1. The Mann uniform shear turbulence model
The IEC Turbulence Simulator is an implementation of the Mann method recommended in IEC61400-1 Ed. 3. It is distributed with Wasp Engineering and, since February 2009, it is also available as a separate tool without a license. The IEC Turbulence Simulator simulates standard turbulence over flat uniform terrain. The simulated turbulence can be used as input to aero-elastic turbine simulators, which in turn can calculate fatigue loads, e.g. as part of the requirements for obtaining an IEC turbine certificate.
The method is based on rapid-distortion theory. For the present application, atmospheric turbulence is modelled as an initially isotropic spectral tensor deformed by the shear of the mean wind profile.
A free Command-Line version of MANN 64bit turbulence generator version 1.0 release date 19.03.2014, may be downloaded from the web site: http://www.hawc2.dk/download/pre-processing-tools
1.1. Direct input
The following input arguments are required to execute the turbulence generator:
\(\Delta x\),\(\Delta y\),\(\Delta z\) |
Distance between grid point in X-direction (wind direction), Y-direction (horizontal and normal to wind direction) and Z-direction (vertical) |
\(Nx\),\(Ny\),\(Nz\) |
Number of grid points in X, Y and Z direction (power of 2) |
\(\gamma=3.9\) |
Gamma is the non-dimensional shear distortion parameter |
\(l\) |
Length Scale parameter, normally in the range \(0.7\cdot\Lambda_1\) to \(0.8\cdot\Lambda_1\) where \(\Lambda_1\) is the longitudinal turbulence scale parameter which for a hub height \(z\) is given by |
\(\alpha\epsilon^{2/3}\) |
AlphaEpsilon is a parameter in Mannâ€™s model. For best agreement with IEC61400-1 it can be calculated according to
where \(\sigma_{iso}^2\) is the un-sheared, isotropic variance. The relation between \(\sigma_{iso}\) and the standard deviation of the wind velocity in the mean wind direction. \(\sigma_{1}\) is to be taken as: \(\sigma_{iso}=0.55\cdot\sigma_{1}\) |
HF Compensation |
If HF compensation is applied, point velocity represents local anemometer measurements. Else, point velocity represents average in grid box. |
The necessary input data are described in IEC61400-1, third edition, ch. 6.3 Wind conditions and Annex B - Turbulence models.
Note that
1. The duration [\(T\)] of the generated turbulent wind is related to the point distance in X, number of points in X and the Mean Wind Speed at hub [\(U\)] as
2. The dimensions of the wind box, e.g. the length, width and height are given by
table(borderlessWhite).
| \(Lx=Nx\cdot \Delta x\)|
| \(Ly=Ny\cdot \Delta y\)|
| \(Lz=Nz\cdot \Delta z\)|
1.2. Derived input
The select box Derived input may be used for easier input specification. In this select box the following applies:
1. The relation between the Length Scale parameter [\(l\)] and the Longitudinal Turbulence Scale Parameter [\(\Lambda_{1}\)] is given by the Length Factor [\(f_{L}\)], i.e. \(l=f_{L}\cdot \Lambda_{1}\)
2. The length of the generated wind box [\(Lx\)] is calculated based on Wind Series Duration [\(T\)] and Mean Wind Speed [\(U\)] by the equation
\(Lx=T\cdot U\)
3. The un-sheared, isotropic variance is calculated according to
4. The AlphaEpsilon parameter is thus calculated according to
1.3. Scaling of generated wind field
The specified Turbulence Intensity [\(TI\)] is used to scale the generated turbulence series. The scale factor is established based on the averaged standard deviation of the wind series in the X direction of the 4 grid points closest to the turbulence box center (at the hub) and the specified standard deviation: \(\sigma_{1}=TI\cdot U\). All wind components are then scaled using the established scale factor.