor average wind speed
• If the wind grid doesn't cover the whole turbine, what wind speeds are applied to the rest of the turbine?

Problem

Is it possible to use turbulent wind files which were generated by Bladed 4.5 in a simulation run in an older version of Bladed?

Solution

Yes, this is possible; wind files are downwardly compatible through different Bladed versions.

Keywords

Wind files; Bladed version; Compatibility

Problem

How you generate a turbulence file to match a measured time history at one point?

Solution

A turbulence file matched to measured data can be defined in the "Turbulence generation - Advanced options" screen.

If you want to specify the flow speed at (say) the hub, make sure that you choose a grid point that is located at the hub location.

Keywords

Turbulence generation; Wind

Problem

What is the definition of the cup anemometer wind speed? How does it differ from hub wind speed, for instance?

Solution

The cup anemometer wind speed is the magnitude of the resultant wind speed in the horizontal plane at hub height, therefore corresponds with the “resultant horizontal wind speed” in the wind data at the hub height location. The hub wind speed is the resultant magnitude of all 3 directions of wind at hub height.

Keywords

Wind; Anemometer

Problem

Turbulence intensity values at individual wind grid points are lower than the value set in the interface.

Solution

Every so often, users of the Mann turbulence model notice a discrepancy between the TI value they have entered in the user interface, and the TI value found from analysing a wind time series at a grid point in the wind field. Such a discrepancy is to be expected when using the Mann model. This article explains why.

The Mann turbulence model is a self-consistent model of the entire three dimensional wind field. All three turbulence components are generated simultaneously and the method accounts for correlation between components (in particular the correlation between vertical and longitudinal turbulence caused by Reynolds stress is captured – which the Kaimal model does not do).

Within the Mann model there is no requirement for the spectral distribution or TI values from the time series at an individual grid point to match that of the whole wind field. This means that variance plotted for an individual grid point will not necessarily lead to the same TI as the whole wind field. However, when averaged over many grid points, the variance will approach a constant value. This will be a slightly lower value than the Kaimal value due to the loss of high frequency variance by the Mann model.

If we compare the Mann model with the Kaimal model, we find that for Kaimal wind files the spectral statistics of the turbulence are satisfied at each individual grid point. This means that for Kaimal wind files, consistent variance numbers for time series from each individual grid point are seen and the TI value for each grid point will match the overall TI value for the whole wind field.

The loss of high frequency variance in the Mann model arises from the limitation on the number of Fast Fourier Transform (FFT) points underlying the turbulence generation. The number of FFT points determines the minimum wavelength in a given direction. Restriction on the maximum number of FFT points limits resolution and results in deficiencies in the high frequency turbulence generation. Mann argued that this is realistic, because it represents averaging of the turbulence over finite volumes of space which is appropriate for practical engineering applications.

It should always be noted that with any turbulence model, using hub wind speed statistics may result in a mismatch in turbulence statistics compared to the specified conditions if there is no wind file grid point at the exact hub location. In this situation the wind data at the hub location is generated by interpolating between the surrounding points.

Keywords

Wind files; Turbulence; Mann; Kaimal

Problem

When running a time domain simulation and using the 3D turbulent time varying wind option, users sometimes find an error where the wind file is too small. In particular, a user may receive an error message that states that the 3D wind file is too small in either the longitudinal, lateral and/or vertical direction.

Solution

At the start of the simulation, Bladed will compute an offset distance to shift the wind file to ensure that the ‘box’ covers the wind turbine structure at the start of the simulation. However, some potential issues may still arise:

• The turbine structure may deflect significantly during the simulation. This may occur due to a poorly defined structural model or some model instability. In this scenario a portion of the turbine may exit the box causing the error.
• If the offset computed by Bladed is not sufficient then a portion of the turbine may not be covered by the wind file ‘box’ at the start of the simulation and a termination error message may occur. If the rotor plane is yawed at the start of the simulation then the offset may not capture this and so the ‘box’ will not cover the whole turbine. Alternatively, the turbine may have become numerically unstable such that large unphysical deflections occur causing it to exit the wind field.

There are two possible ways in which a user can try to overcome this issue:

• In “Calculation Parameters”, specify a large value for “Start time for turbulent wind”.
• Check the stability of the structural model. To understand whether a model is numerically unstable you can run a more simplified time domain calculation by:
  1. Running constant wind rather than 3D Turbulent Wind.
  2. Start outputting results from the start of the simulation by setting the "Time to start writing outputs" option to 0
  3. Set the time step for which results are written to a small value such that the outputs can be viewed early before the simulation terminates
  4. Then check the blade tip flapwise and torsional deflection. In many instances the model goes unstable because a high frequency component is not being resolved by the integrator. The solution could be to reduce the time step to capture the vibration but this will slow down the simulation time. Alternatively, remove the high frequency vibration from the simulation on the basis that it will not be excited.

Keywords

3D wind file; Turbulent; Error; Longitudinal; Lateral; Vertical

Problem

I need to obtain the wind velocity at a given (X,Y,Z,t) - or perhaps need to calculate the entire wind field across the turbine for verification purposes etc. I need to be able to include all additional wind effects such as shear, tower shadow and upstream wake, so just parsing the turbulence file won't do.

Solution

As of Bladed 4.10 you can now do this using the External Loads DLL. See the Bladed 4.10 release notes for a quick overview.

Keywords

Wind; Turbulence; Velocity; External Loads DLL

Problem

There could be several potential ways of getting an average wind speed over the rotor swept area. Which method is actually used in Bladed?

Solution

Rotor average longitudinal wind speed is calculated by the following algorithm:

• Divide each blade into sections where each section is bounded by a blade station at each end. If there are N stations, then there will be N-1 sections.
• Divide rotor disc into annular "strips" swept out by these sections.
• Use normal wind model to determine instantaneous longitudinal wind speed at each blade station (taking into account such factors as wind shear, turbulence, upwind turbine wake etc as per user settings).
• Get the average wind speed for each section of each blade by averaging the two station speeds at the ends of this section.
• Assign each blade section speed a weighting factor corresponding to one-third of the area of the corresponding annular strip (assuming three blades).
• Calculate the weighted sum of all the blade section speeds.
• Divide the result by the rotor swept area to get an overall average.

Keywords

Wind speed; Rotor; Average; Flow speed; Swept area

Problem

When generating a wind turbulence file in Bladed, there's a limit of 49x49 grid cells. For larger turbines and when finer resolution is desired, this can be inadequate.

Solution

This issue is addressed in Bladed 4.16. The wind file generation application windnd has been compiled in 64 bit. As a consequence it can generate wind files with much larger grid sizes in excess of the 49 x 49 limit of previous versions of Bladed. Follow the guidance below for older versions of Bladed.

When using Bladed 4.15 and earlier, you can increase the number of grid points by doing the following:

Once the Wind Turbulence calculation is set up and ready to go, you'll need to create an input file (WINDND.IN file). To do this, first ensure that the option “Warn when starting calculation” is ticked in Tools -> Preferences.

You then need to run your calculation using the “Run Now” button. A “Starting Calculation” info box will pop up (don’t click either "Start" or "Abort" at this stage). You should then navigate to the Bladed installation folder where you can find a sub-folder whose name starts with "$$$$" (similar to the one shown below). In this folder you will find the WINDND.in file. Take a copy of this WINDND.in file to a folder location of your choice.

Once you have this WINDND.IN file, open it in a text editor - you need to alter the lines below to reflect the new number of grid squares in lateral and vertical directions.

Save the file and continue on to the turbulence run. It will generate a wind file with the new larger number of grid points.

Keywords

Wind file; Turbulence; Grid; Resolution; Squares

Problem

How Bladed calculates the rotor average wind speed?

Solution

Bladed calculates the average wind speed over the rotor according to below figure. The rotor is discretized into several smaller areas where the wind speed at each of it is extracted from the field. The discretization is performed as a function of the rotor radius, i.e., the larger the rotor the larger number of smaller areas become (the size of each small area is constant). This is done to ensure that the rotor is discretized properly regardless of the rotor size. A minimum discretization of 3 is set when dealing with a very small rotor. The average wind speed (and direction) is then applied by averaging the values obtained from all smaller areas, see below figure for more detail.

The following table shows all the assumptions made when calculating the average wind speed and direction.

Considered effects when calculating the rotor average wind speed and direction

Keywords

Aerodynamics; Average wind speed; Direction; Method; Environment

Problem

In an example like the one below, the wind grid covers the rotor and part of the tower but it is not high enough to cover the whole tower. What happens between the bottom of the grid and the ground?

In this case the "Best fit for rotor and tower" option was selected on the Time Varying Wind tab. This means the top of the wind grid is aligned to the top of the rotor.

Solution

Ideally we suggest you should make sure the grid is large enough to cover the whole turbine. But if this is not possible for some reason, what happens outside the grid?

There are non-zero wind speeds between the bottom of the grid and the ground. They vary smoothly with height. They are based on the speeds at the bottom of the wind grid, modified by the shear profile. If you remove shear (and tower shadow), then the wind speed at every point P below the grid will be the same as the lowest point on the grid that's vertically above P.

Keywords

Wind; Turbulence