The spectra in IEC61400-1 ed 3 is in inertial subrange

Mann turbulence for the IEC Code Comparison Collaborative (OC3) Kenneth Thomsen, Risø National Laboratory, Update 3 January 24, 2006 Two sets of turbulence fields are created using the IEC Turbulence Simulator in the WAsP Engineering model1, one at Vhub=Vr=11.4 m/s and one at Vhub=18 m/s. Both fields are simulated with the Mann model2 using the requirements from IEC61400-1 ed.3: =3.9 (non-dimensional shear distortion parameter) iso = 0.55 1 l=0.7 1 With a value of Iref =0.14 the following values of 1 are calculated: 11.4 m/s: 18.0 m/s:

1=1.981 m/s 1=2.674 m/s

1=42m and thus l=0.7 1 = 29.4 m The Mann model needs the parameter  2 / 3 which can be calculated from l and iso , see Appendix A: 11.4 m/s: 18.0 m/s:

1 2

 2 / 3 =0.18  2 / 3 =0.33

www.waspengineering.dk Mann, J. Wind Field Simulation, Prob. Engng. Mech., v. 13, n. 4, 1998.

The fields are simulated using 8192 longitudinal points in a 32 by 32 cross-sectional grid and saved in two different formats:  Rectangular format 32x32x8192  Polar format 16x64+1 The fields are simulated in the rectangular grid and afterwards interpolated into the polar grid with 16 radial points and 64 azimuthal points (and one center point). The length of fields corresponds to 700 seconds, i.e. for the 11.4 m/s case 7980m and for the 18 m/s case 12600m. The file formats is described in Appendix B and the echo output files from the simulations are given in Appendix C. For each wind speed, three files are provided: u, v and w component. Note that the turbulence values are saved in the polar format as scaled zero-mean values with a standard deviation of 1.0. This means that the actual values must be scaled to the proper value of turbulence intensity and mean value (for the u-component). Azimuth zero is down. For the rectangular format, the values are zero-mean (appr.) with the simulated standard deviation (close to the input value). Information on the point positions are included in the polar format files. For the rectangular field files, the distance between the points are given for all three directions in Appendix C. Note that the outer most points are separated by (Npoints-1)/Npoints * Fieldsize, i.e. 31/32 * 150 = 145.3m. This corresponds to (Npoints-1)* dL2. The grid points are the center points of the grid segments, which form the fields. Plots of center point signals are given in Appendix D.

Appendix A Note by Hans E. Jørgensen, Risø National Laboratory 2005. The spectra in IEC61400-1 ed. 3 is in inertial subrange described as 2 / 3

 2 l  S1 ( f )  0.4754  f 2 / 3   Vhub  In jakob’s model the spectra are described in wave numbers so V 2 S (k1 )  S ( f )  0.4754  iso l 2 / 3 k12 / 3 2 2 iso

(1.1)

(1.2)

when we compare Mann’s twosided spectra in inertia subrange with (1.2) we have that : 9 0.4754 2 2 / 3  2 / 3   iso l 55 2

(1.3)

 2 / 3 

55 2 0.4754  iso l 2 / 3 18

The parameter Alpha in Basic corresponds to  2 / 3 The parameter Gamma is similar to   3.9 in IEC61400-1 ed3 L is corresponding to 0.7-0.8  in IEC61400-1 ed3

Appendix B: From the WasP Engineering Help File

Turbulence fields Output format to FLEX4 The file output (*.int) is intended for the FLEX program. It has the following file format: month> 5x2-byte integer 40x1-byte character 5x2-byte integer … Nsx4-byte real 4x4-byte real NxNvx2-byte integer

no. of radial stations no. of azimuth stations no. of nodes no. of data points scaling factor for data radial positions [m] time step mean wind speed target turbulence intensity target turbulence length scale



Stored data are deviations from the target mean value and they are compressed to integers by the formula:



Turbulence intensity and length scale of simulations may differ slightly from their targets.

For each of batch simulation of turbulence an ASCII file (*.txt) is generated with the simulation settings. Below is shown an example of such a file:

Output format HAWC and Periodic Similar to the output format from FLEX4 there is *.txt file containing the information of the how the simulation has been generated. An example is shown below with the name "Hawc_sim.txt

The corresponding data output files are stored as binary files for each component i.e. Hawc_sim_xu.bin, where x is the seed number (an integer and) u is the component. The binary format is stored as a 4 byte real and below is described how the files for the components are written in the C language: int NComp; /* is the number of components of the velocity vector to be simulated */ int NPoint[3]; /* NPoint[0] is the number of points in the x-drection, NPoint[1] in the y-direction and NPoint[2] in the z-direction * float ***field3d[3]; /* Is a four-dimensional array containing the simulated field */ Memory for that is allocated by for (i=0;i